Nuclear Safety
NEA/CSNI/R(2016)5
May 2016
www.oecd-nea.org

International Iodine Workshop
Full Proceedings

For Official Use

NEA/CSNI/R(2016)5

Organisation de Coopération et de Développement Économiques
Organisation for Economic Co-operation and Development

30-May-2016________________________________________________________________________________________________________English text only
NUCLEAR ENERGY AGENCYCOMMITTEE ON THE SAFETY OF NUCLEAR INSTALLATIONSNEA/CSNI/R(2016)5
For Official Use
INTERNATIONAL IODINE WORKSHOP: Full Proceedings

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NEA/CSNI/R(2016)5

ORGANISATION FOR ECONOMIC CO-OPERATION AND DEVELOPMENTThe OECD is a unique forum where the governments of 34 democracies work together to address the economic, social andenvironmental challenges of globalisation. The OECD is also at the forefront of efforts to understand and to help governments
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NUCLEAR ENERGY AGENCYThe OECD Nuclear Energy Agency (NEA) was established on 1 February 1958. Current NEA membership consists of
31 countries: Australia, Austria, Belgium, Canada, the Czech Republic, Denmark, Finland, France, Germany, Greece, Hungary,
Iceland, Ireland, Italy, Japan, Luxembourg, Mexico, the Netherlands, Norway, Poland, Portugal, the Republic of Korea, theRussian Federation, the Slovak Republic, Slovenia, Spain, Sweden, Switzerland, Turkey, the United Kingdom and the UnitedStates. The European Commission also takes part in the work of the Agency.
The mission of the NEA is:– to assist its member countries in maintaining and further developing, through international co-operation, the scientific,
technological and legal bases required for a safe, environmentally friendly and economical use of nuclear energy forpeaceful purposes;– to provide authoritative assessments and to forge common understandings on key issues, as input to government
decisions on nuclear energy policy and to broader OECD policy analyses in areas such as energy and sustainable
development.
Specific areas of competence of the NEA include the safety and regulation of nuclear activities, radioactive waste
management, radiological protection, nuclear science, economic and technical analyses of the nuclear fuel cycle, nuclear law
and liability, and public information.
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tasks, the NEA works in close collaboration with the International Atomic Energy Agency in Vienna, with which it has a Cooperation Agreement, as well as with other international organisations in the nuclear field.

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COMMITTEE ON THE SAFETY OF NUCLEAR INSTALLATIONSThe NEA Committee on the Safety of Nuclear Installations (CSNI) is an international committee made up
of senior scientists and engineers with broad responsibilities for safety technology and research
programmes, as well as representatives from regulatory authorities. It was created in 1973 to develop andco-ordinate the activities of the NEA concerning the technical aspects of the design, construction andoperation of nuclear installations insofar as they affect the safety of such installations.
The committee’s purpose is to foster international co-operation in nuclear safety among NEA member
countries. The main tasks of the CSNI are to exchange technical information and to promote collaboration
between research, development, engineering and regulatory organisations; to review operating experienceand the state of knowledge on selected topics of nuclear safety technology and safety assessment; toinitiate and conduct programmes to overcome discrepancies, develop improvements and reach consensus
on technical issues; and to promote the co-ordination of work that serves to maintain competence innuclear safety matters, including the establishment of joint undertakings.
The priority of the CSNI is on the safety of nuclear installations and the design and construction of
new reactors and installations. For advanced reactor designs, the committee provides a forum for
improving safety-related knowledge and a vehicle for joint research.
In implementing its programme, the CSNI establishes co-operative mechanisms with the NEA Committee
on Nuclear Regulatory Activities (CNRA), which is responsible for issues concerning the regulation,
licensing and inspection of nuclear installations with regard to safety. It also co-operates with other NEA
Standing Technical Committees, as well as with key international organisations such as the International
Atomic Energy Agency (IAEA), on matters of common interest.

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ACKNOWLEDGEMENTS

WORKSHOP ORGANIZING COMMITTEE
JACQUEMAIN Didier
KISSANE Martin
VAN DORSSELAERE Jean-Pierre(IRSN, France)(NEA)(NUGENIA/SARNET)

WORKSHOP SCIENTIFIC COMMITTEE(MEMBERS OF THE ORGANIZING COMMITTEE AND)ALBIOL Thierry
DICKINSON Shirley
HERRANZ Luis Enrique
FUNKE Friedhelm
GLOWA GlennGUPTA Sanjeev
HOSHI Harutaka
HOTTA Akitoshi
KARKELA Teemu
LIND Terttaliisa
POWERS Dana
SALAY Michael
SONG JinHo(IRSN, France)(NNL, UK)
(CIEMAT, Spain)(AREVA GmbH, Germany)(CNL, Canada)(Becker Technologies, Germany)(NRA, Japan)(NRA, Japan)(VTT, Finland)(PSI, Switzerland)(SNL, USA)(USNRC, USA)
(KAERI, Korea)

FUNDING
The workshop was co-funded by the European Commission – Joint Research Centre, the French “Institut
de Radioprotection et de Sûreté Nucléaire” and the Nuclear Energy Agency (NEA) of the OECD.

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LIST OF ACRONYMSBDBA
CIEMAT
DBA
BIP
BWR
CNL
CRPPH
CSNI
DF(s)
EC
(F)CVS
FP
HBU
IRSN
ISTP
KAERI
LWR
MCCI
MOX
NEA
NNL
NRA
NPP
OECD
Org-I
PRG
PSI
RCS
SA
SAM(M)SARNETSFP
SNLST

Beyond Design Basis Accident
Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas (Spain)
Design Basis Accident
Behaviour of Iodine Project (a NEA joint project)
Boiling-Water Reactor
Canadian Nuclear Laboratories
NEA Committee on Radiation Protection and Public Health
NEA Committee on the Safety of Nuclear Installations
Decontamination Factor(s) (in FCVS)
European Commission(Filtered) Containment Venting System
Fission ProductHigh Burn-Up
Institut de Radioprotection et de Sûreté Nucléaire (France)
International Source Term Programme (a co-operative research programme)
Korea Atomic Enery Research InstituteLight-Water Reactor
Molten-Core-Concrete Interaction
Mixed OXide Fuel
Nuclear Energy Agency of the OECD
National nuclear laboratory (UK)
Nuclear Regulation Authority (Japan)
Nuclear Power Plant
Organisation for Economic Cooperation and Development
Organic Iodides
Programme Review Group (of the BIP or STEM joint projects)
Paul Scherrer Institute (Switzerland)
Reactor Coolant System
Severe Accident
Severe-Accident Management (Measures)
Severe-Accident Research NETwork
Spent-Fuel Pool
Sandia National Laboratories (USA)
Source Term
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STEM
TGT
THAI
USNRC
VTT
WGAMA
WPNEM

Source-Term Evaluation and Mitigation project (a NEA joint project)
Thermal Gradient Tube
Thermal hydraulics, Hydrogen, Aerosol, Iodine project (a NEA joint project)
United States Nuclear Regulatory Commission
Technical Research Centre of Finland
CSNI Working Group on Analysis and Management of Accidents
CRPPH Working Party on Nuclear Emergency Matters

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EXECUTIVE SUMMARY

The international Iodine Workshop organized by the French “Institut de Radioprotection et de Sûreté
Nucléaire”, the Nuclear Energy Agency of the OECD, the NUGENIA-SARNET network and the European
Commission through the Joint Research Centre (Petten) was held in Marseille (France) from 30 March to 1
April 2015.
The scope and objectives of the workshop were to: review and discuss the outcomes of recently concluded and on-going national and international
research programmes related to iodine and ruthenium behaviour during NPP accidents including DBA(Design Basis Accident) and BDBA (Beyond Design Basis Accident);
 review and discuss the associated progress made in related modelling in NPP accidents;
 review and discuss the implications for source term evaluations and assessment of accident
management measures (for radiological impact reduction through, e.g., FCVS implementation); identify safety-significant issues in the field not yet sufficiently covered that would induce gaps or
potential weaknesses in accident analysis and assessment of accident management measures,
including for the long-term phases of an accident;
 share source-term evaluation results and related uncertainties assessments;
 contribute to the dissemination of knowledge gained in the field and promote the best use ofknowledge updates for accident analysis and management.
The workshop enjoyed the participation of 76 experts from research organizations, industry, safety
authorities and their technical safety organizations from Belgium, Canada, Finland, France, Germany,
Italy, Japan, Poland, Republic of Korea, Spain, Sweden, Switzerland, UK, USA and two internationalorganizations EC and NEA. Thirty-one technical papers were presented.
The workshop objectives relative to reviewing and discussing the state of the art knowledge of iodine
and ruthenium behaviour in NPP accidents gained through research projects and progress made in therelated modelling were well fulfilled. In addition, highlighting remaining open issues and assessing theircoverage through on-going and planned research activities was well considered with, in particular,
discussions concerning the NEA BIP (Behaviour of Iodine Project)-3, STEM-2 and THAI-3 follow-on
proposals and recommendations for future work. The workshop also contributed, and will continue
contributing through publication of the summary report, to knowledge sharing and dissemination in thefield.
However, contributions concerning applications of research results for improving accident analyses
and management were limited in number and scope. This led workshop experts to formulate
recommendations to foster international exchanges and actions to assess more thoroughly implications ofprogress on ST research for NPP accident management; in addition to this, recommendations were also
made for further investigations on some technical issues.

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Main conclusions by topic
Iodine and ruthenium release from fuelThe existing large database on FP release from fuel highlights that volatile FPs (I, Cs, Te) tend towards
complete release in SAs involving significant fuel degradation while release of semi-volatile FPs (Mo, Ba,Ru) is strongly dependent on oxido-reducing conditions, with Mo and Ru release tending to be large inoxidizing conditions. Semi-volatile FP-release modelling has still to be improved. Recently obtained data
are challenging hypotheses used in accident analyses for volatile FPs (notably Cs) release for DBA, BDBA
with limited fuel degradation and for FP release in oxidizing conditions.
Iodine and ruthenium transport in the RCS
Much progress was made on understanding and modelling of gas-phase iodine chemistry in the RCS. SA
system codes benefited from model developments related to the effect of Mo on Cs and I chemistry andtransport. Data were generated to treat the kinetics and thermodynamic modelling of influential reactions
for the Cs, Mo, B, I, H, O chemical system. The kinetic database is being extended to treat Ag, In and Cd
effect on iodine transport and chemistry. All these developments aim at providing better predictions of thegaseous iodine fractions at the RCS break in a SA – which is highly scenario dependent and affected bycarrier gas composition, compounds resulting from control rod degradation and other FPs - and reduce
related uncertainties on iodine ST evaluations.
Some progress in understanding Ru transport was obtained from experimentation. The issue is todevelop models able to calculate Ru re-emission from RCS deposits as Ru rapidly deposits on RCS
surfaces after its release from the fuel. The Ru source to the containment would then be very dependent onsuch re-emission processes. However, experiments closer to reactor conditions are necessary to provide
data for the development of applicable models.
A remaining important issue is the development of a pragmatic research approach to tackle complex
heterogeneous processes (interactions of gas species with surfaces and aerosols in the RCS) and assess theeffect of re-suspension/re-volatilization/decomposition of deposits resulting from mechanical, thermal anddose loadings. These may be important delayed sources of FPs (I, Cs and Ru) to the containment
potentially contributing to the ST in later stages of the accident, notably in case of FCVS use. Limited
experimentation on Cs and Ru re-vaporization processes was and is being performed. This is proposed tobe continued for Ru in the STEM-2 project. However, performing reactor-relevant experiments anddeveloping models still appear challenging due to the complexity of the involved processes and theimportance of dealing with representative surface states and deposits.
A test was performed to investigate gaseous iodine release for SGTR accidents. Calculations haveshown the lack of validation of existing iodine chemistry models in the aqueous phase for DBA conditionsin the RCS (low doses and concentrations, high temperatures, additives effects).
Iodine and ruthenium behaviour in the containment
The knowledge gained on the containment gas phase (aerosols and gases/vapours) during all stages of theaccident should help assess releases through containment leaks and through FCVS. Following PHEBUS
FP, research focused on gas-phase and heterogeneous processes (interaction of gaseous iodine with paints
and organic iodides (Org-I) production, with reactive aerosols, iodine-oxide (IxOy) particle
formation/decomposition, gaseous iodine release by decomposition of deposited aerosols by radiation).
The research was recently conducted in the ISTP, NEA BIP-1 and BIP-2, THAI-1 and THAI-2 and STEM
projects. Gas-phase processes are reasonably well covered by past, on-going and planned research (withthe proposed NEA BIP-3, STEM-2, THAI-3 follow-on projects) with a focus on inorganic gaseous iodine
species, IxOy, Org-I and gaseous ruthenium tetroxide (RuO4) behaviour. Significant progress has been
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made through all these projects on the understanding and modelling of such processes. Part of the gained
knowledge is implemented in SA system codes.
Following the Fukushima accident, considering potential long-term loss of containment heat removal
systems, questions were raised on FP remobilization from deposits on containment surfaces and fromsumps/suppression pools during long-term stages of a SA, notably in relation to assessing FP releaseduring containment venting. This contributed to the definition of research projects intending to increaseknowledge on such processes. This is included in the BIP-3, STEM-2 and THAI-3 projects.
Less work was recently performed on containment aqueous-phase chemistry in SA as the main sourceof volatile iodine was considered to be in the gas phase. The effect of impurities in sumps on iodine
volatility was investigated, notably showing a low effect for chlorine and generating data to model
nitrate/nitride effects. Recently, following Fukushima’s accident, questions were raised as to the effect of
seawater compounds on water-phase chemistry concerning the FP-scrubbing efficiency in suppression
pools and in liquid-type FCVS considering evolving hydrodynamics and chemical conditions during theaccident and considering the long-term aqueous-phase chemistry in relation to long-term accident
management (corrosion reactions and leaching of corium/debris).
The effect of seawater is currently investigated in Japan with possible effects of bromine on iodine
chemistry. Work in this field will continue in the coming years to develop the corresponding models.
The effects of evolving hydrodynamic and chemical conditions on FP pool scrubbing efficiency insuppression pools and FCVS during a SA were, are or will be partly investigated. However, the modellingof hydrodynamics, with existing modelling unable to represent flow instabilities which may strongly affectFP scrubbing efficiencies, has to be improved. There are presently only limited concerted research actionsin the field, with the notable exception of the EU-PASSAM project covering some aspects, and a larger
collaboration would certainly be beneficial to progress on scrubbing modelling.
FP Filtration in FCVS
Important efforts were led in the past for the development and qualification of FCVS but questions remain
as to the efficiency and robustness of such systems in SA conditions as they may be envisaged postFukushima. For some countries where safety criteria associated with releases are more stringent, there is asearch for more efficient filtration to further reduce radiological consequences. Such issues, with theassessment of innovative filtration technologies, are being covered in current research projects. There arealso some specific FCVS national developments notably in Japan and in the Republic of Korea.
The knowledge-base related to “delayed” in-containment re-volatilization and re-suspension processesof radionuclides from RCS and containment surfaces and containment pools is being extended through ongoing and future research programmes.
As for filtration, besides aerosols, specific attention is being given to Org-I and IxOy particles as they
may contribute significantly to the ST in some accidents. Possible contribution of ruthenium-oxide species
to the ST is also under investigation.
Iodine chemistry in the environment
Little attention was given to iodine chemistry in the environment for the assessment of its dispersion andrelated radiological consequences. Due to the complexity of the chemical systems to be treated and the lackof validation of the existing preliminary modelling, the potential impact of iodine chemistry in theenvironmental on radiological consequences has to be further assessed. If the impact is shown to be strong,a pragmatic approach to model it will have to be developed.

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SA system codes ST models validation
Questions were raised about the level of detail required in models for system codes and the effect of
uncertainties in the models on the calculated ST and consequences. For “end-user” applications such asradiological consequence assessments and emergency planning, the models have to be simple andconservative. However, even the apparently simple models have to be based on a good understanding andrepresentation of the iodine behaviour, sometimes at the level treated by the more detailed models, in orderto ensure that the important phenomena are correctly taken into account and scaled.
Reference ST evaluations and emergency response tools
Final objectives of the ST research are to contribute to the consolidation of reference ST calculations used,notably, for design of protection measures and of fast-running calculation tools used to support emergency
response. The question of the methodology of implementation of ST-research outcomes into these tools
and of the assessment of their robustness remains a key issue.
Main recommendations
Experts consider that models able to better predict the kinetics of FP release from fuel would be valuablefor the evaluation of consequences of accidents with limited fuel degradation (incl. DBA) and withoxidizing conditions (incl. SFP) and their management. Analyses of existing data (e.g., VERDON tests)
should be deepened to propose future research in the field.
Experts recommend conducting further assessments of the importance of oxidizing accident scenarios
which could result in significant Ru release to the environment.
Experts recommend designing research to test the validity of aqueous-phase iodine-chemistry models
in DBA conditions.
Experts recommend conducting further assessments to assess the potential effect of “delayed” FP reemission from RCS and containment surface deposits and sumps on ST evaluations and focus the researchin the field to be relevant to reactor applications, integrating existing knowledge, on-going and planned
research.
For scrubbing FCVSs, the effect of evolving hydrodynamic and chemical conditions in SAs on
scrubbing efficiency requires further investigations. Experts also recommend fostering collaborations on
pool-scrubbing model development in relation to assessing FP retention in suppression pools and in FCVSs
in SAs. Such models should also be of relevance for FP retention in SGTR.
Experts recommend strengthening collaborative analytical activities, if possible joined integrated
activities between projects, using the available international programmes results, highlighting that suchactivities were in the last years limited in scope.
Experts recommend organizing further exchanges to deepen the assessment of the validity of STrelated models implemented in SA system codes notably by addressing: the definition of a reference code-validation matrix for iodine behaviour in the RCS and containment; the necessary level of detail for the modelling of the complex iodine behaviour for plant calculations;
 the model validation for plant calculations (scaling issue) by performing, e.g., a SA system-code
benchmarking exercise for defined reactor-like scenarios notably to assess the impact of recent
developments of chemical models.
Experts recommend strengthening knowledge exchange between the ST-research and theenvironmental-chemistry communities.

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Concerning ST calculations, the experts recommend strengthening exchanges on source term
evaluations performed for NPPs in various countries (results and methodologies) and on quantification ofremaining uncertainties on the related modelling, this in view of checking that the ST research is properly
focused and no key issue overlooked.
Concerning fast-running tools for emergency response, the workshop experts recommend
strengthening links between the ST-research community and the emergency response ‘end-users’.

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TABLE OF CONTENTS

LIST OF ACRONYMS ...................................................................................................................................7
EXECUTIVE SUMMARY .............................................................................................................................9TABLE OF CONTENTS ..............................................................................................................................15
BACKGROUND AND SCOPE OF THE WORKSHOP..............................................................................17
PARTICIPATION IN THE WORKSHOP....................................................................................................19
SUMMARY OF TECHNICAL SESSIONS .................................................................................................21
Session 1:main lessons from experimental programmes on iodine and ruthenium
behaviour in a severe accident and research perspectives ........................................................21
Session 2:iodine transfer to aqueous phase, aqueous-phase chemistry......................................................27
Session 3:organic iodide formation and partitioning, chemistry and interactions
with aerosols, droplets and surfaces in the gas phase ...............................................................29
Session 4:filtered containment venting systems ........................................................................................33
Session 5:recent developments in iodine chemistry modelling in severe accidents
and applications to source-term and radiological-consequences evaluations ...........................35
SUMMARY STATUS REPORTS OF NEA BIP-2 AND OECD/STEM PROGRAMMES ........................37
NEA BIP-2 and follow-up - G. Glowa (CNL) ...........................................................................................37
NEA STEM and follow-up - C. Mun (IRSN) ............................................................................................38
SUMMARY, CONCLUSIONS AND RECOMMENDATIONS .................................................................43
APPENDIX 1: WORKSHOP AGENDA ......................................................................................................51
APPENDIX 2: LIST OF PARTICIPANTS...................................................................................................55
APPENDIX 3: PAPERS BY TECHNICAL SESSIONS ..............................................................................65
Session 1:main lessons from experimental programmes on iodine and ruthenium
behaviour in a severe accident and research perspectives ........................................................67
Session 2:iodine transfer to aqueous phase, aqueous-phase chemistry....................................................181
Session 3:organic iodide formation and partitioning, chemistry and interactions
with aerosols, droplets and surfaces in the gas phase .............................................................213
Session 4:filtered containment venting systems ......................................................................................275
Session 5:recent developments in iodine chemistry modelling in severe
accidents and applications to source term and radiological consequences evaluations ..........339

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BACKGROUND AND SCOPE OF THE WORKSHOPThe international OECD/NEA-NUGENIA-SARNET Iodine Workshop was organized jointly by the
French “Institut de Radioprotection et de Sûreté Nucléaire” (IRSN), the Nuclear Energy Agency (NEA) of
the OECD, the NUGENIA-SARNET network and the European Commission through the Joint ResearchCentre (JRC) of Petten. It was held in Marseille (France) from March 30, 2015 till April 1, 2015.
Stakeholders involved in improving understanding of iodine behaviour and mitigating iodine release in a
nuclear power plant (NPP) accident judged appropriate to organize such an event in view of: the recent termination of many research initiatives in the field: the International Source Term
Programme (ISTP), the NEA Behaviour of Iodine Project phase 2 (BIP-2), the NEA Thermalhydraulics, Hydrogen, Aerosol, Iodine Project phase 2 (THAI-2), the European Commission (EC)Severe Accident Research Network of Excellence phase 2 funded within the 7th Framework
Programme (SARNET-2 in FP7), which all ended in 2014, and the NEA Source Term Evaluationand Mitigation Project phase 1 (STEM-1), which will end in June 2015;
 the recent launching, driven notably by the Fukushima accident, of research initiatives related tothe design or improvement of filtered containment venting systems (FCVSs), which provide ameans to preserve the containment and mitigate radioactive releases when an accident at a NPP
results in containment over-pressurization: the EC PASSAM FP7 project, the French MIRE project
which were launched in 2013 and many other national initiatives notably in Europe, Japan,
Switzerland and the Republic of Korea;
 the on-going elaboration of future research international ventures, notably in the frame of the NEA
safety research projects: BIP-3, STEM-2 and THAI-3;
 questions raised after the Fukushima accident relative to iodine releases and related radiologicalconsequences.
The scope and objectives of the workshop were to: Review and discuss the outcomes of recently concluded and on-going national and international
research initiatives (PHEBUS FP/ISTP, SARNET-2, NEA BIP-2, STEM and THAI-2, EC
PASSAM, national projects, etc.) related to iodine – and, for some of them, ruthenium – behaviour
during NPP accidents including design basis accidents (DBA) and beyond design basis accidents(BDBA);
 Review and discuss the associated progress made in iodine and ruthenium chemistry modelling in
NPP accidents;
 Review and discuss the implications of the main research outcomes (experiments and model
development/assessment) for source term evaluations and assessment of accident management
measures (for radiological impact reduction through, e.g., the implementation of FCVS) both for
DBA and BDBA;
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 Highlight open issues of safety significance in the field and confirm that they are well-covered byon-going or planned research activities, notably through the NEA BIP-3, STEM-2 and THAI-3
proposals. Check that these activities are as far as possible conducted in an integrated andoptimized way (e.g., in facilities with complementary capabilities); Contribute to the dissemination of knowledge gained in the field;
 Share recently updated knowledge on iodine and ruthenium chemistry (as well as on chemistry ofother elements - fission products and structure materials - that will affect it) in NPP accidents,
including release from the fuel, transport in the reactor coolant system, behaviour in thecontainment and release to the environment through containment leak paths or containment
venting systems (CVS), filtered or unfiltered; Promote the best use of knowledge updates for accident analysis and management; Share source term evaluation results made in various countries (e.g., as part of deterministic or
PSA level 2 studies) and related uncertainties assessments;
 Identify safety-significant issues in the field not yet sufficiently covered that would induce gaps or
potential weaknesses in accident analysis and assessment of accident management measures,
including the consideration of the long-term phases of an accident.
Because one of the objectives of the workshop was to review and discuss the main outcomes of thecompleted NEA BIP-2 project and the finishing STEM project (last test completed in April 2015) as wellas the pertinence of the follow-up research initiatives, the BIP-3 and STEM-2 projects, the workshop
should also fulfil the objectives of the final summary workshop for the BIP-2 and STEM projects.

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PARTICIPATION IN THE WORKSHOP

Thirty-one technical papers were presented in five technical sessions (see the agenda in
Appendix 1). Authors were members of research organizations, industry, safety authorities and theirtechnical safety organizations. The workshop enjoyed the participation of 76 experts from Belgium,
Canada, Finland, France, Germany, Italy, Japan, Poland, Republic of Korea, Spain, Sweden, Switzerland,
United Kingdom, United States of America and two international organizations EC and NEA (participants
are shown in Appendix 2). The technical papers are provided in Appendix 3.

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SUMMARY OF TECHNICAL SESSIONS

Session 1: main lessons from experimental programmes on iodine and ruthenium behaviour in asevere accident and research perspectives
Session Chairs: T. Albiol (IRSN), H. Hoshi (NRA)
Session 1 consisted of ten presentations. Main outcomes are reported hereafter as well as chairmen viewson pending issues.
1.1 – Main outcomes relative to iodine behaviour from the VERDON/ISTP programme - Y. Pontillon(CEA) et al. Four VERDON FP release tests were successfully performed by CEA from 2011 to 2014 in
the frame of the ISTP programme. The programme objectives were to study (1) FP release from high burnup UO2 and MOX fuels, in particular for low volatile FP and (2) impact of air ingress on FP behaviour
which is expected to enhance the release of highly radiotoxic FP, such as ruthenium, as well as to revolatilise volatile FP (iodine, caesium) previously deposited in the RCS. Three VERDON tests covering
the first topic were performed: VERDON-1 with UO2 fuel at 72 GWd/t in reducing conditions, VERDON3 and VERDON-4 on a 60 GWd/t MOX fuel respectively in steam (oxidizing) and hydrogen (reducing)conditions. The VERDON-2 test, covering the air ingress topic, was performed on a 60 GWd/t MOX fuel,using a more complex experimental circuit including thermal gradient tubes (TGTs) downstream thefurnace where the fuel sample is heated. The main outcomes of the experimental programme related toiodine and caesium behaviour are:
 earlier and faster releases of highly volatile FP (I, Cs) – also for semi-volatile FP such as Mo andBa - for MOX and high burn-up (HBU) UO2 fuel compared to average burn-up UO2 fuel - releases
are the fastest with MOX fuel with in particular larger than expected caesium releases measured at
the 1200°C temperature plateau in VERDON 3 and 4 tests;
 faster releases of highly volatile FP with increasing oxygen potential of the carrier gas – the carriergas oxygen potential also strongly affects semi-volatile FP releases such as that of Mo and Ba, Mo
release being promoted in oxidizing conditions (Mo behaviour is underlined there as Mo isaffecting Cs and I chemistry in the RCS); re-vaporisation of iodine deposited in the TGT during the air ingress phase of VERDON 2;
 low gaseous iodine fractions (~ 1% of the iodine inventory) were measured in VERDON-3 and 4(MOX fuels) where release started at very low temperature (~ 300°C) and accelerated above ~600°C.
An additional air ingress test, VERDON-5, will be performed in 2015 with HBU UO2 fuel (same fuelas VERDON-1) with similar experimental arrangements as the VERDON-2 test, to compare the ruthenium
release and FP re-volatilisation with those obtained for MOX fuel in air.
These results of the VERDON programme are presently under analyses. However, they raisequestions as to the hypotheses one should consider for the volatile FP releases for design basis accidents
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NEA/CSNI/R(2016)5(DBA), beyond design basis accident (BDBA) with limited fuel degradation (e.g., when core cooling canbe recovered before significant fuel melting) and accidents in oxidizing conditions (including loss ofcooling in spent fuel pools (SFPs)).
1.2 – Research on the transport and chemistry of iodine in the primary circuit and containment conditions
at VTT - T. Karkela (VTT) et al. The behaviour of iodine in a SA is studied at VTT as regards RCS
chemistry, containment chemistry and filtration (in relation to FCVS use). It was shown that FP deposits
on RCS surfaces can act as a source of gaseous iodine (long-term source), that gaseous iodine (inorganicand organic) can transform in the containment to aerosol particles due to oxidation (by reacting with air
radiolysis products), that particle deposits on containment surfaces can act as a source of gaseous iodine(even in the absence of radiation) and that particulate and gaseous iodine can be filtered with a modified
Wet Electrostatic Precipitator (WESP) filter. All those processes that may significantly impact the iodine
source term are still the subject of research.
1.3 – Main findings of the IRSN experimental programmes performed on iodine chemistry in severe
accident conditions – A.C. Grégoire (IRSN) et al. IRSN performed the CHIP programme in the ISTP frame
to study iodine chemistry in the RCS during a SA. The programme was terminated in 2014 and was
conducted notably to explain the unexpected gaseous iodine fractions measured in the PHEBUS FP tests inthe containment during the fuel degradation. The results supported the development of a comprehensivemodel for the homogeneous gas phase chemistry of the (I, Cs, B, Mo, O, H) system able to reproduce
PHEBUS tests observations. A proper estimation of the gaseous iodine fractions in the RCS necessitatestreatment of kinetic limitations, of the effect of B and Mo on I and Cs chemistry (Mo playing a major rolein oxidizing conditions by the formation of CsMoO4) and of the effect of the carrier gas composition –notably the relative H2/H2O content. All this is now included in the ASTEC system code and partly in othersystem codes (MELCOR). On-going work concerns the effect of Ag-In-Cd control rod elements on iodine
chemistry in the RCS (CHIP+ programme which started in 2014).
IRSN performed experiments in the frame of the ISTP and NEA STEM programmes in the EPICUR
facility to study iodine chemistry in the containment during a SA. Experimentation in the STEM
programme related to iodine has recently been completed. These programmes were conducted to gain
knowledge on organic iodides and iodine oxides formation/decomposition in the containment atmosphereand on radio-induced iodine re-volatilisation from aerosols deposited on containment walls. These
processes were identified from PHEBUS FP tests analysis as potential major contributors to the gaseousiodine remaining in suspension in the containment. The results supported the development of a model forthe organic iodides formation/decomposition which is now included in the ASTEC system code. On-goingand future work are now focusing on the effect of paint ageing on organic iodides formation, on thedeposited aerosols and iodine oxides stability under radiation (NEA STEM2 proposal, see 1.10) and on
medium-term iodine releases by re-vaporisation of RCS deposits (French MiRE programme).
1.4 – THAI Experiments on volatility, distribution and transport behaviour of iodine and fission productsin the containment - S. Gupta (Becker) et al. The THAI facility provided a significant amount of data on
coupled-effects tests investigating thermal-hydraulics, hydrogen combustion risk and mitigation, aerosol
and iodine behaviour in a LWR containment. Based on THAI experimental results, progress has been
demonstrated in modelling of aerosol and iodine behaviour and their coupling with containment thermal
hydraulics in severe accident analysis codes, such as COCOSYS-AIM, ASTEC-IODE, MELCORINSPECT, and others. The improved models have demonstrated reliable simulation of complex
experiments, e.g., analysis of NEA THAI2 test investigating molecular iodine interaction with non-reactive(SnO2) and reactive (Ag) aerosols, and EU-SARNET/SARNET2 code-benchmark exercises involving
THAI data on iodine/surface interactions, iodine mass transfer, iodine oxides behaviour and iodinetransport in multi-compartment behaviour.

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A follow-up of the NEA THAI2 project is currently under discussion. Hydrogen-related investigations
on PAR performance in counter-current flow conditions and hydrogen deflagration tests in twocompartment system are proposed. Additionally, in light of the Fukushima Daiichi accident, experimental
investigations are foreseen to study the re-entrainment of fission products (aerosols and gaseous I2) fromwater pool at elevated temperature due to continuous heat-up of pool or depressurization-induced (venting)boiling. Another planned experiment is related to “delayed” source term in order to investigate resuspension of aerosols as well as iodine deposits from steel/painted surfaces due to hydrogen combustion.
1.5 – A summary of the PSI investigations on iodine chemistry in the presence of impurities and additives T. Lind (PSI) et al. PSI experimental methods include lab-scale facilities (also in hot cell), lab-scale bubble
column and intermediate and full height Filtered Containment Venting System (FCVS) facilities. PSI
experimental programmes investigated iodine retention in FCVS, organic iodides (CH 3I) decomposition byhydrolysis and radiolysis and effect of impurities (N2O, NO3-, Cl-) on the iodine volatility. Data on organic
iodides decomposition were used to develop/validate the corresponding models for system codes. Data on
effect of N2O, NO3- on iodine volatility were reported in the SARNET frame and are considered adequate
to develop the corresponding modelling. Chloride ions were shown to have little or no effect on the iodine
volatility. The latest research focused on the use of additives for improved retention of organic iodides inliquid FCVS.
1.6 – Recent Findings on Ruthenium Chemistry in a Severe Accident - T. Karkela (VTT) et al. An
overview of recent experimental results from VTT, IRSN, CEA, MTA EK and CHALMERS (integral,semi integral and separate effect experiments) was presented. The chemistry of radiotoxic ruthenium in asevere nuclear power accident has actively been investigated especially during the last decade. The Ru
studies have covered the release from the fuel, the transport in the primary circuit and the behaviour in thecontainment building. The gathered experimental data have been used to understand the key parameters
governing the Ru chemistry in a severe accident (SA) and to develop/validate models for SA analysis
codes. The collaboration on the international level in the frame of SARNET and SARNET-2 has been
intensive on the subject. Detailed studies on separate phenomena have been conducted, e.g., as part of
ISTP and NEA STEM programmes. Furthermore, PHEBUS FP tests have produced valuable data on
integral phenomena.
The large-scale integral and semi-integral experiments have confirmed that Ru release depends
strongly on carrier gas composition. Ru may be largely released from an irradiated fuel sample whenconditions result in significant fuel oxidation, this may occur in particular when steam-rich gas or air is
involved. In addition, the oxidation of UO2 fuel seems to lead to a higher and faster Ru release than in caseof MOX fuel as evidenced by VERDON results. As mentioned earlier, an additional test, VERDON5 will
be performed in 2015 to confirm this difference.
If Ru is released from the fuel in significant amount, it was shown both from analytical and integral
tests to interact and deposit fast on surfaces offered by RCS piping (with and without FP deposits) andaerosol particles (e.g., Ag containing particles). In the containment building, most of the ruthenium is
expected to end up as deposits on the containment surfaces and in the sump. However, even if the majorpart of Ru is deposited or in particulate form after its release from the fuel, analytical experiments
evidenced higher gaseous ruthenium fractions (in the form of RuO4, stable at low temperature) than what
can be expected form thermodynamic equilibrium calculation. Oxidizing conditions in the RCS (presenceof air) and in the containment (presence of oxidant air radiolysis products) favour ruthenium oxidation tohigher oxidation states, including the gaseous RuO4 form.
Additional investigations are proposed as part of the NEA STEM2 proposal (see 1.10) to quantify
whether significant fractions of gaseous ruthenium (RuO4) may be transferred to the containment andreleased to the environment through leaks or FCVS and have a significant radiological impact.

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1.7 – Thoughts on Ruthenium – M. Salay (USNRC) et al. Research on ruthenium focus on its transport
once released from the fuel and provides considerable information on RuO4 behaviour. However, it has yet
to be demonstrated that Ru release under prototypic conditions could substantially affect source term.
Research seems to be lacking in some areas such as detailed analyses of scenarios that could lead to air
ingress, transport of air to fuel rods and fuel itself and clad and fuel relocation behaviour.
1.8 – The Main Outcomes of the NEA Behaviour of Iodine Project – G. Glowa (CNL) et al. The main fieldof investigation of the phase 2 of the BIP programme which ended in 2014 was the interaction of iodinewith paint and organic iodides formation in the containment during a severe accident. In the first phase of
the project, adsorption onto epoxy paint – that are commonly used as coatings in NPPs - and formation of
CH3I from irradiated epoxy paint were investigated. The important role of water in these processes wasevidenced. BIP-2 investigated the same processes, with emphasis on developing a more mechanistic
understanding of organic iodide production during irradiation of iodine-loaded paint. Epoxy paint is acomplex mixture of polymeric compounds and it is difficult to know where and how iodine is interacting
and how methyl iodide is formed.
It can be concluded from BIP-2 results that while iodine seems to preferentially associate with thenitrogen-containing functional groups (amines, amides) in the paint, reactions with many parts of thecomplex paint structure are possible. Obtained results can help explain differences in sorption behaviour
between various epoxy paints. While iodine may react with various chemical compounds on and within
the paint, the resulting organic iodides can break down under radiation to form methyl iodide. The methyl
iodide formation does not appear to be a specific reaction associated with a particular bonding site orfunctional group, but rather seems to be a common degradation product during a complex free radical
degradation process. These results can be used to guide the development of computer models and provide
data for their validation.
The data shows that there are similarities in the iodine-paint interaction between the epoxy paints
studied, which allows some optimism for the development of a generic iodine-paint model. However, theresults also suggest that the treatment of the paint (e.g., pre-irradiation), and differences in the chemical
makeup of the paint (i.e., type of hardener) can affect its behaviour with respect to iodine. It is proposed toinvestigate remaining issues regarding organic iodides formation in the containment in the frame ofcomplementary NEA BIP-3 and STEM-2 initiatives (see 1.9 and 1.10).
1.9 – The Behaviour of Iodine Project: A Proposal for BIP-3 – P. Yakabuskie (CNL) et al. Recently, there
have been several NEA initiatives to study the behaviour of iodine in containment: the THAI (THAI-1 and
THAI-2), STEM and BIP (BIP-1 and BIP-2) projects. Despite considerable advancements in theunderstanding of this topic over the past decade, there are still outstanding gaps in knowledge that warrant
further investigation. Based on acquired knowledge, CNL is proposing in a phase 3 of the BIP project tofurther explore the effects of paint ageing and pre-irradiation, total dose and dose rate, humidity andwetting of painted surfaces and the presence of competing species (e.g., NOx, Cl2) on adsorption anddesorption processes. Radiolytic studies relating to CH3I production and degradation in the gas phase areas well proposed. Stronger analytical activities than in the BIP-2 project are foreseen to promote the use ofexperimental data for model development/validation and ST computations. The BIP-3 project is expected
to run for a 3-year term from mid-2015 to mid-2018.
1.10 – NEA STEM Project and its Follow-up STEM-2 – C. Mun (IRSN) et al. IRSN is proposing a phase 2
for the STEM project focusing on unresolved relevant Source Term (ST) issues after STEM 1, BIP-2 and1

It was agreed, prior to the STEM project start, to split the project into two phases, as the initial proposal was toolarge to be covered in 4 years. The present STEM phase 2 proposal considers the initial proposal and outcomes of
STEM phase 1 and BIP and THAI phase 1 and 2 projects.

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NEA/CSNI/R(2016)5
THAI-2 projects completion: effect of paint ageing (normal operation and pre-irradiation on iodine
trapping
andreleaseunder
irradiation(proposed
testingcomplementarytoBIP-3); effect of irradiation on stability/destruction of iodine aerosols (iodine oxides as well as multicomponents iodine aerosols); ruthenium re-vaporisation from RCS deposits with more prototypic
conditions than in the previous STEM phase (study of oxidizing conditions and representative RCS
surfaces to develop predictive models for ST evaluations). An Analytical Working Group should be set up
to allow stronger analytical activities than in the previous STEM phase and to promote the use of theexperimental results in some ST computations. This STEM follow-up project is expected to run for a fouryear term from mid-2015 to mid-2019.
Conclusion and recommendations from the Chairmen
Knowledge on iodine and ruthenium chemistry in a severe accident has progressed considerably throughvarious experimental programmes in the last decade. A large data set has been produced that is not yet
fully integrated by analysts. One limitation is the restricted access to some of the produced data.
Collaborative work for the development/validation of related models for severe-accident system codes –such as ASTEC, MAAP, MELCOR - and for the assessment of gains in accuracy of source-term
evaluations should be reinforced. The necessary level of detail for the modelling of the complex behaviourof iodine and ruthenium and the model validity for plant application (scaling issue) should be further
addressed. It could be recommended to develop a standard code-validation matrix related to iodine
behaviour in a severe accident. Sharing the outcomes of performed and future international collaborative
programmes as well as knowledge transfer to new generations appears to be of key importance.

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Session 2: iodine transfer to aqueous phase, aqueous-phase chemistry
Session Chairs: F. Funke (AREVA GmbH), J. Song (KAERI)
Session 2 consisted of three presentations, one about fission product transport in the containment in wet
conditions, and the other two about formation of volatile iodine by seawater radiolysis in containment
sumps in severe accident conditions, an issue raised by the Fukushima Daiichi accident.
2.1 – Iodine and Silver Wash-Down Modelling in COCOSYS-AIM by Use of THAI Results –G. Weber (GRS) et al. G. Weber (GRS) presented new and revised models of fission products transportand reactions at wet walls or in sumps, using data from THAI projects. The models are already or will be
implemented in COCOSYS. I2 deposition onto wet paint during steam condensation is described by a newwater layer model. The model considers all relevant chemical reactions in the water layer as well as iodine
adsorption onto paint covered by water layer either in the form of film or rivulets. The model was
successfully checked on the THAI test Iod-24, where two different wall condensation rates on two painted
surface areas were established at the same time. For the wash-down on non-soluble aerosol particles bydraining condensate, the new model AULA was developed, based on a model for sediment transportapplied in geology. Particles erode and are washed down when a critical flow velocity is exceeded. Twoflow types are considered, rivulets and films. For a first validation, laboratory tests performed in support ofthe large-scale THAI test AW-3 were analysed with AULA. The new AULA model is able to calculate
erosion and wash-down of non-soluble aerosols by rivulets and closed films. First results indicate that
wash-off from floors is not efficient in accident conditions. It was concluded that further wash-down tests
are required with a realistic reactor typical aerosol of compact particles consisting of soluble and insoluble
materials. The reaction model of silver with molecular iodine (I2) in sump was enlarged by implementing a
new parameter, the reactive surface of silver particles in a sump. The reactive surface is reduced when Ag
particles are deposited on the sump bottom. A post-test calculation of the THAI test AW-3, where
suspended and precipitated silver particles were contacted with I2, agreed well with the results.
Highlights from the discussion were (i) the need for an appropriate nodalization of zones to model therivulets and films, and (ii) that the fission-product behaviour at the PHEBUS FP tests condensers should
also be analysed in the future using the new wash-off models.
2.2 – Effect of Constituents of Seawater on Formation of Volatile Iodine by Aqueous Phase Radiation
Chemistry – K. Hata (JAEA) et al. K. Hata (JAEA) presented model calculations on the effects ofconstituents of seawater on the formation of volatile iodine (I2) by aqueous phase radiation chemistry. The
aqueous model was extended to include the reactions of the seawater constituents. The gamma-radiolysisof seawater model solutions at ambient temperature and doped with iodide (I-) was calculated to simulate
effects on iodine volatility. It was comprehensibly demonstrated that the production of I2 is affected by anumber of constituents. In particular, bromide promotes the production of I2, but chloride does not.
Hydrogen carbonate influences the I2 production through its role in determining the pH value. The H2
production is promoted by I-. The pH of the seawater effectively determines the I2 yields. It was concluded
that the injection of seawater into a pool containing iodide can have a significant impact on the iodine
source term.
In the discussion it was commented that the chloride result was consistent with previous work, andthat borate could be relevant in radiolytic I2 formation due a catalytic influence. Organic constituents ofseawater, which could be precursors of organic iodides and which were not included in this study, could beof importance in future work, but the organic iodide production could also be suppressed by the scavenging
of oxidizing radicals from water radiolysis by organic molecules.

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2.3 – Formation and Release of Molecular Iodine in Aqueous Phase Chemistry during Severe Accident
With Seawater Injection – K. Kido (JAEA) et al. K. Kido (JAEA) used the KICHE code to perform
calculations on the radiolytic formation of gaseous I2 as function of the mixing ratio of RCS water withseawater, to simulate the situation of seawater injection into a degraded core. Calculations show that the I2
production increases with the fraction of seawater. Effects from bromide and hydrogen carbonate weresimilar to those found in the previous presentation by Hata et al. Dissolved oxygen drastically reduced theI2 yield by catalytically consuming hydroxyl radicals, while dissolved carbon dioxide promoted the I2
production by reducing the pH value. The calculated effect of chloride, which was also treated in thediscussion, seems to be small as in the previous paper by Hata et al.

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Session 3: organic iodide formation and partitioning, chemistry and interactions with aerosols,
droplets and surfaces in the gas phase
Session Chairs: G. Glowa (CNL), M. Salay (USNRC)
Six interesting papers, contributed by Chalmers, NNL, AREVA, and KAERI, were presented during
Session 3 dealing with containment gas phase processes. These papers covered both experimental andmodelling efforts on the subjects of the formation and destruction of organic iodides, adsorption ofmolecular iodine onto paints and aerosols, the volatilization of molecular and organic iodine from sprays,
and the development of materials that could reduce the gaseous iodine such as silver impregnated silica
gels or paints modified to produce a lower quantity of organic iodides. The topics covered by these papers
differed significantly, and as such, will be discussed separately.
3.1 – Chalmers Project on Volatile Organic Iodine Species Behaviour and Retention under Severe Nuclear
Accident Conditions in LWR – S. Tietze (Chalmers University).
3.2 – OIPHA-I and II Model – Mathematical Models to Describe the Partitioning and Hydrolysis
Behaviour of Organic Iodides Between Gaseous and Aqueous Phase - S. Tietze (Chalmers University).
The first two papers were presented by Chalmers University. The first paper covered experimental
work in the areas of formation and chemistry of organic iodides from paint constituents. It reviewed thechemistry of paint and the potential mechanisms for organic iodide formation along with discussing
potential modifications to the paints, namely increasing the use of ring structures and aromatics, that would
be expected to result in a lower rate of organic iodide formation. It presented experimental results invarious areas including sorption and leaching of organic iodides and looked at different approaches toimproving wet containment filtration methods. The second presentation described the early developmentof the OIPHA code. This code calculates the mass transfer of organics between the gaseous and aqueous
phases along with hydrolysis. Models of gas-phase and aqueous-phase radiolysis and iodine-surface
interactions with paints are planned.
3.3 – The Radiolysis of Gaseous Methyl Iodide in Air – S. Bowskill (NNL). National Nuclear Laboratories
presented two papers concerning the gas phase radiolysis of iodine species in air. The 3rd paper presented
experimental results while the 4th paper (below) discussed the model development and simulations ofseveral experimental series. The experiments involved the measurement of the radiolytic decomposition of
methyl iodide as a function of O2(g) concentration, temperature, liquid water volume, dose rate, andsurface area to gas volume ratio. The decomposition rates were found to be comparable to that of previous
work. No significant effect of temperature was observed, but increasing the concentration of O 2, thesurface-area to gas volume, and the water vapour concentration all decrease the decomposition rate. The
decomposition exhibited a square-root dependence on dose rate. The rate of decomposition of ethyl iodide(C2H5I) was found to be similar to that of methyl iodide. Overall, no significant sensitivities were observed
so that the measured rates are considered to be applicable to containment situations. The results of theseexperiments have been used to develop a mechanistic model of methyl iodide decomposition described in
the fourth paper.
3.4 – The IODAIR Model for Radiolysis of Gaseous Iodine Species in Air: Data
Comparison and Predictions – S. Dickinson (NNL). The fourth paper described the IODAIR model andprovided the simulations of several experiments including the Harwell experiments discussed above,
PARIS tests, as well as BIP tests. The model gives good agreement with experimental measurements ofgaseous I2 decomposition during irradiation at relatively high starting concentrations. However, at lower
concentrations the extent of decomposition is overestimated. The model also gives good agreement withmeasurements of the radiolytic destruction of CH3I in air, and reproduces the observed increase in rate at
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low O2 concentrations. Mechanisms are proposed to account for the experimentally-observed dose ratedependence, and for the effect of humidity. Although it has been reported in the literature, the model could
not reproduce the formation of CH3I from mixtures of CH4 and I2.
3.5 – Interaction of Gaseous I2 with Painted Surfaces and Aerosols in Large-Scale THAI Tests –F. Funke (AREVA GmbH). The fifth paper detailed recent experiments in the THAI experimental facility(no radiation) regarding the interaction of molecular iodine with wet and dry paint, and with aerosols ofdiffering reactivity. German Gehopon epoxy paint was used for these experiments and was aged by heat
treatment to a 15 year equivalent. In agreement with BIP tests, the adsorption of molecular iodine on dry
paint exhibited a large effect of humidity, and the iodine was mainly chemisorbed and therefore strongly
bound. Molecular iodine was observed to adsorb onto wet paint at a similar overall rate to dry paints
exposed to high relative humidity. This observation helped in the development of a water-layer model.
Molecular iodine adsorption on different aerosols was measured. Both high-reactivity (Ag) and lowreactivity (SnO2) aerosols were used to represent potentially bounding reactivities for nuclear aerosols.
Although little adsorption was observed on the non-reactive aerosol, the reactive aerosol absorbed
molecular iodine at rates comparable to paint. The THAI experiments are used to help develop AIM-3, theiodine code within COCOSYS.
3.6 – Effect of Water-droplet Sizes on the Migration of Volatile I2 and CH3I – H.-J. Im (KAERI). The final
paper in session 3 primarily addressed the effect of water droplet sizes on molecular iodine and methyl
iodide volatilization but also addressed techniques for making filter materials. The transfer of both
molecular and methyl iodine was lower for smaller droplets. Silver nanoparticles were successfully
embedded in silica gels for potential use as iodine adsorbents in filter systems. These adsorbents are now
ready for testing.
Importance of the research
Due to the integrated nature of the various iodine phenomena that lead to the prediction of the chemical
form and concentration of iodine in the containment environment, the implications for the researchpresented in session 3 are closely linked to the conclusions of the workshop.
The iodine concentration directly affects the amount of iodine that can be released from thecontainment to the environment, and thus the dose to workers and the public. The chemical form of theiodine in containment depends on multiple processes that affect the location of iodine in containment and,thus, the concentration of iodine in the containment atmosphere.
Understanding the mechanisms for the formation and destruction of organic iodides and adsorption ofiodine onto paints and aerosols are, therefore, crucial to understanding not only the chemical form, but also
the concentration of iodine in the containment atmosphere subject to release.
After the Fukushima Daiichi accident, many countries are installing or upgrading FilteredContainment Venting Systems (FCVS) that can relieve containment pressure while minimizing fission
product release. Understanding the processes ongoing in containment filters along with the speciationentering the filter are needed to predict the reduction in dose that the filtration systems provide because
they exhibit different performance for different iodine chemical forms. Session 3 had a strong focus on the
behaviour of organic iodides, which are considered to have low decontamination factors in several types offiltration systems.
The models that are being used to fully capture all the observed phenomena are quite complex and involve
hundreds of reactions. These full reaction sets are far too complex to be practically included in a system

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code of the type used to calculate releases for reactor accidents. Therefore, simplified or reduced equation
sets that capture the dominant trends must be developed for use in system codes.
Conclusion and recommendations from the chairmen
Despite the long history of post-accident iodine behaviour research, and the complexity of the full models,
gaps remain in understanding of some of the fundamental phenomena that govern iodine behaviour incontainments. Some of the areas requiring further research were discussed in Session 3:Formation of organic iodides from paint constituents;

Formation of CH3I in the gas phase from CH4;

Interaction of I2 with paints and aerosols;

Gas phase CH3I and I2 radiolysis;Potential mitigation strategies.

Several of these items are the subjects of proposed OECD projects including THAI-3, STEM-2 andBIP-3. It is recommended that these international projects receive continued support. Not only do theseprojects generate experimental data required to develop and validate SA codes computer models and to
reduce uncertainties on ST evaluations, but they also provide a forum in which experts throughout theworld can exchange knowledge and ideas.

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NEA/CSNI/R(2016)5
Session 4: filtered containment venting systems
Session Chairs: T. Lind (PSI), L.E. Herranz (CIEMAT)
A total of six papers were presented in the session. The sources of those papers were diverse: 2 fromindustry (Westinghouse and AREVA), 3 from research institutes (KAERI and PSI) and 1 from a joint
project of the 7th Framework Programme of EURATOM (PASSAM). Accordingly, the major messages sent
across might be grouped in three categories: progress in industrial devices optimization and/or
development, aspects to consider when coming to an effective implementation of FCVS and potential
improvement of mitigation from research.
4.1 – Westinghouse Molecular Sieve Technology – A. Andren (Westinghouse). Westinghouse presented
the molecular sieve technology as the means chosen to trap effectively volatile Organic Iodides (Org-I).
Different configurations of this technology have been built (i.e., cylindrical and box types) to couple it totheir traditional wet and dry filters. A number of tests have been carried out in the presence of steam,radiation and high temperatures and they seem to anticipate that DFs from 5 to 50 would be reachable for
Org-I, depending on zeolite bed depth, in FCVS working conditions. No degradation of zeolite material has
been observed whenever they are kept in dry conditions by keeping a N2 atmosphere.
4.2 – Improved Iodine Retention at Filtered Containment Venting – P. Zeh (AREVA GmbH). AREVA
outlined the improvement achieved in their system performance by adding a new stage for gaseous iodine,particularly volatile Organic Iodine (Org-I). The system consists of three stages: a Venturi scrubber(aerosols and I2 retention), a metal-fibre filter (fine particles re-suspended and/or escaping from the pool)and a sorbent section (trapping of remaining volatile iodine of any kind). The last experiment conductedfocused on Org-I retention under different temperature, pressure and steam fractions; after more than 50
tests, they reported DFs higher than 50. The whole system has been tested at large scale to avoid any
scaling effect in their results.
4.3 – Design on the Performance Test of Filter Containment Venting System – K.-S. Ha (KAERI).
4.4 – Thermal-hydraulic Effect on Iodine Retention in the Filtered Containment Venting System –Y.-S. Na (KAERI). KAERI highlighted many related aspects to potential source term to environment: therelevance of containment integrity, the importance of bypass sequences, the close link between thermalhydraulics and any FCVS performance, etc. In the final part of their presentation, they presented thecurrent status in Korea concerning the development of an enhanced FCVS system, the ongoing studies
concerning implementation, with emphasis placed on the component performance when scaled up. A
number of innovative ideas were introduced that are still under investigation, like the use of two filtering
vessels or the external cooling of the scrubber systems. On line with this work they have already started
modelling of FCVS performance with the MELCOR code in a OPR 1000 NPP. Even though the resultspresented were still preliminary, this approach might assist in assessing the thermal-hydraulic boundary
conditions on a wet scrubber performance.
4.5 – Investigation of Iodine Retention in a Filtered Containment Venting System in the VEFITA Test
Programme – D. Suckow (PSI). PSI showed their progress in the investigation of an alternative means for
capturing volatile Org-I in water pools based on finding the right combination of sodium thiosulfate(Na2S2O3) and Aliquat336, the second one being responsible for accelerating Org-I mass transfer to thewater and fixing of iodide ions. After getting successful results in lab scale tests, in the near future they will
determine the novel method performance in more representative conditions of an FCVS through using a
1:1 in height facility called VEFITA. A test matrix has already been built and analytical work tocharacterize major FCVS thermal-hydraulic variables is ongoing.

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4.6 – The European PASSAM Project on Atmospheric Source Term Mitigation: Half-Way Progress andMain Results – T. Albiol (IRSN). The PASSAM project is investigating, within the 7th Framework
Programme of EURATOM, the potential optimization and enhancement of Source Term mitigation systems
in case of a severe accident. Coordinated by IRSN, it empirically explores both traditional (pool scrubbing
and sand filters) and innovative (acoustic agglomeration, high pressure sprays, wet electrostatic
precipitators and advanced zeolites) methods, in addition to combined methods (i.e., wet + dry filtration).
Some results have been already obtained, but it is still too early to set any firm conclusion regarding thesystems under investigation. Particular emphasis is being placed on understanding the phenomena
responsible for retention in realistic severe-accident conditions in the short and long term of a severe
accident.
In summary, the session delivered new and innovative results for the FCVS design andimplementation. Commercial FCVSs can be equipped with a filtration stage for retention of volatileorganic iodides with the possibility of tailoring the decontamination factor for each application. The
operation of such filtration stages has been experimentally proven in a wide range of conditions. Openissues remain the source term of organic iodides to such filters, as well as the long-term behaviour of thefilters and the retained iodine species. Alternative methods for retention of organic iodides are being
investigated. In addition, several innovative FCVS designs and source term mitigation methods were
presented, and further research is on-going to determine the feasibility, scale-up potential, and thecollection efficiency of such systems.

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Session 5: recent developments in iodine chemistry modelling in severe accidents and applications tosource-term and radiological-consequences evaluations
Session Chairs: S. Dickinson (NNL), D. Jacquemain (IRSN)
This session comprised two papers on the development of models for iodine chemistry in the containment,
and four papers in describing the application of models of varying degrees of complexity to calculate thereleases or radiological consequences from design basis and severe accidents.
5.2 – Iodine Behaviour in the Circuit and Containment: Modelling Improvements in the Last Decade andRemaining Uncertainties – L. Bosland (IRSN).
5.3 – Containment Iodine Chemistry – M. Salay (USNRC).
The modelling development papers presented by Bosland and Salay focused on the specific aspects of the
behaviour of iodine where the authors consider that additional data and models are needed to complete anadequate representation of the behaviour of iodine in containment. These included modelling of chemistry
in the RCS to improve prediction of the gaseous iodine source to the containment, and a number ofheterogeneous phenomena in the containment that have an impact on the concentration and chemical
speciation of iodine in the containment atmosphere in the medium term. In some cases, experimental
programmes are underway or planned to address the issues; in others, models are being developed on thebasis of information available from other fields of study. The discussion of these papers raised questions
about the level of detail required in the models, and the effect of uncertainties in the containment chemistrymodels on the calculated source terms and consequences.
5.1 – Iodine Chemistry in Design Basis Faults – A. Zodiates (EDF Energy).
5.4 – Application of the COCOSYS Iodine Chemistry Model AIM-3 to PWR Containment Analyses Using
New Validation Results of RTF and CAIMAN Experiments – H. Dimmelmeier (AREVA GmbH).
5.5 – Iodine Source Term Computations with ASTEC, Link with PSA Level 2 Tools and Fast Running
Source Term Tools For Emergency Organisation – K. Chevalier-Jabet (IRSN).
The papers presented by Zodiates, Dimmelmeier and Chevalier-Jabet described the application ofmodels to calculate iodine releases from containment. These papers highlighted the differences incomplexity of the models, depending on their application, and in particular the importance of simplicity
and conservatism for “end-user” applications such as radiological consequence assessments and emergency
planning. However, even the apparently simple models have to be based on a good understanding of the
behaviour of iodine, at the level treated by the more detailed models, in order to ensure that the importantphenomena are taken into account and correct extrapolations are made from experimental data. Data fromPHEBUS FP, SARNET and recent NEA programmes such as BIP, STEM and THAI are thus used to
inform the development of the higher-level codes. An example was highlighted in which the application ofdata measured for severe-accident conditions may not be conservative in design-basis conditions,suggesting the need for additional data on iodine volatility from solutions at low dose rate, lowconcentration and high temperature.
5.6 – In What Extent the Iodine Reactivity in the Atmosphere Can Impact the Radiological Consequences– J. Trincal (IRSN).
The final paper of the session, presented by Trincal, showed that the complex chemistry of iodine
continued after it was released to the environment, and could have an important impact on the dispersion
and deposition behaviour controlling the biological uptake of radioiodine. Furthermore, there is a large
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overlap between the gas-phase chemistry provoked by the radiation field within the containment, and by
UV radiolysis in the atmosphere. This paper provoked discussion about the different drivers for predicting
the gaseous iodine species in containment, since these calculations implied that the environmental fatewould be the same for organic and inorganic iodine. In contrast, the distribution between organic andinorganic iodine is an important consideration in the efficiency of mitigation systems.

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SUMMARY STATUS REPORTS OF NEA BIP-2 AND OECD/STEM PROGRAMMESAt the NEA BIP-2 and STEM Programme Review Group (PRG) meetings in September 2014, it was
decided that the upcoming OECD/NEA-NUGENIA/SARNET International Workshop on iodine behaviour
would be an excellent forum to present and discuss the final outcomes of the two projects and the proposal
for the follow-ups. The final summary status reports of both projects are presented hereafter as well as the
proposed topics for the follow-ups and the related discussions.
NEA BIP-2 and follow-up - G. Glowa (CNL)
The primary objective of the NEA BIP project operated by CNL (formerly AECL) was to investigate theinteractions between iodine and containment paints, including iodine sorption behaviour and thesubsequent formation of organic iodides. The first phase of BIP (BIP-1, July 2007 - March 2011) focusedon the interaction between iodine and epoxy paints. Epoxy paints are commonly used surface coatings
within nuclear reactor containment buildings throughout the world. In particular, experiments
investigating the role of water on the iodine deposition velocities and the formation of methyl iodide from
irradiated paint showed that adsorbed water strongly promoted both the sorption and organic iodide
formation processes.
BIP-2 (July 2011 – September 2014) investigated the same processes as BIP-1, with emphasis on
developing a more mechanistic understanding of organic iodide production during irradiation of iodineloaded paint. Epoxy paint is a complex mixture of polymeric compounds and it is difficult to know where
and how iodine is interacting and how methyl iodide is formed. To isolate the influence of various
functional groups present in epoxy paint, polymer samples having simple, but representative, structures(such as amide groups) were used as model compounds for comparative testing. The samples were
exposed to I2(g) and irradiated to determine the rate of methyl iodide formation. In addition to the polymer
tests, tests were performed with epoxy paint at 80°C (for comparison to EPICUR tests) with different
iodine loading schemes (to help our understanding of the effect of water) and with paint that was preexposed to Cl2 or NO2 (to simulate the effect of reactive species expected within containment).
Several spectroscopic techniques were utilized in BIP-2 to investigate iodine-paint surface interactions.
Scanning electron microscopy was used to determine coupon surface morphology, energy dispersive x-ray
analysis measured the depth of penetration of the iodine into the paint layer, and gas chromatography-mass
spectroscopy was used to study solvent release from 20-year-old epoxy paint.
BIP-2 members have indicated that the BIP-2 project has been used to further their general
understanding of post-accident iodine behaviour and aid in the interpretation of integral tests such as
PHEBUS FP. The data are also used for the development and validation of iodine models within severe
accident system codes such as MELCOR, ASTEC-IODE, COCOSYS, and RAIM.
It was also highlighted at this workshop that the organic iodide source term in containment is required
as input for designers of filtered containment venting systems (FCVS). Organic iodides have a low
decontamination factor in such systems, so they are relatively important contributors to environmental
releases as compared to aerosol forms and molecular iodine (I2).

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It should be noted that, while the inclusion of a detailed, mechanistic model to describe iodine-paintinteractions is not a realistic objective for many of our iodine codes, the ability to qualitatively describe theunderlying chemical processes provides support for the use of empirical models.
In addition to presenting the final results of the BIP-2 project, a preliminary proposal for a follow-up
project (BIP-3) was presented. The proposal contained topics that were previously identified and ranked
by the BIP-2 PRG and focused on: Improving our ability to simulate iodine adsorption and desorption behaviour over a range ofconditions:
Paint ageing (“natural”, thermal, and irradiation)Long irradiations
 Predicting CH3I behaviour (formation and degradation) in accident conditions:
Paint ageing
Wet/dry cycling
 Investigating the effect of contaminants (e.g., NOx, Cl2).
The complementarity with the STEM-2 proposal topics was also considered in the project proposal.
The workshop allowed further comment and discussion by a wider group of iodine experts. Thisrigorous process provided an opportunity to evaluate the technical scope of the BIP-3 programme while
ensuring the efficient use of our combined resources. Specific workshop discussions related to BIP-3
topics included:
Importance of gas-phase reactions - It is clear that gas-phase reactions are important for thedetermination of the speciation and concentration of iodine in the gas phase. Items related to the effects ofgas phase contaminants such as Cl2 and NOx within the BIP-3 programme will be retained. The presenceof CO(g) due to the molten corium concrete interaction has been identified as a reactive species incontainment. The prospect of studying aspects of CO reactions with iodine species will be discussed by
the BIP members.
Gas phase methyl iodide degradation - It is clear that this process is important for the determination of
the gas phase methyl iodide concentration. New information presented at this workshop suggests that theremaining uncertainty in the degradation rate is not significant for the calculation of iodine source term.
As a result, this topic will be removed from the BIP-3 proposal.
A code-comparison exercise - It was reiterated at this workshop that the sharing of analytical work was
not a strong component of BIP-2. Furthermore, it was noted that an iodine code comparison exercise has
not been performed in a while. In the absence of new appropriate experimental data, a comparison of asimple theoretical scenario could provide some guidance to ongoing experimental and code development
work. The BIP-3 analytical group may be an appropriate venue for such an exercise.
In conclusion, this international OECD/NEA-NUGENIA/SARNET Iodine Workshop provided an
excellent opportunity to share the results and discuss the path forward for the BIP project. A modified
proposal will be submitted through the NEA for expressions of interest in BIP-3.
NEA STEM and follow-up - C. Mun (IRSN)The STEM (Source Term Evaluation and Mitigation) NEA project operated by the French “Institut deRadioprotection et de Sûreté Nucléaire” (IRSN), was launched mid-2011 in order to improve theevaluation of Source Term (ST) for a severe accident (SA) on a nuclear power plant and to reduce
uncertainties on specific phenomena related to the chemistry of two fission products which may be major
contributors to the radiological consequences: iodine and ruthenium. Concerning these two radionuclides,
three main issues were addressed: i/ middle-term iodine behaviour in the containment with specific38

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attention to the stability of deposited iodine aerosol particles under radiation (radiation induced
decomposition producing gaseous iodine species), ii/ short and middle-term iodine-paint interactions underradiation (organic iodides production), iii/ ruthenium transport chemistry in order to determine thespeciation of Ru, in particular the partition between gaseous and condensed forms, during its transport
through the Reactor Cooling System (RCS).
Concerning the STEM iodine research, semi-integral experiments were carried out in the EPICUR
facility. These tests were first focused on the releases of volatile molecular iodine (I2) and organic iodides(Org-I) from representative epoxy painted coupons loaded with molecular iodine and also with iodine
aerosols species (CsI/IxOy) to assess their thermal and radiolytic stability. Main outcomes of the
programme are:
 New data from long duration irradiation tests which can be used to develop accurate kinetic models
of gaseous iodine (molecular and organic iodides) release from paints in the containment withconsideration of short-term and “delayed” releases. New models based on STEM data were already
implemented in the ASTEC SA system code (IRSN) and in the RAIM iodine chemistry code
(KINS);

 Demonstration that iodine aerosol (CsI, IxOy) decomposition processes in the containment are
major processes affecting the volatile iodine concentrations in suspension in the containment andthat a proper modelling of these processes has to be developed. Some kinetics data of thedecomposition processes were obtained and a preliminary modelling was implemented in the

ASTEC code.
Recent ASTEC calculations performed by IRSN with the new modelling developed based on STEM
data showed a better understanding of the iodine behaviour in the containment for the integral PHEBUS FP
tests series with an iodine volatility which would be essentially driven by organic iodides
formation/decomposition at paint and in the gas phase, IxOy formation/decomposition in the gas phase andother iodine aerosols decomposition.
The additional knowledge and improvements for calculation tools derived from the STEM project
should allow a more robust assessment of radioactive releases and consequences in a SA. Indeed, the
STEM project contributed to fill some gaps of knowledge on production of gaseous iodine in thecontainment from important processes – these processes were previously identified through an IRSN study
coupling ASTEC and SUNSET (uncertainty evaluation tool) as major contributors to the remaining
uncertainties on ST evaluations - which were not earlier thoroughly investigated.
As underlined during the workshop, the gained knowledge is expected to allow a better appreciationof releases by containment leaks and through FCVS during all phases of an accident. A specific emphasis
was put on iodine oxides and organic iodides filtration since these species are expected to have a rather low
decontamination factor in FCVS (both French sand bed filters and pool-scrubbing filters) and may be main
contributors to iodine environmental releases.
Concerning the STEM ruthenium research, a series of vaporization and re-vaporization analyticalexperiments were performed in the START test facility. The effect of the oxygen potential on the deposits
formation was assessed, as well as the kinetics of Ru transport after vaporization and the kinetics of revaporization from the deposits in a transport tube simulating the RCS.
As already demonstrated by previous international or national experimental programmes, ruthenium
species released from the core in oxidizing conditions can reach the containment and form volatile
ruthenium tetroxide. The transport in the RCS is one of the key process driving gaseous ruthenium
formation and eventually its release to the environment. The STEM ruthenium research allowed gaining
knowledge on the key parameters impacting the transport kinetics, the partition between gaseous RuO4 andparticles of RuO2 and the extent of the re-vaporization process of deposited species in a transport tube. It
was shown in particular that, despite the fact that most of the volatilised ruthenium deposits fast on the39

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transport tube, a few % of it, produced either by vaporisation or re-vaporisation, may eventually betransported as a gaseous fraction downstream to the tube exit (simulating the break to the containment).
Based on STEM results, a preliminary Ru transport model through the RCS was developed andimplemented in the ASTEC system code. Considering these developments, a preliminary PSA level 2
assessment was made by IRSN to appreciate Ru contribution to the radiological consequences for a French
1300 MWe NPP (presented at the 2015 ERMSAR conference). It was shown that, even when gaseous
ruthenium fractions reaching the containment represents only a few % of the core inventory (order ofmagnitude of STEM data), Ru isotopes are with iodine ones major contributors to the radiologicalconsequences.
Following the presentation of the main outcomes of the STEM project, a preliminary proposal for afollow-up project (STEM-2) was presented and discussed. It was reminded that initially two phases for the
STEM project were defined at the STEM expert meeting held in October 2010: the first one from mid2011 till mid-2015 and the second one (STEM-2) till mid-2018 to complete the first phase. Based on recent
progresses made in the ST area, notably based on the NEA BIP-2 and STEM programmes, topics for
STEM-2 proposal could be identified or consolidated and ranked by the STEM PRG members with theobjective to fill main remaining knowledge gaps affecting ST evaluations. The complementarity with the
BIP-3 proposal topics was also considered in the project elaboration.
Following the last STEM PRG meeting discussions, the second phase would possibly be performed
from mid-2015 to mid-2019 focussing on:
 The effect of the ageing of paints on iodine behaviour: i.e., irradiation tests (medium and longterms irradiation periods) on paints aged in representative reactor conditions and loaded withiodine. Some limited data reported in the literature indicate that paint ageing may strongly impact
iodine adsorption/desorption kinetics and organic iodides production – and consequently
potential iodine releases; The study of iodine aerosol stability:-

iodine oxides radiolytic decomposition;-

iodine oxides decomposition by carbon monoxide;-

radiolytic oxidation of multi-components iodine aerosols.

As the first phase of the project demonstrated, these processes may strongly affect the iodine fraction(amount and speciation) in suspension in the containment – and consequently potential iodine
releases. Data generated in the first phase of the project have to be analysed to develop kinetics
models able to reproduce aerosol decomposition processes for all phase of the accident (including
the MCCI phase).

 Complementary tests on ruthenium re-vaporization from RCS stainless steel surfaces for highly
oxidizing conditions. Data generated in the first phase of the project have to be completed for more
representative conditions.
The workshop brought further discussion and debate and thus induced an additional evaluation of thetechnical scope of the STEM-2 proposal leading to consolidation & update of the proposal:
Iodine oxide decomposition by carbon monoxide:
The study of the effect of CO on iodine oxides decomposition appeared as an important issue as COconcentrations in the containment can be several orders of magnitude higher than volatile iodine
concentrations during the MCCI phase. It also appeared during the discussion that the hydrogen generated
during the accident (by the Zircaloy oxidation during early phases and by MCCI later) may also affect theiodine oxides decomposition despite possible hydrogen consumption by mitigation means (PARS or

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igniters) when these are implemented. The effect of both gases on iodine oxide stability has to be further
assessed.
Prototypical conditions for ruthenium re-vaporisation tests:
While the probability of “air ingress” accident scenarios was debated between experts during theworkshop, providing more conclusive answers relative to the ruthenium issue – which is considered bysome experts as an important objective by itself – requires the performance of more representative tests
than in the first phase of the STEM project. This may be a challenge to achieve as one would like toconsider RCS representative surfaces for deposition, the presence of representative aerosols particles andproper gas composition including oxidant conditions. This will require a careful design of the experiments
for that specific investigation.
Analytical work:
It was also underlined during the workshop that analytical work to provide a final comprehensive analysisof the results with implications for ST evaluations including mitigation aspects should be promoted.
In conclusion, the international OECD/NEA-NUGENIA/SARNET Iodine workshop provided an
excellent opportunity to present the main outcomes of the first phase of the STEM project and to discuss,between all experts in the field, the main issues that should be investigated during the second phase of the
project. Following the experts’ remarks expected on an initial STEM-2 proposal distributed at theWorkshop, an updated STEM-2 proposal will be submitted as a basis for launching the STEM-2 project.

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SUMMARY, CONCLUSIONS AND RECOMMENDATIONS

Fulfilment of Workshop objectives
It was recognized that the workshop objectives relative to reviewing and discussing the state of the art
knowledge of iodine and ruthenium behaviour in NPP accidents gained through research projects andprogresses made in the related modelling were well fulfilled. Also, highlighting remaining open issues andassessing their coverage through on-going and planned research activities was well considered with inparticular discussions concerning the NEA BIP-3, STEM-2 and THAI-3 proposals and recommendations
for future works reported hereafter. The workshop also contributed, and will continue contributing throughthe summary report publication, to knowledge sharing and dissemination in the field.
However, it was underlined that contributions concerning research results applications for improving
accident analyses and management were limited in number and scope, with the notable exception of an
IRSN presentation of the status of different tools used for source term evaluations and emergency planningpresented in the last session. This led workshop experts to formulate some general recommendations tofoster international exchanges and actions to assess more thoroughly research progresses implications for
NPP accident management. This comes in addition to recommendations on technical issues which need,according to these experts, additional research efforts in the future.
Conclusions and recommendations
Iodine and ruthenium release from fuelThe existing large database on FP release from fuel experiments (CRL, HI/VI, PHEBUS FP, VEGA,VERCORS, etc.) was recently completed by the ISTP/VERDON test series focusing on HBU and MOX
fuel. VERDON results confirm that highly volatile FP (I, Cs, Te) should tend to completely release in a SA
resulting in significant fuel degradation and that semi-volatile FP (Mo, Ba, Ru) release is strongly
dependent on oxido-reducing conditions, with Mo and Ru release tending to be large in oxidizing
conditions – Ru release requiring fuel oxidation and being lower for MOX fuel than for UO2 fuel.
Results of the VERDON programme are still under analysis. However, they raise questions as to the
hypotheses one should consider for the volatile FP release (notably for Cs) for DBA, BDBA with limited
fuel degradation (e.g., when core cooling can be recovered before significant fuel melting) and for
accidents in oxidizing conditions (including loss of cooling in spent fuel pool (SFP) with interest, in such asituation, in assessing margins before intervention).
The existing FP release database was used by the past to develop simplified or more mechanistic FP
release models. These models should be tested against VERDON tests results in the near future. However,in order to progress significantly in the existing modelling and to incorporate the major effects on FP
release kinetics (such as fuel BU effect and oxidizing potential effect), a detailed mechanistic approachrequiring substantial development would be necessary.
For the time being, except for the VERDON-5 additional test, which should be performed in 2015 tocomplete the existing VERDON database (release from HBU fuel with an air oxidation phase, boron effecton FP transport), no other international initiative to address the above listed issues is planned for the near43

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future. An initiative proposing the development of a mechanistic approach relating fuel microstructure
evolutions and FP release mechanisms in accident transients was proposed within the last EC H2020 call
(INFORMS project) but it was not funded by the EC.
Workshop experts discussed the usefulness of simplified analytical testing (using e.g., fuel simulants)
to progress in the field. No strong recommendation was made at the workshop concerning research foriodine and ruthenium release from the fuel considering that: analyses of the VERDON test series has still to be completed; filling knowledge gaps concerning FP behaviour (Cs, I, Ru) in the RCS and the containment aswell as in FCVS is expected to provide more gain on ST evaluations for SA; also, as already mentioned, for Ru, the importance one should attribute to oxidizing accident
scenarios, which would result in significant Ru release and transport to the containment, should befurther assessed. This resulted in a recommendation formulated when discussing research for Ru
transport in the RCS (see next section).
Nevertheless, it is recognized that models able to predict FP release kinetics from fuel would bevaluable tools for the evaluation of consequences of accidents with limited fuel degradation (includingDBA) and with oxidizing conditions (including SFP) and their management. Analyses of existing data(in particular VERDON) should be deepened to design future research in the field.
Iodine and ruthenium transport in the RCS
a. Homogeneous chemistry and heterogeneous process (reactions with surfaces and aerosols) in thegas phase in a SA
Much progress was reported at the workshop in the understanding and modelling of homogeneous
iodine gas phase chemistry in the RCS, notably from ISTP/CHIP data analyses and involving in somecases supporting theoretical chemistry. System codes such as ASTEC and MELCOR benefited from model
developments in that respect related to the effect of Mo on Cs and I chemistry and transport. Data weregenerated to treat the Cs, Mo, B, I, H, O chemical system (CHIP/ISTP) and models are implemented in
ASTEC considering both kinetics and thermodynamics modelling of influential reactions. The database isbeing extended by IRSN to treat Ag, In and Cd effect on iodine transport and chemistry (CHIP+
programme and theoretical developments). All these developments aim at providing better predictions of
the gaseous iodine fractions at the RCS break in a SA – which may be highly scenario dependent asevidenced by PHEBUS FP observations (with strong effect of carrier gas composition, of compounds
resulting from control rod degradation (including Ag, In, Cd, B elements), of other FPs (Mo)) - and reduce
related uncertainties on iodine source term evaluations.
Presently, SA system codes have implemented with different levels of detail the gained information.
ASTEC in its latest version includes a revised modelling of kinetics and thermodynamics of the Cs, Mo, B,
I, H, O system which was validated against CHIP and PHEBUS FP data with significant progress obtained
notably in reproducing the iodine behaviour for PHEBUS FP tests. The approach used was not to
implement kinetics for all chemical reactions but only for the most influential reactions. This way, code
calculations times are not significantly impacted despite detailed calculations of the chemical system.
The required level of detail of the modelling of RCS chemistry, notably for SA system codes, wasdiscussed. Even if some uncertainty evaluations performed prior to these developments using coupled
ASTEC/SUNSET tools indicated that calculated iodine releases may be in some accident scenarios
strongly dependent on gaseous iodine fractions at the RCS break, comparative reactor-like (BWRs- PWRs)
SA system code calculations in their present stage of development for typical accident scenarios would
provide some input to such a debate and help further assessment of the gain for ST evaluations. The
workshop experts recommended that some SA system code benchmarking exercise be organized for44

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defined reactor-like scenarios to assess the effect of recent chemical-model developments on ST
evaluations. This recommendation was raised on several occasions during the workshop as it does not
relate solely to assessing the progress made in RCS chemistry modelling.
Some progress in understanding of Ru transport from experimentation performed at VTT, MTA EK
Chalmers University and IRSN was obtained. The issue is to develop models able to calculate Ru reemission from RCS deposits as Ru is known to deposit very fast on RCS surfaces after its release from thefuel. The Ru source to the containment would then be very dependent on such re-emission processes.
However, experiments in more representative conditions (RCS surfaces and oxidizing conditions) arenecessary to provide data for the development of models applicable to the reactor case. This is proposed asa part of the STEM-2 project.
As discussed earlier, the importance of the Ru issue was debated as some experts pointed out that theimportance of oxidizing accident scenarios, which could result in significant Ru release and transport to thecontainment, should be further assessed. Others consider that the potentially important contribution of Ru
to radiological consequences even when considering a limited release of it (few % of the core inventory) –as evidenced by some L2 PSA evaluations - is sufficient to justify research efforts to either close the issue
or provide tools to assess possible ruthenium contribution to radiological consequences. The workshop
experts recommended conducting further assessments as to the importance of oxidizing accident
scenarios which could result in significant ruthenium release to the environment.
More generally, an important issue that was underlined during the workshop discussions relates to thenecessity to develop a pragmatic research approach to treat complex heterogeneous processes involving
interactions of gas species with surfaces and aerosols in the RCS and considering possible resuspension/re-volatilization/decomposition of deposits resulting from mechanical, thermal and dose
loadings. These may be important delayed source of FP (I, Cs and Ru) to the containment - whensignificant deposits have occurred in the RCS during release phases (probably more relevant for Cs and
Ru, Cs re-volatilization was evidenced in some PHEBUS FP tests) – potentially contributing to the ST inlater phases of the accident, notably in case of FCVS use. It is indeed generally hypothesized in
calculations supporting ST evaluations that radioactive aerosols and gas species are depleted by fast
deposition processes in the containment leading to substantial reduction of suspended concentrations in thefirst hours following their release to the containment.
Limited experimentation on Cs and Ru re-vaporization processes is being performed in the French
MiRE and the NEA STEM projects. This is proposed to be continued for Ru in the STEM-2 project.
However, doing experiments and developing models relevant to reactor situations still appear challenging
notably due to the complexity of the involved processes and the potential importance of dealing withrepresentative surface states and deposits.
The workshop experts recommended conducting further assessments to appreciate the potential
effect of FP re-emission from RCS deposits on ST and to focus the experimental research in the field –to be relevant to the reactor case - integrating existing knowledge, on-going and planned research.
b. Aqueous phase chemistry in DBA
Specific testing was performed as part of the NEA THAI-2 programme to investigate gaseous iodine
release for the case of SGTR accidents (flashing conditions at the break induced by RCS depressurization).
Calculations performed for the test preparation have evidenced the lack of validation of existing iodine
chemical models in the aqueous phase for RCS conditions relevant for DBA (low doses and low
concentrations, elevated temperatures (> 200°C), additives effects). This was also concluded frominvestigations conducted by EDF Energy. The workshop experts recommended designing research to test
the validity of iodine chemical models for aqueous phase chemistry in DBA conditions.

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Iodine and ruthenium behaviour in the containmenta. Homogeneous chemistry and heterogeneous process (reactions with surfaces and aerosols) in thegas phase in a SA
In that field, gained knowledge on containment gas phase content (aerosols and gas) is expected to help
appreciate releases through containment leaks and through FCVS during all phases of the accident,
including long term phases (e.g., MCCI), considering changing gas phase composition (H 2, H2O, CO andCO2 production and consumption), effects of dose, of SAMM (e.g., steam production resulting fromcorium cooling in and ex-vessel, sprays) and of energetic events (e.g., hydrogen combustion) on FP
remobilization from deposits on containment walls.
Following PHEBUS FP, research on iodine containment chemistry focused on gas phase processes(interaction of gaseous iodine with paints and organic iodides (Org-I) production, interaction with reactive
aerosols (e.g., Ag-containing particles), iodine oxides (IxOy) formation/decomposition, gaseous iodine
release by radiolytic decomposition of deposited aerosols) as the PHEBUS tests analysis indicated they
determined the gaseous iodine evolution - whereas a lot of research efforts were earlier put on aqueous
phase chemistry (gaseous iodine was initially assumed to be essentially produced by radiolytic processes inthe sumps following iodine aerosols deposition there). The research was recently notably conducted in the
ISTP/Ru, ISTP/EPICUR, NEA BIP-1 and BIP-2, THAI-1 and THAI-2 and STEM. The above processes
are considered reasonably well covered by past, on-going and planned research (notably with the proposed
NEA BIP-3, STEM-2, THAI-3) with a focus on inorganic gaseous iodine species, iodine oxides particles(IxOy), organic iodides (Org-I) and gaseous ruthenium tetroxide (RuO4) behaviour. Significant progresses
have been made through all these programmes on the understanding and modelling of Org-I, IxOy, RuO4
production/decomposition. Part of the gained knowledge is implemented in SA system codes. As for RCS
chemistry, the workshop experts recommended that some SA system code benchmarking exercise beorganized for defined reactor-like scenarios to assess the effect of recent chemical models developments
on ST evaluations.
Following the Fukushima accident and considering potential long term loss of containment heatremoval systems, questions have been raised on possible FP remobilization from deposits on containment
surfaces and sumps/suppression pools during long term phases of a SA and notably in relation to assessing
FP release during containment venting. This resulted in the definition or reorientation of research projects
intending to increase knowledge on such processes. This is in particular included in the NEA BIP-3,
STEM-2 and THAI-3 projects.
Complementary BIP-3, STEM-2 and THAI-3 projects have been presented and discussed at theworkshop. Proposals for BIP-3 and STEM-2 have been distributed to the workshop participants andoffered for comments. They were already discussed and reviewed by BIP-2 and STEM partners. The
THAI-3 proposal was already extensively discussed at an expert meeting in November 2014. These threecomplementary projects will be focusing on:
 formation/decomposition of Org-I through homogeneous and heterogeneous processes - including
paint ageing effect – (BIP-3/STEM-2);
 reemission of gaseous iodine from iodine aerosols deposits under radiation (STEM-2);
 formation/decomposition of IxOy including CO effect (MCCI phase) and assessment of H2 effect(STEM-2);
 effect of impurities (Cl2, NOx) on gas phase reactions involving iodine (BIP-3);
 remobilization of gaseous iodine from containment sumps or suppression pools either due tocontinuous heat-up or due to depressurization-induced (venting) boiling (THAI-3);
 effect of hydrogen deflagration on FP remobilization from wall deposits (THAI-3).

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The workshop experts recognize that the follow-on proposals are well targeted to tackle some of theraised issues. Some comments were made which helped improve the focus of the proposals as reported
earlier in the programmes summary section. Proposals remained open for comments till the end of April
2015 with the objective of having the project proposals finalized by the operators (CNL for BIP-3, IRSN
for STEM-2 and Becker Technologies for THAI-3) by mid-2015 after taking account of the workshop
discussions and any additional relevant expert comments. The operators intend to launch the projects overthe second half of 2015 (BIP-3, STEM-2) or early 2016 (THAI-3). It is also recognized that the proposed
experiments will only partly cover the issues listed above. There are also some specific national
developments that will tackle such issues with, e.g., new containment facilities under development in Japanand Republic of Korea with the intention to investigate more challenging conditions than studied inexisting facilities (e.g., containment gas-phase temperatures of 200°C or higher as measured during the
Fukushima’s accident).
The workshop experts recommended conducting further assessments to assess the potential effectof “delayed” FP re-emission from containment surface deposits and sumps on ST evaluations and focus
the research in the field – to be relevant to the reactor case - integrating existing knowledge, on-goingand planned research.
The workshop experts further recommended to strengthen collaborative analytical activities, if
possible joined integrated activities between projects, using the available results of international
programmes, highlighting that such activities were in recent years limited in scope.
b. Aqueous-phase chemistry in SA
As discussed earlier, following the PHEBUS FP tests, less work was performed on aqueous-phasechemistry in SA as the main source of volatile iodine was considered to be in the gas phase. The effect of
impurities in containment sumps on iodine volatility was investigated at PSI, notably showing the low
effect of chlorine content and generating data to tackle nitrate/nitride effects. Recently, following the
Fukushima accident, questions were raised as to the effect of seawater compounds on water-phasechemistry during the accident concerning the FP-scrubbing efficiency in suppression pools and in liquidtype FCVS considering evolving thermal-hydraulic and chemical conditions during the accident andconcerning the long-term aqueous-phase chemistry in relation to long-term accident management(corrosion reactions and leaching of corium/debris).
The effect of seawater is currently investigated in Japan as presented at the workshop with possible
effects of bromine on iodine chemistry. Work in this field will continue in Japan in the coming years todevelop the corresponding models.
The effects of changing thermal hydraulic (changing flow hydrodynamics) and chemical conditions(composition, doses, pH, oxido-reducing potential) on FP pool-scrubbing efficiency in suppression pools
and FCVS during a SA were, are and will be partly investigated in research programmes (e.g., EC
PASSAM, NEA THAI-3 and national initiatives on pool scrubbing experimentation and modelling inFrance, Germany, Japan, Republic of Korea, Spain and Switzerland). Analytical work performed to date on
scrubbing experiments shows the necessity to improve modelling of the hydrodynamics of scrubbing, withexisting modelling unable to represent flow instabilities which may strongly affect FP scrubbing
efficiencies. There is presently only limited concerted research actions on pool scrubbing in the PASSAM
project, and in the future in the THAI-3 project, and a larger collaboration in the field would certainly bebeneficial to progress faster on scrubbing modelling. At a later stage, modelling FP scrubbing will requirea coupling between hydrodynamic models with modelling of chemical reactions in the liquid phase.
Investigations of the long-term aqueous-phase chemistry in relation to the long-term accident management(corrosion reactions and leaching of corium/debris) are being or will be conducted in Japan. There was no
specific recommendation in this field as the subject was not thoroughly discussed.

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NEA/CSNI/R(2016)5
The workshop experts recommended fostering collaborative initiatives on pool-scrubbing models
development in relation to assessing FP retention in suppression pools and in FCVS in SA. Suchmodels should also be of relevance to dealing with FP retention in SGTR with flooded secondary side.
FP Filtration in FCVS
FCVSs implemented before Fukushima were mainly designed to manage long-term pressure build-up in
the containment; new FCVS may perhaps be designed to deal with more challenging conditions(management of early phases of an accident, cycling or long-term use in SA conditions). The robustness,the safe use and the reliability of FCVS for such conditions should be further assessed either to improveexisting systems or to propose upgraded design requirements for future systems.
The well-known existing filtration technologies, e.g., scrubbers, deep-bed filtration and different
sorption systems were presented at the workshop. Part of the information concerning the existing filtrationsystems performance and qualification is proprietary and was not disclosed by FCVS designers. However,two major aspects can be underlined concerning existing systems:
 most of the available systems were designed based on knowledge existing in the late 1980s; somehave been updated depending on the system design and implementation (based on consideration of
the results of relevant research and plant safety reviews notably additional filtration stages tomodify the overall filtration efficiency); and given the possible extension of the domain of FCVS use, the demonstration of the systems’
performance should be extended to more challenging conditions.
In different countries, ST evaluations are factored into accident analysis, used for FCVS regulatory
assessment and as a guide for design and operation requirements of FCVS. ST evaluations in general are apart of PSA level 2 studies. These studies typically include the FCVS performance in reducing the ST,
though the system is generally not separately examined. It is worth stressing, as discussed earlier, that theknowledge-base related to “delayed” in-containment re-volatilization and re-suspension processes forradionuclides/aerosols from RCS and containment surfaces and containment pools (sumps and suppression
pools) is being extended through on-going and future research programmes (notably in the NEA BIP,
STEM and THAI projects and the EC PASSAM project).
As for FCVS filtration, besides aerosols, specific attention is being given to organic iodides andiodine-oxide particles as they may contribute significantly to the ST in some accidents (notably, in the
NEA BIP, STEM and EC PASSAM projects). Possible contribution of ruthenium-oxide species to the ST
is also under investigation in on-going research programmes (notably in NEA STEM).
Important efforts were made for the development and qualification of FCVS by industry but questions
remain as to the efficiency and robustness of such systems in SA conditions as these may be envisaged
post-Fukushima. For some countries where safety criteria associated with releases are more stringent, there
might be a search for more efficient filtration to further reduce radiological consequences. Such issues,
with the assessment of innovative filtration technologies, are being covered in the EC PASSAM and
French MiRE project. There are also some specific FCVS national developments notably in Japan and in
the Republic of Korea.
Except for the specific recommendation on the need to foster collaborative efforts on FP scrubbing
modelling in FCVS (see previous section), experts considered that on-going research on FCVS is wellfocused.
Recommendations on research tackling “delayed” re-emission processes in relation to ST assessment with
FCVS use were listed in earlier sections. In particular for scrubbing FCVS, the effect of changing
chemical conditions in SA on scrubbing efficiency requires further research efforts.

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NEA/CSNI/R(2016)5
Iodine chemistry in the environment
If a lot of development had been made to model iodine chemistry in the RCS and in the containment in a
SA for ST applications, little attention has been given to iodine chemistry in the environment, i.e., after its
release from the containment, for the assessment of its dispersion and related radiological consequences.
Preliminary work based on existing knowledge of atmospheric iodine chemistry indicates that theradiological consequences due to iodine may be strongly dependent on the iodine chemistry in theenvironment. Due to the complexity of the chemical systems to be treated, it is necessary at this stage tofurther assess the potential impact of iodine chemistry in the environment on radiological consequencesand define, if the impact is demonstrated to be strong, a pragmatic approach to model it. As a first step, theworkshop experts recommended strengthening knowledge exchange between the ST researchcommunity and the environmental-chemistry community.
SA system codes ST models validation
The discussions at the workshop raised questions about the level of detail required in the models, and theeffect of uncertainties in the RCS and containment chemistry models on the calculated source terms andconsequences. For “end-user” applications such as radiological consequence assessments and emergency
planning, the models have to be simple and conservative. However, even the apparently simple models
have to be based on a good understanding and representation of the behaviour of iodine, at the level treatedby the more detailed models, in order to ensure that the important phenomena are correctly taken into
account and scaled.
It was noted that international code benchmarking exercises had been performed in the past on
specific RTF, THAI and PHEBUS FP tests and provided important lessons and that further benchmarking
exercises are proposed for some THAI tests in the NUGENIA/SARNET frame. However, in order toaddress more specifically the scaling issue, benchmarking exercises for well selected reactor-like scenarios
would be of higher value.
Experts recommended organizing further exchanges to deepen the assessment of the validity of ST
related models implemented in severe accident system codes – such as ASTEC, MAAP, MELCOR –notably by addressing: the definition of a reference code-validation matrix for iodine behaviour2 in RCS andcontainment; the necessary level of detail for the modelling of the complex iodine behaviour for plantcalculations;
 the model validation for plant calculations (scaling issue) by performing, e.g., some SA systemcode benchmarking exercise for defined reactor-like scenarios notably to assess the effect ofrecent chemical models development.
Reference ST evaluations and emergency response tools
Final objectives of the ST research is to contribute to the consolidation of reference ST calculations usednotably for protection measures design and of fast running calculation tools used as support to emergency
response. The question of the methodology for the implementation of ST research outcomes into reference
ST calculations and into fast running tools and of the assessment of their robustness was judged a key
issue.

2

As was done through an NEA exercise for containment thermal hydraulics.

49

NEA/CSNI/R(2016)5
Concerning ST calculations, the experts recommended strengthening exchanges on source term
evaluations performed for NPPs in various countries (results and methodologies) and on related
quantification of remaining uncertainties on the related modelling, this in view of checking that the ST
research is properly focused and that no remaining important issue has been forgotten. The phase 2 of the
NEA BSAF project which will start mid-2015 will concentrate on the assessment of radioactive substances
releases and dispersion in the environment during the Fukushima Daiichi’s accident and will contribute tothis objective.
Concerning fast-running tools for emergency response, the workshop experts recommended
strengthening links between the ST research community and the emergency response community.
Initiatives in this field should be promoted following the conclusions of the NEA FASTRUN
benchmarking exercise conducted for fast-running emergency response tools that was organized by the
WGAMA and WPNEM (report NEA/CSNI/R(2015)19). Also, they should complement work that will be
performed through the FASTNET project, which will start mid-2015 and is funded in the EC H2020 frame,
dealing with methodological developments of more advanced prognostic/diagnosis emergency response
tools.

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APPENDIX 1: WORKSHOP AGENDA

MONDAY MARCH 30, 2015
Registration

8:30-9:00

Welcome, opening remarks(D. Jacquemain, IRSN; J.P. Van Dorsselaere, NUGENIA/SARNET, N. Blundell, OECD/NEA)

9:00-9:20

Session 1 Main lessons from experimental programmes on iodine and ruthenium
behaviour in a severe accident and research perspectives
Session Chairs: T. Albiol (IRSN), H. Hoshi (NRA)
1.1 Main Outcomes Relative to Iodine Behavior from the VERDON/ISTP
Programme (Y. Pontillon, CEA)
1.2 Research on the Transport and Chemistry of Iodine in Primary Circuitand Containment Conditions at VTT (T. Karkela, VTT)
1.3 Main Findings of the IRSN Experimental Programmes Performed on
Iodine Chemistry in Severe Accident Conditions (A.C. Grégoire, IRSN)
1.4 THAI Experiments on Volatility, Distribution and Transport Behavior of Iodine
and Fission Products in the Containment (S. Gupta, Becker Tech.)

10:35-11:00

Coffee Break

11:00-11:30

9:20-9:45
9:45-10:10
10:10-10:35

1.5 A Summary of the PSI Investigations on Iodine Chemistry in the Presenceof Impurities and Additives (T. Lind, PSI)
1.6 Recent Findings on Ruthenium Chemistry in a Severe Accident(T. Karkela, VTT)
1.7 Thoughts on Ruthenium Research (M. Salay, USNRC)

11:55-12:20
12:20-12:45

Lunch

12:45-14:00

1.8 The Main Outcomes of the OECD Behavior of Iodine Project (G. Glowa, CNL)
1.9 The Behavior of Iodine Project: a Proposal for BIP3 (P.A. Yakabuskie, CNL)
1.10 OECD/STEM Project and Its Follow-up STEM2 (C. Mun, IRSN)

14:00-14:25
14:25-14:50
14:50-15:15

Coffee Break

15:15-15:45

Session 2 Iodine transfer to aqueous phase, aqueous-phase chemistry
Session Chairs: F. Funke (AREVA GmbH), J. Song (KAERI)
2.1 Iodine and Silver Wash-Down Modelling in COCOSYS-AIM
by Use of THAI Results (G. Weber, GRS)
2.2 Effect of Constituents of Seawater on Formation of Volatile Iodine by
Aqueous Phase Radiation Chemistry (K. Hata, JAEA)

51

11:30-11:55

15:45-16:10
16:10-16:35

NEA/CSNI/R(2016)5
2.3 Formation and Release of Molecular Iodine in Aqueous Phase Chemistryduring Severe Accident With Seawater Injection (K. Kido, JAEA)

16:35-17:00

End of day 1

17:00

TUESDAY MARCH 31, 2015
Session 3 Organic iodide formation and partitioning, chemistry and interactions with aerosols,
droplets and surfaces in the gas phase
Session Chairs: G. Glowa (CNL), M. Salay (NRC)
3.1 Chalmers Project on Volatile Organic Iodine Species Behaviour and Retention
under Severe Nuclear Accident Conditions in LWR(S. Tietze, Chalmers Univ.)
9:00-9:25
3.2 OIPHA-I and II Model – Mathematical Models to Describe the Partitioning andHydrolysis Behaviour of Organic Iodides Between Gaseous and Aqueous Phase(S. Tietze, Chalmers Univ.)
9:25-9:50
3.3 The Radiolysis of Gaseous Methyl Iodide in Air (S. Bowskill, NNL)
9:50-10:15
Coffee Break

10:15-10:45

3.4 The IODAIR Model for Radiolysis of Gaseous Iodine Species in Air: Data
Comparison and Predictions (S. Dickinson, NNL)
3.5 Interaction of Gaseous I2 with Painted Surfaces and Aerosols in
Large-Scale THAI Tests (F. Funke, AREVA GmbH)3.6 Effect of Water-droplet Sizes on the Migration of Volatile
I2 and CH3I (H.-J. Im, KAERI)
Lunch

10:45-11:10
11:10-11:35
11:35-12:00
12:00-13:15

Session 4 Filtered containment venting systems
Session Chairs: T. Lind (PSI), L.E. Herranz (CIEMAT)
4.1 Westinghouse Molecular Sieve Technology (A. Andren, Westinghouse)
4.2 Improved Iodine Retention at Filtered Containment Venting(P. Zeh, AREVA GmbH)
4.3 Design on the Performance Test of Filter Containment Venting System(K.-S. Ha, KAERI)
4.4 Thermal-hydraulic Effect on Iodine Retention in the Filtered Containment
Venting System (Y.-S. Na, KAERI)
4.5 Investigation of Iodine Retention in a Filtered Containment Venting Systemin the VEFITA Test Programme (D. Suckow, PSI)
4.6 The European PASSAM Project on Atmospheric Source Term Mitigation:
Half-Way Progress and Main Results (T. Albiol, IRSN)

15:20-15:45

Coffee Break

15:45-16:15

13:15-13:40
13:40-14:05
14:05-14:30
14:30-14:55
14:55-15:20

Session 5 Recent developments in iodine chemistry modelling in severe accidents and applications tosource term and radiological consequences evaluations
Session Chairs: S. Dickinson (NNL), D. Jacquemain (IRSN)
5.1 Iodine Chemistry in Design Basis Faults(A. Zodiates, EDF Energy)
16:15-16:40
52

NEA/CSNI/R(2016)55.2 Iodine Behaviour in the Circuit and Containment: Modelling Improvementsin the Last Decade and Remaining Uncertainties (L. Bosland, IRSN)
5.3 Containment Iodine Chemistry (M. Salay, USNRC)

16:40-17:05
17:05-17:30

End of day 2

17:30

WEDNESDAY APRIL 1, 2015

Session 5 cont. Recent developments in iodine chemistry modelling in severe accidents andapplications to source term and radiological consequences evaluations
Session Chairs: S. Dickinson (NNL), D. Jacquemain (IRSN)
5.4 Application of the COCOSYS Iodine Chemistry Model AIM-3 to PWR
Containment Analyses Using New Validation Results of RTF and CAIMAN
Experiments (H. Dimmelmeier, AREVA GmbH)
9:00-9:25
5.5 Iodine Source Term Computations with ASTEC, Link with PSA Level 2 Tools
and Fast Running Source Term Tools For Emergency Organisation(K. Chevalier-Jabet, IRSN)
9:25-9:50
5.6 In What Extent the Iodine Reactivity in the Atmosphere Can Impact theRadiological Consequences (J. Trincal, IRSN)
9:50-10:15
Coffee Break

10:15-10:45

Session 6 General Discussion – conclusions of the workshop (all)Session Chairs: L. E. Herranz, D. Jacquemain, M. Kissane

10:45-12:30

Lunch

12:30-13:45

End of day 3

13:45

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NEA/CSNI/R(2016)5

APPENDIX 2: LIST OF PARTICIPANTS

BELGIUM
Dr ADORNI Martina
BEL V
Rue Walcourt 148
Brussels
BELGIUM
Phone: +3225280384
E-mail: martina.adorni@belv.be

Ms OURY Laurence
TRACTEBEL ENGINEERING (GDF Suez)
Avenue Ariane 71200 Bruxelles
BELGIUM
Phone: +3227737775
E-mail: laurence.oury@gdfsuez.com

CANADAMr GLOWA GlennCANADIAN NUCLEAR LABORATORIESChalk River Laboratories
286 Plant Road
Chalk River, Ontario, K0J 1J0
CANADA
Phone: +16135848811 ext. 46052
E-mail: glenn.glowa@cnl.ca

Dr YAKABUSKIE Pamela
CANADIAN NUCLEAR LABORATORIESChalk River Laboratories
286 Plant Road
Chalk River, Ontario, K0J 1J0
CANADA
Phone: +16135843311 ext 43146
E-mail: pam.yakabuskie@cnl.ca

FINLANDMr AUVINEN Ari
VTT TECHNICAL RESEARCH CENTRE
OF FINLAND
Biologinkuja 7
P.O. Box 1000
FI-02044 VTT, Espoo
FINLAND
Phone: +358207225785
E-mail: ari.auvinen@vtt.fi

Mr KÄRKELÄ Teemu
VTT TECHNICAL RESEARCH CENTRE
OF FINLAND
Biologinkuja 7
P.O. Box 1000
FI-02044 VTT, Espoo
FINLAND
Phone: +358207225718
E-mail: teemu.karkela@vtt.fi

55

NEA/CSNI/R(2016)5
FRANCEDr EZZIDI Alexandre
AREVA NP
1 place Jean Millier
92084 Courbevoie
FrancePhone: +33134967733
E-mail: alexandre.ezzidi@areva.com
Mr DUCROS GérardCEA
CE Cadarache, bâtiment 315
13108 Saint-Paul-lez-Durance Cédex
FrancePhone: +33442256150
E-mail: gerard.ducros@cea.fr
Dr HANUS Eric
CEA
CE Cadarache
13108 Saint-Paul-lez-Durance Cédex
FrancePhone: +33442254566
E-mail: eric.hanus@cea.fr
Mrs BOSCHIERO Marie-Hélène
EDF
12-14 avenue Dutrievoz
69628 Villeurbanne Cédex
FrancePhone: +33472827163
E-mail: marie-helene.boschiero@edf.fr
Mrs MAURICE Aubélia
EDF
12-14 avenue Dutrievoz
69100 VilleurbanneFrancePhone: +33472827375
E-mail: aubelia.maurice@edf.fr

Dr GALLAIS-DURING Annelise
CEA
DEN/CAD/DEC/SA3C/LAMIRCE Cadarache, bâtiment 315
13108 Saint-Paul-lez-Durance Cédex
FrancePhone: +33442254993
E-mail: annelise.gallais-during@cea.fr
Mr PONTILLON Yves
CEA
DEN/CAD/DEC/SA3C/LAMIRCE Cadarache, bâtiment 315
13108 Saint-Paul-lez-Durance Cédex
FrancePhone: +33442257226
E-mail: yves.pontillon@cea.fr
Mr FORTIN Matthias
EDF
SEPTEN, TE/CE
12-14 avenue Dutrievoz
69628 VilleurbanneFrancePhone: +33472824764
E-mail: matthias.fortin@edf.fr
Ms ROZEL Christelle
EDF
SEPTEN
12-14 avenue Dutrievoz
69628 VilleurbanneFrancePhone: +33472827065
E-mail: christelle.rozel@edf.fr

Dr TOURNIAIRE Bruno
EDF
SEPTEN
12-14 avenue Dutrievoz
69628 VilleurbanneFrancePhone: +33472827884
E-mail: bruno.tourniaire@edf.fr
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NEA/CSNI/R(2016)5
Mr ALBIOL Thierry
IRSN
PSN-RES/SEREXCE Cadarache, bâtiment 328
BP 313115 Saint-Paul-lez-Durance Cédex
FrancePhone: +33442499794
E-mail: thierry.albiol@irsn.fr

Dr BOSLAND LoïcIRSN
PSN-RES/SAG/LETR
CE Cadarache, bâtiment 702
BP 313115 Saint-Paul-lez-Durance Cédex
FrancePhone: +33442199446
E-mail: loic.bosland@irsn.fr

Mr CANTREL Laurent
IRSN
PSN-RES/SAG/LETR
CE Cadarache, bâtiment 702
BP 313115 Saint-Paul-lez-Durance Cédex
FrancePhone: +33442199450
E-mail: laurent.cantrel@irsn.fr

Mrs CHEVALIER-JABET Karine
IRSN
PSN-RES/SAG/LETR
CE Cadarache, bâtiment 702
BP 313115 Saint-Paul-lez-Durance Cédex
FrancePhone: +33442199364
E-mail: karine.chevalier-jabet@irsn.fr

Mr COUSIN Frédéric
IRSN
PSN-RES/SAG/LETR
CE Cadarache, bâtiment 702
BP 313115 Saint-Paul-lez-Durance Cédex
FrancePhone: +33442199479
E-mail: frederic.cousin@irsn.fr

Dr FLEUROT JoëlleIRSN
PSN-RES/SAG/LETR
CE Cadarache, bâtiment 702
BP 313115 Saint-Paul-lez-Durance Cédex
FrancePhone: +33442199521
E-mail: joelle.fleurot@irsn.fr

Dr GERYES TonyIRSN
BU-DCI
31 avenue de la division Leclerc
92260 Fontenay-aux-Roses
FrancePhone: +33787106500
E-mail: tony.geryes@irsn.fr

Dr GREGOIRE Anne-CécileIRSN
PSN-RES/SEREX/L2EC
CE Cadarache, bâtiment 328
BP 313115 Saint-Paul-lez-Durance Cédex
FrancePhone: +33442199740
E-mail: anne-cecile.gregoire@irsn.fr

Dr HASTE Tim
IRSN
PSN-RES/SAG/LETR
CE Cadarache, bâtiment 702
BP 313115 Saint-Paul-lez-Durance Cédex
FrancePhone: +33442199567
E-mail: tim.haste@irsn.fr

Dr JACQUEMAIN Didier
IRSN
PSN-RES/SAG
CE Cadarache, bâtiment 702
BP 313115 Saint-Paul-lez-Durance Cédex
FrancePhone: +33442199565
E-mail: didier.jacquemain@irsn.fr

57

NEA/CSNI/R(2016)5
Mrs MONSANGLANT-LOUVET CélineIRSN
Centre de Saclay
BP 68
91192 Gif-sur-Yvette Cedex
FrancePhone: +33169082913
E-mail: celine.monsanglant-louvet@irsn.fr

Dr MORIN Sandrine
IRSN
PSN-RES/SEREX/L2EC
CE Cadarache, bâtiment 328
BP 313115 Saint-Paul-lez-Durance Cédex
FrancePhone: +33442199620
E-mail: sandrine.morin@irsn.fr

Dr MUN ChristianIRSN
PSN-RES/SEREX/L2EC
CE Cadarache, bâtiment 328
BP 313115 Saint-Paul-lez-Durance Cédex
FrancePhone: +33442199624
E-mail: christian.mun@irsn.fr

Mr TRINCAL Julien
IRSN
PSN-RES/SAG/LETR
CE Cadarache, bâtiment 702
BP 313115 Saint-Paul-lez-Durance Cédex
FrancePhone: +33442199187
E-mail: julien.trincal@irsn.fr

Mr VAN DORSSELAERE Jean-Pierre
IRSN
PSN-RESCE Cadarache, bâtiment 250
BP 313115 Saint-Paul-lez-Durance Cédex
FrancePhone: +33442199709
E-mail: jean-pierre.van-dorsselaere@irsn.fr

Mr VEILLY Edouard
IRSN
31 avenue de la division Leclerc
92260 Fontenay-aux-Roses
FrancePhone: +33158357162
E-mail: edouard.veilly@irsn.fr

Mr VOLA Didier
IRSN
PSN-RES/SAG
CE Cadarache, bâtiment 702
BP 313115 Saint-Paul-lez-Durance Cédex
FrancePhone: +33442199714
E-mail: didier.vola@irsn.fr

Dr STREKOWSKI Rafal
MARSEILLE UNIVERSITY
Université Aix-Marseille
Campus Saint-Charles
3 place Victor Hugo
13331 MarseilleFrancePhone:
E-mail: rafal.strekowski@univ-amu.fr

GERMANYDr DIMMELMEIER Harald
AREVA GmbH
Henri-Dunant-Strasse 50
D-91058 Erlangen
GERMANY
Phone: +49913190095035
E-mail: harald.dimmelmeier@areva.com

Mr FUNKE Friedhelm
AREVA GmbHPaul-Gossen-Strasse 100
91052 Erlangen
GERMANY
Phone: +49913190097681
E-mail: friedhelm.funke@areva.com

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NEA/CSNI/R(2016)5
Dr ZEH Peter
AREVA GmbHPaul-Gossen-Strasse 100
91052 Erlangen
GERMANY
Phone: +49913190097690
E-mail: peter.zeh@areva.com
Dr GUPTA Sanjeev
BECKER TECHNOLOGIES GmbH
Koelner strasse, 06
65760 Eschborn
GERMANY
Phone: +496196936115
E-mail: gupta@becker-technologies.com

Mr POSS Gerhard
BECKER TECHNOLOGIES GmbH
Koelner strasse, 06
65760 Eschborn
GERMANY
Phone: +496196936101
E-mail: poss@becker-technologies.com

Mrs BECK Sara
GRS
Schwertnergasse 1
50667 Köln
GERMANY
Phone: +492212068722
E-mail: sara.beck@grs.de

Mr LOVASZ Liviusz
GRS
Boltzmannstr. 14
85748 Garching
GERMANY
Phone: +498932004458
E-mail: liviusz.lovasz@grs.de

Dr WEBER Gunter
GRS
Forschungszentrum
Boltzmannstrasse 14
D-85748 Garching
GERMANY
Phone: +498932004506
E-mail: gunter.weber@grs.de

Dr WEBER SebastianGRS
Boltzmannstr. 14
85748 Garching
GERMANY
Phone: +498932004438
E-mail: sebastian.weber@grs.de

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NEA/CSNI/R(2016)5
INTERNATIONAL ORGANIZATIONSDr SIMOLA Kaisa
EUROPEAN COMMISSIONJOINT RESEARCH CENTRE
P.O. Box 2
NL 1755 ZG Petten
The Netherlands
Phone: +31224565180
E-mail: kaisa.simola@ec.europa.eu
Mr BLUNDELL NeilOECD NUCLEAR ENERGY AGENCY
Le Seine St-Germain
12 boulevard des iles
9213 Issy-les-MoulineauxFrancePhone: +33625043143
E-mail: neil.blundell@oecd.org

Dr KISSANE Martin
OECD NUCLEAR ENERGY AGENCY
12 boulevard des îles92130 Issy-les-MoulineauxFrancePhone: +33145241054
E-mail: martin.kissane@oecd.org

ITALY
Mr GONFIOTTI Bruno
UNIVERSITY OF PISA
Largo Luciano Lazzarino
56122 Pisa
ITALY
Phone: +393331716086
E-mail: bruno.gonfiotti@for.unipi.it

JAPAN
Dr HATA Kuniki
JAPAN ATOMIC ENERGY AGENCY
2-4 Shirane Shirakata
Tokai-mura, Naka-gun
Ibaraki 319-1195
JAPAN
Phone: +81292826778
E-mail: hata.kuniki@jaea.go.jp
Dr HOSHI Harutaka
NUCLEAR REGULATION AUTHORITY
Roppogi-First Bldg
1-9-9 Roppongi, Minato-ku, Tokyo
JAPAN
Phone: +81351142224
E-mail: harutaka_hoshi@nsr.go.jp

Dr KIDO Kentaro
JAPAN ATOMIC ENERGY AGENCY
2-4 Shirane Shirakata
Tokai-mura, Naka-gun
Ibaraki 319-1195
JAPAN
Phone: +81292826159
E-mail: kido.kentaro@jaea.go.jp
Mr NUKATSUKA Shigehiro
ENERGIS Co., Ltd.
16-12, Shimbashi 6-Chome
Minato-ku, Tokyo 105-0004
JAPAN
Phone: +81368802312
E-mail: shigehiro_nukatsuka@mshi.co.jp

60

NEA/CSNI/R(2016)5
POLAND
Ms SKOLIK Katarzyna
AGH UNIVERSITYOF SCIENCE AND TECHNOLOGY
al. A. Mickiewicza 30
30-059 Cracow
POLAND
Phone: +48662631850
E-mail: kskolik@agh.edu.pl
Mr MALICKI Mateusz
PAA
NATIONALATOMICAGENCYul. Krucza 36
00-522 Warszawa
POLAND
Phone: +48226959924
E-mail: malicki@paa.gov.pl

Mr WLOSTOWSKI Mateusz
ENERGY PAANATIONALATOMICAGENCYul. Krucza 36
00-522 Warszawa
POLAND
Phone: +48226959724
E-mail: wlostowski@paa.gov.pl

ENERGYREPUBLIC OF KOREADr HA Kwang Soon
KOREA ATOMIC ENERGYRESEARCH INSTITUTE
989-111 Daedeok-daero
Yuseong-gu
Daejeon, 305-353
REPUBLIC OF KOREA
Phone: +82428688653
E-mail: tomo@kaeri.re.kr
Dr NA Young Su
KOREA ATOMIC ENERGYRESEARCH INSTITUTE
989-111 Daedeok-daero
Yuseong-gu
Daejeon, 305-353
REPUBLIC OF KOREA
Phone: +82428688522
E-mail: ysna@kaeri.re.kr
Dr LEE Jongseong
KOREA INSTITUTE OF NUCLEAR SAFETY
62 Gwahak-ro, Yuseong-gu,
Daejeon 305-338
REPUBLIC OF KOREA
Phone: +82428680670
E-mail: jongseong@kins.re.kr

Dr IM Hee-Jung
KOREA ATOMIC ENERGYRESEARCH INSTITUTE
Nuclear Chemistry Research Division
Deokjin-dong
Yuseong-gu
Daejeon, 305-353
REPUBLIC OF KOREA
Phone: +82428684740
E-mail: imhj@kaeri.re.kr
Dr SONG JinHo
KOREA ATOMIC ENERGYRESEARCH INSTITUTE
989-111 Daedeok-daero
Yuseong-gu
Daejeon, 305-353
REPUBLIC OF KOREA
Phone: +82421041849396
E-mail: dosa@kaeri.re.kr
Mr LEE Seongnyeon
KOREA INSTITUTE OF NUCLEAR SAFETY
62 Gwahak-ro, Yuseong-gu,
Daejeon 305-338
REPUBLIC OF KOREA
Phone: +82428680817
E-mail: adult@kins.re.kr

61

NEA/CSNI/R(2016)5SPAIN
Dr HERRANZ Luis E.
CIEMAT
Head of Unit for Nuclear Safety Research
Division of Nuclear Fission
Department of Energy
Avenida Complutense, 40
28040 MADRID, SPAIN
Phone: +34913466219
E-mail: luisen.herranz@ciemat.es
SWEDENDr ANDREN Anders
WESTINGHOUSE ELECTRIC SWEDEN AB
SE 721 63 Västeras
SWEDEN
Phone: +46732367765
E-mail: andrenal@westinghouse.com
Dr EKBERG Christian
CHALMERS
UNIVERSITYTECHNOLOGY
Chemical and Biological Engineering
Energy and Materials
Kemivägen 441296 Göteborg
SWEDEN
Phone: +46317722801
E-mail: che@chalmers.se

Mr KAJAN Ivan
CHALMERS UNIVERSITY of TECHNOLOGY
Kemivägen 441296 Göteborg
SWEDEN
Phone: +46723709988
E-mail: kajan@chalmers.se
Mrs TIETZE Sabrina
OF CHALMERS
UNIVERSITYOFTECHNOLOGY
Nuclear Chemistry
Department of Chemical and BiologicalEngineering
Kemivägen 441296 Göteborg
SWEDEN
Phone: +46735214168
E-mail: sabrina.tietze@chalmers.se

Dr GLANNESKOG Henrik
VATTENFALL AB
Gullbergs Strandgata 6
se-401 27
Gothenburg
SWEDEN
Phone: +46706252487
E-mail: henrik.glanneskog@vattenfall.com
SWITZERLANDMr ZIEGER Tobias
CCI AG
Itaslenstrasse 98362 Balterswi
SWITZERLAND
Phone: +41792764155
E-mail: tobias.zieger@imi-critical.com
62

NEA/CSNI/R(2016)5

Dr LIND Terttaliisa
PAUL SCHERRER INSTITUTOHSA / C11
CH-5232 Villigen PSISWITZERLAND
Phone: +41563102650
E-mail: terttaliisa.lind@psi.ch

Dr SUCKOW Detlef
PAUL SCHERRER INSTITUT
5232 Villigen PSIOHSA/C04
SWITZERLAND
Phone: +41563104145
E-mail: detlef.suckow@psi.ch

UNITED STATESDr SALAY Michael
U.S.
NUCLEARREGULATORYCOMMISSION
MS: C3-A07M
11555 Rockville Pike
Rockville, MD 20852-2739
U.S.A
Phone: +13012517543
E-mail: michael.salay@nrc.gov

UNITED KINGDOMDr DOUGALL AlexEDF ENERGY
Barnett Way
Barnwood, Gloucester GL4 3RS
UNITED KINGDOM
Phone: +441452652694
E-mail: alexandra.j.dougall@edf-energy.com

Mr PRIOR Robert
R.P. SAFETY CONSULTING LTD
2 Elwick Road
Ashford TN23 1PD
Kent
UNITED KINGDOM
Phone: +447827292920
E-mail: bob.prior@rpsafetyconsulting.com

Ms BOWSKILL Susannah
NATIONAL NUCLEAR LABORATORY
B168, Harwell Oxford Science Campus
Didcot, OX11 0QT
UNITED KINGDOM
Phone: +441925289995
E-mail: susannah.r.bowskill@nnl.co.uk

Dr DICKINSON Shirley
NATIONAL NUCLEAR LABORATORY
B168, Harwell Oxford Science Campus
Didcot, OX11 0QT
UNITED KINGDOM
Phone: +447894598707
E-mail: shirley.dickinson@nnl.co.uk

Mr IHARA Takafumi
TOKYO ELECTRIC POWER COMPANY
Berkeley Square House
Berkeley Square
London W1J 6BR
UNITED KINGDOM
Phone: +447917816226
E-mail: ihara.takafumi@tepco.co.jp

Dr ZODIATES Anastasios
EDF ENERGY
Barnett Way
Barnwood, Gloucester GL4 3RS
UNITED KINGDOM
Phone: +441452653915
E-mail: tasos.zodiates@edf-energy.com

63

NEA/CSNI/R(2016)564

NEA/CSNI/R(2016)5

APPENDIX 3: PAPERS BY TECHNICAL SESSIONS

65

NEA/CSNI/R(2016)5

66

NEA/CSNI/R(2016)5

Session 1: main lessons from experimental programmes on iodine and ruthenium behaviour in asevere accident and research perspectives

67

NEA/CSNI/R(2016)5
MAIN OUTCOMES RELATIVE TO IODINE BEHAVIOUR FROM THE VERDON/ISTP
PROGRAMME
Y. Pontillon*, S. Bernard, A. Gallais-During, B. Gleizes, E. Hanus and G. Ducros
CEA, DEN, DEC, F-13108 Saint-Paul-lez-Durance, France*Corresponding author, tel: (+33) 442257226, Email:yves.pontillon@cea.fr

Abstract – Following the TMI-2 accident in 1979, numerous programmes have been launched in France
and abroad, in order to quantify the release or fission products (FP) from irradiated fuel and to better
assess the source term under severe accident conditions. In France, the CEA has conducted the VERCORS
programme, composed of 17 separate effect tests (SET), which is considered to be complementary to theintegral-type PHEBUS-FP tests programme. After the successive shutdowns of the VERCORS (2002) and
PHEBUS-FP (2004) facilities, a thorough identification of the remaining uncertainties in source term
assessment and modelling was established by CEA, EDF and IRSN. This work led to the launch of theInternational Source Term Programme (ISTP).
The ISTP is composed of SET, taken in charge by IRSN and CEA. The CEA part of this programme is the
VERDON programme dedicated to: (1) FP release from high burn-up UO2 and MOX fuels, in particular
for low volatile FP and (2) impact of air ingress on FP behaviour, which is expected to enhance therelease of highly radiotoxic FP, such as ruthenium, as well as to re-volatilise volatile FP (iodine, caesium)
previously deposited in the primary circuit. This VERDON programme has been carried out in a newlaboratory of the CEA Cadarache, where two dedicated hot cells and one glove box have been built for thispurpose. Three VERDON tests covering the first topic (FP release) were performed: VERDON-1(September 2011) on UO2 at 72 GWd/t in reducing conditions, VERDON-3 (April 2013) and VERDON-4(October 2014) on a 60 GWd/t MOX fuel respectively under steam oxidising and hydrogen reducing
conditions. The VERDON-2 test (June 2012), covering the air ingress topic, was performed on a 60 GWd/t
MOX fuel, using a more complex experimental circuit including in particular thermal gradient tubes
downstream the furnace where the fuel sample is heated.
The main outcomes relative to the iodine behaviour identified from this experimental programme arepresented, focusing in particular on the following aspects:
 Release kinetics
o observed for high burn-up UO2 fuel and MOX fuel, compared to the kinetics measured onstandard UO2 fuel in the previous VERCORS programme,o at lower temperature simulating a LOCA,
 The iodine deposits and re-volatisation observed in VERDON-2, following the air injection, The gaseous iodine fraction measured at the outlet of the VERDON circuit, simulating the entrance
in the containment.

68

NEA/CSNI/R(2016)5
I. INTRODUCTION
One of the most important areas of researchconcerning a hypothetical severe accident in a
Light Water Reactor (LWR), on both a Frenchand an international level, is determining thesource term, i.e. quantifying the nature, release
rate and global released fraction of fission
products (FP) and other radioactive materials.
This is in great part due to the consequences of
the Three Mile Island (1979), Chernobyl (1986)
and more recently Fukushima accidents. In thecourse of this type of scenario, the chain of
events can effectively result in primary coolant
boiling and draining, meaning that it is no longer
cooling the core. One direct result is the coremelting and the release of FP and structural
and/or activated control rod elements (e.g.
activation products, AP) into the containmentbuilding. If there is a failure in the various
protective barriers, the FP/AP can leak out of thecontainment building and be released into theenvironment.
A large numbers of research programmes havethus been undertaken on this subject in various
countries. In line with this approach, IRSN
(France) has been the driving force and has
conducted programmes specifically focusing on
determining the source term, with particularefforts devoted to understanding themechanisms that lead to the release of FP, since
only a very complete knowledge of thephenomena governing the behaviour of FP/AP
under such constraints will make it possible todefine the actions that need to be planned (and/or
performed) to minimise emissions and optimise
the protection of both the people and theenvironment. The HEVA [1]/VERCORS [2]programmes were thus initiated by the CEA.
VERCORS has considerably broadened the fieldof application by exploring higher temperaturesand by testing a wider range of fuels (UO2,
MOX, debris bed configurations, high burn-ups)
in a more complex experimental installation withimproved instrumentation. It was composed of
17 tests which were conducted over 14 years, inaccordance with 3 experimental phases. A firstseries of six tests (VERCORS 1 to VERCORS 6)
was conducted between 1989 and 1994 on UO2
fuels in a higher temperature range than thepreceding HEVA phase, close to the fuelrelocation [3]. This series made it possible tointegrate certain FP with low volatility into the
HEVA results database. Two series of tests –VERCORS HT and RT– were then conducted
alternately throughout 1996-2002 allowing thedata base extension up to the less volatile FP.
These analytic experiments simulating severe
LWR accidents were financed jointly by IRSN
and EDF. Their aim was to quantify the released
fraction and release rates of FP from irradiatednuclear ceramics (UO2 or MOX, typically three
PWR pellets in their original cladding), quantify
the nature of the gases and aerosols emitted(particle
size
analysisandchemicalcomposition), and understand how the fueldegrades. These experimental sequences werecarried out in a very high activity cell and werecommonly considered to be complementary to
the PHEBUS FP [4] integral tests andcomparable with certain tests carried out abroad:
HI/VI [5] in the United States, VEGA [6] inJapan or the programme conducted at the CRL in
Canada [7]. The experimental results of thisprogramme are used to (a) define the envelope
values for released fraction within the scope ofassessing reference source terms for all French
PWRs, and (b) validate the semi-empirical ormechanistic models regarding FP release andtransport while qualifying the simulation codes
by integrating these models [8], [9], [10], [11].
However, major uncertainties still remain insome fields, concerning the assessment of risks
for populations and the environment [12]. As aconsequence, it has been decided to build a cooperative research programme between teams
involved in severe accident (SA) phenomenology
all over the world (US-NRC, IRSN, CEA, EDF,
GDF Suez, PSI, European Commission, EACL,KAERI), based on separate-effect experiments
and called “International Source Term Program(ISTP)”. The results of these separate-effectexperiments would allow improving models used
for Source Term evaluation studies. Four main
R&D research axes have been included in thisprogramme: (1) iodine behaviour in the RCS andthe containment, (2) study of the boron carbide
effect on fuel degradation and FP release, (3)study of the air effect on fuel and FP behaviour

69

NEA/CSNI/R(2016)5
and (4) study of the fission product release fromthe fuel.
As far as the source term quantification isconcerned, four VERDON tests were considered.
They are devoted to FP release from high burnup UO2 fuel, MOX fuels and air ingress scenario.
They were performed in the new VERDON
laboratory at the CEA Cadarache Centre. After
having described this new VERDON laboratory(section II), an overview of the four ISTP
VERDON tests is proposed (section III). In thelast part of the paper (section IV), the main
outcomes relative to the iodine behaviour
identified from this experimental programme will
be presented, focusing in particular on three main
aspects:

the fluid injection system, (b) the furnace, (c) an
aerosol filter located directly on the top of thefurnace (Its filtering part is constituted ofstainless steel poral which function is to stop all
the fission products under aerosol forms. The
aerosol filter is heated at 150°C10%), (d) a
May-Pack filter, filled with zeolite (impregnatedwith silver) in order to trap potential moleculariodineand heated at 150°C10% to avoid
condensation, (e) a condenser the function ofwhich is to condense steam from theexperimental gas and to recover the water for
analysis, (f) a final safety filter, which filtering
part is constituted of stainless steel poral, inorder to stop any residual trace of fission
products (other than gaseous Xe, Kr). Upstream
from the condenser, the circuit is constituted ofstainless steel tubes heated at 150°C10%.
Downstream, the condenser is linked to the final
safety filter. Outside of the cell, a “linking line”
is used to make the junction between the C5 celland the gloves box.

 Release kinetics
o observed for high burn-up UO2 fuel and
MOX fuel, compared to the kinetics
measured on standard UO2 fuel in theprevious VERCORS programme,o at lower temperature simulating a
LOCA type accident, The iodine deposits and re-volatisation
observed in VERDON-2, following the air
injection, The gaseous iodine fraction measured at theoutlet of the VERDON circuit, simulating
the entrance in the containment.

C4
C5

Glove box

II. THE VERDON LABORATORYThe four ISTP VERDON tests were conducted
between 2011 and 2014 in an entirely newlaboratory at the CEA Cadarache Centre in the
LECA-STAR facility [13]. It is constituted of
two high activity cells (called C4 and C5) and a
gloves-box, as illustrated in Figure 1. The C4 cellis dedicated to the sample reception, pre/posttests gamma scanning and loop elements storage.
The C5 cell contains the experimental circuit
itself (i.e. VERDON loop, including two
configurations). It is dedicated to the accidental
sequence
realisation
andtoon-linemeasurements. The glove-box main functions are
to analyse and store the fission and carrier gases.

Figure 1: Schematic view of the VERDON
laboratory

The VERDON loop in its release configuration is
illustrated in Figure 2. This experimental loop is
constituted of (along the path of gas flow): (a)70

NEA/CSNI/R(2016)5
Downstream from the TGTM, the fluid goes
through a 150°C heated Inconel section andgoes successively to: (a) an aerosol filter or (b)a cascade impactor composed of a successionof 6 impaction stages designed to collect theaerosols on drilled discs with decreasing
diameter openings from 1.2 mm to 0.2 mm, two
successive batteries of beds designed to collectsubmicronics aerosols and finally a Poral filter(grade-3). The impactor operates for few
minutes at the end of the test in order toquantify the particle size distribution of theaerosols. It is heated to allow 150°C in theimpactor. The May-Pack filter and thecondenser are similar to those of the releaseloop and located downstream of the aerosols
filter and cascade impactor. Downstream, thecondenser is linked to a P4VP filter made of“polyvinylpyridine” in order to trap the
potential gaseous ruthenium tetra-oxide
potentially produced during the experiment.

Furnace & filter g sightings
Aerosol filter
Induction furnace

C5 VERDON

May-Pack filter

Release Experimental Loop
AEROSOLFILTER

Filtersighting
150 C

150 C

150 C
VACUUM

CONDENSERHELIUMSUSCEPTORFURNACE
Plan 08 11 07_01

Four VERDON

Fourniture
CEA

300 C

CEA
LAMIR
DEC-SA3CCadaracheschoopage

Tol
Souduresétanches: à discuter

EchelleEntrées gaz et vapeur d'eau chauffée
générales: 1 en format A4 Quantité : 1

Vérification étanchéité avant
Vérification étanchéité avantschoopage

Composants :Tuyau 6 x 4 type 304 ou 316

3 raccords Staubli
à souderx/y "- 01 030
2 générateur de-vapeur
06-2 antiretour
Swagelock - SS 56SW6T

P.P.M algouyres
Vérificateur: Gildas07/11/2007
Courtois

Schoopage--Après
- Schoopage
test d'étanchéité.
cuivre épaisseur 2 mm
final alumine (isolation thermique)

Furnacesighting

50 mm

SoudureétancheSoudureétancheWater steam

6060
mm
mm

RR== 1212
190 +mm 20 mm

Zone
dedeschoopage
Zone
cuivreTube 6 x 4
ultérieur
ultérieur(e(e=2= mm)2 mm)+ +schoopage
schoopagealumine

EXPERIMENTAL GAS

150 C(He, H2)

150 C

Swagelockgénérateur
vapeursans
lesrepésenté- retour2 anti

SS 56SW6T

coquilles de cuivre

Soudures étanches

Soudures étanches
Soudures étanches

Soudures étanches
Soudures étanches

MAY-PACK
30 C

Pyromèter

P

May-packsighting

Glove Box

Figure 2: The so-called release experimental
loop (CER)
The VERDON loop in its transport configuration
is illustrated in Figure 3. In addition to FP releaseobjective, this experimental loop is also
dedicated to the study of FP deposits and theirpotential re-volatisation. It is constituted above
the furnace of: (a) a section heated at 700°C; (b)a multiple thermal gradient tubes (TGTM) in
which the temperature along the outlet of four
Inconel tubes (700 mm length, ~20 mm
diameter) is linearly decreasing from 730°C to
140°C.

In both configurations, the VERDON furnace isbased, as the previous VERCORS one, on
induction technology. Schematically, it isconstituted of a coil surrounding a tungsten
susceptor tube, which is the heating componentof the furnace. A high frequency power supply
generates a current in the coil. By electromagnetic coupling, a current is generated into thesusceptor tube and the corresponding electricenergy is converted to thermal energy by Jouleeffect, leading to the heating of the susceptor
tube. Then, the fuel sample, located at the centreof the susceptor tube, is heated, mostly bythermal radiation.

TGTM: 4 rotating thermal gradient tubes
to simulate the FP deposition in RCS

C5 VERDON

700 C
elbow
sighting

May-Pack filter

Transport Experimental Loop
TGTM
700 C

150 C

CONDENSEUR

700 C

VACUUM

150 C

FOUR
HELIUM
HYDROGENE

Furnacesighting

30 C

AEROSOLFILTER

IMPACTOR
300 C
STEAM
EXPERIMENTAL
GAS

150 C

P
Pyrometer

MAY-PACK
Aerosolfiltersighting

Glove Box

Figure 3: The so-called transport
experimental loop (CET)FP release kinetics is measured by means ofthreecomplementary
on
line
gamma
spectrometry stations and one micro gas
chromatography apparatus.
One gamma station is aimed directly at the fuelsample and used during all the test. This gamma
station makes it possible to qualitatively measure
the FP remaining in the fuel as a function oftemperature, which explains why a relatively
imprecise quantification of the release kinetics
was obtained3. The two advantages of this station
3

71

At least 10% release have to be recorded by thisstation to guarantee a significant value, particularly

NEA/CSNI/R(2016)5
come (i) from its ability to measure directly at
the source (all the FP were measured, unlike at
the other stations where deposits upstream could
occur) and (ii) for its ability to indicate theprecise moment when the fuel relocates bydetecting the disappearance (or significant
decrease) in the signal from non-volatile FP. This
last point was well illustrated in the case of the
VERCORS series [2, 14]. This gamma station is
also used in order to perform pre and post
qualitative gamma scanning of the sample inside
the furnace respectively before and after theaccidental sequence.
One other gamma station is aimed at the largecapacity aerosol filter. It provides a very precise
measurement of the FP deposited at this point,where most of the volatile FP were found. It ishighly complementary with the previous station(fuel sight).
The last gamma station is aimed at the May-Pack(CER configuration only and allows measuringpotential gaseous iodine during the test.
The gas analysis can be also performed on-line
by a micro gas chromatograph (µGC) localized
inside the glove box. The µ-GC extends theanalysis of active gases to all the gases.

effects, are measurable by using on-line gamma
spectrometry during the experiment. The fuelsample used for VERDON-1 was a high burn up
UO2 fuel very similar to VERCORS-RT6 (samefuel assembly, same power history, and very
close burn up). For VERDON-2 to -4, thesamples were made up of a fuel section taken at
the same span of a MOX fuel rod so that they are
identical. The main parameters exploredthroughout the 4 tests during the program weremainly the maximal temperature, the oxidizingreducing conditions of the carrier fluid, the highburn-up and the nature of the fuel sample (UO2or MOX). For the four tests, an intermediate
temperature plateau was performed at 1500°C (ina steam and hydrogen atmosphere for VERDON1 and -3, pure steam for VERDON-2 and -4) inorder to fully oxidize the cladding before thetemperature ramps up to the final phase of thetest. The duration of this intermediate plateau
was about 10-15 minutes, except for VERDON-1
for which it was 50 minutes long.
During the first VERDON-1 test, the good
performances of the VERDON loop in terms oftightness, thermal-hydraulics, furnace ceramics
behavior, etc.… and of the gamma scanning andsighting have been clearly demonstrated. As aconsequence it was asserted that the VERDON
facilitywas
technology-approved.
The
comparison with VERCORS RT6 (same fuel,same thermal history, same atmosphere, etc…)
has been possible and conclusive: similar FP
released fractions at 1500°C. The VERDON loop
was then qualified in “release configuration”, incontinuity with VERCORS experiments.
The VERDON-2 test concerns high burn up
MOX fuel behavior - and corresponding FP
release and transport - under air ingressconditions. One of the main goals of this test
concerns the transport/re-volatilization study ofruthenium and previously deposited volatile
fission products in the RCS of a nuclear power
plant, thanks to the TGTM of the Transportconfiguration.

III. THE FOUR VERDON ISTP TESTSFor the four VERDON-ISTP tests (Table 1), thesample was taken on a UO2 or a MOX fuel rod,
previously irradiated in a PWR operated by the
French operator EDF. The sample consists of
two irradiated pellets in their original cladding
and two half-pellets of depleted (and unirradiated) uranium dioxide located at each endof the sample and held there by crimping thecladding so that the cladding is not fully sealed.
Before the experimental sequence, the sample
was re-irradiated at low linear power (15 to 20
W/cm) in the OSIRIS material testing reactor for
about a week, in order to recreate the short halflife FPs without any in-pile release. As aconsequence, these FPs (i.e. 99Mo, 132Te, 133I, 131I,
140
Ba…), important for their radiobiological

VERDON-3 and -4 are two complementary
experiments. Their main objective is thequantification of FP release from a high burn up
MOX fuel under steam oxidizing conditions for
VERDON-3 and hydrogen reducing conditions
for VERDON-4. Both tests were conducted atas the changes in the object geometry measured
during heating (swelling, fracturing, then fuelcollapse, etc.) significantly complicates thequantitative use of the measurement, just like theaxial migration of the FP.

72

NEA/CSNI/R(2016)5
high temperature but without reaching global
fuel relocation and with a follow-up of semivolatile fission products in order to expect
interesting post-test micro-analysis. They were
performed with the Release Circuit.



Table 1: Main characteristics of the 4 VERDONISTP tests



As far as UO2 fuels are concerned, according to
the released fractions measured by on-line
gamma station and thanks to the data obtained
via pre and post-test gamma scannings, the FP
general classification determined during the
VERCORS series, in relation to their released
fractions and specific behaviours, is confirmed
here with:
 Volatile FP (including fission gases, iodine,caesium, antimony, tellurium, cadmium,rubidium and silver) all have a high or evenalmost complete release for temperatures
representative of a SA. The nature of the test
(fuel type, initial geometry, atmosphere at
the end of the test, etc.) essentially affects therelease kinetics of these species and has little
effect on the released fraction once thistemperature level has been reached during
the test.
 Semi-volatile FP such as molybdenum,rhodium, barium, palladium and technetium
have released fraction that can attain 50% to
100%, but their redeposit are close to theemission point. In addition, the highsensitivity of the kinetics and released
fraction regarding the oxidising conditions of
the tests – already demonstrated during the
VERCORS programme – were confirmedwith VERDON. For instance, Mo release
increased under oxidising conditions throughthe formation of volatile species. On thecontrary, the release for rhodium and bariumincreased under reducing conditions. Thislatter point seems to be less pronounced
during VERDON tests on MOX fuels.
Low volatile FP such as ruthenium, niobium,
strontium, yttrium, lanthanum, cerium andeuropium have low, yet significant, released
fraction of around 3% to 10% on average,
but these values can attain 20-40% in thecase of some FP under particular conditions,e.g. oxygen potential or high burn-up. In
addition, the FP in this category are
essentially re-deposited in the hightemperature section of the test loop, i.e. near
the fuel (emission point). Furthermore, it also
appears that reducing conditions encourage
the release of strontium, cerium, europium
and lanthanum, whereas oxidising conditions
enhance the ruthenium release.
Non-volatileFP
includezirconium,
neodymium and praseodymium. Theirreleased fractions are too low to be measured
by gamma spectrometry under even theseverest of the test grids used here.

Besides, one has to note that VERDON program
highlights some FP specific release behaviours
from MOX fuels. In particular:







The relatively low release rate of Ru during
VERDON-2 (air ingress scenario),High release kinetics, beginning at rather low
temperature, for volatile (I, Cs, Te) and semivolatile (Mo) FP,
Unexpected Ba behaviour during VERDON
2 and 3 with practically the same release (i.e.
whatever the atmosphere of the test),
Specific iodine behaviour, as described in thefollowing section.
IV. MAIN OUTCOMES RELATIVE TO THE
IODINE BEHAVIOUR

The main outcomes relative to the iodine
behaviour identified thanks to the ISTP
“VERDON” programme are principally: Release kinetics
o observed for high burn-up UO2 fuel and
MOX fuel, compared to the kinetics
measured on standard UO2 fuel in theprevious VERCORS programme,
73

NEA/CSNI/R(2016)5
o at lower temperature simulating a LOCA
type thermal history.
 The iodine deposits and re-vaporisation
observed in VERDON-2, following the air
injection, The gaseous iodine fraction measured at theoutlet of the VERDON circuit, simulating
the entrance in the containment.
These three main
successively below.

aspectsare- on the release kinetics. It was also possible
to identify the influence of one of theseparameters on another. From a general pointof view, the release rate increases with theburn up, in oxidizing condition and with
MOX compared to UO2 fuels.
To illustrate the impact of the burn-up on therelease kinetics of iodine and caesium, Figure 4acompares tests VERDON-1 (UO2 6 cycles) andVERCORS RT1 (UO2 4 cycles which can beconsidered as reference test with a moderate
burn-up). An unequivocal effect on the rate ofiodine’s and caesium’s release kinetics is visible.
At the end of the cladding oxidation plateau,caesium release was around four times higher for
VERDON-1 than for RT1.
The influence of the nature of the fuels (MOX
versus UO2) is also perfectly highlighted by thecomparison between tests VERDON -2, -3 -4(MOX fuels at 60 GWd/t) and VERCORS RT1.
Whatever the atmospheres of the tests and up to
the end of the cladding oxidation plateau, thevolatiles (Cs/I) FP release kinetics is faster for
MOX compared to UO2 fuels (Figure 4b).
The influence of the atmosphere (oxidizing or
reducing) is also perfectly highlighted by thecomparison between tests VERDON-3 andVERDON-4 during the so-called oxidation
plateau. These tests are effectively similar fromthe viewpoint of both the fuel used (identicalfather rods) and the temperature ramp history up
to end of the 1500°C plateau. The comparisons
are thus direct. A marked difference in therelease for VERDON-3 (Mixed H20/H2
atmosphere) was measured in relation to
VERDON-4 (more oxidizing conditions, steam),with respectively around 40% and 60%
respectively for VERDON-3 and VERDON-4.

described

Release kinetics of Iodine (and Caesium) and
Iodine release (versus Caesium) at low
temperature
Iodine and caesium FPs are of great importancewith regard to the radiological consequences
following a severe accident in a LWR core. They
are composed of isotopes with very different
half-lives:

Short half-life for iodine (from 1 hour for 134I
to 8 days for 131I); the short-term radiologicaleffects are very high in the first few days
following an accident, but are negligible after
1 month. Iodine carries 15% of the core's
decay heat 1 day after the emergency
shutdown; In addition, iodine is of highconcern according to its ability to be released
and/or produced under gaseous forms in thecontainment.
Long half-life for caesium (30 years for
137Cs); the radiological effects, which are low
in the short term (there are nevertheless 138Cs
and 136Cs with respective half-lives of 30 min
and 13 days) stretch into long term over
several decades.

Although the global release of these two FP did
not depend (or only very little) on the test
conditions at temperatures of more than 2,300°C,
it was quite different for their release kinetics.
Inter-comparison of all the tests in the
VERCORS/VERDON grid makes it possible tohighlight: that for temperature superior than 1200°C,
the release kinetics of Cs and I are almost
identical,
 the influence of key parameters - such as theburn-up, atmosphere, fuels (MOX or UO2),
and its initial geometry (intact or debris bed)

The “LOCA” plateau (15 min at 1200°C),
performed for VERDON 3 and 4, makes it
possible to highlight a discrepancy between theiodine (131I, 133I) and caesium (134Cs, 137Cs)
behaviour. At this stage, a 10% release has beenrecorded for the caesium whereas the iodine
seems not to be released. This can be interpreted
as a difference in the behaviour between long(caesium) and short (iodine) half-life species:74

NEA/CSNI/R(2016)5

Released fraction at 1500°C plateau

70%

60%

50%

40%

30%

20%

10%

0%
RT1, UO2 47 GWd/t

VERDON 2, MOX 60GWd/t

VERDON 3, MOX 60GWd/t

VERDON 4, MOX 60GWd/t(b)
Figure 4 : (a) I and Cs Release kinetics for VERDON-1 andVERCORS RT1 (b) Release at 1500°C (after 10 minutes
plateau) during VERDON -2, -3, -4 and VERCORS RT1.
120%

2500
134Cs

VOLATILES FP:~ TOTAL RELEASE~ 100%

133I
131I

100%

137Cs
Fuel center temperature

2000

80%

1500
60%

40%
1000

Temperature (°C)

Long half-life nuclides are quasi uncreatedduring the irradiation in the OSIRIS MTR.
Thus, their repartition inside the MOX
pellets is mainly determined by the fuelthermal history during base irradiation in
EDF nuclear power plant. In the case of thecaesium, diffusion occurs to the grainboundaries and its distribution inside the
MOX pellet includes inter- and intra-granular
parts. In addition, a strong axial and radialmigration leading to an accumulation at thepellets boundaries has been observed on thisparticular rod. This behaviour can explain theearly release recorded during the “LOCA”
plateau.
Short half-life nuclides, unlike the previous
ones, completely disappear from the MOX
pellets a few days after the end of the power
plant irradiation. Thus, their repartition
inside the VERDON fuel sample is only due
to the MTR irradiation and they are mainly
located in intra-granular position of the
MOX pellets. This repartition inside the fuelmicrostructure can explain the delay in therelease of the short half-life iodine species
compared to the long half-life caesium ones.

Released fraction (% initial inventory))



20%

500
0%

-20%

3000

0
0

5000

10000
Elapsed time (s)

15000

20000

90,00%

Figure 5 : I and Cs release rate at low temperature simulating
LOCA conditions (VERDON-3).

2500

Cs137
Cs_RT170,00%

Temperatue-VERDON 1

2000

Temperature_RT1

50,00%
1500

30,00%
1000

10,00%

Temperature (°C)

released Fraction (% intial inventory) .

I133

500

-10,00%0,05000,010000,015000,020000,025000,030000,0One can note that this is the first time that thisspecific behaviour of I compared to Cs isrecorded at this level of temperature. That canhave a significant impact, regarding the source
term, in LOCA type conditions according to thecorresponding half-life.
Iodine deposits re-volatilization during
VERDON-2
The VERDON-2 thermal-hydraulic sequence isdisplayed on Figure 6a. Phase 1 and 2 weresimilar to VERDON-1 test without H2 injection.
Phase 3, under steam conditions, was dedicatedto the measurement of release, transport anddeposit of FPs without air injection. The first
TGTM rotation occurred (tubes 1-2 to tubes 2-3)
at the end of this phase. Phase 4, under air andsteam conditions (molar ratio of 1 between steam
and air) has been maintained for 70 minutes.
Phase 5 was then performed up to 2100°C. The

035000,0Elapsed time (s)

(a)

75

NEA/CSNI/R(2016)5
2100°C plateau lasted for 90 minutes. During
this plateau, the TGTM was turned after 60
minutes (tubes 2-3 to tube 3-4) and after therotation, the tubes 3 and 4 stayed during 30
minutes under the same atmosphere to be able tostudy the FP deposits and their potential revolatilisations.
The transport of volatile FPs (caesium andiodine) along the TGTM is very different. For
Cs, the deposit profile is qualitatively similar ineach tube (almost the totality of caesium wasreleased at 2000°C), showing no effect of air
injection (Figure 6b), whereas for iodine, one canobserve (Figure 6c) that the highest 131I deposits
were located in the middle of tube 1 and at theoutlet of tube 2.

2,5E+17

Tube 1

Cs 137

Tube 22,0E+17

Tube 3

Atoms number

Tube 4
1,5E+17

1,0E+17

5,0E+16

0,0E+00
0

100

200

300

400

500

600

700

500

600

700

Distance from tube entry (mm)(b)
1,8E+14

Tube 1
1,6E+14

I 131

Tube 2

1,4E+14

Tube 3
1,2E+14

Atoms number

Tube 4The central deposit observed on tube 1 has been
completely re-volatilized during the air injection
since it no longer appeared on tube 2. Deposits
on tube 3 are very low, which shows that
deposits at the outlet of tube 2 have been also revolatilized at the end of the test. This latter point
clearly highlights the difference, in terms oftransport, which exists between Cs and I,
difference which can impact chemistry occurringin the RCS during a severe accident.

1,0E+148,0E+13

6,0E+13

4,0E+13

2,0E+13

0,0E+00
0

100

200

300

400

Distance from tube entry (mm)(c)
Figure 6 : (a) VERDON-2 thermal-hydraulic sequence, 137Cs
and 131I deposit along the four TGTM tubes respectively for (b)and (c)1

TGTM 22
3

2400

Fuel center
temperature

2000

Total flow rate (Heequivalent)Gaseous iodine fraction monitored during
VERDON tests.

30

H2O steam

Air

1600

3
4

IMPACTOR

Gas samplingGS3-1997 C

25

GS4-2000 C

H2O

Temperature (°C)

800

20

GS2-1495 C

400

GS1-1033 C

0
0

3600

7200

10800

14400

18000

21600

25200

15
28800

-400
cladding oxidation peak
1495 C

-800

10

-1200
Air

-1600

5

-2000

Phase 1

Phase 2Phase 3-2400

Phase 4Phase 50Time (s)(a)

76

H2O, Air and Total flow rate (mg/s)

1200

As explained above, the May-Pack gamma
station is dedicated to the iodine behaviour andmore particularly to its gaseous species as far as
only gaseous FP remain in the experimental loop
downstream the aerosol filter. As displayed in
Figure 7, continuous accumulation of gaseousiodine is recorded by this gamma station during
VERDON-3 (similar behaviour has been also
recorded latter during VERDON 4). It is
important to notice that only iodine has been
detected in the may-pack filter, no other nuclides
are present. It is the first time along the
VERCORS and VERDON programs that iodine
has been recorded on-line on the May-Pack filter(only post-test measurements previously),highlighting here a very early arrival of gaseousiodine. Based on a rough comparison with
VERDON-2, the total amount of gaseous iodine
trapped in the May-Pack filter of VERDON-3

NEA/CSNI/R(2016)5
can be estimated to around 1% of the iodine
initial inventory.
1,0%Normalized deposit fraction

133I

0,6%

600

Fuel center temperature

Fuel center
temperature

0,4%

400

150
6,00E+13

200

2000
position (mm)
250
300
350

400

450

500

131I
133I
5,00E+13

80,0%
200

0,0%

0
0

2000

4000

6000

8000

10000

Elapsed time (s)60,0%4,00E+13

1500

3,00E+13

131I

Temperature (°C)

0,2%count rate (arbitrary unit)

Normalized deposit fraction

Besides, thanks to the high quality of the results
gained from this experimental loop together withthe fact that the VERDON laboratory is uniquein the world for studies performed on reirradiated nuclear fuels up to temperature
representative of a SA it is possible to investigate
new field of research such as for instance: LOCA
type experiments under air/steam atmosphere, airingress scenario at moderate and low
temperature, coupling between release and fueldegradation. These new topics are currently
under evaluation.

131I
800

133I
100,0%

2500

1000

131I
0,8%

Temperature (°C)

120,0%(same high burn-up fuel as VERDON-1), using
the Transport Circuit and similar atmosphereconditions as for the VERDON-2 test.

133I

2,00E+13

1000
1,00E+13

40,0%
0,00E+00

400 C

20,0%

500

inlet
0,0%

0
0

5000

10000

15000

20000

Elapsed time (s)Figure 7 : Gaseous Iodine behaviour during VERDON3

V. CONCLUSION/PERSPECTIVESREFERENCES

Four ISTP-VERDON tests were conducted
between 2011 and 2014 in an entirely newlaboratory at the CEA Cadarache Centre in the
LECA-STAR facility. These tests were dedicatedto: (1) FP release from high burn-up UO2 and
MOX fuels, in particular for low volatile FP and(2) impact of air ingress on FP behaviour, whichis expected to enhance the release of highly
radiotoxic FP, such as ruthenium, as well as tore-volatilise volatile FP (iodine, caesium)
previously deposited in the primary circuit. After
having described this new VERDON laboratory,
an overview of the four ISTP VERDON tests
was proposed. Then the main outcomes relative
to the iodine behaviour identified from thisexperimental programme have been presented.

[1] Leveque et al., 1994. "The HEVA experimental
programme". Nucl. Technol. 108, 33-44[2] Y. Pontillon, G. Ducros, P.P. Malgouyres, 2010.
"Behaviour of fission products under severe
PWR accident conditions: the VERCORS
experimental programme - Part 1: Generaldescription of the programme - Part 2: Releaseand transport of fission gases and volatile fission
products - Part 3: Release of low-volatile fission
products and actinides", Nucl. Eng. Des. 240(2010) 1843-1881.
[3] Ducros et al., 2001. "Fission Product release
under severe accidental conditions; general
presentation of the program and synthesis of
VERCORS 1 to 6 results". Nucl. Eng. Des. 208,
191-203

From a general point of view, the main insights
gained from the VERDON tests are: (1) highrelease kinetics, beginning at low temperature,for volatile (I, Cs) and even for semi-volatile(Mo, Ba) FPs, for high burn-up UO2 and MOX
fuels; (2) re-volatilization of iodine deposited in
the thermal gradient tubes has been observed
during air injection in VERDON-2 test; (3)gaseous iodine has been quantified downstream
the VERDON circuit simulating its release into
the containment for VERDON 3 and 4.

[4] Schwarz et al., 2013. ”PHEBUS-FP final seminar,
special issue”, Annals of Nuclear Energy 61(2013) 1-230.
[5] Lorenz R.A., Osborne M.F., 1995. “A summary of
ORNL fission product release tests withrecommended release rates and diffusion
coefficients”. Report ORNL/TM-12801 Nureg/CR-6261[6]A. Hidaka, 2011. “Outcome of VEGA Program onRadionuclide Release from Irradiated Fuelunder Severe Accident Conditions”, Journal of
Nuclear Science and Technology, vol. 48,(2011), 85-102

To complement the experimental data concerning
FP release and transport from high burn-up fuelunder air ingress conditions a new test is
considered. This “Air ingress” test, called
VERDON-5, will be performed on UO2 fuel77

NEA/CSNI/R(2016)5[7] Lui Z. et al., 1994. “A summary of CRL fission
product release measurements from UO2
samples during post-irradiation annealing(1983-1992)”. Report COG-92-377[8] Veshchunov, et al., 2003. "Development of themechanistic code MFPR for modelling fission
product release from irradiated UO2 fuel (part1). Development and validation of new models".
In: Proceedings of ENS Topfuel 2003,
Würzburg, Germany[9] Dubourg, R., Nicaise, G., 2003. "Development of
the mechanistic code MFPR for modellingfission product release from irradiated UO2 fuel(part 2). Application to integral tests VERCORS
4–5 and PHEBUS FPT0". In: Proceedings of
ENS Topfuel 2003, Würzburg, Germany[10] Brillant G., Marchetto C., Plumecocq W., 2010.
"Ruthenium release from fuel in accident
conditions", Radiochim. Acta, 98(5), p.267-275,
2010[11] Beuzet E. et al., 2012. "Ruthenium releasemodelling in air and steam atmospheres under
severe accident conditions using the MAAP4code", Nucl. Eng. Des. 246 (2012) 157-162[12] Clement, B., “The Phebus Fission Product andSource Term International Programs”, Int. Conf.
Nuclear Energy for New Europe 2005 Bled,
Slovenia, September 5-8, 2005
[13 ] M.P. Ferroud-Plattet et al., 2009, “CEAVERDON Laboratory at Cadarache: new hot
cell facilities devoted to studying irradiated fuelbehaviour and fission product releases undersimulated accident conditions”, Proc. HotLabInt. Conf., Prague, Czech Republic, 2009.
[14] Pontillon, Y., et al., 2005. “Study of the activerole played by UO2–ZrO2–PF interactions on
irradiated fuel collapse temperature fromVERCORS tests”. J. Nucl. Mater. 344, 265-273.

78

NEA/CSNI/R(2016)5

RESEARCH AT VTT ON THE TRANSPORT AND CHEMISTRY OF
IODINE UNDER PRIMARY CIRCUIT AND CONTAINMENT CONDITIONS
Teemu Kärkelä(1)*, Mélany Gouëllo(1), Jarmo Kalilainen(1), Pekka Rantanen(1), Ari Auvinen(1)(1)
VTT Technical Research Centre of Finland, Espoo, Finland*Corresponding author, tel: (+358) 20 722 5718, Fax: (+358)20 722 7026, Email:teemu.karkela@vtt.fi

Abstract –The behaviour of fission products (FPs) during a severe nuclear power plant accident has beenstudied at VTT for the past 30 years. The focus has been on the transport and chemistry of gaseouscompounds and aerosols in the primary circuit and the containment. In the past containment aerosols havebeen studied e.g. with intermediate scale facilities AHMED and VICTORIA. In primary circuit the studies
have included resuspension and revaporisation phenomena as well as retention of FPs in steam generators. In
recent years the main interest has been on ruthenium and iodine, because of their high radiotoxicity andpossibility to form gaseous compounds.
FPs deposited on primary circuit surfaces may be released back into the gas phase, even after a long timefrom the beginning of a severe accident. Therefore high temperature chemistry of iodine has been studied with
EXSI-PC facility. It was found out that when using CsI as a precursor with Ag, B 2O3 or MoO3 the release of
gaseous iodine even at 400 °C was significant. Boron seemed to react with caesium forming a solid, glassy
compound and thus the release of gaseous iodine was enhanced. The effect of chemical reactions on primarycircuit surfaces on the transport of FPs is not well-considered in current severe accident codes.
Gaseous iodine reacts with air radiolysis products, such as ozone, in the gas phase of containment. The
radiolytical oxidation of elemental and organic iodine was investigated using EXSI-CONT facility. The
formation of iodine oxide aerosol particles in air by UV(c) radiation was detected on-line in the experiments.
The main gaseous reaction products from the radiolytical oxidation of CH3I were methanol andformaldehyde. Further studies on the oxidation by beta radiation with BESSEL facility verified the formationof iodine containing particles. During a severe accident, a part of the nucleated iodine oxide particles in theatmosphere will deposit on the various surfaces of containment. It was found that the desorption of iodine
from the particles deposited on painted surface was enhanced by gamma radiation.
The filtration of gaseous and particulate iodine has been investigated using a wet electrostatic precipitator(WESP) technique. The filtration efficiency of a modern WESP can be higher than 99.9 % for the particles. To
trap gaseous iodine in this study, additional ozone is fed to the gas flow in order to oxidize all gaseous iodine
to iodine oxide particles. The newly formed particles are mixed with a spray of water droplets. Inside thefiltration unit of WESP, large droplets and particles are charged and driven to the collection electrode. The
droplets are very efficient in trapping small particles, for which charging efficiency may otherwise be too low
for effective filtration. As a result of the first filtration experiments, 95 % of iodine was filtered with the
WESP. The remaining 5 % of iodine was transported through the filter in a gaseous form.

79

NEA/CSNI/R(2016)5
I. INTRODUCTION

influence the iodine partition in the gas phase. In
addition, iodine concentration in the gas phaseincreased when the sump was condensing anddecreased when it was evaporating. One would
expect exactly opposite behaviour, if the sourceof the iodine is from the sump. In addition, silver
iodide precipitated in the sump, when AIC(AgInCd) control rod was applied in the tests.
With B4C control rod most iodine released into
the model containment was in gaseous form.

In Fukushima Daiichi nuclear plant cooling of
the reactor cores at units 1, 2 and 3 was lost due
to station black out. Since the cooling could not
be restored in time, fuel damage took place in all
three reactors and fission products were partly
released from the core. As expected in a suchsevere accident, the highest contribution to thesource term to the environment was partly fromiodine isotopes.

Based on the results from Phébus FP program anumber of hypothesis on iodine behaviour were
formulated [2]. It was suggested that:

Traditionally, it has been assumed that in asevere accident most iodine would be released
from the fuel. Release to the containment would
take place mostly as aerosol particles withgaseous/vapour compounds fraction of about 5%[1]. Particulate iodine is expected to be removed
from the gas phase of the containment byengineered safety systems as well as by natural
aerosol
processes
likesettling
anddiffusiophoresis. Particulate and gaseous iodine
would finally end up in sump waters and remain
there if alkaline conditions are maintained.
However, most iodides (except AgI, TlI and CuI)
are very water soluble. Unfortunately, iodine
water chemistry is also very complicated as it
can assume several different oxidation states. In
case of acidification of the water e.g. from nitricacid formation and other radiolytic processes,
molecular iodine would partition back to theatmosphere. It might also form volatile organic
iodides. Rate of release from water would
increase as the sump temperature would
approach the boiling point.

1) Either the painted surface or the steel walls
acted as the source of the persistent gas phase
iodine species in the tests.
2) Radiolytic processes destroyed gas phasemolecular iodine and organic iodide to form
iodine oxide or iodine nitroxide particles.
These particles further coagulated and settled
or were removed by other natural processes.
3) The source of gaseous iodine from the circuit
was either chemical reactions in the gas
stream or on the surfaces of the tube walls.
Only by mechanistic understanding of iodine
behaviour the consequences of a severe accident
can efficiently be mitigated.
II. FISSION PRODUCT TRANSPORTSTUDIES AT VTT
The behaviour of fission products (FPs) during asevere nuclear power plant accident has beenstudied at VTT for the past 30 years. The focus
in experiments has been on the transport andchemistry of FPs in the primary circuit and thecontainment.

Phébus FP program provided opportunity to test
these expectations in iodine chemistry withrealistic
configurations
andchemicalenvironment. In Phébus FP tests iodine wasindeed mostly released to the model containment
as aerosol particles with gaseous iodine species
making up few percent of the overall release. The
release rate of iodine as well as aerosol
sedimentation were also consistent with current
severe accident modelling. Contrary to the
expectations, gaseous iodine depleted from theatmosphere much faster than expected in theearly phase of the tests. More alarmingly, asteady-state concentration of iodine incontainment atmosphere was reached in all
Phébus FP tests. The sump pH did not seem toIn the past containment aerosols have beenstudied e.g. with intermediate scale facilities
AHMED [3] and VICTORIA [4]. The objective
was to study the behaviour of hygroscopic andinert aerosols at different relative humidities andtemperatures. In studies at primary circuit
conditions the experiments have included
resuspension and revaporisation phenomena as
well as retention of FPs in steam generators [5,6].
In recent years the interest in experiments has
been on ruthenium and iodine, because of their80

NEA/CSNI/R(2016)5
high radiotoxicity and possibility to formgaseous compounds. In these studies have been
focused on to find out in which form FPs aretransported. In addition, the effect of various
parameters, such as temperature, humidity andradiation, on the speciation of fission products
have been studied. The adsorption and desorptionof iodine from iodine oxide aerosol deposition on
painted containment surface has also been
investigated. The recent activities of VTT on
iodine behavior in a SA are described in thefollowing chapters. The focus in the work has
been e.g. to test the hypothesized mechanisms
formulated on the basis of Phébus FP program(see above).

CsI + B2O3
650
A, B and C
CsI + B2O3
650
A, B and C
CsI + B2O3
400
A, B and C
CsI + Cd
650
A and C
Cd
650
A and CCsI + Cd
400
A, B and C
Cd
400
A, B and C
CsI + CsOH
650
A and C* The composition of atmosphere is explained inTable 2.

The crucible containing the precursors washeated up to 400 °C, 550 °C or 650 °C in thereaction furnace. The furnace tube used in theexperiments was made of alumina or stainlesssteel (AISI 304), which was pre-oxidized in a
steam flow before the experiment. The reaction
product particles were collected on PTFE filterswith 5 µm pore size, and the gaseous reaction
products were trapped in two consecutive
bubbling bottles (0.2 M NaOH and 0.02 M
Na2S2O3 water solution), which locateddownstream the aerosol filters. Additional
toluene trap was used in some experiments. The
elemental composition of both samples wasanalyzed with Inductively Coupled Plasma Mass
Spectrometer (ICP-MS). Experiments included
three different gaseous atmosphere conditions,with different fractions of steam, argon andhydrogen, shown in Table 2.

III. PRIMARY CIRCUIT CHEMISTRY
OF IODINE
After fission products have been released fromthe overheated and molten fuel, they aretransported through the reactor coolant systemand FPs will reach areas at lower temperature. As
a consequence, vapour condensation and particle
nucleation processes takes place in the gas flow.
If vapour condensation takes place close to thesurfaces of primary circuit, a layer of condensate
can be formed on the circuit surface. Particles in
the gas flow may also deposit on the circuitsurfaces together with control rod and structural
materials. The release of gaseous iodine fromprecursor mixtures simulating deposits inprimary circuit conditions have been investigatedwith EXSI-PC facility [7-12]. A list of therecently conducted experiments is presented inTable 1.

TABLE IIGas flow rates (NTP conditions 0 °C and 1013 mbar) andvolume fractions (at 100 °C and 1013 mbar) fed to thereaction furnace.

Argon

TABLE I: recently conducted experiments on
primary circuit chemistry of iodine.
Precursors
Temperature
Atmosphere*
[°C]
CsI
650
A, B and C
CsI
650
A, B and C
CsI
550
A, B and C
CsI
400
A, B and C
CsI + MoO3
650
A, B and C
CsI + Mo
650
A, B and C
CsI + Mo
650
A, B and C
CsI + Ag
650
A, B and C
CsI + Ag
400
A, B and C
AgI
650
A, B and C
AgI
400
A, B and C
CsI+Ag+MoO3
400
A, B and C

Steam
H2

A
3.3

Condition
B
C
3.2
2.9

Flow rate[l/min, NTP]
Gas vol-%
86.7 83.9
Massflow 0.3
0.3
rate [g/min]
Gas vol-%
13.3 13.5
Flow rate
0
0.1[l/min, NTP]
Gas vol-%
0
2.6

76.1
0.3
13.4
0.4
10.5

In experiments with only CsI precursor, theoverall release of iodine decreased significantly
when temperature was decreased to 400 °C. As aconsequence, the transport of iodine containing81

NEA/CSNI/R(2016)5
particles (CsI) was very low. Nevertheless, asmall amount of gaseous iodine was observed tobe released from the precursor even at 400 °C. In
that specific case, the fraction of gaseous iodine
was higher than the fraction of aerosols. The
addition of Ag to CsI precursor at 650 °C
resulted in the similar release of CsI aerosol aswith pure CsI precursor, whereas the transport ofgaseous iodine decreased. Some amount of silver
was found on filter in aerosol form, probably as
AgI, which could explain the observed decreasein gaseous iodine transport. At 400 °C reaction
temperature, the release of gaseous iodine wasmuch higher compared to the experiment at
650 °C and almost no iodine containing aerosol
particles were formed, see Figure 1. Most likely,
the lower reaction temperature reduced
significantly the vaporization of Cs, I and Ag
compounds. Similar behaviour was observed
when CsI precursor, mixed with additional
MoO3, was heated to 400 °C. Addition of
metallic Mo seemed to increase the release of
gaseous iodine as well.

Fig. 2. Iodine and caesium mass concentrations(aerosol and gas) at 650 °C with caesium to iodine
molar ratio of (a) 1.0 and (b) 1.6. Conditions A and C
only. [12]The transport of cadmium was significantly
increased by the presence of caesium iodide,whereas CsI aerosol transport was decreased
only slightly. 10 to 60 times more cadmium
reached the sampling lines than in the experimentwith only cadmium. As it was observed, most of
cadmium was condensed on the facility’s
surfaces before the sampling lines in thecadmium experiment. Thus, it is highly probablethat the vaporised cadmium had condensed on
caesium iodide particles and transported to thefilter. A fraction of cadmium was transported as
cadmium hydroxide Cd(OH)2, that was verified
by XRD. Addition of Cd to CsI precursor seemed
to result in a higher transport of gaseous iodine,
but the formation of Cd-I compound was notdetected.

The formation of vitreous caesium borate
compound was noticed, when a mixture of CsIand B2O3 was heated to 650 °C. This could
contribute to the formation of possible blockagesin the circuit or containment. Such a boron-richpartial blockage was observed in the rising line
before the steam generator inlet in Phébus FPT3
test. Also as a result of boron trapping most of
caesium in the crucible, a high fraction of
gaseous iodine was transported to the bubbling
bottle. The release of gaseous iodine remained
high even at 400 °C.

The amount of gaseous iodine was barely
detectable when the initial Cs/I molar ratio washigher than 1 (addition of CsOH to CsI
precursor), see Figure 2. The effect of theatmospheric composition was noticed as well;when hydrogen was present in the carrier gas(condition C), the amount of aerosols (caesium
and iodine) released from the crucible wassignificantly decreased.
IV. CONTAINMENT CHEMISTRY OF
IODINE
Volatile iodine can form several compounds withother fission products, control rod and structural
materials in the primary circuit. Therefore, e.g. incase of a tube break, iodine can be transported tothe containment atmosphere both in gaseous andparticulate forms. It is very likely that iodine at

Fig. 1. Iodine, cesium and silver mass concentrations,transported in gaseous and particulate forms, in theexperiments at (a) 650 °C and (b) 400 °C. Conditions
were in order from C to A. [9]82NEA/CSNI/R(2016)5
least partly deposits on painted surfaces of areactor containment building during a severe
accident. In such a case, molecular iodine can bereleased from these deposits (e.g. particle
deposits) back to the gas phase under varying
temperature, chemical and radiation conditions ina severe accident. Furthermore, iodine may also
react with painted surfaces to form organic
iodine species. These organic species are anotherpossible source of volatile iodine, which may
increase the fraction of iodine in the gas phase.
Therefore, it is important to study the transport ofiodine in containment conditions. This is being
investigated in collaboration between VTT and
Chalmers University of Technology (Sweden).
First results have shown, that the release of
gaseous iodine from IOx and CsI deposits on
various metal and aged paint surfaces increased
with the temperature and the release wasenhanced by gamma radiation. Another questionis, whether these released iodine compounds aretransported as gaseous molecules or as aerosol
particles resulting from reactions with air
radiolysis products. To answer this last question
several experiments were conducted in cooperation with Chalmers University ofTechnology [13, 14]. In experiments moleculariodine and methyl iodide were fed into theVTT´s EXSI-CONT facility in an air mixture. In
some experiments the flow contained also a low
fraction of water vapor. The reactions took place
in a quartz tube heated either to 50 °C, 90 °C or
120 °C. UV-light was used as a source ofradiation to produce ozone from oxygen. A
separate generator was also applied to reachhigher ozone concentrations. VTT has carried out
further studies on the radiolytical oxidation of I2
and CH3I by beta radiation in oxygen at 20 °C
with a new BESSEL facility [15, 16]. In
experiments gaseous sample was irradiated for 1
hour to 4 days and afterwards the formed
gaseous and aerosol reaction products wereanalysed with online devices.

detected while the concentration of ozone wasincreased. The measured gaseous reaction
product species in experiments at 50 °C to
120 °C are presented in Figure 3. Similarly, themass concentration of aerosols increased as well,thus aerosol nucleation was enhanced, see Figure
4. Also, increase in temperature seemed toincrease the aerosol mass concentration. This isprobably partly due to more efficient
decomposition of gaseous CH3I and subsequentaerosol formation with ozone. The formation of
gaseous reaction products was efficient in thisstudy, even though the residence time of the flow
inside the facility was only approx. 7 seconds. It
seemed that thermal decomposition reactions
further enhanced the formation of reaction
products.
400

300
250
200
150
100
50
120 C

0
90 C

Temperature[C]

Transported aerosol mass
[mg/m^3]

350

10
5

50 C

1
0

Ozone generator power setting [#]Fig. 4. The measured aerosol mass concentration while thetemperature of EXSI-CONT facility and the power setting of ozone
generator were varied. [14]As a result of CH3I experiments with EXSICONT, there was a clear trend in the formationof gaseous reaction product species. The main
gaseous reaction products were methanol andformaldehyde. Especially at elevated temperatureother reaction products, such as formic acid andmethyl formiate, became important as well. Increasing amount of reaction product species were

Fig. 3. The gaseous reaction products (in ppm) from the oxidationof methyl iodide with ozone. The temperature of EXSI-CONT
facility and the power setting of ozone generator are presented onvertical axis. [14]In the beta irradiation experiments with BESSEL
facility, it was found out that the concentration
83

NEA/CSNI/R(2016)5
precursors to beta radiation in oxygen atmosphere at 20 °C.
The irradiation period ranged from 1 hour to 4 days. The
initial vapour pressure of CH3I was approx. 4 kPa, whereas
the vapour pressure of molecular iodine ranged from 2.5 Pa
to 30 Pa.

of formed particles (ca. 10 nm to 50 nm indiameter) decreased very slowly with increasing
irradiation time, see Figure 5. It seemed that an
equilibrium was reached between gas phase
iodine compounds and iodine species deposited
on wall surfaces. At that equilibrium the rate of
new particle formation was low. When thefacility was purged with oxygen, a newformation of particles was observed in every
CH3I experiment. It suggested that the radiolysis
reaction products were limiting the particle
formation. Oxygen, a precursor of ozone whenirradiated, was also needed for the nucleation totake place, since the new particle formation was
not observed without irradiation or when theatmosphere was pure nitrogen. The formed
particles were highly water soluble and volatile.
These findings could also partially explain theconstant concentration of iodine, which wasobserved in the gas phase of containment at theend of every Phébus FP test.

V. FILTRATION OF GASEOUS AND
PARTICULATE IODINE
In the containment atmosphere, part of gaseousiodine compounds is radiolytically decomposed
mainly by gamma rays and beta particles
originating from fission products. As a result ofradiolytical oxidation, gaseous iodine can betransformed to e.g. iodine oxide particles. Due tothe high contribution of radiotoxic iodine to thepossible source term, the retention of iodine
containing compounds and thus the prevention of
their release to the environment is of primary
importance. Currently, VTT is studying thefiltration of both gaseous and particulate iodinewith a new filtration system. This technique canbe used to filter e.g. other FP aerosols as well.
The efficiency of the system is being evaluatedin EU PASSAM project, which aims atdemonstrating the ability of innovative systems
to achieve larger source term attenuation.

(a)

The basic studies on iodine oxidation at VTT
have produced a significant amount of data andknowledge on the behaviour of iodine in a severe
accident. The knowledge is being used toenhance the filtration efficiency of the wet
electrostatic precipitator (WESP) technique. In
the conventional ESP technique, particles arecharged with ions and in the electric fieldcharged particles drift to the collection electrode.
For small particles, less than 1 µm in diameter,
charging efficiency may be too low for effective
filtration. These particles are filtered with an ion
wind principle. Momentum of the ions travellingto the collection electrode drives small aerosol
particles there as well.

(b)

The conventional ESP technique can also be used
to filter gaseous pollutants when the gas flow is
pre-treated before the filtration unit. The water
droplets fed into the system adsorb gaseous
impurities. The effect can be enhanced byinjecting additives with the water. Inside thefiltration unit large droplets are charged anddriven to the collection electrode. The droplets
are also very efficient in trapping small particles.

Fig. 5. The number concentration of the formed aerosols
after the exposure of gaseous (a) CH3I [15] and (b) I2 [16]84NEA/CSNI/R(2016)5
The gaseous compounds can also be oxidized toform solid particles which are filtered with the
ESP technique. Both proposed methods are being
studied now for the decontamination of gaseous
species in containment conditions. Gaseousiodine is oxidized with additional ozone andwater droplets are fed to the gas flow just before
the filtration unit of WESP.

high temperatures and various steam fractions ina gas flow.
VI. SUMMARY AND CONCLUSIONS
Phébus FP program raised a number of questions
that could not be answered by the current severe
accident analysis tools. Based on theexperimental observations several hypothesis e.g.
on iodine behaviour in the primary circuit as wellas within the containment were formulated.
These hypotheses must be experimentally tested
and not just parameterized.

The first experiments were carried out with TiO2
and IxOy aerosols, see Figure 5. The applied
electric voltage between the electrodes, theresidence time of particles inside the ESP
chamber and the injection of water droplets
before the ESP chamber were varied in theexperiments. The injection of water droplets
significantly increased the trapping efficiency ofTiO2 particles for applied electric voltage lessthan 15 kV (negative). In the iodine experiments,
a gas flow containing elemental iodine (2.2l/min, 16 ppm) was mixed with ozone (3 l/min, >1000 ppm) in order to oxidize all gaseous iodine
to iodine oxide particles. The gas flow was
further diluted and the total flow rate though the
WESP was 86 l/min (20 s residence time). As aresult of the first filtration experiments, close to
100 % of the formed IxOy particles was filteredwith the WESP when the applied electric voltage
was in a range from -10 kV to -25 kV.

Some of these hypotheses have been
experimentally verified by VTT. One of theuncertainties was the effect of reactions on
primary circuit surfaces on the fraction andtiming of iodine release into the containment
atmosphere. In the experiments, a difference inthe release of iodine in the primary circuit
conditions at 400 °C, 550 °C and 650 °C
temperatures was detected. Iodine seemed to bereleased almost completely in gaseous form atlower temperature despite the used precursor
mixtures. This surprising observation suggeststhat the FP deposits on the surfaces of primarycircuit may act as a source of volatile iodine for along period since the beginning of severe
accident. However, an excess amount of Cs to I
seemed to reduce the formation of gaseous iodine
at 650 °C. Results on the radiolytical oxidation of
gaseous molecular and organic iodine by beta
radiation, and complementary experiments withozone and UV radiation, verified the formationof aerosol particles, which was one of theformulated hypotheses after Phébus FP
experiments. The diameter of nucleated particles
ranged from ca. 10 nm to 50 nm. These verysmall particles were highly water soluble andvolatile.
Recent studies by VTT have produced new
information on the chemistry and transport ofiodine in severe accident conditions. These
results are needed e.g. in order to explain theobserved behaviour of iodine in the experimentsof Phébus FP program. However, the mostimportant impact of these results will be on thedevelopment of severe accident analysis codes.
Previously, the lack of information has prevented
to taking into consideration the FP deposits on

Fig. 5. Evolution of the fraction of trapped TiO2 and IxOy
particles with the WESP filter as a function of the applied
electric voltage between the electrodes. [17]Currently, the main focus is on testing thetrapping of gaseous molecular (I2) and organic(CH3I) iodine with the WESP filter. In thoseexperiments the filter will also be exposed torepresentative containment conditions, such as

85

NEA/CSNI/R(2016)5“State of the art report on iodine chemistry”,
NEA/CSNI/R(2007)1 (2007).

primary circuit surfaces and iodine oxide
particles both in the containment gas phase andon the containment surfaces as a short and longterm source of volatile iodine. The new
information on the iodine chemistry developed
have been utilized e.g. in the development ofelectric filtration technique for gaseous andparticulate iodine species. The efficiency of thisFinnish innovation is currently evaluated in EU
PASSAM project.

2. R. Y. Lee and M. Salay, “Findings on iodine
behaviour in design basis and severe
accidents”, Presented to the Advisory
Committee on Reactor Safeguards 8.5.2008,
U.S.NRC
3. J. Mäkynen, J. Jokiniemi, P. Ahonen, E.
Kauppinen,
R.
Zilliacus,“AHMEDexperiments on hygroscopic and inert aerosol
behaviour in LWR containment conditions:experimental results”, Nuclear Engineeringand Design, 178, pp. 45-59 (1997).

ACKNOWLEDGMENTS
The financial support of VTT TechnicalResearch Centre of Finland, The Finnish
Research Programme on Nuclear Power Plant
Safety (SAFIR2014), Nordic Nuclear Safety
Research Programme (NKS-R) and Severe
Accident Research NETwork of excellence(SARNET) is acknowledged.

4. J. Mäkynen, J. Jokiniemi, E. Kauppinen, H.
Tuomisto, T. Routamo, “LWR aerosol
experiments
at
VICTORIA
model
containment”, Journal of Aerosol Science,
28, pp. 715 (1997).

NOMENCLATURE
AHMED = Aerosol
measurement device

and

heat

5. T. Raunio, “Experimental Study on Fine
Particle Resuspension in Nuclear Reactor
Safety”, Master’s Thesis (2007)transfer

BESSEL = Beta irradiation vessel

6. D. Bottomley, R. Dickson, T. Routamo, J.
Dienstbier, A. Auvinen, N. Girault,“Revaporisation issues: On Overview”, Proc.
of the 2nd European Review Meeting on
Severe Accident Research, ERMSAR 2007,
Germany (2007).

EXSI-CONT = Experimental study on iodine
chemistry – containment
EXSI-PC = Experimental study on iodine
chemistry - primary circuit
ESP = Electrostatic precipitator

7. T. Kärkelä and A. Auvinen, “Experimental
study on iodine chemistry (EXSI) – Facility
for primary circuit experiments”, VTT-R02791-09 (2009).

FP = fission product
ICP-MS = Inductively coupled plasma mass
spectrometer
NTP = Normal temperature and pressure

8. J. Kalilainen, T. Kärkelä, P. Rantanen, J.
Forsman, A. Auvinen and U. Tapper, Part
“Primary circuit chemistry of iodine,” Proc.
Of the SAFIR2010, The Finnish Research
Programme on Nuclear Power Plant Safety
2007-2010, Final report, 312-320 (2011).

PTFE = Polytetrafluoroethylene
UV-light = Ultraviolet light
VICTORIA = Facility for studies on LWR
containment aerosol behaviour
WESP = Wet electrostatic precipitator
XRD = X-ray diffraction

9. J. Kalilainen, P. Rantanen, T. Kärkelä, M.
Lipponen, A. Auvinen and J. Jokiniemi,“Effects of molybdenum and silver on iodinetransport in primary circuit on severe nuclear
accidents”, VTT-R-00425-12 (2012).

REFERENCES
1. B. Clément, L. Cantrel, G. Ducros, F. Funke,
L. Herranz, A. Rydl, G. Weber, C. Wren,
86

NEA/CSNI/R(2016)5of the ICONE17, Brussels, 2, 719 – 727(2009).

10. J. Kalilainen, T. Kärkelä, R. Zilliacus, U.
Tapper, A. Auvinen and J. Jokiniemi,“Chemical reactions of fission productdeposits and iodine transport in primarycircuit conditions”, Nuclear Engineering and
Design 267, 140-147 (2014).

14. T. Kärkelä, J. Holm, A. Auvinen, R.
Zilliacus, T. Kajolinna, U. Tapper, H.
Glänneskog and C. Ekberg, ”Gas phasereactions of organic iodine in containment
conditions”, In Proc. Of the ICAPP2010, San
Diego, 2, 1084 – 1091 (2010).

11. M. Gouëllo, J. Kalilainen, P. Rantanen, T.
Kärkelä, and A. Auvinen, ”Experimental
study on the behaviour of CsI on primarycircuit surfaces and effects of cadmium on
iodine transport during a severe nuclear
accident”, VTT-R-00630-14 (2014).

15. T. Kärkelä, A. Auvinen, T. Kekki, P.
Kotiluoto,J.
Lyyränen
andJ.K.
Jokiniemi, ”Radiolytical oxidation of
gaseous iodine by beta radiation”, Submitted
to Radiochimica Acta (2015).

12. M. Gouëllo, J. Kalilainen, T. Kärkelä and A.
Auvinen, “Experimental Study on the
Behaviour of CsI on Primary Circuit
Surfaces: Effects of Cadmium and Caesium
Hydroxide on Iodine Transport during a
Severe Nuclear Accident”, VTT-R-00424-15(2015).

16. T. Kärkelä, M. Gouëllo, T. Kekki, P.
Kotiluoto and A. Auvinen, “Formation of
HNO3, O3 and IOx particles by beta
radiation”, VTT-R-00641-15 (2015).
17. M. Gouëllo, T. Kärkelä, J. Hokkinen and A.
Auvinen,Presentation
in
the
5th
management team meeting of EU PASSAM
project (2015).

13. T. Kärkelä, J. Holm, A. Auvinen, C. Ekberg,
H. Glänneskog, U. Tapper and R.
Zilliacus, ”Gas phase oxidation of elemental
iodine in containment conditions”. In Proc.

87

NEA/CSNI/R(2016)5

MAIN FINDINGS OF THE IRSN EXPERIMENTAL PROGRAMMES PERFORMEDON IODINE CHEMISTRY IN SEVERE ACCIDENT CONDITIONSColombani J, Grégoire A.C *, Morin S.
Institut de Radioprotection et de Sûreté Nucléaire, Saint-Paul Lez Durance, France*
Corresponding author, tel: (+33) 442199740, Fax: (+33) 442199163, Email:annececile.gregoire@irsn.fr

Abstract – Over the past two decades, IRSN has sustained large efforts on Severe Accident (SA) research
dedicated to the evaluation of the “Source Term” (ST). The overall aim of this ST work is to better predict
potential releases of fission products into the containment atmosphere and finally into the environment,with a special attention to iodine species.
Large efforts achieved so far have clearly improved the understanding of iodine behaviour in SA
conditions. Iodine is known to be almost entirely released from the degraded core, with significant early
releases during the accident transient. Depending on the chemical composition of the flow passing throughthe Reactor Coolant System, a significant amount of volatile iodine species is potentially transported fromthe primary circuit up to the containment building. In the containment atmosphere, main iodine species(molecular iodine, organic iodides, and iodine aerosols species) have been identified as well as their mainprocesses of formation, deposition and decomposition: volatile iodine species are quickly adsorbed onto
the containment surfaces and/or transferred into the sump, they may also react with the air/steam
radiolytic products, yielding soluble iodine oxides particles that are then transferred to the sump or that
settle on containment surfaces. The adsorbed species may also lead again to the formation of volatile
species. All these processes are linked and have been evidenced experimentally.

88

NEA/CSNI/R(2016)5

I. INTRODUCTION

This facility [5], scaled down by a factor 5000
relative to a 900MWe pressurized water reactor,
allowed the study of fuel degradation, the release of
FPs and their transport and behaviour through the
RCS and in the containment building with a
specific attention to iodine radiochemistry (seeFig.1). Experiments FPT-0 to FPT-3 used a 1 mhigh bundle of 21 fuel rods including a control rod
in the central position. Radioactive materials exiting
the bundle during its degradation were transported
to a single inverted U-tube simulating a steam
generator and then to a 10 m3 vessel simulating areactor containment. The 1/5000 scaling ratio isalso valid for the volume of the containment
atmosphere, the surface area of the painted
condensing surfaces and the interfacial area
between the sump and the atmosphere.

Due to its high volatility and radiotoxicity, iodine isa major contributor to the short and middle term
radioactive releases to the environment in case of
Severe Accident (SA) on a Nuclear Power Plant(NPP). In the 80-90’s, after the Three Mile Island
accident, a vast amount of research dealt withsevere accident source term. As far as iodine isconcerned, the main findings indicate that iodine
releases from the reactor core could amount to 6070% of its core inventory in case of SA and that it
would be almost exclusively transported as CsI in
the Reactor Coolant System (RCS) [1].
Since then, many experimental programmes havebeen performed by IRSN in cooperation withinternational partners. The understanding of the“SA processes” considerably evolved after having
performed the series of Phébus Fission Products(FP) integral tests that were undertaken toinvestigate key phenomena governing NPPs SA
from core degradation to the late phase andparticularly the iodine chemistry [2].

The main features of the test matrix are displayed inTable I. Concerning the core, the main parameters
were the nature of the coolant (steam-rich or steampoor with a high or a low steam flow rate) and thematerial composition of the control rod: SilverIndium-Cadmium (SIC) or Boron Carbide (B4C).
These variables potentially had a direct impact on
the iodine behaviour in the primary circuit. Indeed,
the redox conditions prevailing in the RCS varied
strongly with the extent and the duration of themain hydrogen phase due to Zircaloy-cladoxidation. Furthermore, iodine reactivity may bedirectly influenced by the release of the other FPs(Cs, Mo, Te) and control rod materials (Ag, In, Cdor B). Concerning the containment andsubsequently the late phase iodine chemistry, themain parameters were the sump pH and its thermal–
hydraulic conditions.

To complete and better understand the volatile
iodine formation, deposition and decomposition
main processes highlighted in the Phébus tests, theInternational Source Term Program (ISTP), waslaunched in 2005 [3]. Small scale experiments havebeen performed at IRSN to examine the behaviourof iodine in the RCS (CHIP-program) and in thereactor containment building (EPICUR program).
Then, the OECD/STEM (Source Term Evaluationand Mitigation) program was launched in 2011.
II. IODINE BEHAVIOUR IN THE PHEBUS
FPT0/1/2/3 TESTS – MAIN FEATURESTABLE I: Phébus test matrix [5]The Phébus FP programme was initiated in 1988
and four tests were performed in a bundle
configuration in the Phébus facility [4].

Test

Fuel /degradation
conditions

Primary circuit

FPT-0

Fresh Fuel/ SIC rod/steam-rich conditions(2g/s)
BR3 fuel, 23 GWd/tU/
SIC rod/ steam rich
(2g/s)As FPT-1 but steam
poor conditions (0.5g/s) and low boric acidflow
As FPT-1 with B4C rod
and steam poor (0.5g/s)

non-condensing steam pH5
generator

FPT-1
FPT-2

FPT-3

As FPT-0

Containment

pH5

As FPT-1 with effect pH9/ evaporatingof boric acidsump

As FPT-0

pH5/ evaporating
sump

Each test started with a pre-conditioning phaseconsisting of a re-irradiation of the fuel bundle togenerate a representative short lived FP inventory.
This was followed by a transition period to adjust
the circuit thermal hydraulic conditions. The
experimental phase itself started with thedegradation phase (4-5 hours long) during which

Fig. 1 Schematics of the experimental circuit in the Phébus FP bundle tests FPT0/1/2/3 [5].

89NEA/CSNI/R(2016)5

fuel degradation and material release to thecontainment were studied. After containment
isolation, it was followed by a “long term” phase
divided into an aerosol phase (2 days) dedicated tothe analysis of aerosols deposition mechanisms, awashing phase (20 min) which aimed at collecting
aerosols into the sump, and a chemistry phase (2days) devoted to the analysis of iodine chemistry.

iodine deposition are characterized by a sharp
thermal gradient at high temperatures [7]. Thus, theiodine deposition fractions were the highest in thesteam generator rising section, accounting for atleast 50% of the total iodine deposited in the circuit.
A second main significant zone of iodine retention
was observed at the bundle outlet characterized by a
sharp thermal gradient from ~1600 down to 700 °C.

II.1 Iodine behaviour in the circuit (degradation
phase)

Three main iodine forms (aerosols, vapours andgases) have been transported in the Phébus primarycircuit with proportions that varied depending on
the tests (linked to redox conditions, material
release kinetics). Iodine vapours forms condense
between 700 and 150 °C whereas the gaseous forms
may be transported up to the containment vessel.

Iodine is considered as a volatile FP with an
integral release fraction ranging from 78 to 90% of
its initial fuel inventory (i.i.) [6]. The releasekinetics gained from on-line measurements in the
hot leg indicates that it is closely linked to thehydrogen release for FPT0/1/2 with an early releaseconcomitant with the first Zircaloy oxidation phase(characterized by a strong hydrogen release) [7]. On
the contrary, for FPT3, the first significant iodine
release (also observed for xenon) was delayed until
after the hydrogen peak. This difference may be
accounted for a different timing in fuel degradation
events or for the formation of different iodine
species [7] - fig.2.

Iodine was mainly transported in vapour form at
700 °C for FPT0/1/2 [7]. Analysis of thermal
gradient tube samplings and thermal transition lines(700-150°C) provided complementary informationon the nature of these vapour species. CsI
condensation peaks were highlighted in several
FPT2/FPT3 samplings. Besides this species, otheriodine condensations peak were clearly identified
indicating that this species was not the only iodine
vapour species transported through the circuit hotleg [7].

Fig. 2 Iodine mass flow rate in hot leg during FTP2/3 tests [7].

The iodine transport kinetics in the cold leg wasstrongly correlated to the flow rate determined in
the hot leg in FPT0/1. On the contrary, in FPT2/3,
the transport kinetics in the cold leg was muchsmoother, owing to a significantly reduced steam
injection in FPT2/3 compared to FPT0/1 and toenhanced iodine retention in the steam generator.
Iodine retention in the Phébus circuit is moderate[7]: the transported iodine fraction from the bundle
to the containment vessel was the highest for the
FPT1/2 tests accounting for respectively 72% and
77% of the released iodine. For FPT3, this fraction
was significantly lower (43%), following the large
deposition of boron-containing materials andsubsequent partial blockage in this region [6, 7]
before the late oxidation phase. The main areas of90

NEA/CSNI/R(2016)5
TABLE II
Overview of iodine behaviour in Phébus tests [ 6], [7], [8]TestRelease from bundle (%i.i)

FPT0
>91
28
UP +VL: n.m.
HL +SG : 76
CL+UP+VL: balance

FPT1
90
26
UP +VL : 21
HL +SG : 76
CL: balance

Main iodine forms transiting the RCSb
(aerosol/vapour/gas %/total I over the transient)

HL > 700 °C :Aerosol : few%
Vapour + gas : dominantCL ~ 150 °C :Aerosol : <98
Gas > 2d

HL > 700 °C :Aerosol : 6-12
Vapour + gas :88-94
CL ~ 150 °C :Aerosol : dominant
Gas : negligibled

Injection into the Containment (%i.i.)
gaseous iodine (% c.i.)c

63
33d
0.1
n.m
n.m.
n.m.
n.m.
16
n.m.

RCS deposits (% i.i)
distribution in the RCS (%/ total deposited iodine)

Containment distribution at the end of the test Gaseous (% c.i.)
Stainless Steel Wall (% c.i.)
Elliptic floor (% c.i.)
Sump (% c.i.)
Painted condenser dry (% c.i.)
Painted condenser wet (% c.i.)
Painted sump coupon (% c.i.)

64
4d

FPT2
72
15.7
UP +VL : 27
HL +SG : 35-62a
CL: balance
HL > 700 °C :Aerosol :11-44
Vapour 56-89
Gas < 0.3
CL ~ 150 °C :Aerosol : ~99.9
Gas : 0.1 d
56.7
0.6d

FPT3
79
44.9
UP +VL: 13
HL +SG : 17-51a
CL: balance
HL > 700 °C :Aerosol :13
Vapour : 42
Gas : 45
CL ~ 150 °C :Aerosol :5
Gas :95
34.1
80

~0.14
2
5.6
92.3
0.2
1
1.2

~0.02
7
54
28
0.4
3.2
1.9

~0.09
13.7
0
17
15.8
38.2
1.3

RCS = model Reactor Cooling System comprising: UP = Upper Plenum, VL = Vertical Line, HL = Hot Leg, SG = Steam Generator, CL = Cold Legarespectively lower bound and upper bound retention in the SG from measured iodine deposition in the rinsing section of the SG and from circuit mass balance between the HL and the CL; b Circuit data; FPT0/ 1/2
tests: trends and value ranges;. for FPT3 test : quantitative data; c containment data, c.i. containment inventory; d : discrepancies between circuit and containment data concerning gaseous fraction : circuit data may
underestimate the transported gaseous iodine fraction.

91

NEA/CSNI/R(2016)5

This observation is also consistent with caesium
found transported mainly in an aerosol form at thislocation. The FPT3 test results, for which half of
the iodine was transported through the hot leg-point
in a vapour form and half in a gaseous form, clearly
depart from the previous Phébus test results. The
use of a B4C control rod in FPT3, instead of SIC,
could be an explanation in the formation of gaseousiodine forms. Indeed both the favoured combinationof caesium with boron (that may also have occurredin FPT2 where a boric acid steady flow was
injected) and the absence of materials such as
silver, indium and cadmium (that are known to havea great affinity for iodine) could promote thepersistence of gaseous iodine species such as HI or
I2.

residual concentrations are reached. Depletionmechanism is dominated i/ by aerosol gravitational
deposition on the elliptic floor for FPT0/1/2 tests
where aerosols were the dominant form of iodine ;ii/ to a lesser extent, by aerosols deposition on thestainless steel surfaces for FPT2 due to smaller
aerosol sizes compared to FPT1; iii/ and byadsorption/absorption on the atmospheric painted
condensers (and to a lesser extent on the stainlesssteel walls) for FPT3 where high gaseous iodine
fraction was injected into the containment.

In the cold leg only iodide aerosols and gaseousiodine are expected. In the FPT0/1/2 circuit coldleg, most of the iodine was transported in an aerosol
form towards the containment vessel, with a very
low gaseous iodine fraction. In FPT3, as expectedfrom iodine forms measured in the hot leg, iodine
was found to be transported in the circuit cold leg
mainly in gaseous form (95%).

The low residual concentrations of gaseous iodine
are similar in all tests (ranging from 8 10-10mol(I)/L for FPT1 to 2 10-11 mol(I)/L for FPT0)
indicating an equilibrium between source and sinkmechanisms. The most efficient depletion
phenomenon during the late phases of the tests, wasfound to be the sump chemistry.

At the end of the test, the fraction deposited on thecontainement surfaces represent about 3-10% c.i.
for FPT1-2 and 70% c.i. for FTP3. Low amounts ofiodine remained on the elliptic floor except for
FPT2 due to a non-efficient washing.

II.3Gaseous iodine speciation

II.2 Atmospheric Iodine behaviour and depositionin the containment

For FPT0/FPT1, if at short and midterm phases,
both molecular and organic iodine concentrations
where similar, the organic iodide became dominant
after the washing phase and throughout the longterm phase. On the contrary for FPT2/FPT3, thedominant form of gaseous iodine was always
molecular.

In all tests, more than half of the iodine fuelinventory reached the containment (Table II).
During the fuel degradation, an early presence ofgaseous iodine in the containment was observed in
all tests with a maximum observed during or justafter the main Zircaloy oxidation phase for
FPT0/1/3 tests and during the second oxidation
phase for FPT2 [8]. Though the potential chemicaltransformation of iodides entering the containment(where specific conditions prevailed dose rate andoxyden concentration) cannot be excluded, thisgaseous iodine may be also likely formed in the
RCS, probably linked to non-equilibrium chemical
effects. In FPT3, the gaseous iodine fraction wasmuch higher pointing to the essential role of control
rod material on iodine chemistry in the RCS. The
lower gaseous iodine fraction measured during the
FPT2 main Zircaloy oxidation phase as compared
to previous FPT0/1 tests might be explained by thelower steam injection that both implies slower andmore progressive fuel degradation as well as higher
transit times of fission products in the primarycircuit. Whatever the iodine fraction and nature(aerosol or gas) injected into the containment
atmosphere, the efficiency of the various sinks
finally leads to a significant depletion of
atmospheric iodine until very low and constant

The painted atmospheric surfaces were the main
source of organic iodides. But even in the FPT3 test
characterized by a high amount of iodine adsorbed
on paints, the organic iodide fraction did not exceed
20% gaseous iodine and its concentration
significantly decreases throughout the test down tovery low constant concentration in the long termby
radiolytic destruction. The immersed painted
surfaces apparently did not act as a significant
organic iodide source.
Organic iodides formed preferentially in tests where
there was no evaporation from the sump and no
steam condensation on the containment surfaces(FPT0/1). Tests performed with sump evaporationand condensation on containment surfaces (FPT2/3)
yielded reduced organic iodide fractions. Thisobservation is consistent with a reduced organic
iodide formation from wet painted surfaces.

92

NEA/CSNI/R(2016)5

II.3 Iodine behaviour in the sump

materials (B, Cd, In, Ag) and to evidence possible
kinetics limitations.

At the end of test, iodine sump inventory
represented a significant fraction of the containment
inventory for all tests (see Table II). The solubilityof iodine in the sump decreased from the earlyaerosol phase towards the final washing phase,indicating the formation of insoluble species.
Indeed, silver originated from control rod wasfound to have played a key role in iodine sumpchemistry in irreversibly binding the iodine to form
an insoluble iodine species (AgI). Iodine was foundto be more insoluble when the Ag/I ratio was highand the pH acidic as in FPT0/1 tests [8] so that very
little water soluble iodine species was detected at
the end of those tests. In FPT2, a smaller insoluble
iodine fraction (~1/3 of final iodine sumpinventory) was found linked to a lower Ag/I molar
ratio in the sump compared to previous tests. In
FPT3, despite the very low Ag/I ratio (0.35), a low
fraction of non soluble iodine forms were also
detected after the washing phase.

III.1 Test facilities
In the CHIP facility [10] and the small scale test
bench, the elements are transported in a controlled
thermal gradient. These benches are designed as anopen system with a continuous flow of reagents(see Fig.2).
The chemical elements, including the carrier gas(H2/H2O/inert gas), are heated at 1600°C in a High
Temperature (HT) zone. At this temperature, all the
species are under gaseous form at thermodynamic
chemical equilibrium. Downstream of the HT zone,
the fluid is cooled down in the so-called “transportzone” where chemical reactions take place
producing aerosols and gases. Particles and gases
are then collected and separated before being
analysed by offline chemical techniques. The main
test parameters are: i/the molar flow rate or theconcentration of the elements injected in the line,
ii/the residence time of the flow and the thermal
gradient profile of the fluid in the transport zone,iii/ the composition of the carrier gas (mixture of
steam/He or steam/He/H2).

No significant iodine relovatilization from the sump
was observed. The reaction of iodine with silver in
FPT0/1 (irreversible formation of insoluble species)
and the sump alkaline conditions in FPT2 (chemicalhydrolysis of volatile iodine species into nonvolatiles iodates) may have prevented iodine
revolatilization for those tests. In FPT3, the low
iodine inventory and the lower dose rates in thesump may have also prevented iodine
revolatilisation.

The steam/carrier gas mixture is directly fed at the
inlet of the high temperature alumina tube. The
reagents of interest are vaporised in external
generators connected to the main line.

To assess the main remaining uncertainties on
iodine source term evaluation the CHIP and STEM
experimental research programs on iodine
chemistry are still on-going.
III. EXPERIMENTAL WORK PERFORMED
IN THE CHIP FACILITY
Based on the Phébus-FP experiments as reported in
section II, the presence of early gaseous iodine in
the containment vessel can possibly be attributed topre-existing gaseous iodine in the primary circuit.
The release of gaseous iodine from the RCS may be
due to a more complex iodine chemistry than
expected or even to possible kinetics limitations as
the chemical system undergoes very strong thermohydraulic changes in SA conditions. In order todevelop and validate models of FP behaviour in the
primary circuit, the ISTP/CHIP experimental
program [3] was launched to obtain more
information on iodine speciation under different
circuit boundary conditions in presence of different
fission products (Cs, Mo, I, …) and control rod

Fig. 3 CHIP PL facility – General design.

Gaseous iodine is obtained by sublimation ofmolecular iodine pellets heated at 25°C and swept
by a low argon flow. Molecular iodine is thenreleased in the HT zone with a mass flow rate
representative of SA conditions (~5 10-8 to 1 107
mol/s). Caesium vapours are produced byvaporization at 500 °C of initially dehydrated
caesium hydroxide; the vapours are thentransported in an argon flow. Molybdenum vapours
are obtained by vaporization at 750 °C of
molybdenum trioxide, in a low argon/steam flow.
Boron is delivered by a steam generator producing
93

NEA/CSNI/R(2016)5

either a mixture of steam/carrier gas or a mixture of
steam/H3BO3/carrier gas by vaporization of a boric
acid aqueous solution. In the steady state, thesegenerators produce a stable mass flow rate of eachreagent. The vaporised reagents are mixed together
in the HT zone.

the CHIP line is obtained. Several sampling lines
are implemented at the main line outlet (150°C).
They are similar to the organic lines of the CHIP
facility, except that here, all the flow transiting themain line is directed to a given sampling line. Test
conduct and post-test operations are the same as for
the CHIP facility.

The main line is equipped with an integral quartz
aerosol filter and by gas scrubbers filled with asodium hydroxide solution (1 M) in order to trap
the gaseous iodine species. The pressure in the
CHIP facility usually set around 2 bars, is regulated
by a control valve located downstream of the main
filter.

III.2 Mains results
III.2.1 IOH and CsIOH system

the Hot Leg sampling line (HL) devoted to fluid
sampling at 500-700 °C (inlet of thedownstream tube) and corresponding to hot leg
break conditions;the Cold Leg sampling line (CL) devoted to
fluid sampling in the 150 °C (outlet of thedownstream tube) and corresponding to coldleg break conditions;
the Organic Line (CL-OL) implemented as abranch on the cold leg sampling line (150 °C)
and devoted to gaseous iodine speciation.
After the test, the facility is dismantled andeach part is leached to establish a reliable
element distribution from the high temperature
mixing zone down to the outlet gas scrubbers.
As iodine can only be stabilised in alkaline
media, the leaching is performed with diluted
NaOH. Caesium and boron species are also
quantitatively recovered by these operations
but molybdenum oxides are only partly
recovered. The solutions are then analysed by
ICP MS (Inductively Coupled Plasma – Mass
Spectrometry) for elemental quantification. A
selection of samplings (sequential filters andaerosols deposits in the main line) is preserved
for aerosols characterisation (determination ofmorphology, composition and speciation).

A first series of tests was performed in the small
scale bench in oxidising (steam) or reducing(hydrogen) atmosphere with the injection ofmolecular iodine only. At high temperature, I2 iscracked to form mainly atomic iodine (calculation
at thermodynamical equilibrium). Recombination
occurs at lower temperatures in the transport zone
and lead to the formation of more stable species as
I2 or HI. Summary of the results are displayed inTable III. For both atmospheres, significant iodine
deposition was observed (around 35%). As
expected from calculations at thermodynamic
equilibrium, the carrier gas composition clearly
influences the nature of the released of gaseousiodine. Molecular iodine is expected with oxidizing
conditions whereas the release of hydrogen iodide
is expected with reducing conditions. Kinetics
limitations were clearly evidenced in bothconditions. Indeed, a measurable fraction of
released iodine is under HI form (~10%) in
oxidizing condition and under I2 form (~20%) in
reducing condition. The main parameter thatgoverns the extent of the kinetics limitations seems
to be the H2O or H/I ratio, as the other thermal
hydraulic parameters that could influence kinetics
limitations (residence time or temperature gradient)were similar. In oxidising conditions, the remaining
HI fraction at 150°C (representative of cold leg
break) decreases as the H2O/I ratio increases
indicating that the system is drawn towards
thermodynamic
equilibrium.
In
reducing
conditions, the I2 released increasing with the H/I
ratio indicates also kinetics limitations (paper inprogress [9]).

The principle of the small scale bench is similar to
CHIP facility. It consists of a high temperature
furnace equipped with an alumina tube andimplemented with lines allowing injection of I2 andCsOH vapours in a given carrier gas flow. I2
vapours were generated with the same device as for
the CHIP line and CsOH vapours were directly
generated in the main line by vaporisation of
dehydrated CsOH placed in a crucible. In thetransport zone, a similar temperature profile as for

Knowing the IOH system, the influence of caesium
on gaseous iodine persistence was studied. Small
scale tests were performed with an excess of Cs
relative to I (to be representative of SA conditions).
No gaseous iodine was observed in conditions of acold leg break whatever the atmosphere condition(reducing or oxidizing, cf. Table III). The thermodynamical modelling at equilibrium correctlypredicts the experimental results, indicating there isno kinetic limitation for iodine/caesium reactivity.

Three sampling lines have been implemented,
which can be sequentially operated:






94

NEA/CSNI/R(2016)5

III.2.2 Mo and B CsIOH system
Some elements are identified as caesium trap andthus can prevent the formation of CsI, such as
molybdenum or boron. It was thus decided to study
more complex systems involving additional
elements to the initial CsIOH system. The aim is to
identify the systems leading to the persistence ofsignificant gaseous iodine fraction at the break. Mo
is present in large quantities in the fuel, in excess
relative to Cs and then can be released in significant
amount particularly under RCS oxidizing condition[11-12].
A first series of tests was performed in the small
scale bench with concomitant injection of CsI andMoO3 in presence of steam [11]. It has been
observed that the presence of molybdenum, even in
low amounts, significantly increases the fraction of
released gaseous iodine.
Complementary experiments were performed in the
CHIP facility allowing thus an excess of Cs relative
to iodine as observed in the Phébus tests. The CHIP
tests main results are presented in Table III. The
influence of the atmosphere was explored with tests
performed in oxidising conditions (steam) and on
tests performed in more reducing conditions.
In oxidizing condition, with a low excess of Mo
relative to Cs, up to 90% of iodine is released asgaseous species mainly as I2 and HI. The aerosols
collected at 150°C were composed of molybdenumoxides and of various forms of caesium molybdates(Cs2MonO3n+1, n2) as already observed in similar
tests on small bench. No CsI particles weredetected. Molybdenum reacts with caesium toproduce caesium molybdates, so caesium is notavailable to react with iodine, and the gaseousiodinefraction
remainshigh.

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NEA/CSNI/R(2016)5
TABLE III
Overview of the CHIP program tests [9], [10],[12]IOH
Small bench
Oxidising
H2O/Ar50/50

Small bench
Reducing
Ar/H2
97.3/2.7

O/I~15000

H/I~9900--

Cs/I~2I
Cs
Mo or B

33%
-

36%
-

Ig
Iaerosol

67%
-

I2
HI

9/10
1/10

SystemTest loop
Carrier gas composition (%Vol.)
Element ratio HT zone
Element ratio sampling lines

CsIOH
Small bench
Oxidising/ Reducing

MoCsIOH
CHIP
Oxidising
H2O/He/Ar
51/44/5
Cs/I~4
Mo/Cs~3.2
Cs/I~ 2.7
Mo/Cs~3.3

CHIP
Reducing
H2O/H2/Ar
2.5/2.5/95
Cs/I~4.2
Mo/Cs~3.7
Cs/I~3.2
Mo/Cs~3.1

BCsIOH
CHIP
Oxidising
H2O/Ar50/50
Cs/I~2.3
B/Cs~19
Cs/I~2
B/Cs~35

50%
70%
-

0%
27%
25%

38%
52%
66%

30%
35%
2%

64%
-

0%
50%

90%
10%

1%
61%

1%
69%

2/10
8/10

-

~2/3
~1/3

0
-

1
0

H2O/Ar or Ar/H2
Cs/I~2 to 4

Deposition in the main line

Iodine form

Iodine gaseous speciation

96

NEA/CSNI/R(2016)5

As soon as the atmosphere becomes more
reducing, gaseous iodine released strongly
decreases even in presence of MoO3. CsI
remains the dominant form of transported iodine.
Molybdenum was significantly reduced to the(+V) and (+IV) oxidation states and caesium
polymolybdates were also detected. .

gives an overview of the results of theexperiments carried out so far to investigate theformation of molecular iodine (I2) and organic
iodide species (RI) from the sump or frompainted surfaces in the containment atmosphere
under various conditions.

On the contrary, large excess of B leads to therelease of only ~1% of gaseous iodine (BCsIOH
test , cf Table III). Molecular iodine has been
identified as the main species, but the existenceof other gaseous species (HI, HOI) cannot be
excluded. The weak influence of B on gaseousiodine persistence at the cold leg break was also
observed during the Phébus FPT2 test, performed
with a boric acid injection in excess relative toCs (B/Cs ~19). It seems that only very large
release of boron (B >> Cs) may lead to asignificant gaseous iodine fraction at the cold leg
break. Even if the formation of caesium
metaborate may be expected, these species could
not be directly evidenced [10].

The experimental set-up used to study the releaseof volatile iodide species consists of a loop
containing a panoramic irradiator, an electropolished stainless steel irradiation vessel (4.8l),
connected through electro-polished stainlesssteel tubes to a selective iodine filtration system,
called Maypack (Fig. 3).

IV.1 Experimental set-up

Cadmium, indium or silver may also contributeto a strong lowering of gaseous iodine at thecircuit cold leg break, more CHIP tests are in
progress performed in order to study theirinfluences.
IV. EPICUR EXPERIMENTS

Fig. 3: Simplified view of the experimental EPICUR
loop

Based on the Phébus experiments as reported in
section II, several iodine species (moleculariodine, aerosols) are transferred into thecontainment building during a SA. The
objectives of the EPICUR tests performed in theframework of ISTP and STEM programs are toobtain results on formation, deposition anddecomposition processes of iodine species
released in the containment vessel. The
ISTP/EPICUR experiments were particularly
dedicated to study the behaviour under radiationof iodine species transferred into the sump (in thecontainment sump, iodine in soluble compounds
is dissolved as non-volatile iodide: I-). The
STEM/EPICUR tests were dedicated to study therelease under radiation of iodine species(molecular iodine or iodine aerosols species)
deposited onto painted surfaces in thecontainment atmosphere.
After a brief
description of the EPICUR facility, this section

The irradiation vessel subjected to the dose ratefrom the irradiator simulates the reactor
containment. In the first set of experiments, thesump is simulated by an aqueous iodine solution.
In the second set of experiments, the surfaces of
the containment are simulated by an EPOXY
painted stainless steel coupon placed in thegaseous phase of the irradiation vessel.
The panoramic irradiator containing 60Co
sources is designed to deliver an average dose
rate of several kGy.h-1 to represent the effect ofradiation related to the presence of fission
products in the reactor containment during a SA.
The sump solution or the painted coupons areplaced in the temperature-controlled irradiation
vessel connected through stainless steel tubing to
the May-pack. Via this device the volatile
species produced in the irradiation vessel are
97

NEA/CSNI/R(2016)5
transferred to different species-selective filters inorder to trap molecular iodine I2 (Knit-mesh filter
with silver clad copper) and organic iodide RI(KI impregnated charcoal filter). Thanks to theuse of 131I to label iodine used to load the sumpor the painted coupons (see § IV.2), continuous
γ-measurements are performed on the May-packfilter stages (NaI counters are placed above eachfiltration stage of the May-pack device) in orderto follow on-line the release of the different
species volatilized and trapped on the selective
filters.

stainless steel coupon before the loading withlabelled iodine and the irradiation phase.
The coupons were loaded either with moleculariodine (I2) or aerosol species (CsI). Moleculariodine was produced from the Dushman reaction(1) in presence of labelled iodine (131I) using asodium iodide and a sodium iodate solution:
IO 3  5I   6H   3I 2  3H 2 O (1)The CsI aerosols were produced via an aerosol
generator from a CsI solution prepared using acommercial powder solubilized in water and inpresence of labelled iodine (131I).

At the end of the irradiation phase, the iodine
filtration system is recovered. The different
filters, the loaded sump or coupon and the rinsing
solutions of the irradiation vessel and of theexperimental loop are also g-counted after thetest in order to compare these post-test
measurements with the on-line measurements
and to calculate the iodine mass balance.

Then, the 131I loaded coupon was introduced in
the irradiation vessel and placed on theexperimental loop.
IV.3 Experimental conditions of radiation
The experimental conditions of the EPICUR tests
are detailed in Table IIV. The objectives of thesetests were to study the impact of several
parameters under radiation on the formation oforganic iodide and molecular iodine from iodine
species transferred into the sump (dissolved in
the sump or deposited onto immersed painted
surfaces) or from iodine species deposited onto
painted surfaces in the containment atmosphere.
The gases used for these experiments (to transfer
volatile species produced in the irradiation vessel
towards the Maypack) were argon or high purity
air.

IV.2 Samples preparation
Two kinds of tests were performed on thereleases of iodine species from the sump or
painted coupon, loaded with iodine.
The sump was simulated with a solution of
sodium iodide (Na127I) prepared in a boric acidsolution (0.2 mol.l-1) and labelled withradioactive iodine (131I). The pH was adjusted
to 5.0 or 7.0 by adding H2SO4 or NaOH. 2 litresof this solution were introduced in the irradiation
vessel and placed on the experimental loop.

Table IIV: experimental conditions of the EPICUR tests

The painted coupons were 56 mm in diameterand 3 mm thick (total geometrical surface of 50
cm2). They were made of black steel andrecovered with 2 layers of Centrepox N (40 µm)
and 1 layer of Hydrocentrifugon (50 µm) paints.
Centrepox N and Hydrocentrifugon paints are
epoxy type paints used in the French nuclear
plants. Centrepox N is used as a sub-layer toavoid the corrosion of the surfaces. The painted
coupons were preheated at 130°C in a dry
atmosphere for 96 h prior to the labelling phaseand the irradiation phase.

Phase orsupport

Iodinespecies

T
(°C)

[I]i
(mol.l-1
or mol.m2)

Liquidphase
Sump

Painted surfaces
in thecontainment
atmosphere

Some tests were performed using quartz or
stainless steel coupons. These coupons were firstcleaned with demineralized water then withalcohol and then dried. No other treatment (preheating phase) was performed on the quartz or
98

Liquidphase +
immersedpaint

10-4 or
10-5

I-

Paint

I2 or
CsI

Quartz or
stainless steel

CsI

80
or
120
10-2 or10-3 or
10-4

NEA/CSNI/R(2016)5
IV.4 Results and discussions
IV.4.1 Sump
experiments

radiolysis–

order to study the aerosols decomposition
without paint interactions. The effect ofparameters of main interest (temperature, relative
humidity, initial concentration of iodine
deposited on the sample surface) on the releasesof I2 and RI and of the global volatilization(defined as the difference between the initialquantity deposited on the coupon before
irradiation and the quantity of iodine remaining
on the coupon after irradiation) are presented anddiscussed below.

ISTP/EPICUR

Iodine behaviour under radiation in the sump has
been extensively studied in the past three decades[13-19]. Parameters such as dose rate, pH,
solutes concentration, presence of oxygen or
temperature were investigated as they can modify
the yield of primary radiolysis products and thus
the volatility of iodine. Results obtained in
ISTP/EPICUR experiments focused on themolecular iodine release from the sump (iodide
in liquid phase) under radiation were presented inan earlier paper [20]. Results indicated that themolecular iodine release decreased strongly withincreasing pH in the studied experimental
conditions. Sump temperature and oxygen
content had a less important impact on themolecular iodine release. Finally, the influence of
the initial iodine concentration in the liquid phase
was also investigated. Results suggested that its
influence on the molecular iodine transfer rate
was rather small in the iodine concentration
range 10-4 - 10-5 mol.l-1. The observations made inthis article were consistent with those of previous
researchers. Further ISTP/EPICUR experiments
focused on the release of organic iodides underradiation from immersed painted surfaces were
performed (liquid phase + immersed paint).
These experiments showed no significant releaseof volatile organic iodides from painted coupons
immersed in a liquid phase.

IV.4.2.1Releases from painted coupons loadedwith I2
Figure 4 shows the general trend of the releases
kinetics of I2 and RI from epoxy painted coupons
under radiation. Figure 4 shows two kinetics: thereleases are rapid during the first hours of theirradiation phase and then kinetics decrease.

Fig. 4: General trend of the RI and I2 releases

The study of the impact of initial iodine surface
concentration on the coupon on the releases
showed that the fraction of organic iodide
released was stable for initial iodine surface
concentrations lower than 10-3 mol(I).m-2 then it
decreased for higher initial iodine surface
concentrations; whereas the fraction of moleculariodine released and the global volatilization
increased when the initial iodine surface
concentration on the coupon increased. These
results may be explained taking into account themolecular iodine interaction with the Epoxy
paint during the loading phase. Different
chemical sites are suspected for iodine-paintinteraction [22]:IV.4.2 Painted surfaces in the containment
atmosphereradiolysis
STEM/EPICUR
experiments.
The STEM/EPICUR experiments focused on thereleases of volatile molecular iodine (I2) andorganic iodides (RI) from coupons loaded withmolecular iodine [21] or iodine aerosols species(CsI) and placed in the gaseous phase of theirradiation vessel, under a long duration period (≥30 hours) to evaluate medium term releases.
Six tests were performed on the releases from
epoxy painted coupons loaded with I2. Seven
tests were performed on the releases from epoxy
painted coupons loaded with CsI aerosols but
also with quartz or stainless steel coupons, in

99chemical sites responsible for the I2 releaseduring the irradiation phase (the reactions or

NEA/CSNI/R(2016)5

interactions between Iodine and paint are
reversible);chemical sites responsible for the RI releaseduring the irradiation phase (the reactions orinteractions between iodine and paint are notreversible, mechanisms and chemical
recombination from the iodine bounded tothe paint lead to the formation of RI).

the decrease of the CsI concentration by a factorof 24 and/or of the CsI particles size (by a factor
of about 10) on the epoxy painted coupon leads
to a decrease of the releases of iodine species by
a factor of 3: an important decrease of thefraction of molecular iodine volatilized and amore moderate decrease of the fraction oforganic iodides volatilized were observed. Thisdecrease of the volatilization of iodine species
could be due to the interactions between CsIaerosols and epoxy paint but this hypothesis
cannot be confirmed at this stage of the study.

The trend towards an increase in the RI fraction
released at lower initial iodine surface
concentration on the coupon would suggest that alarger fraction of iodine is bounded to thechemical sites responsible for RI release when
the surface concentration is decreased. This inturn indicates a preferential interaction of iodinewith these sites, rather than with reversible sites
as discussed above. Saturation of these preferredsites could have been reached after a certain
initial iodine surface concentration on the
coupon. Further adsorption would then occur onother chemical sites identified above, leading to a
relative decrease of the RI release.

The irradiation of aerosols of CsI deposited on aquartz or a stainless steel coupon led to an almost
total volatilization of iodine species whereas theirradiation of aerosols of CsI deposited on apainted coupon (epoxy paint) led to a partial
volatilization. This difference of behaviour may
be due to different interaction processes between
CsI aerosols and the surface of coupons ofdifferent nature (epoxy paint or quartz or
stainless steel).

When the relative humidity increased from 20 to
60%, the organic iodide release and the global
volatilization decreased, the molecular iodine
release decreased moderately. This may be
explained as follows: water vapour may polarize
iodine and make easier its interaction with polargroups of paint (chemisorption) which could
therefore lead to lower releases.

The increase of relative humidity from 20% to
50% promoted molecular iodine release from aquartz coupon whereas the increase of thetemperature from 80°C to 120°C did not affectsignificantly the fraction of molecular iodine
released from a stainless steel coupon, taking
into account all experimental uncertainties.
V. SUMMARY AND PERSPECTIVES

When the temperature increased from 80 to
120°C, the global volatilization and themolecular iodine release decreased drastically;
whereas the organic iodide release was not
significantly affected. A possible explanation isthat iodine interaction with paint is enhancedwith a higher temperature, favouring chemical
reactions with higher activation energy, leading
to a decrease in the molecular iodine release at
higher temperature [22].

Short term iodine processes are well predicted
for various accident scenarios. Indeed, from a“circuit” point of view, the CHIP program
highlighted the fact that in certain conditions, a
direct gaseous iodine release in the containmentbuilding cannot be excluded – as alreadyobserved in at least one of the Phébus tests. The
presence of fission products as Mo or B and thenature of the atmosphere (oxidizing or reducing)have a strong influence on this release.
Moreover, kinetic limitations may also play arole as evidenced from the study of the gas phase
iodine reactivity.

IV.4.2.2 Releases from coupons loaded with CsI
The general trend of the I2 and RI releases from
epoxy painted coupons loaded with CsI underradiation showed only one kinetics: the releasesof I2 and RI are constant during the irradiation
phase.

Regarding the iodine behaviour in thecontainment, the ISTP/EPICUR tests highlighted
that molecular iodine could be released from thesump and that molecular iodine and organic
iodides could be released from painted surfaces
according to various phenomena and kinetics.

The study of the impact of the CsI concentration
loaded on the epoxy painted surface showed that100

NEA/CSNI/R(2016)5
Gesellschaft für Anlagen – und Reaktorsicherheit(Germany), the Korea Atomic Energy ResearchInstitute, the Korea Institute for Nuclear Safety,
The US Nuclear Regulatory Commission.

CHIP and STEM/EPICUR programs haveproduced numerous results which help to greatly
improve the understanding of iodine behaviour in
Phébus program and therefore in severe accident
conditions. The ongoing STEM/EPICUR tests
are focused on the iodine oxide aerosols stability
under radiation and might show a significant
volatilization of molecular iodine from these IOx
species. Phenomenological models have beendeveloped and implemented in simulation tools
like the ASTEC severe accident code.

REFERENCES
1. L. SOFFER, et al., “Accident Source Term
for Light Water Nuclear Power Plants”,
NUREG-1465 (1995).
2. M. SCHWARZ, et al., Phebus FP: A severe
accident research program for current andadvanced light water reactors. Nuclear
Engineering and Design, 187, 47. (1999).

However, qualifying the potential mid-term
releases of volatile iodine species (fission
products revaporisation from the RCS, radiolytic
degradation of iodine aerosols in thecontainment) still needs further knowledge.
Therefore, experimental works on iodine
chemistry in SA conditions are still ongoing at
IRSN.

3. B. CLEMENT and R. ZEYEN, The Phebus
Fission Product and Source Term
International Programs. In : Proceedings ofInternational Conference on Nuclear Energy
for New Europe, Bled, Slovenia, 5-8
September. (2005).

The CHIP program is extended as CHIP +focusing on the influence of control rods
materials (silver, cadmium, and indium) on
iodine chemistry in the RCS. Within the
mitigation and outside releases in case of SA(MIRE) project, the mid-term potential iodine
releases due to revaporisation of RCS deposits isinvestigated. An extension of the current
OECD/STEM program is foreseen incollaboration with an extension of the
OECD/BIP2 program. The effect of paint ageingon iodine trapping and release under irradiationand the effect of irradiation on stability of iodine
aerosols will be the main subjects on iodine
behaviour to be studied by IRSN.

4. B. CLEMENT and R. ZEYEN “Theobjectives of the Phébus Experimental
program and the main lessons learned,
Annals of Nuclear Energy, 61, 4-10”(2013)
5. P. MARCH and B. SIMONDI TEISSEIRE“Overview of the facility and experiments
performed in Phébus FP”, Annals of Nuclear
Energy, 61, 11-22 (2013).
6. A.C. GREGOIRE and T. HASTE “Material
release from the bundle in Phébus FP”,
Annals of Nuclear Energy, 61, 63–74 (2013).
7. N. GIRAULT and F. PAYOT, “Insights into
iodine behaviour and speciation in the
Phébus primary circuit”, Annals of Nuclear
Energy, 61, 143–156(2013)

ACKNOWLEDGMENTS
Authors acknowledge the partners of the Phébus
and the International Source Term Program:
Electricité de France, the Commissariat àl’Energie Atomique, the European Community,
the US Nuclear Regulatory Commission, theAtomic Energy of Canada Limited, SuezTractebel and the Paul Scherrer Institute.

8. B. SIMONDI TEISSEIRE et al., “Iodine
behavior in the containment in PHébus FP
tests”, Annals of Nuclear Energy, 61, 157169 (2013).
9. A.C. GREGOIRE, et al. “Studies on thethermokinetic of the IOH system in theconditions representative of the RCS in
nuclear severe accident conditions”, inprogress.

Authors acknowledge also the OECD hosting the
STEM project. Authors acknowledge for theirsupport the Electricité De France, the Atomic
Energy of Canada Limited, the Teknologian
tutkimuskeskus VTT (Finland), The Nuclear
Research Institute (Czech Republic), the10. A.C.
GREGOIRE,
H.
MUTELLE,“Experimental study of the [B, Cs, I, O, H]
and [Mo, Cs, I, O, H] systems in the primary101NEA/CSNI/R(2016)5
17. Lucas M., “Radiolysis of cesium iodide
solutions in conditions prevailing in apressurized water reactor severe accident”,
Nuclear Technology, 82, 157-161 (1988).

circuit of a PWR in conditions representativeof a severe accident”, Proceedings ofInternational Conference on Nuclear Energy
for New Europe, Ljubljana, Slovenia, 5-7
September (2012).

18. G.J. EVANS et al., “The volatilization ofiodine species over dilute iodide solutions”,
Canadian Journal of Chemical Engineering,
71, 761-765 (1993).

11. M. GOUELLO, et al. « Analysis of theiodine gas phase produced by interaction of
CsI and MoO3 vapours in flowing steam”,
Nuclear Engineering and Design, 263, 462472 (2013).
12.

19. F. TAGHIPOUR et al., “Iodine behavior
under conditions relating to nuclear reactor
accidents”, Nuclear Technology, 137, 3, 181193 (2002).

A.C. GREGOIRE et al., « Studies on the roleof molybdenum on iodine transport in the
RCS in nuclear severe accident conditions,Annals of Nuclear Energy, 117-129 (2015).

20. S. GUILBERT et al., “Radiolytic oxidationof iodine in the containment at hightemperature and dose rate”, Proceedings ofthe International Conference Nuclear Energy
for New Europe, Portorož, Sept. 10-13,
Slovenia (2007).

13. C.B. ASHMORE et al., “Measurements of
the radiolytic oxidation of CsI using sparging
apparatus: part II”. AEA-TSD-0516, AEA
Technology (1995).
14. C.B. ASHMORE et al., “Measurements of
the radiolytic oxidation of aqueous CsI using
a sparging apparatus”, Nuclear Technology,
129, (3), 387-397 (2000).

21. J. COLOMBANI et al., “Experimental studyof organic iodide volatilization from painted
surfaces present in the containment during asevere accident”, Proceedings of the 6th
European Review Meeting on Severe
Accident Research, Avignon, 2-4 October,France (2013).

15. C.B. ASHMORE et al., “Measurements of
the radiolytic oxidation of CsI using sparging
apparatus: part III”, AEAT-1268, AEA
Technology (1997).

22. BOSLAND L. et al., “Towards amechanistic interpretation of the iodine –paint interactions”, Proceedings of the 4th
European Review Meeting on Severe
Accident Research, Bologna, 11-12 May,
Italia (2010).

16. W.G. BURNS et al., “The radiolysis ofaqueous solutions of caesium iodide andcaesium iodate”, AERE-R-13520, Harwell(1990).

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THAI EXPERIMENTS ON VOLATILITY, DISTRIBUTION AND TRANSPORTBEHAVIOUR OF IODINE AND FISSION PRODUCTS IN THE CONTAINMENT
S. Gupta(1), *, F. Funke(2), G. Langrock(2), G. Weber(3), B. von Laufenberg(1), E. Schmidt(1), M.
Freitag(1), G. Poss(1)1Becker Technologies GmbH, Koelner Strasse 6, 65760 Eschborn, Germany, 2AREVA GmbH; PaulGossen-Straße 100, 91052 Erlangen, Germany, 3Gesellschaft für Anlagen- und Reaktorsicherheit(GRS)mbH, Forschungsgelände, 85748 Garching, Germany*Corresponding author, Tel: (+49) 6196936115, Fax: (+49) 6196936100, Email:gupta@beckertechnologies.com

Abstract – In case of a hypothetical severe accident in a nuclear light water reactor, the kinetics ofreactions of iodine species entering from primary side to the containment is complex and mainly influenced
by the thermal hydraulic conditions and the type of gaseous, aqueous and solid materials available forinteraction, e.g. gas components, decontamination paint, aerosols, and water pools. Resulting physical andchemical processes have an influence on the iodine volatility and subsequently on the iodine and aerosol
inventory in the containment atmosphere. The availability of experimental data investigating abovementioned effects in large scale experimental facility is very sparse and thus desirable for the further
development and validation of iodine and aerosol models incorporated into integral accident analysis
codes.
In this context, 49 experiments conducted so far in the THAI facility provide a basis to understand thesource term relevant iodine and aerosol behaviour inside the containment. The THAI test facility allows
investigating various accident scenarios, ranging from turbulent free convection to stagnant stratified
containment atmospheres and can be combined with simultaneous use of hydrogen, iodine and aerosol
issues. Main component of the THAI test facility is a 60 m³ stainless steel vessel, 9.2 m high and 3.2 m indiameter, with exchangeable internals for multi-compartment investigations. The maximum designpressure of the vessel is 14 bar at 180 °C which allows to conduct H2 deflagration tests. The facility isapproved for the use of radiotracer I-123 which enables the measurement of time resolved iodine behavior
down to very low concentration levels.
THAI experiments related to iodine and aerosol issues cover a wide spectrum of accident scenarios andprovide relevant data for validation and development of iodine and aerosol models implemented in thesevere accident analysis codes. The adsorption and desorption of iodine on steel and painted surfaces
under different thermal-hydraulic conditions in single- and multi-compartment volumes, the mass
exchange of iodine between gaseous and liquid phases, interaction between iodine and airborne aerosol
particles, dry and wet resuspension of aerosols from surfaces and water pools, iodine and aerosols
interaction with an operating PAR, formation of iodine oxides by reaction between iodine and ozone, andiodine release from a flashing jet are examples of the phenomena and processes which have been
investigated in the THAI tests.
Based on THAI experimental results, important progress has been demonstrated in modelling of aerosol
and iodine behaviour and their coupling with containment thermal hydraulics in severe accident analysis
codes, such as COCOSYS-AIM, ASTEC-IODE, MELCOR-INSPECT, and others. The improved models
have demonstrated reliable simulation of complex experiments, e.g. analysis of NEA THAI 2 test
investigating molecular iodine interaction with non-reactive (SnO2) and reactive (Ag) aerosols, and EUSARNET/SARNET2 code-benchmark exercises involving THAI data on iodine/surface interactions, iodine
mass transfer, iodine oxide behaviour and iodine transport in multi-compartment behaviour.
A follow-up of NEA THAI 2 project is currently under discussion. Hydrogen related investigations on PAR
performance under counter-current flow conditions and hydrogen deflagration tests in two-compartment

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NEA/CSNI/R(2016)5
system are proposed. Additionally, in light of Fukushima Daiichi accident, experimental investigations areforeseen to study the release of fission product (aerosols and gaseous I2) from water pool at elevatedtemperature due to continuous heat-up of pool or depressurization (venting) induced boiling. Anotherplanned experiment is related to “delayed” source-term in order to investigate re-emission of aerosols aswell as iodine deposits from steel/painted surfaces due to hydrogen combustion.
The present paper provides an overview of the THAI experiments related to iodine and aerosol issues
performed in the frame of national and NEA joint projects. From the comprehensive THAI experimental
database, a selection of typical results is presented to illustrate the multi-functionality of the THAI facility
and the broad variety of iodine and aerosol related experimental investigations.

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NEA/CSNI/R(2016)5
I. INTRODUCTION

condensation. Experiments on iodine masstransfer between gas space and sump have been
carried out under well controlled thermal
hydraulic test conditions, stratified or well-mixed
sump, and for different convection states of sump
water motion. Another example out of amultitude of possibilities is the interaction ofmolecular iodine, and/or aerosols with safety
devices, such as, spray, passive autocatalytic
recombiners (PAR), pressure suppression pool,
installed inside the containment as accident
mitigation systems.

The local iodine concentration and iodine species
behaviour in the containment is governed byseveral chemical and physical processes andmainly determined by the presence of radiation,
thermal hydraulic conditions and the type of gas
and materials available for interaction, such as
gas-mixtures, decontamination paint, water pools
and their chemical boundary conditions e.g. pH,
surfactants. Estimation of fission product sourceterm requires precise information about amount,composition and release time of the volatile
radioactive species which could be present insidecontainment and become available for release to
the environment during an accident either due toa containment leakage or via controlled release
pathways (e.g. containment venting).

In the present paper, from the comprehensive
THAI experimental database, a selection of
typical results is presented to illustrate the multifunctionality of the THAI facility and the broad
variety of iodine and aerosol related experimental
investigations.

Safety analysis tools based on coupled thermal
hydraulic, aerosol and iodine codes (e.g.
COCOSYS-AIM, ASTEC-IODE, MELCORINSPECT, and others) are employed to simulate
iodine and aerosol transport and distribution
behavior in the containment and the source term
to the environment. The performance of thesesafety analysis codes for a real reactor
application is often validated through large scale
experiments reasonably representative ofdominant accident sequences or conditions.
Large scale experiments used for validation
purpose need to consider the impact of physicalprocesses which govern the mixing ofcontainment atmosphere such as, forced
convection, natural convection, condensation for
an accurate prediction of iodine and aerosol
transport and distribution behavior.

II. THAI TEST FACILITY
THAI is a technical-scale containment test
facility designed and built to address hydrogenand fission product issues under Design Basis
Accident (DBA) and Severe Accident (SA)conditions. Main component of the facility is a
60 m³ stainless steel vessel, 9.2 m high and 3.2 m
in diameter, with exchangeable internals for
multi-compartment investigations. At the lower
end of the vessel a sump compartment is
attached. Figure 1 depicts the THAI test vessel
with its standard internals: a 4 m high inner steel
cylinder (divided into two 2 m sections, ID 1380
mm, 10 mm thick wall), open at its upper andlower end, supported by six columns (915 mmlong pipes of
60 mm OD), and four
condensate trays fixed in the annulus between
vessel wall and inner cylinder at elevation
H = 4 m. These internal structures are removable.
The vessel is designed for a maximum
overpressure of 14 bar at 180 °C and canwithstand moderate hydrogen deflagrations. The
facility is approved for the use of I-123
radiotracer which enables the measurement oftime resolved iodine behavior.

In this context 49 experiments related to iodine
and aerosol issues have been conducted in the
THAI (Thermal-hydraulics, Hydrogen, Aerosols,and Iodine) test facility. An overview of theiodine and aerosol related tests conducted up to
now in the THAI test facility is given in Table 1.
THAI offers the opportunity for the coupling ofdifferent processes or phenomena which may
occur under accident conditions. Thermal
hydraulic and iodine effects, for example, arecoupled for an investigation of iodine
distribution in a steam-air-hydrogen containment
atmosphere with natural and/or forced
convection and with or without wall

The cylindrical part of the test vessel is equippedwith three independent heating/cooling mantles
over the height for controlled wall temperature
conditioning by means of external thermal oil
circuits. The heating/cooling power of each105

NEA/CSNI/R(2016)5
jacket is determined from measurements of oil
mass flow and inlet/outlet temperature
difference. Top and bottom of the vessel can beheated electrically. The sump water can beheated and recirculated at different flow rates.
The outer sides of the vessel are thermallyinsulated by
12 cm mineral wool to minimize
the heat losses. Typical heat losses are in theorder of 8 kW at 110 °C operation temperature.
A large top flange of 1.4 m diameter and two
man holes provide access to the interior of thevessel for modifications of internals, installationof test components, e.g. PAR, andinstrumentation. Measuring flanges on five levels
and at five circumferential positions allow installation of in-situ optical and conventional
instrumentation, and of sampling lines.

LiNO3, SnO2, etc.) at specified thermal-hydraulicconditions and aerosol mass concentration up to
4 g/m3.
Instrumentation
Conventional thermal hydraulic instrumentation
is provided for pressure, fluid and wall
temperature, relative humidity, feed mass flow,
wall heating/cooling power, water level andcondensate mass flow measurements. It is
supplemented by a thermal conductivity basedgas sampling system for continuous light-gas(H2, He) concentration monitoring and withoxygen concentration analyzers based onelectrochemical detection principle. Furthermore,a spectral photometer (FASP) is available for insitu measurements of fog droplet size andairborne liquid water content.
For flowmeasurements THAI can be equipped with vane
wheeltransducers
for
point
velocity
measurements,
a
2-D
Laser
Doppler
Anemometer (LDA) for velocity profile
measurement, and a Particle Image Velocimetry(PIV) for velocity field measurement.

The size of the test vessel is large enough toestablish inhomogeneous atmosphere conditions(e.g. stratification) as well as natural convection
flow loops by controlled wall heating andcooling. The vessel dimensions, its modular
multi-compartment configuration and the wellcontrolled boundary conditions facilitate
coupling of flow distribution studies with fission
products to investigate the influence ofindependent
thermal-hydraulicconditions
prevailing in different compartments on fission
product deposition and resuspension behaviour inthe presence of steel, painted surfaces and water
pools.

Iodine distribution measurements are mainly
based upon radioactive iodine I-123 which isused as a tracer for inactive iodine, and liquid orgas samples are taken at numerous locations for
immediate gamma-ray evaluation. To avoid
errors by adsorption of gaseous iodine within thesampling lines several small gas scrubbers are
installed at the measuring points inside the test
vessel. They are filled with an aqueous iodine
absorber (alkaline sodium thiosulphate) which
retains the iodine from the gas sample sucked
through the liquid. Filling, draining and purging
of the scrubbers, and gas flow control aremanaged from outside. The iodine concentrationin the vessel atmosphere is determined from theamount of I-123 tracer in the absorber liquid andfrom the gas flow through the scrubber. In
addition, external Maypack filters are applied todiscriminate molecular, organic and aerosolborne iodine in the test vessel atmosphere.
Samplings from condensates in trays, collecting
tanks, and from sump water at different heights
allows offline evaluation of I-123. Furthermore,
I-123 sensitive scintillation detectors are focused
onto deposition coupons mounted inside thevessel and are monitoring online the iodine
deposits through glass windows. The material of

Feed systems
Feed systems are available for injection of steam,
air, gas (e.g. N2, O2, He, H2, O3), iodine andaerosol at variable positions. A 108 kW steam
generator provides up to 36 g/s saturated steam
for release into the test vessel. Elemental iodine(I2) is labelled with I-123 radiotracer by melting
the solid I2 together with I-123 placed inside asmall glass flask. The injection of gaseous radiolabelled I2 into the gas phase of the THAI vesselis then performed by hot carrier gas (e.g.
synthetic air, helium). Injection of radio-labelled
I2 into the sump water is carried out by using acold aqueous I2 solution in a pressure vessel
placed inside a glove box, which can be directly
connected to the vessel sump. The THAI facility
is equipped with various aerosol generation
techniques to produce severe accident
prototypical poly-disperse aerosols (CsI, Ag,
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NEA/CSNI/R(2016)5
the surface of the deposition coupons is chosen
to be representative for the tests objectives, suchas steel, paint or aerosol covered. Coupons canbe heated or cooled by independent steam andwater based circuits to study iodine desorption
and surface wash-down behaviour. Additionally,
specific chemical trace analyses of iodine(including iodine speciation measurements) are
performed on-site or at AREVA's Radiochemical
Laboratory, Erlangen, Germany.

hydraulic conditions. On the basis of THAI
experimental
data,
appropriate
modelsconsideringboth
physisorption
andchemisorption reactions have been developedand implemented in the COCOSYS/AIM code[1]. For example, results obtained in several
single-compartment THAI iodine tests with dry
air and steel environment facilitated optimizationof the iodine depletion velocity constants and theimplementation of a resuspension term in
COCOSYS/AIM. The model was extended byuse of the data from two steel samples measured
in the NEA BIP programme [2]. Figure 2 shows
good agreement achieved in the comparison of
COCOSYS results with one of the THAI I2/steel
interaction test Iod-18, indicating that allimportant aspects necessary for evaluating
iodine/steel interaction under dry and wet test
conditions are considered in the model. In thistest, I2 interaction with steel was measured asfunction of relative humidity over a broad rangeof relative humidity from dry air up to
superheated steam/air mixture. The influence of
natural and turbulent atmosphere convection
states on iodine deposition and resuspension
behavior was separately studied in THAI test
Iod-22. The test data indicate overall negligible
influence between natural and turbulent
convection states on I2/steel deposition andresuspension behaviour under the investigatedexperimental conditions.

Aerosol concentration is measured by filter
samplers which are installed outside the THAI
vessel. Additionally, an in-line aerosol
concentration measurement is performed by laser
light extinction measurement, which provides acontinuous and non-intrusive measurement.
Low-pressure cascade impactors are used for
determining
theaerosolparticle
size
distribution. A condensation nuclei counter and adifferential mobility analyzer are available formeasuring aerosol particles in sub-micron range.
III. SINGLE AND MULTI-COMPARTMENTIODINE DISTRIBUTION AND BEHAVIOUR
Iodine is transported in the containment by gas
and water flows under stratified or mixed
boundary conditions. There will be locallydifferent chemical and thermal-hydraulicconditions for processes such as chemical
reactions of iodine, iodine deposition andresuspension at walls and iodine mass transfer toand from sumps. As a consequence iodine will be
distributed non-homogeneously, and gaseousconcentrations, wall deposits or concentrations insumps will be different in different
compartments of reactor containment.

Iodine/paint model in COCOSYS/AIM which
has also been partly developed based on THAI
data also describes the interaction of I2 with dry
painted surfaces based upon physisorption
followed by chemisorption analogously to theiodine/steel model. Figure 3 shows thecomparison of the COCOSYS/AIM prediction
with experimental data of THAI test Iod-15. This
test was conducted at 100 °C and with 45 %relative humidity. A good agreement between theanalyses and the experiment is confirmed.
However, when the same model was applied
against THAI test
Iod-17 which had been
conducted with a large variation in relative
humidity (60-70 % in the initial test phase and
15-20 % in a late test phase), the modelled iodine
desorption was found to be too strong as
compared to the experimental results. The effectof relative humidity was not yet considered in themodel, but the availability of more recent THAI

THAI experiments cover both single- and multicompartment iodine distribution experiments.
Experiments conducted in a single compartment
with homogeneously mixed atmosphere may lackcoupling of above-mentioned phenomena withlocally varying thermal-hydraulic conditions as
expected in a multi-compartment containment
structure but provide useful database for
optimization of a particular phenomenon orprocess in iodine models. An extensive THAI
database is available on iodine interaction withsteel and painted surfaces in single- compartment
configuration under a wide spectrum of thermal107

NEA/CSNI/R(2016)5
tests suggests a significant dependency of iodine
deposition on paint from very dry to nearlysaturated conditions [3].

steam injection were operated to mix up theatmosphere, and the temperature and relative
humidity conditions were maintained in a third
test phase over longer time until end of the test.
The distribution behaviour of gaseous I2 and its
homogenisation over the whole vessel during
these test phases was monitored.

In continuation to the single-compartment iodine
tests, so far seven multi-compartment iodine tests
have been conducted in THAI. Varied test
parameters were relative humidity (alsoincluding partly condensation at walls) andsurface types (stainless steel from vessel walls
and internals or also additional painted surfaces),and coupling with aerosol behaviour. In THAI
multi-compartment tests, the iodine distribution
by atmospheric flows is measured in the THAI
vesselsub-divided
into
5-compartments
consisting of a dome, upper and lower annulus,
sump and a dead end room (inner cylinder
covered at the top). A general sketch of iodine
multi-compartment tests is given in Figure 4.

The vertical temperature profiles as measured
during different test phases of Iod-11 test are
shown in Figure 5. At the beginning of thestratified phase, about 1 g of gaseous I2 wasinjected into the dome within
10-15
minutes. The beginning of injection was the startof the test time (t = 0). Test results indicate thatpart of the I2 was adsorbed on the steel surface
but almost no I2 reached the lower compartments
during thermal stratification phase. The iodine
inventory accumulated in dome can be assumed
as a source of iodine for the subsequent test
phases. In the transient phase the mixing of thevessel atmosphere was stimulated by a controlled
injection of heat, helium and steam. The vessel
atmosphere was heated by the middle and lower
vessel jackets and the sump water was heatedelectrically. Helium injection was carried out inthe bottom compartment to support theatmospheric mixing and to serve as a tracer foratmospheric mixing. The released helium was
rapidly distributed by the convective flows andafter three hours it was completely mixed (Figure6). In contrast, iodine behaved differently andindicated
inhomogeneousconcentration
distribution in the vessel atmosphere. This
inhomogeneous mixing is due to I2 adsorption
and desorption processes onto/from the steel
surfaces which delays the transport of gaseous I2
between the compartments. This delayed I2
transport lasted until the end of the test. In themixed phase the well mixed conditions weremaintained by keeping atmospheric and sumptemperatures nearly stationary. The I2
distribution in the vessel changed only gradually,
but the concentration decreased in allcompartments due to chemisorption at the steel
surfaces. At the end of the mixed phase the I2
concentration in the dome was still one order ofmagnitude higher than in the lower
compartments. In the lower compartments the I2
concentration differed up to a factor 2, which isclearly above the measurement error of ± 30 %.

The first four iodine multi-compartment tests(Iod-10, Iod-11, Iod-12, and Iod-19) wereconducted in an almost identical test
configuration and with stainless steel(163m2) environment. The first iodine multicompartment test, Iod-10 was conducted in a dry
air atmosphere with helium tracer and in
5-compartment geometry, whereby a low air
humidity was set through a rather cold sumpcompartment. The next two THAI tests Iod-11
and Iod-12 investigated the influence of humidity
on the iodine resuspension from the wall. The
distribution behaviour of iodine in a steam/air
atmosphere with a rather high relative humidity,
yet without wall condensation, was investigatedin test Iod-11. In the third multi-compartment
test Iod-12, additionally the influence of wallcondensation was investigated. Test Iod-11 wasrepeated as Iod-19 after a time interval of three
years in order to check the reproducibility and apossible change of the stainless steel wall withrespect to iodine reactions. Thermal hydraulicand iodine results could be reproduced very well.
It was also concluded that the steel surface of the
THAI vessel had not changed significantly withrespect to interactions with iodine during othertests performed meanwhile.
In multi-compartment iodine tests, the test
strategy was generally to start with a stratified
phase with high temperature in the dome
compartment and low temperatures below. In thesubsequent mixing phase, wall heating, helium or
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The THAI multi-compartment tests Iod-11 andIod-12 have been analysed and interpreted in the
THAI benchmark performed in the frame of
SARNET2 [4]. In addition to the large user effectobserved on the results, the benchmark results
also highlighted the need of detailed modellingof iodine adsorption/desorption reactions at steel
surfaces. The iodine/steel model is alreadyavailable in COCOSYS-AIM 3. Under wet
conditions, i.e. with wall condensation, in
addition to the improved iodine/steel model atdry surfaces, optimization of iodine models withregard to iodine wash-down behavior to precisely
model I2 reaction with wet painted surfaces was
also required, and this was investigated in THAI
tests Iod-21 and Iod-24. Experimental results andtheir application for iodine model validation anddevelopment are discussed in [3, 5]. The need for
detailed thermal-hydraulic modelling to calculate
the correct relative humidity and atmosphericflow rates was also identified. Relative humidity
provides a basis for the correct application of theiodine/steel model and thus to accurately
simulate the I2 transport in a multi-compartment
geometry. Similar iodine multi-compartment
tests with partly painted surface were alsosuggested.

IV. I2 MASS TRANSFER BETWEEN SUMPAND GASI2 mass transfer between the sump and theatmosphere is an important process whichgoverns the iodine equilibrium concentrations in
the containment atmosphere. Mass transfer ofvolatile iodine species (I2, organic iodide) can besignificantly influenced by the prevailingcontainment
thermal-hydraulics,
chemicalconditions in sump, and sump water flow
regimes, e.g quiescent or agitated sump.
Volatility of iodine species from sump increases
as water temperature increases or pH in the waterpool decreases. Under condensing conditions,some iodine species deposited over containment
surfaces will be washed into the sump. Some ofthese processes have been subjected to detailed
investigation in THAI tests.
In the Iod-9 mass transfer test conducted in the
THAI test facility, test phases were designed toaddress the following main processes: I2
adsorption onto and desorption from the stainlesssteel vessel walls with and without wallcondensation, the mass transfer of moleculariodine from the gas into two different sumps, andthe iodine transport into the sumps duringcondensing conditions at the walls. The mainsump with a volume of 0.625 m3 was stagnant,then recirculated. A second sump was small, flat
(52 liter/1m2) and unstirred. Gaseous iodine wasinjected at the start of the test into a superheatedsteam/air atmosphere (relative humidity 3040 %) at 1.5 bar pressure and 90 °C temperature
with dry walls. Repartitioning of the iodine wasobserved in the gaseous phase at the wall and in
the sumps. In the final washing test phase steam
was injected at different mass flow rates to study
the I2 deposition at wet steel walls. The resulting
locally measured minimum and maximum
condensate rates from defined surface area were
1.8E-5 kg/m2s and 2.8E-4 kg/m2s, respectively.
In addition to the atmospheric iodine
concentration, the simultaneous iodine transportwith the falling wall water film into the sump
was also measured.

In the meantime, THAI multi-compartment
iodine tests Iod-27A, Iod-28 and Iod-30
including painted surfaces have been conducted.
At the time of I2 injection, relative humidity inthe thermally stratified dome compartment
varies from < 1 % (Iod-27A) to > 90 % (Iod-28
andIod-30). In Iod-30, multicompartment iodine behaviour is studied in thepresence of high relative humidity (but without
wall condensation) and with silver aerosol. The
effect of I2 removal from containment
atmospheres by interaction with aerosols has
been studied in two THAI tests using inert
aerosol SnO2 (Iod-25) and reactive aerosol Ag(Iod-26). Main experimental findings on reactionof I2 interaction with dry and painted surfaces
and aerosols are discussed in [3].

Test Iod-9 was analysed within the frame of theEuropean SARNET project and valuable insights
were obtained for severe accident analysis codeswith integrated semi-empirical approach based
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iodine models (COCOSYS/AIM, ASTEC/IODE)
as well as mechanistic approach based iodine
model (LIRIC) [6]. Measured and simulated
iodine concentration in gas atmosphere is shownin Figure 7. Employed codes fairly wellsimulated the iodine transport and behaviour in
THAI test Iod-9 in the gaseous and water phases,
and also on the surfaces in the gaseous phase.
Figure 7 also shows that wall condensation with
fast wet iodine adsorption caused fast iodine
depletion in the atmosphere. As in other THAI
tests, the distribution of gaseous I2 was strongly
inhomogeneous because of local steam injection
and locally different wall condensation. Analysis
results demonstrated the need to improvemodelling of I2 mass transfer with particularfocus on coupling of iodine mass transfer
coefficient with sump flow conditions and iodine
wash-down behavior from surfaces. The
importance of a correct modelling of I2/steel
reactions under dry and wet conditions bydifferent codes was highlighted. Under wet
conditions, i.e. with wall condensation, iodine
wash-down has to be considered. Codes withinadequate modelling capability on iodine washdown behavior and wrongly predictedcondensate mass flow rates indicated wrongly
predicted iodine concentration in the sump.

V. FISSION PRODUCT WASH DOWNBEHAVIOUR
Depending on the accident scenario and theprevailing thermal hydraulic conditions, aerosol
entering into reactor containment may consist ofparticles of various sizes and may be composedof more than one species. Aerosol particles
deposited on floors and walls in the containmentare washed off by water flow (condensate) into
puddles or small water pools in the containment
sub-compartments. From there they are washeddown via existing drains and other paths into thesump. Only a few per cent of the aerosol mass istransferred directly into the sump bysedimentation. The potential of a particularfission product to be re-released from thecontainment sump during a severe accident may
depend on several factors, such as, accident
sequence, other chemical substances such as
aerosols, surfactants present in the sump,chemical boundary conditions, e.g. sump water
redox potential, pH, and the prevailing thermalhydraulic conditions. Fission product transport inthe containment atmosphere dominates the
distribution of dose rate, decay heat and hence
humidity in the containment which in turn has alarge influence on the aerosol depletion
behaviour. Therefore, understanding of iodine
and aerosol wash-down behaviour is importantfor source term assessment. Wash-down ofsoluble aerosols like CsI from surfaces iscomparatively simple as the aerosol particles are
dissolved in the condensate and drained
downwards inside the containment. Morecomplex is the behaviour of insoluble materials,
such as SnO2 and Ag as wash-down processdepends
oncondensate
andaerosolcharacteristics among others. In case of iodine
wash-down there is an additional influence ofchemical reactions with the decontamination
paint in the presence of wall condensate.

Mass transfer of molecular iodine at the waterpool-gas interface as a function of water motion
has been investigated in THAI test Iod-23. In thetest, I2 was initially dissolved in a water
volume and the amount of gaseous iodinetransferred from water to gas side wasmeasured by instantaneously collecting thegas-borne iodine on a filter. From the iodine
accumulation on the filter, the iodine mass
flow and the mass transfer coefficient could be
evaluated. Two different states of water
motion were repeatedly established: a stagnant
pool, and an agitated pool with well-defined
flow distribution, in order to cover a wide
range of conditions for I2 mass transfer. The
measurements show that the mass transfer
coefficient is strongly dependent upon thewater motion, with much higher values for theagitated pool. The experimental data can also
be correlated by means of a water surface film
renewal model superimposed to the two-film
theory [7].

The THAI aerosol wash-down (AW) tests
addressed the main phenomenon related to thewash-down process of soluble and insoluble
aerosols deposited under superheated thermal
hydraulic conditions on flat surfaces or in a small
pool (initially dry during aerosol injection anddeposition phase). The thermal-hydraulicconditions were established in such a way that
wash-down of deposited aerosols occur only by
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NEA/CSNI/R(2016)5
steam condensing over the vessel mantles and novolume condensation takes place during steam
injection. The condensate drained out from theplates and the puddle was analysed separately for
determining the aerosol concentration. Tests
were conducted with single component aerosolsof different solubility (CsI, Ag), and aerosol
mixture (CsI/SnO2).

For soluble aerosol CsI, test results indicate that
wash-down from horizontal surfaces wascomplete but aerosol concentration in the puddle
runoff water decreased much slower and shows
draining out of CsI even after a day-longwashing period. In other words, the puddle water
acts as an intermediate storage of dissolved CsI,
which leads to a considerable delay in the aerosol
transport. In test AW2 conducted with CsI/SnO2
aerosol mixture, results on wash-down behaviourof soluble CsI aerosol were in agreement with
the AW test. However, wash-down of nonsoluble aerosols (SnO2, Ag) remained far frombeing complete in tests AW2 and AW3. Test
results both for soluble and insoluble aerosols
indicate an initial concentration spike implying
higher aerosol removal followed by a longtransient of continuous low aerosol removal.
Decrease in aerosol concentration does not occur
smoothly but exhibit timely peaks which could
be linked to the rivulet formation and dissolution
mechanisms observed during the tests.

For the wash-down tests, standard internals(inner cylinder and four lateral condensate trays)
had been removed to allow free unobstructedenvironment for aerosol deposition on speciallydesigned plate sections and small puddle. Steamcondenses and runs down vertical THAI vessel
walls. In the lower part of the THAI vessel,
horizontal deposition surfaces for aerosols were
installed in the form of interconnecting sections
made of stainless steel. Out of the total 20
stainless steel sections used to occupy the THAI
vessel cross section, 16 sections form a flat
surface (plate section) and 4 sections form awater puddle (39 mm deep, volume: 26.9 litre).
The stainless steel sections and the puddle wereinclined with a downward gradient of about 2°.
The surface area of each stainless steel section is0.37 m2. The material of horizontal deposition
surfaces was stainless steel in AW test, whereas
in AW2 & AW3 tests horizontal deposition
surfaces were coated with artificially aged
decontamination paint with an estimated age of
15 years.

In AW2 test, considering complete wash-downof aerosols from the vertical surfaces, theestimated total wash-down efficiency of theaerosol mixture from deposition surfaces was ofthe order of 50 % consisting to a large extent of
the soluble CsI aerosol. The unwashed aerosol
mass which remained on the surfaces consistedof 95 % of the non-soluble SnO2 aerosol.
The main processes which were found to have an
influence on the observed aerosol wash-down
behaviour included condensation induced flow
patterns, such as rivulets or closed water films, incase of rivulets – which was the prevailing caseduring the tests – their formation and dissolution
mechanisms, partial aerosol wash-down ascompared to closed water-film, and strong
retention of even soluble aerosols in the water
filled puddle [5, 8]. The THAI tests on aerosol
wash down were accompanied by two lab-test
programs in which parameter variations likeaerosol loading, condensate mass flow, particle
sizes and surface coating were investigated. A
major outcome of these tests was the observationof long-lasting stability of rivulets in thepresence of non-soluble aerosols surface
loadings. Furthermore, the width of theindividual rivulets scales with surface
inclination, but not with condensate mass flow.

In the first test phase aerosol injection was
carried out and sufficiently long time wasallowed to let the aerosol particles depositing
over surfaces. The mass median diameters of theairborne aerosols were in the range of
1.0 2.0 µm with geometric standard deviations in
between 1.5 - 2.0 for the three tests. At the endof the aerosol deposition phase, the achieved
horizontal surface loadings were 80 g/m2 CsI(AW), 30 g/m2 CsI/60 g/m2 SnO2 (AW2), and 35
g/m2 Ag (AW3), respectively. The specificsurface loading of the vertical walls was about 10
times lower than on the horizontal surfaces,
which is in accordance with the ratio of verticalto horizontal surface area in THAI vessel(approximately 10). The mass flow rate of steam
was kept constant at 14 g/s (AW) and 17 g/s(AW2 & AW3).

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The wash-down behaviour of molecular iodine
from vertical surfaces was first studied during a
dedicated test phase of the THAI test Iod-9. Tests
results indicate an increase in the aqueous iodine
concentration due to steam condensation on themain sump surface and the surrounding walls.
However, the doubling of the steam injection ratefrom 15 g/s to 30 g/s in the test phase dedicatedto study wash-down behaviour did not lead to an
additional increase in the iodine concentration.
The iodine wash-down behaviour and its
influence on iodine distribution was not very
well understood by the iodine codes used for Iod9 post-test analysis [6].

the optimisation and further development of
iodine and silver wash-down models in
COCOSYS/AIM-3 code.
VI. IODINE-OZONE INTERACTION AND
IODINE OXIDE BEHAVIOUR
Radiolytic
processes
occurringinsidecontainment have the potential to convert gas
phase molecular iodine and organic iodide into
iodine oxide or iodine nitroxide particles(“IOx”). This gas to particle conversion has been
investigated in the THAI tests Iod-13 and Iod-14[10]. As no radiation source is available in
THAI, the radiation field was simulated bydirectly injecting ozone (O3) as a representativeof air radiolysis products into the THAI vessel,
where it reacted with the gaseous I2. The two
tests were conducted at an atmospherictemperature of 100 ºC and a relative humidity of
60 – 70 %. In test Iod-13 ozone was injected into
the I2-loaded atmosphere. In Iod-14 test, ozone
was injected first to a high concentration level
expected to immediately oxidise all I2. The test
results indicate that iodine behaviour in Iod-13 ismainly governed by the gaseous iodine, as theozone concentration was low. However, in Iod14 test performed with ozone-saturated
conditions, I2 is rapidly converted into IOx andthe behaviour of the atmospheric iodine isgoverned by iodine oxides as aerosol particles.
The observed iodine oxides form submicronparticles with mean volume related diameter ofabout 0.35 µm. Over time, agglomeration ofparticles is observed but the IOx concentration
depleted slowly (indicating efficient diffusive
deposition rather than sedimentation) under theinvestigated test conditions and in the absence ofother nuclear aerosols.

Later on, two THAI tests Iod-21 and Iod-24 wereconducted for detailed investigation on iodine
wash-down behaviour. The parameters varied
included dry or wet painted surfaces andvariation in condensate rate over the painted
surfaces. One of the main outcomes from Iod-24
test was that the iodine adsorption onto paint canbe higher with a low condensation rate than highcondensate rate. The iodine wash-down versus
condensate rate behaviour observed in Iod-24 aswell as in Iod-9 could be explained by limited
wash-down efficiency which could be achieved
with stable rivulets and also due to the fact thatthe wet I2 adsorption velocity does not rise
linearly with the condensation rate.
Furthermore, in the THAI experiment AW3,
investigation on wash-down behaviour of thenon-soluble
aerosolwasextendedbyinvestigating the effect of washed down Ag
aerosol on the fixation of volatile iodine (I2) inthe sump water in a second part of the test.
Regarding the I2-Ag reaction in sump, thespecific surface area of the silver particles settled
at the bottom of the sump, relevant in theiodine/silver model has been identified as one of
the influential uncertain parameters determinedin the uncertainty and sensitivity study on a
COCOSYS-AIM calculation on PHEBUS test
FPT1 [9]. In THAI test AW3, the I2/Ag reaction
rate in the case of I2 being dissolved in stagnant
water above a layer of settled Ag particles wasmeasured, first results and modelling areprovided in [5].

Uncertainty still remains regarding the effect ofgas composition on IOx compounds and related
iodine volatility, which are partly considered forinvestigation in the planned NEA STEM2 project(see paper by C. Mun in this workshop).
Different conditions depending on reactor design
as well as during an accident can be expected
such as inert versus non-inert containment
atmospheres, presence of CO from fires and
MCCI or H2 from zircaloy oxidation. Additional
experiments under reprehensive accident
conditions will be required to understand the

The THAI experimental investigations related toiodine and aerosol wash-down as well as I2-Ag
reaction in sump have provided relevant data for112

NEA/CSNI/R(2016)5
effect of gas composition on conversion of IOx
compounds back into gaseous iodine species.

Conversion (%)  PAR in
100
CCsI aerosol( PARin )  PAR out
CI gas( PARout )(1)VII. INTERACTION BETWEEN FISSION
PRODUCTS AND AN OPERATING PASSIVEAUTOCATALYTIC RECOMBINER (PAR)The evaluation with Eq. 1 considers that no
gaseous iodine enters the PAR. This has also
been confirmed by gaseous iodine concentration
measurements in the vicinity of the PAR inlet.

During an accident, the prevailing thermal
hydraulic conditions and the extent of fission
products interaction with PAR units may either
jeopardize their performance or lead to constitutean additional potential source term. For the
purpose of safety analysis and the managementof mitigation measures these interaction
processes must be known by means ofexperiments under realistic test conditions. Suchexperimental data are required to be subjected todetailed plant scale analysis for evaluating thepossible impact on source term to theenvironment in the event of an accident. In thiscontext, two THAI experiments HR-31 and HR32 were specifically designed to study the fission
product interaction with an operating PAR.

The measured particle Sauter mean diameter at
the PAR inlet was in the range of 0.6 - 0.8 μm.
Considering the PAR inlet cross-section
dimension and the measured inlet velocity,
residence time of about 0.15 sec was estimated in
between two catalyst plates. The test results
indicate the conversion rates of CsI aerosol togaseous iodine in the range of 1 - 3 %.
Another important result of THAI experiments
HR-31 was the shifting in aerosol particle
distribution to lower Sauter mean diameter at the
PAR outlet.
The second experiment (HR-32) was designed toinvestigate the poisoning effects of fission
product on PAR performance. The PAR
poisoning test considered challenging test
conditions which are conceivable during a severe
accident. Subjected conditions for the test
included PAR exposure to fission products atvery low inlet H2 concentration (< 2 vol %),elevated concentration of realistic aerosol
mixture (hygroscopic and inert aerosols), use ofreactive molecular iodine, and saturated steam
conditions prevailing inside the test vessel. Test
phases were defined to ensure fission products
arrival at PAR inlet before the onset of PAR
recombination occurs.

The HR-31 test series consisted of two tests forinvestigating the thermal conversion of iodine
aerosol (CsI) to gaseous iodine (I2) while passing
through an operating PAR. Related PAR researchbased on thermochemical analysis indicated thatCsI aerosol particles in a size range typicallypresent in the containment atmosphere can bevaporized
attemperatureof 800 ˚C within residence times of 0.1s - 0.5s
which are typical of a PAR operation [11].
Test boundary conditions were established inparticular to achieve high catalyst temperatures(> 800°C) to maximize CsI conversion, which
requires high hydrogen concentrations (8 to 9
vol %) at the PAR inlet. The gas mixture in thevessel atmosphere was rendered inert by adding
steam (> 60 %) to avoid H2 ignition risks during
the experiment. The conversion yield of aerosolborne CsI to gaseous iodine has been calculated
according to the Eq. 1, by using the measured
concentration of gaseous iodine at PAR outlet
and the relationship of gas densities at PAR inlet
and PAR outlet as follows:

During the tests, the PAR was exposed to highconcentrations (1.5 g/m3 - 2.5 g/m3) of insoluble
tin oxide (SnO2) aerosol, highly hygroscopic
lithium nitrate (LiNO3) droplets, steam andradio-labelled molecular iodine (injected amount
1.7E-6 g/l). Test results indicate that PAR H2depletion efficiency remained unaffected andcomparable to the results of the other PAR tests
with the identical thermal-hydraulic conditions
but without aerosols and iodine. The onset ofrecombination was delayed and occurred at inlethydrogen concentration of about
4.0 vol %and under saturated steam conditions. However,the delay in H2 recombination onset was also113

NEA/CSNI/R(2016)5
observed for other PAR-tests conducted withcomparable thermal hydraulic conditions in thepresence of saturated steam content but without
fission products.

obtained during THAI test cases confirmed
former REST laboratory-scale test data withentrainment factors varying between 1.5E-05 and1.0E-04 for the investigated range of gas
superficial velocity from 0.044 m/s to 0.185 m/s[12]. As shown in Figure 9, a decreasing trend of
the resuspension fraction for insoluble particles
was found as the superficial velocity increases.
The effect of primary particle sizes on theresuspension fraction was found to be rather
small compared to the effect of the superficial
velocity. The largest values obtained at asuperficial velocity of 0.024 m/s wereapproximately 4.1E-4 and reduce to 1.9E-5 forhigh superficial velocities of about 0.13 m/s. Test
results indicate that mainly very small aerosols in
the range of 0.06 µm - 0.11 µm (mass median
diameter) are released from a boiling pool, which
in turn might remain airborne for a long time.
Such a small particle size represents
unfavourable value for the design of the accident
management measure filtered containment
venting because filters normally show aminimum efficiency in this particle size range(“Filter gap”).

VIII. AEROSOL RESUSPENSIONIn the event of a severe accident, aerosols
deposited onto containment walls or trapped into
water pools may be re-entrained or re-suspended
to containment atmosphere depending on an
accident transient. Aerosol “wet” resuspension
may occur either due to extreme sump water
heating, e.g. by core melt, or in case of rapid
depressurization, e.g. due to containment
venting. Other processes which may result into“dry” resuspension of already deposited aerosols
into the containment atmosphere include
atmospheric currents induced by hydrogendeflagrations or by other phenomena like steam
explosions. Among other effects, resuspension
may result into an increase in aerosol
concentrations in the containment atmospherewhich in turn may have an influence on theradiological source term from containment to theenvironment.

The varied test parameters in the THAI “wet”
aerosol resuspension tests did not consider theeffect of chemical boundary conditions, e.g. pH,
surfactants concentrations on the fission productscrubbing and release behaviour from water
pools. Further experiments will be required as
chemical boundary conditions may havesignificant impact on the fission product releasefraction either by modifying the fission productspeciation or by changing the bubble dynamics inwater pool and the splashing process at the water
surface.

Wet resuspension of aerosols from boiling sumphas been investigated in seven tests (TH-14 –TH-17 and TH-25.1 – TH-25.3) conducted so far
in the THAI test facility. The tests wereconducted by using aerosol material of different
chemical nature: CsCl, KI, Cs2SO4, and Li2SO4
salts as soluble aerosols and non-soluble aerosol
suspension of calcium carbonate (CaCO3) withtwo different primary mass-median diameters of
0.065 µm and 0.9 µm. The test configuration for
wet aerosol resuspension tests and main steps
followed during test progression are shown in
Figure 8. The resuspended fraction refers to thefission products in droplets and is defined as a
product of entrainment factor (mass ratio of
released droplets and released gas) and aerosol
enrichment factor (ratio of concentration in
droplets and in sump water). Resuspension was
effected by injecting steam at the bottom of thesump. The investigated superficial velocities(defined as the volume flow of the released gas
and steam bubbles per unit area of the pool)
ranged from 0.024 m/s to 0.13 m/s with higher
values representing churn turbulent flow regime.
For soluble aerosols, the entrainment factor

Experimental investigations on fission productpool scrubbing and water pool hydrodynamics
for selected accident scenarios are underway inthe currently running projects, e.g. EUPASSAM. The THAI water pool hydrodynamics(WH) tests [14] have been focused oninvestigating fluid-dynamic features produced bya gas/steam discharge in water pools. The
experimental investigations covered various
phenomena, such as complete and incomplete
steam condensation, thermal stratification in thewater pool, bubble-column induced gas-liquidhydrodynamics, and air blowdown behaviour in
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the water pool to quantify dynamic pressureloadings.

its speciation were not considered in the THAI
aerosol resuspension tests. However, THAI PAR
tests (see section VII) as well as previous
experimental investigations reported in [15] oninteraction between hydrogen combustion andmetal iodides showed a conversion yield of up to
75 % depending on the investigated H2concentrations, aerosol compositions and aerosol
particles size. However, the influence of various
parameters (aerosol concentration, aerosol size,
co-aerosol effect, and influence of steam content)
was mentioned to be not very well understoodand therefore additional experiments are required
to further investigate potential impact ofhydrogendeflagrationinducedthermalconditions on oxidation behaviour of metal
iodides as well as on iodine speciation undercontainment typical geometry and gas
atmosphere (e.g. paint, aerosols, steam).

Dry aerosol resuspension experiments in THAI
test facility investigate the interaction of aerosols
with hydrogen deflagrations. The experimental
configuration and the test methodology used forthe three THAI tests AER-1, AER-3 and AER-4
investigating dry aerosol resuspension is shownin Figure 10. In the first step, a layer of aerosol
depositions has been prepared on a floor close to
the vessel bottom by injecting CsI aerosol into
the test vessel and subsequent settling over a 24
hours period. Then the 5.5 m long vertical
deflagration tube has been filled with ahydrogen-air mixture. In step 2, after ignition at
the lower end of the deflagration tube thehydrogen-air mixture burns in upward direction
towards the closed top end. The burnt gases
expand and escape through the bottom pipe andare directed by a slit nozzle in horizontal
direction over the aerosol deposits. The escaping
gas jet velocities over the deposition plate are in
the range of 17 m/s to 67 m/s. Significant
portions of the deposited aerosol become resuspended,andtheresultingaerosolconcentration in the gas atmosphere is measured
by bulk-filter sampling. Before initiation of thedeflagration, majority of the aerosol particles had
been deposited over surfaces and only very fineparticles contribute to the measured low massconcentration
in
the
gas
atmosphere.
Immediately after the deflagration, the measured
particle size shows a large fraction of coarse
particles. The test conditions before and after
ignition in AER-1, AER-3, and AER-4 tests arecompiled in Table 2.

IX. CONCLUSION AND PERSPECTIVES
The main objectives of the experiments in the
THAI test facility are to supply safety-relevant
information in the area of hydrogen and fission
product transport under prototypical severe
accident conditions. The THAI dimensions andits operational features facilitate providing
database on phenomena oriented and coupledeffects experiments for validation and further
development of coupled thermal hydraulic,aerosol and iodine codes used for severe accident
analyses. In the present paper, an overview has
been provided on the THAI experiments
investigating volatility, distribution and transport
behaviour of iodine and aerosols in thecontainment.

By using THAI experimental data, a source term
calculation was performed with COCOSYS [13].
For the calculation purpose, a hydrogendeflagration was assumed in a KONVOI typereactor. The performed COCOSYS calculations
show that high air velocities produced byenergetic deflagration may resuspend alreadydeposited aerosols resulting into an increase of
the released aerosol mass up to a factor of 10
higher than aerosol mass available for release
without resuspension.

THAI experiments fill gaps and also enlarge theexisting knowledge on iodine behaviour in
gas/liquid phase under severe accident typicalthermal hydraulic conditions and interactions
with exposed steel/painted surfaces, gas
mixtures, radiolysis by-product ozone, water
pools, and aerosols. Based on THAI
experimental findings, important progress has
been achieved in aerosol and iodine modelling
and their coupling with containment-typicalthermal hydraulics. The improved models havedemonstrated reliable simulation of complex
experiments,
e.g.
hydrogen
distribution
experiment (ISP-47 and NEA THAI code

The potential impact of hydrogen deflagration onoxidation behaviour of metal iodides (e.g. CsI)
and any effect on iodine volatility by changing
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NEA/CSNI/R(2016)5
benchmark), hydrogen combustion behaviour(ISP-49), hydrogen mitigation by PARs (NEA
THAI-2 code benchmark), iodine/surface
interactions, iodine mass transfer, and iodinetransport and multi-compartment behaviour (EUSARNET and EU-SARNET2).

REFERENCES

In the currently running national project,extension of the THAI test facility to two-vesselgeometry is planned. The extended THAI facility
will retain its unique experimental features, e.g.
use of H2 and iodine I-123 tracer, differential
wall heating/cooling. Moreover, by establishing
independent desired flow and temperature
conditions in two vessels, it will be possible toapproach realistic similarities to reactor
scenarios. Experimental investigation on washout behaviour of iodine by water spray is alsoplanned to be tested as a function of the spraywater chemistry, i.e. the difference of spraying
with fresh water and recirculated sump water.
A follow up of the recently concluded NEA
THAI 2 project is currently under discussion toresolve remaining open issues in the fields ofthermal hydraulics, hydrogen deflagration,
Passive Autocatalytic Recombiner performance,
aerosols and iodine. Some of the proposed
experiments will benefit from the extended
THAI test facility. The fission product topics areproposed to include aspects of fission productrelease from hot water pools and of gaseousiodine in complex boundary conditions including
the presence of aerosol and the effect ofhydrogen deflagration. The coupled-effects
nature of the proposed fission productexperiments would be valuable in improving theprediction capabilities of the severe accident
analysis tools.
ACKNOWLEDGMENTS
THAI experimental research programme isfunded by the German Federal Ministry for
Economic Affairs and Energy, on the basis of adecision of the German Bundestag (Projects
number 1501218, 1501272, 1501325, 1501326,
1501361, 1501420, 1501455). The sponsorshipby the countries of the NEA THAI and NEA
THAI2 is gratefully acknowledged.

116

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G. Weber, B. Krzykacz-Hausmann, F. Funke, W.
Klein-Hessling, “Uncertainty and Sensitivity
Analysis on the Iodine Model in the Containment
Code COCOSYS”, NUTHOS-10 Conference,
Okinawa, Japan, December 14-18 (2014).

NEA/CSNI/R(2016)5
10. F. Funke, G. Langrock, T. Kanzleiter, G. Poss, K.
Fischer, A. Kühnel, G. Weber, H.-J. Allelein,“Iodine oxides in large-scale THAI tests“,
Nuclear Engineering and Design, 245, 206-222(2012).
11. J.C. Sabroux and F. Deschamps, “Iodine
chemistry in hydrogen recombiners”, Eurosafe
forum on Nuclear safety, Brussels (2005).
12. H. Bunz, M. Koyro, B. Propheter, W. Schöck, W.
Wagner-Ambs,“Resuspension of fission products
from sump water”, Final Report, European
Commission EUR 14635 EN (1992).
13. H. Nowack, and H.-J. Allelein, “Dry aerosol
resuspension after a hydrogen deflagration in thecontainment”, Proceedings of international
conference on nuclear energy for new Europe,
Slovenia, September 10 – 13 (2007).
14. S. Gupta, B. Balewski, K. Fischer, G. Poss,“Experimental investigations of BWR pressure
suppression pool behaviour under loss of
coolant accident conditions”, Proceedings of
ICAPP-11 conference, Paper 11389, Nice,
France (2011).

15. Committee on the Safety of Nuclear
Installations, OECD Nuclear Energy
Agency, “Effects of hydrogen combustion on
fission products and aerosols”, Technicalnote by a group of experts, Report number
NEA/CSNI/R (93) 6 (1993).

Figure 1: THAI test vessel with standard internals

Table 1: Overview of THAI tests
investigating iodine and other fission productrelated issues. Tests not specifically marked
have been conducted in the frame of national
THAI projects (2000-2015).

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NEA/CSNI/R(2016)5

Figure 2: Comparison of COCOSYS I2 /steel model with data fromTHAI Iod-18

Figure 3: Comparison of COCOSYS I2 /paint model with data fromTHAI Iod-15

Figure 4: An example of THAI test configuration used in
Iod-30 iodine multi-compartment test conducted in thepresence of high relative humidity and aerosol

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NEA/CSNI/R(2016)5

Figure 5: Vertical profile of gas temperatures measured during
different test phases in Iod-11 test
THAI Iod-11: Measured gaseous iodine and helium
heating

Iodine, GS6 8.3m - dome
Iodine, GS5 7.0m - dome
Iodine, GS4 5.3m - upper annulus
Iodine, GS3 4.3m - inner cylinder
Iodine, GS2 3.7m - lower annulus
Iodine, GS1 1.8m - sump compartment
Helium, 8.7m - dome
Helium, 3.1m - lower annulus
Helium, 1.7m - sump compartment
Heating started
Heating terminated

1E-05

20

15

1E-06
10

1E-07

Helium concentration [Vol-%]Gaseous iodine concentration [g/l]

1E-04

Figure 8: THAI test configuration as used in wet aerosol
resuspension tests

5

detection
limits

1E-08

0
0

5

10

15

20

25

Time after iodine injection [h]Figure 6: Iodine and helium concentrations measured during
different test phases in Iod-11 test

Figure 9: Calcium carbonate resuspension fractions derived fromgas scrubber based measurements

Figure 7: Comparison of measured iodine concentration ingas atmosphere in Iod-9 test with simulation results of
COCOSYS, ASTEC and LIRIC [6]119NEA/CSNI/R(2016)5
Table 2: Dry aerosol resuspension tests:
selected test conditions before and after the
deflagration… Deflagration
Aer-1
Surface loading
g/m²
14.4
before…
Air-borne aerosol
g/m³
0.0044concentration before …
MMD d50.3 before …
µm
0.938
Temperatur°C
875
Blown-out area
mm x mm 500 x
1000
Air-borne aerosol
g/m³
0.0807concentration after …
MMD d50.3 after …
µm
1.526

Figure 10: THAI test configuration and main steps of the test
procedure used in dry aerosol resuspension tests

120

Aer-3
19.8

Aer-4
10.6

0.00350.0013

0.814
0.715
810
750
600 x 1300 500 x 500
0.24190.0424

1.857

1.547

NEA/CSNI/R(2016)5
A SUMMARY OF THE PSI INVESTIGATIONS ON IODINE CHEMISTRY IN THE
PRESENCE ON IMPURITIES AND ADDITIVES
T. Lind*1, B. Jäckel1, D. Suckow1, S.Guentay2
Paul Scherrer Institute, Villigen-PSI, Switzerland*Corresponding author, tel: (+41)310 2650, Email terttaliisa.lind@psi.ch2
Presently at: Innovative Technology Development GmbH, Switzerland
1

Abstract – An extensive experimental research project was carried out at PSI in the late 1990s´ until 2008
to investigate the behaviour of different iodine species in water pools. The first part of the project wasdevoted to generation of an easy and effective preparation of labeled organic iodide aqueous solution,
iodine speciation analysis techniques, and experiments to study the basic decomposition mechanism of
methyl iodide CH3I by water hydrolysis and radiolysis. In the later stages, the effect of different impurities
in the water on the iodine volatility was investigated. The latest research had its emphasis on the use ofadditives for improved retention of organic iodides in the water pools, and the mitigation of the release ofiodine species under conditions relevant to filtered containment venting systems. In addition, a dedicatedtool, PSIodine, was developed for interpretation of the experimental results.
Many of the results of the investigations have been already reported in the open literature and in the
research programs such as SARNET. This work will be shortly summarized here. Some unpublished work
mainly related to the iodine and organic iodide retention in the filtered containment venting systems arepresented, and conclusions are made based on the results.

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I. INTRODUCTION

Intermediate scale tests were carried out tostudy the effect of gas flow through the water
pools with realistic residence times at two
different facilities. The retention of iodine andorganic iodide in FCVS was investigated in 1:1
height scale experimental facility.

PSI has conducted several experimental andmodelling research programs on iodine chemistry
and iodine / iodide retention since the 1990’s.
Experimental investigations have covered therange from very small-laboratory scale
investigations to 1:1 height scale experiments forfiltered containment venting systems (FCVS),and from inactive species to irradiation
experiments in the PSI Hot laboratory. A
mechanistic PSIodine code has been developed
to interpret the results of the experimental
investigations. In this paper, we will give an
overview of the developments mainly after year
2002. The latest work on characterization of the
FCVS operation as well as development of amethod for improved organic iodide retention in
the FCVS are presented in a separate paper inthis publication (Suckow et al., 2015).

Here we describe the facilities and experiments
in two different scales, the small laboratoryscale and the 1:1 height scale.
Small laboratory-scale set-up
Two types of reaction vessels were employed
for the experiments in the small laboratoryscale. A specially-designed reaction vessel, andcommercially available small bottles which areemployed in the pharmaceutical industry wereused.

At PSI, the research on iodine chemistry
after the year 2002 has mainly concentrated on
the effect of impurities on the volatility of thedifferent iodine species in aqueous solutions,decomposition of methyl iodide in water
solutions in the presence of additives, andcharacterization of the retention of iodine andiodides in the FCVS. This work has beenpreceded and accompanied by fundamental
investigations devoted to generation of an easy
and effective preparation of labeled organic
iodide aqueous solution, iodine speciation
analysis techniques, as well as experiments
studying the basic decomposition mechanisms of
CH3I by water hydrolysis and radiolysis. The
results have demonstrated the repeatability of theliterature results and extended the database toother conditions, such as in-situ beta-irradiation
effects. Special emphasis has been tocharacterize the behaviour of organic iodides,
mainly CH3I, and to investigate the effects ofdifferent additives with the aim to increase theCH3I decomposition rate.

The specialized reaction vessel in theirradiation chamber of the gamma cell, Figure
1., in the PSI’s Hot Laboratory was used forseveral tests involving irradiation of mixturesof initially non-volatile iodide and various
accident-relevant compounds. It was also used
to pre-irradiate investigated reagents. The
vessel contains a frit near its inside bottom topass bubbles of a selected gas through theirradiated or unirradiated solution (sparging).
The gas then passes out of the vessel and into a
glass tube containing a solid-phase absorberselected for either I2 or CH3I absorption for
activity measurements. The gas flow rate waselectronically controlled. Such a facility alsoenabled determination of the mass transfer
coefficients of I2 and CH3I for the particular
reaction vessel in use. The reaction vessel was
also jacketed to perform experiments at higher
temperatures.
The majority of experiments were carried out inglass pharmaceutical bottles of capacity 37 cm3.
The bottle was closed by a PTFE
(Polytetrafluoroethylene)-lined rubber septum
cap. This cap enabled experiments with stilland sparged test solutions. Two syringe needles
pierce the septum cap. They are connected to
the regulated gas flow and tube containing thesolid phase material to trap volatile iodine
species respectively. Thus sparging either

II.EXPERIMENTAL METHODSThe experimental work was carried out indifferent facilities representing different scales.
Fundamental investigations of iodine chemistryin water pools was carried out in smalllaboratory-scale devices and in the hot cells.

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NEA/CSNI/R(2016)5
during or after irradiation or after a pre-defined
period for the thermal (unirradiated) reactions
can be carried out.
For the tests, aqueous solutions were preparedfrom analytical grade reagents, e.g., CsI, KNO3,
KNO2, CsCl, B(OH)3 without further
purification. Water was doubly distilled fromdemineralised water. The sparging gases usedwere argon, nitrous oxide and air of 99.999%
purity. For air-sparged experiments, synthetic
air was used (80% N2, 20% O2). A tracer 131-Iwas used to determine the distribution of iodine
species in the gas phase and in the solution by
activity measurements. For this purpose, gas
phase species were collected in an activatedcarbon filter impregnated with tri-ethylenediamine (TEDA).
To control the pH of the water, test solutions
were usually buffered. The required initial pH
from 7 to 9 was obtained by boric acid andsodium borate mixtures, or by sodiumhydroxide addition to boric acid solution. The
higher initial pH was obtained by either sodiumhydroxide or sodium carbonate addition. For
tests in the highly acidic range of pH 2 to 4,
sulphuric acid or nitric acid were used at
appropriate concentrations.

Figure 1. The reaction vessel shown within theirradiation chamber of PSI’s γ-irradiation cell.

1:1 height scale FCVS facility

More detail of the facility, methods andmeasurements are given by Cripps et al., 2010.

The retention on iodine (I2) and methyl iodide in
the FCVS was investigated in the 1:1 heightscale, reduced diameter facility, Figure 2. Inorder to represent the most importanthydrodynamic features of the FCVS, the facility
had all the internals of a commercial FCVS,
including the injection nozzle, water pool withmixing elements, and different components ofthe droplet separators. The water level above theinjection orifice could be varied.
The full height of the facility enabled a similar
residence time for the gas as in the real scale
FCVS. The volume and flow rate scaling factorof the facility was 1:124. The facility dimensions
are given in Table 1. The operation pressure of
the facility was 5 bar(abs), and temperature
200oC. The carrier gas was air / nitrogen, steam,or a mixture of steam and air / nitrogen.

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NEA/CSNI/R(2016)5
The experiments for iodine / CH3I retention in
the FCVS were carried out using the relevant
additives in the water, namely, Na2S2O3 forretention of iodine compounds, and Na2CO3 tocontrol the pH.

Depending on the experiment one or bothcomponents of the iodine was mixed in the liquid
feed tank. The liquid mixture was then pumped
into a two-component spray nozzle to spray thewater in form of extremely fine droplets. The
spray nozzle was attached to the mixing chamberof the DRAGON II facility. The gas needed fordisintegrating the water jet into fine droplets was
hot nitrogen. Additional nitrogen flow washeated and directly fed into the mixing chamber.
The walls of the mixing chamber were heated up
to avoid steam condensation as well as providing
additional energy for droplet evaporation. The
desired organic iodide or elemental iodine
concentration in the steam-nitrogen mixture flow
was adjusted by selecting a feasible combinationof spray flow rate and the dissolved organic
iodide or elemental iodine concentration in thesprayed water.
The retention of iodine and methyl iodide in thetest section was determined by measuring
simultaneously iodine / CH3I concentration at thetest section inlet and outlet. Iodine concentration
was measured with selective iodide ion
electrodes. For the electrodes, the sample gas
was bubbled through a scrubber filled with water
doped with ascorbine acid. The methyl iodide
concentration was measured with a mass
spectrometer.
Table 1. FCVS facility dimensions and operation
conditions
Inner diameter [m]Height [m]Cross sectional flow area [m2]
Number of injection nozzles
Average gas velocity [m/s]
Pressure [bar,abs]
Gas temperature [oC]Gas flow rate [kg/h]

Figure 2. The 1:1 height scale FCVS facility
used to determine retention of iodine and
CH3I
The gaseous elemental iodine was generated bydissolving iodine crystals in about 100 ml
alcohol. The resultant solution was then poured
into the feed tank to provide a desired elemental
iodine concentration in water. Similarly theorganic iodide was generated by mixing organic
iodide solution in a known amount of
demineralised water. The mixer in the 70 l tanksecured a homogenous mixture of water andmethyl iodide; otherwise a segregation of thephases might take place.

0.27-0.40
6.0
0.06-0.13
1
0.26-0.57
4.9
152
350

III. RESULTS OF THE EXPERIMENTAL
PROGRAMMES
Several experimental programmes were carriedout at PSI in the different scales as described in
the previous section. The focus of the work waspartly on the FCVS efficiency for different
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iodine compounds, and partly on understanding
the iodine chemistry on a more general context
with a recent focus on the effect of impurities on
iodine volatility. In this section we give an
overview of the activities in these areas.

The measured decontamination factors for iodine
were around 300-400 with the water level of 1.2
m above the orifice at the initial pH of 11.25 (pHat 20oC). The higher water level would increase
the decontamination factor (DF). Consequently,
it can be seen that given a sufficient water level
above the injection orifice and a high pH in thewet scrubber, high retention of iodine in the wet
scrubber can be achieved.

Iodine and organic iodide retention in the
FCVS
Iodine (I2) and organic iodide (CH3I) retention in
the FCVS was investigated in five tests in which
either I2, CH3I, or both were used. CH3I was used
to represent the organic iodides as the mostvolatile of them. The temperature in the tests wasvaried in the range 70-120oC, and the FCVS inlet
pressure was 4.4 bar (abs). The steam massfraction in the gas flow was approximately 0.70.8, and the total flow rate 325-350 kg/h. The
additives relevant to FCVS were used in thewater in the wet scrubber, i.e., Na-thiosulfate forretention of iodine, and Na2CO3 for pH control in
the tests when high pH was used. No otheradditves were used in the water. The
experimental conditions are given in Table 2.

Organic iodide (CH3I) retention wasinvestigated in three experiments. The retention
was found to depend on the water level andtemperature. When the water temperature
decreased to 70oC and below, and the water level
above the injection orifice was approximately 1m or less, the CH3I retention was low with thedecontamination factors 2-10. With the waterlevel of 1.6 m and more, and T = 120oC,
decontamination factor was more than 10. The
effect of pH on the retention of CH3I was notinvestigated in this experimental program.
Based on the tests, it was concluded thatwith the existing FCVSs, sufficient retention of
gaseous elemental iodine can be achieved when
the water level in the wet scrubber can be
maintained at a high level, and the pH is high.
However, the retention of CH3I without any
further actions is relatively modest. It has to be
noted that other organic iodides than methyl
iodide were not investigated. They are typicallyless volatile than methyl iodide, and are therefore
considered to pose less challenges in terms ofenvironmental
releases.
Following
this
investigation, it was decided to concentrate the
PSI efforts in the iodine research on finding
solutions to retain CH3I more efficiently in the
FCVS and other water reservoirs, such as
suppression pools in BWRs.

Table 2. Experimental conditions in the I2 andCH3I retention tests in the FCVS.
Test #
B01
B02
B03
B04
B05

I2
Yes
Yes
Yes

CH3I
Yes
Yes
Yes
-

Na2S2O3
Yes
Yes
Yes
Yes
Yes

Na2CO3
Yes
Yes
Yes
-

Table 2. continues. Experimental conditions in
the I2 and CH3I retention tests in the FCVS.
Test # Water level
[m]
B01
0.9
B02
0.9
B03
1.2
B04
0.75-1.6
B05
2.1

Water T
[oC]
120
120
120
70, 120120

pH
4.5-5.5999
4.5-5.5

The experiments showed the dependence ofgaseous elemental iodine retention on the pH of
the water in the wet scrubber, and the water level
above the injection orifce, Figure 3. All theiodine retention tests were carried out at thewater temperature of about 120oC, see Table 2.
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NEA/CSNI/R(2016)5
shows that the total estimates of first and second
order CH3I hydrolysis rate constants at up to pH
9 are very slow at 25oC, but increase rapidly up
to 90oC. For example, at pH 9, after 6 days at
22oC, only about 4% of the initial CH3I was
decomposed, but 13% was decomposed after
only 45 minutes at 70oC. When comparing thereaction rate constants at pH 5 and pH 9 with thepredicted rates, determined using the Borkowski
equation [Borkowski, 1985], shown in Figure 4,
and with literature data [Parsly, 1971], ourmeasured data is in agreement with the literature
data within the scatter and uncertainties of both
sources. The literature data suggest that at
temperatures from 70oC to 150oC, the hydrolysis
rate increases further by a factor of about 103.

Figure 3. Retention of elemental iodine in the
FCVS 1:1 height scale facility (tests B01, B03,
B05, Table 2). The dependence of thedecontamination factor on the water level above
the injection orifice and the pH.

The effect of temperature on the hydrolysisof CH3I was also investigated with added
Na2S2O3 in the water to apply the results to
FCVS. Figure 5 shows the comparison withliterature predictions. The discrepancy at higher
temperatures was attributed by the authors to theexperimental difficulties at high temperatures.

Small-scale experiments for thedecomposition of CH3I
PSI has participated in the iodine researchby performing basic investigations devoted togeneration of an easy and effective preparation oflabeled organic iodide aqueous solution, iodine
speciation analysis techniques, and experiments
studying the basic decomposition mechanism of
CH3I by water hydrolysis and radiolysis. The
results have demonstrated the repeatability of theliterature results and extended the database toother conditions, such as in-situ beta-irradiation
effects. The effects of different additives were
investigated with the aim to increase the CH3I
decomposition rate.
In this section, we give examples of the PSI
work on decomposition of CH3I by waterhydrolysis and radiolysis. The use of additives toincrease CH3I retention in water pools will bepresented separately. For the hydrolysis, theeffect of i) temperature, ii) pH, and iii) initial
CH3I concentration was studied. The results are
given in the form of decomposition rate of CH3I
as determined based on the laboratory
experiments [Cripps et al., 2010]. Tests were also
carried out with additives in the water, such as
Na2S2O3.

Figure 4. CH3I hydrolysis rates at pH 9 vs.
temperature, comparison of the literature and PSI
data.

The effect of temperature on thedecompositon of CH3I is shown in Figure 4. It

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NEA/CSNI/R(2016)5

-2

0

10

10

.

-1

.

-2

.

3

-1

Decomposition rate (s )-1

Decomposition rate [s ]10-2[THS]: 3.8 10 - 7.4 10 mol/ dm.
-3.
3[THS]: 4.0 10 mol/ dmo
T: 22 C

-2

10

-3

10

Predictions based on model [Parsly.1971/Hasty.1969]
PSI data.
-2.
3[THS]: 6.3 10 mol/ dm
.
-5
. -5
.
3
[CH3I]: 1.2 10 - 4.0 1 mol/ dm

-4

10

-4

10

-5

10

pH9

-5

-3

10

0

10

10

20

30

40

50

60

70

80

90

2

4

6

100

8

10

12

14

pH

o

Temperature ( C)

Figure 6. CH3I decomposition rates vs. pH
for different Na2S2O3 concentrations at 22 ºC.

Figure 5. CH3I decomposition rates vs.
temperature with Na2S2O3 added in the water,comparison of the prediction based on literatureand PSI experimental data.

Further tests were carried out to determine
the dependence of the CH3I decomposition rateon the concentration of CH3I and Na2S2O3.
Figure 7 shows that within the experimental
uncertainty, the CH3I decomposition rate does
not depend on the initial CH3I concentration.

Several experiments were conducted todetermine the effect of pH on the hydrolysis rate.
The data show that the hydrolysis reaction rates
at pH 5 and pH 9 are relatively similar. These
small rate differences are approximately repeatedat different temperatures. From pH 9 to 14, thedecomposition rates become higher: by about1000 times larger at pH 14 than that at pH 9.
This trend is in agreement with the predicted rate
constants at 25oC. The increased decomposition
rate is presumably due to the high OH─
concentration at pH >11. For experiments withpH ≥ 11, sodium bicarbonate was also used,since it is used in containment venting filter
solutions of some Swiss NPPs.

-2-1

Decomposition rate (s )

10

-3

10

-4

10

.

The pH dependence was also determinedwith the added Na2S2O3 in the water, Figure 6.
The decomposition of CH3I does not show
significant dependence on the pH except forsome reduced decomposition at a very low
pH<2.

-3

.

3

[THS] 4.0 10 mol/ dm.
-23[THS] 6.2 10 mole/dm
pH9
0C
T: 22

-5

10

-6

10

-6

10

-5

10

-4

10

-3

-2

10

10
3

Initial CH3I Concentration (mol/dm )Figure 7. CH3I decomposition rates vs. CH3I for
two Na2S2O3 concentrations at 22oC.

CH3I decomposition by radiolysis wasinvestigated with and without additives in thewater. At pH = 9 and T = 22oC, thedecomposition increases with the cumulateddose, Figure 8.

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NEA/CSNI/R(2016)5[Cripps et al., 2011], and therefeore, only a shortsummary of the main results is given here.

100

The experimental study consisted of smallscale irradiations of CsI, boric acid and tracer
containing aqueous solutions. Baseline tests werefirst carried out without nitrate or nitrite ions to
obtain results to confirm expected iodine
behaviour by correlation with predicted results,
which were generated by concurrently developed
code PSIodine. The solutions were sparged with
N2O, argon and air to provide different net
oxidation systems and to remove volatile iodine
for measurement. The baseline experiments
showed the expected results and the prediction
with the model agreed mainly well with theexperimental data. Therefore it was concluded
that sufficient baseline results showed that theexperimental facility and the iodine reactions
modelled in the PSIodine code provided a usefulbasis to investigate the effects of impurities in
irradiated solutions on iodine release.

Decomposition fraction (%)

80
60

40

pH9
o
Temp.: 22 C
.
-5
.
-5
.
3
[CH3I]: 1 10 - 4 10 mol/ dm

20

0.1

1

10

Dose (kGy)

Figure 8. CH3I decomposition fraction versus
radiation dose.
Effect of impurities on iodine behaviour

Both experimental and predicted results
clearly showed that nitrate or nitrite ions inargon-sparged and irradiated iodide solutions
containing boric acid lower iodine volatility up to
an initial NO3─ concentration of 5.0 x 10-3 mol
dm-3, Figure 9. Using a low CsI concentration(4.0 x 10-5 mol.dm-3) estimated in containment
sump during a postulated severe-accident, an
initial nitrate concentration (10-3 mol.dm-3) andpH 7.1, 3.6 % volatile iodine was produced byargon sparging at a dose of 20.5 kGy. In contrast,
80% volatile iodine was formed at a dose of 12
kGy in the absence of nitrate ions. Since theradiation chemistry of N2O-saturated iodide
solutions is well established and the measured
volatile iodine yields and final pH correlated
very well with the PSIodine code predictions, theeffect of added nitrite ions to these solutions
provided results which support the correct choiceof relevant reactions for inclusion in the nitratemodel. As a result, the authors concluded that thenitrate and nitrite reactions could be included in
the iodine chemistry codes.

In the containment atmosphere and sump during
a postulated severe reactor accident, many
elements, fission product compounds andstructural materials, collectively known as‘impurities’ are anticipated and measured ingaseous, dissolved or solid form [Hanniet-Giraultand Repetto, 1999; Jacquemain et al., 2000]. Viavarious transport pathways, these consitituentsmay end up in the sump, suppression pool, or
FCVS. Once resident in aqueous solution, thepotential effects of these ions as well as theeffects of their products on iodine chemistry
should be understood. To increase thisunderstanding, PSI investigated the effect of the
impurities nitrate, nitrite and chloride ions andassociated species on iodine volatility by
performing experiments (both sparged andunsparged) and by modelling chemical reactions.
The species for investigation were as follows:
I. nitrate ions;II. nitrite ions;III. chloride ions;IV. modelling of other dissolved fission products
measured in the sumps of Phébus FP tests.
This work has been reported in the
SARNET frame in a nine part report [Cripps etal., 2010] and appeared as a scientific publication
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NEA/CSNI/R(2016)5

Figure 10. Measured and predicted I2 fractions
when using chloride and nitrate (R5029) or
chloride only (R5001) as the impurities in thesolution at pH 2.5.

Figure 9. Predicted volatile iodine yields versus
dose (kGy) as a function of the initial nitrite
concentration (Cripps, 2010).
Chlorine ions in the water were found to havelittle or no effect on the iodine volatility. There
was one unresolved issue of a small increase incumulative volatility of iodine at a very low pH
of 2.5 when both chloride and nitrate were
present in the solution [Cripps et al., 2010].
Using only chloride as the impurity, the highest
cumulative volatile iodine fraction wasobserved at the pH of 4.0. Figure 10 showswith the initial pH of 2.5 the fraction of I2
reaching almost 90% in the test when bothchloride and nitrate were used, as well as thecorresponding modelling result. When pH wasincreased in the tests with both chloride andnitrate, the cumulative I2 fraction was
decreased.

IV. PSIODINE CODEA mechanistic computer code PSIodine wasdeveloped to interpret the results obtained frombench-scale, gas-sparged and irradiated iodide
solutions [Cripps et al., 2011]. The code models
reactions for the iodine oxidation states −1 to +5
in solution under strong (N2O-saturated) andweak oxidising (argon- and air-saturated)conditions. An empirical model was developed totransport I2 and other species from solution to thegas space by gas bubbles (sparging). By using
measured I2 mass transfer rates for specificreaction vessels, the need to apply assumptions,e.g., uniform and estimated bubble sizes andconcentration, diffusion coefficients, wascircumvented. By using the same I2 transfer rate
for irradiation of CsI solutions with and without
additional ions, data for volatile iodine yields for
initial chemical conditions can be compared.
Reaction rate changes due to solution
evaporation are also modelled. The predicted andexperimental data (I2 fractional releases, pHchanges and H2O2 formation) correlate well for
initial CsI concentrations from 4.0 × 10−5 to 1.0 ×
10−3 mol dm−3 and for pH 4.6–7.1 in weakoxidising systems (argon- and air-sparged
solutions). Data correlations for strong oxidisingconditions (N2O-saturated CsI solutions) are alsosatisfactory. However, the authors recommendthat further experiments and model development
are necessary to improve data correlations under

The authors noted that as the experiments weremainly carried out with very low concentrationsof the reacting species, some the experimental
results should be considered as tentative. Also,
many of the tests were not repeated, andtherefore, for firm conclusions, more tests
should be carried out to show the
reproducibility of the results.
The experimental and predicted results of thiswork give sufficient evidence to underline theimportance of the effects of nitrate and nitrite
ions in irradiated CsI solutions.

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conditions particularly relevant to postulated
severe accidents in containment.

NOMENCLATURE

More details on the PSIodine model is given
by Cripps et al., 2011.

BWR = Boiling Water Reactor
DF = Decontamination Factor
FCVS = Filtered Containment Venting
System

V. CONCLUSIONS
Methods were developed for experimental iodine
and CH3I investigations in different scales. The
small laboratory-scale methods were tested bycomparing with literature data for CH3I
decomposition by hydrolysis and radiolysis.
Further experiments were carried out under
severe accident and FCVS relevant conditions.

REFERENCES
1. D. Suckow, J. Yang, M. Furrer, T. Lind.
„Investigation of Iodine Retention in a
Filtered Containment Venting System in the
VEFITA
test
program”.
OECDNUGENIA/SARNET
workshopon“Progress in Iodine behaviour for NPP
Accident Analysis and Management”,
Marseille, March 30-Aptil 1, 2015.

The small laboratory-scale investigations on
the effect of impurities on iodine volatility under
irrdiation have shown that both nitrite and nitrateions in the water decrease the formation ofvolatile iodine. Chlorine in the water has little
effect on the iodine volatility. Models for thiseffect can be implemented in the codes forprediction of iodine behaviour.

2. Borkovski, R. “Untersuchungen zum
chemischen Verhalten des Methyliodides bei
schweren
Störfällen
in
Druckwasserreaktoren”, KfK Report 3968,
Karlsruhe (1985), Germany.

The retention of iodine and methyl iodide in
the FCVS was investigated in a 1:1 height scale
facility. If sufficient water level and high pH canbe maintained in the wet scrubber, the retentionof iodine is high. The retention of methyl iodide
in the wet scrubber using only the chemicals
which are typically present in the water of thewet scrubber, e.g., Na2S2O3 and NaOH / Na2CO3,
is limited even at high water temperatures.
Consequently, additional substances in the water,or other methods should be used to increase
retention of organic iodides in FCVSs.

3. R. Cripps, B. Jäckel, H. Bruchertseifer, S.
Güntay, 2010. Experimental Results andModel Predictions of Nitrate, Nitrite andChloride Effects on Iodine Radiolysis
Reactions Part 1: Baseline Experiments.
Report SARNET2-ST-P3, PSI TM-42-10-03,
PSI, Villigen, Switzerland.
4. R. Cripps, B. Jäckel, H. Bruchertseifer, S.
Güntay, 2010. Experimental Results andModel Predictions of Nitrate, Nitrite andChloride Effects on Iodine Radiolysis
Reactions Part 2: N2O Sparged CsI Solutions
Containing Nitrite. Report SARNET2-STP4, PSI TM-42-10-04, PSI, Villigen,
Switzerland.

ACKNOWLEDGMENTS
This work has been carried out during many
years by a large group of scientists many ofwhom have already retired. A large fraction of
the work was carried out by Robin Cripps andHorst Bruchertseifer which is respectfully
acknowledged. Over the years, the funding for
the work came from various sources, and it is notpossible to list here all of them, but we sincerely
thank them all. Last, we would like to thank the
personnel of the PSI Hot laboratory for providing
laboratory resources for this work.

5. R. Cripps, B. Jäckel, S. Güntay, 2010.
Experimental Results and Model Predictionsof Nitrate, Nitrite and Chloride Effects on
Iodine Radiolysis Reactions Part 3: ArSparged CsI Solutions Containing Nitrate.
Report SARNET2-ST-13, PSI TM-42-10-07,
PSI, Villigen, Switzerland.

130

NEA/CSNI/R(2016)5
6. R. Cripps, B. Jäckel, S. Güntay, 2010.
Experimental Results and Model Predictionsof Nitrate, Nitrite and Chloride Effects on
Iodine Radiolysis Reactions Part 4: ArSparged CsI Solutions Containing Chloride.
Report SARNET2-ST-14, PSI TM-42-10-08,
PSI, Villigen, Switzerland.

with argon, air, or nitrous oxide. Nucl. Eng.
Design, 241, pp. 4306-4325.
13. Parsly, L.F., 1971. “Chemical and physicalproperties of methyl iodide and its
occurrence under reactor accident conditions(A summary and annotated bibliography)”,
ORNL-NSIC-82, (1971).

7. R. Cripps, B. Jäckel, S. Güntay, 2010.
Experimental Results and Model Predictionsof Nitrate, Nitrite and Chloride Effects on
Iodine Radiolysis Reactions Part 5: AirSparged CsI Solutions Containing Nitrate.
Report SARNET2-ST-15, PSI TM-42-10-09,
PSI, Villigen, Switzerland.

14. N. Hanniet-Girault and G. Repetto, FPT0
Final Report, IPSN/DRS/SEA/SEA/1/99,
Phebus report PH-PF IP/99/423, 1999.
15. D Jacquemain, S Bourdon, A de Bremaeker
and M Barrachin, FPT1 Final Report,
IPSN/DRS/SEA/1/00, Phebus report PH-PF
IP/00/479, 2000.

8. R. Cripps, B. Jäckel, S. Güntay, 2010.
Experimental Results and Model Predictionsof Nitrate, Nitrite and Chloride Effects on
Iodine Radiolysis Reactions Part 6: ArSparged CsI Solutions Containing Nitrateand nitrite. Report SARNET2-ST-16, PSI
TM-42-10-10, PSI, Villigen, Switzerland
9. R. Cripps, B. Jäckel, S. Güntay, 2010.
Experimental Results and Model Predictionsof Nitrate, Nitrite and Chloride Effects on
Iodine Radiolysis Reactions Part 7: ArSparged CsI Solutions Containing Nitrate
and Chloride. Report SARNET2-ST-17, PSI
TM-42-10-11, PSI, Villigen, Switzerland.
10. R. Cripps, B. Jäckel, S. Güntay, 2010.
Experimental Results and Model Predictionsof Nitrate, Nitrite and Chloride Effects on
Iodine Radiolysis Reactions Part 8: AirSparged CsI Solutions Containing Nitrateand / or Chloride. Report SARNET2-ST-18,
PSI
TM-42-10-12,
PSI,
Villigen,
Switzerland.
11. R. C. Cripps, B. Jäckel, S. Güntay, 2011. On
the radiolysis of iodide, nitrate and nitrite
ions in aqueous solution: An experimental
and modelling study. Nucl. Eng. Design,
241, pp. 3333-3347.
12. R. C. Cripps, B. Jäckel, S. Güntay, 2011. The
PSIodine Code: A computer program tomodel experimental data on iodine and otherspecies in irradiated CsI solutions sparged
131

NEA/CSNI/R(2016)5

RECENT FINDINGS ON RUTHENIUM CHEMISTRY IN A SEVERE ACCIDENT
Teemu Kärkelä(1)*, Christian Mun(2), Gérard Ducros(3), Nóra Vér(4), Marie-Noëlle Ohnet(2), Ivan
Kajan(5), Sidi Souvi(2)(1)

VTT Technical Research Centre of Finland, Espoo, Finland, (2)Institut de Radioprotection et de Sûreté
Nucléaire, Saint-Paul Lez Durance, France, (3)Commissariat à l'énergie atomique et aux énergies
alternatives (CEA), Saint-Paul Lez Durance, France, (4)Centre for Energy Research (EK), Hungarian
Academy of Sciences (MTA), Budapest, Hungary, (5)Chalmers University of Technology, Göteborg, Sweden*Corresponding author, tel: (+358) 20 722 5718, Fax: (+358) 20 722 7026, Email:teemu.karkela@vtt.fi
Abstract – The chemistry of radiotoxic ruthenium in a severe nuclear power accident has actively been
investigated especially during the last decades. The Ru studies have covered the release from a fuel, thetransport in the primary circuit and the behaviour in the containment building. The gathered experimental
data have been utilized to understand the key parameters governing the Ru chemistry in a severe accident(SA) and to check the ability of the existing models of SA analysis codes to explain the experimental results.
To further increase the knowledge on Ru behaviour, the collaboration on international level has been
intensive. Lately, the widest and most active networks have been EU SARNET and EU SARNET2. The
valuable effort of these networks on sharing information of e.g. national programs and on interpreting theexperimental results is continued in EU NUGENIA program. More detailed studies on separate phenomena
have been conducted e.g. as part of NEA STEM/START and ISTP/VERDON programs. Furthermore,
Phébus FP tests have produced valuable data on integral phenomena.
The large-scale integral and semi-integral experiments have confirmed that Ru release depends strongly on
carrier gas. Ru is significantly released from an irradiated fuel sample under oxidizing conditions, inparticular when air is involved. In addition, the oxidation of UO2 fuel seems to lead to a higher Ru releasethan in case of MOX fuel. Ruthenium can be transported to the containment atmosphere both in gaseousand particulate forms. The small-scale separate-effect experiments gave a detailed view on Ru transport. A
high fraction of ruthenium was detected as particles at the outlet of the model primary circuit in an air
atmosphere. However, the observed gaseous Ru fraction is higher than what could be expected based onthermodynamic equilibrium calculations. Further studies on the effect of flow residence time in atemperature gradient for the equilibrium of Ru oxides have been conducted.
The effect of other fission products in the gas phase, as well as FP deposits on the surface of primarycircuit, on the Ru transport has been investigated. For example, caesium containing deposits seemed totrap gaseous ruthenium effectively. Similarly in case of control rod residues, silver particles in the gas
phase of the circuit acted as a sink for gaseous Ru. In an air ingress accident, the effect of air radiolysis
products on the Ru chemistry becomes important. As the main air radiolysis products can be considered asoxidizing agents, their ability to oxidize the lower oxides of Ru to higher oxidation state has been
examined.
Most of Ru in the containment building ends up as deposits on the containment surfaces and in the sump.
Experiments on the radiolytical revaporisation of ruthenium deposits on the epoxy paint surface indicatedthe release of gaseous ruthenium and it was enhanced under humid atmosphere and elevated temperature.
It appeared that the products of air radiolysis caused by γ-radiation promoted the formation of gaseous
ruthenium from Ru oxide deposits on paint in a higher amount than could be expected by pure ozone
action. Concerning the irradiation tests of perruthenate aqueous solutions, they indicated the formation of
gaseous Ru by γ-radiolysis products in solution.

132

NEA/CSNI/R(2016)5
I. INTRODUCTION

up UO2 and MOX fuels. The semi-integral
VERDON-2 test was in particular dedicated to
study the impact of air ingress following a lower
head failure on the ruthenium release from thefuel and its transport/revaporisation in the
primary circuit.

In case of a severe accident (SA) in a nuclear
power plant (NPP), when the coolant is lost, fuelwill heat-up rapidly leading to a release of someof the fission products that have accumulated in
the fuel during a normal operation of NPP. In
oxidizing conditions, the release of Ru is
probable by formation of volatile rutheniumoxides. The radiotoxicity of these oxides is highboth in the short and the long term due toisotopes: 103Ru with a half-live of 39.3 days and106
Ru with a half-live of 373.6 days. In order tolimit the possible source term, it is of interest tounderstand the behavior of ruthenium oxides
when they are released from the fuel, transported
through the reactor coolant system (RCS) or
carried along to the atmosphere of containmentbuilding.

The transport of Ru in primary circuit conditions
has been studied in separate-effect tests withsmall-scale experimental facilities as a part of
SARNET [4-7] and NEA STEM/START
programs [8, 9]. Ruthenium was oxidized at hightemperature under dry or humid air atmosphereclose to atmospheric pressure with specificattention being paid to the transport of Ru
through a thermal gradient tube (TGT). The
effect of other fission products and control rod
materials on the transport of Ru was also
investigated. A newly initiated NKS-ATR
activity [10], under the Nordic Nuclear Safety
Research (NKS) program, produces informationon the effect of aerosols and air radiolysis
products on the transport of Ru.

To increase the knowledge on Ru behavior in SA
conditions, the collaboration on international
level has been intensive. The effect of oxidizing
atmosphere on source term has been one of thehigh priority open issues in the framework of EC
SARNET and SARNET2 (Severe Accident
Research NETwork of Excellence) projects [1].
These networks acted as a forum for sharing
information of national programs and on
interpreting the experimental results. This
international collaboration is continued in theframe of EU NUGENIA program. More detailed
studies on separate phenomena have been
conducted e.g. as part of NEA STEM/STARTand ISTP/VERDON programs. Furthermore,
Phébus FP tests have produced valuable data on
integral phenomena.

As part of SARNET projects, an extensive set ofexperiments was carried out to understand the
behaviour of ruthenium tetroxide when reacting
and thus depositing on containment paint andmetal surfaces. Furthermore, the revaporisationof Ru from the deposits, the distribution of
gaseous RuO4 between gas and liquid phases, aswell as the revaporisation of Ru from the sumpof containment building, were examined. [11, 12,
13]This paper summarizes the main results of theserecent experimental studies on the behaviour of
Ru in severe accident conditions. Also the main
phenomena governing Ru deposition in circuit
conditions are presented.

The large-scale integral tests of the Phébus FP
program [2] studied the degradation of real fuelbundle and the release, transport and depositionof FPs, e.g., ruthenium, as well as structural
materials and control rod materials in the modelof primary circuit and containment building. The
tests were conducted under steam-rich or steampoor atmospheres and under low pressures (~ 0.2
MPa). The interpretation of the huge amount ofexperimental data is ongoing.

II.Ru RELEASE FROM FUEL
II.A PHÉBUS FP - RELEASE
The overall objective of the well-known
international Phébus FP programme [2] was tosimulate phenomena starting from the meltdown
of an irradiated UO2 fuel assembly in steam flow
up to the releases outside of the containment
through the transfer of radioactive products
coming from the RCS and their subsequent

The four VERDON tests [3] of the International
Source Term Program (ISTP) were devoted to thestudy of fission products release from high burn-133

NEA/CSNI/R(2016)5
behaviour in the containment. The release ofruthenium occurred mainly in the late oxidation
phase where temperatures are the highest, as
expected for low-volatile elements (here fueloxidation plays a role). Ru release from the test
section was low [14]. However, thisunderestimates the release from the fuel itself, as
Ru can be released from high temperature zones
in the bundle and deposit downstream on cooler
surfaces (upper parts of the fuel rods as well as in
the circuit). The low steam injection rate in sometests (~0.5 g/s) specifically favoured significant
deposition of Ru in the upper sections of thedegraded fuel bundle. With the higher steam
injection rate (~2.0 g/s) used in other tests
deposition occurred rather in the downstream
sections of the circuit. This indicates that thepartition of deposits between the fuel bundle andthe circuit proper depends on the prevailingthermal hydraulic conditions, which may changewith time, thus one cannot consider the circuitand bundle regions independently when
considering how much Ru would reach thecontainment.

circuit. Gaseous iodine is trapped in the MayPack filter on successive sites of this specificfilter according to its chemical forms (molecular
or organic) and RuO4 in a dedicated filter madeof polyvinylpyridine. After a security filter, theinjected and produced non-condensable gas (He,H2, air …) and the released fission gases are
directed outside the cell up to the glove box
where they are finally analyzed and stored in a
3m3 storage vessel.
The accident sequence is simulated by heating
the fuel sample in a high frequency furnace underconditions representative of a severe accident, ina fluid flow, which could be a mixture of helium,
water steam, hydrogen and air. The fuel sample
consists of two irradiated pellets in their original
cladding and two half-pellets of depleted (andun-irradiated) uranium oxide placed at each endof the sample and held there by crimping thecladding so that the cladding is not fully sealed.
Two different and complementary circuits areavailable:
1) The Release Circuit (Figure 1a), which isdevoted to the precise characterization of the FP
release by means of a total aerosol filter heated at
150°C and being located just above the furnace,
allows the precise quantification of the released
fraction by gamma spectrometry and/or chemical
analysis directly after the test. VERDON-1, -3
and -4 were performed with this circuit.

II.B ISTP/VERDONThe International Source Term Program has beenlaunched in 2005 with the aim to reduce theuncertainties relative to Source Term assessment
under LWR severe accident conditions. Amongthe R&D research axes addressed in thisprogram, four VERDON tests devoted to thestudy of Fission Products (FP) release from highburn-up UO2 and MOX fuels were performed
from 2011 to 2014. The VERDON-2 test was inparticular dedicated to study the impact of air
ingress following a lower head failure on theruthenium release from the fuel and its
transport/revaporisation in the primary circuit.

2) The Transport Circuit (Figure 1b), whichenables the study of FP transport in the primarycircuit of a nuclear power plant, through a seriesof 4 thermal gradient tubes (TGTMs) that can be
operated sequentially with two in parallel inorder to study FP deposit as well as
revaporisation [15]. In order to be representativeof the hot leg and cold leg temperatures under
severe accident conditions, the temperature along
these tubes linearly decreases from 700°C at theentrance to 150°C at the exit, and is monitored
by a series of thermocouples located every 100
mm. Downstream from the TGTM, the fluid istransported at 150°C to an aerosol filter or to acascade impactor where the particle size
distribution of the aerosols is quantified. The air
ingress VERDON-2 test was performed with thiscircuit.

The four VERDON tests of the ISTP wereconducted in a new hot cell at the CEA
Cadarache center, specially built for this purpose(Figure 1) [3].
In both configurations, downstream of the filter,the fission gases (Xe, Kr) and the potential
gaseous forms of iodine or ruthenium aretransported at 150 °C by the fluid flow along the134

NEA/CSNI/R(2016)5150°C

Filtersighting

volatile FP deposits (mainly iodine, tellurium,
caesium) and their potential revaporisationa)following the air ingress phase.

150°C

AEROSOLFILTERCONDENSER

VERDONFURNACE

Furnacesighting

2) The second phase under mixed air-steam(50%-50% molar ratio): the 2000°C plateau has
been maintained for additional 70 minutes. Since
the Ru release had not reached the “stopcriterion” of 60% released, another plateau at
2100°C was performed for 90 minutes. During
this plateau, the TGTM was turned after 60
minutes (tubes 2-3 to tube 3-4) and after theb)rotation,
the tubes 3 and 4 stayed during 30
minutes under the same atmosphere to be able tostudy the potential revaporisation of RuO2
deposits.

VERDON
Release Loop

Up to2600°C

Steamgenerator

300°C

H2O
EXPERIMENTAL GAS
He, H2

150°C

30°C

MAY-PACK

Pyrometer

Gas platform May-packsighting

C5 Cell

700°C

TGTM

Glove
Box

150°C

700°C

CONDENSER

VERDONFURNACE

300°C

H2O
EXPERIMENTAL GAS
He, H2

Pyrometer

Gas platform

Aerosolfiltersighting

FILTER

Steamgenerator

AEROSOL

VERDON
Transport Loop

Up to2600°C

Furnacesighting

MAY-PACK

150°C

IMPACTOR

Heatedsectionsighting

The Ru release started with the air ingress phaseduring the 2000°C plateau, as shown in Figure 3,
and reached about 70% of the initial inventory at
the end of the sequence. The release rate slightly
increased between the 2000°C and 2100°C
plateaus. From a general point of view, theruthenium kinetics seems to be relatively low
compared to what could be expected due to thefinal temperature and atmosphere of the test (2.5hours under mixed steam-air conditions, 1 hour
at 2000°C and 1.5 hour at 2100°C). This low
value can be the consequence of the MOX natureof the fuel, as it was confirmed in VERDON-3(see chapter II.D).

30°C

Glove
Box

Fig. 1. VERDON hot cell: a) Release Circuit, b)
Transport Circuit

II.C Ru FINDINGS FROM THE VERDON2 TESTThe VERDON-2 test was performed using a
MOX fuel at 60 GWd/t. The thermal-hydraulicsequence (Figure 2) was composed of two main
phases [16]:100,00%2500

RF(% i.i.)-Zr95
RF(%i.i.)-Ru103
RF(%i.i.)-Mo99
RF(%i.i.)-Cs137
RF(%i.i.)-Ba140
Fuel Center Temperature (°C)

80,00%

Released Fraction, RF (% of i.i.)

Before the experimental sequence, the sample isre-irradiated at low linear power (15 to 20 W/cm)
in the OSIRIS material testing reactor for about aweek, in order to recreate the short half-life FPs
without any in-pile release. As a consequence,these FPs (i.e. 99Mo, 103Ru, 132Te, 133I, 131I,
140
Ba…), important for their radiobiologicaleffects, are measurable by using on-line gamma
spectrometry during the experiment.

60,00%

2000

1500

40,00%

1000
20,00%

500
0,00%-20,00%1) The first phase under pure steam, including anoxidation plateau at 1500°C, followed by asecond 15 minute plateau at 2000°C, simulating
SA conditions before the lower head failure. The
first TGTM rotation occurred (tubes 1-2 to tubes
2-3) at the end of this phase, in order to study the00,05000,010000,015000,020000,025000,0Elapsed time (s)Fig. 3. Ru release kinetics, along with Cs(volatile FP), Mo, Ba (semi-volatile FP) and Zr(non-volatile FP)

135

Temperature (°C)

C5 Cell

NEA/CSNI/R(2016)5

1

TGTM 2

2
3

3
4

2400

30
Fuel Center
Temperature

2000

Total Flow Rate
(He equivalent)

1600

H2O steam25
GS3-2000°C

H2O

GS4-2000°C

Air
800

20

Temperature (°C)

GS2-1500°C

400
GS1-1000°C

0
0

3600

-400

7200

10800

14400

18000

21600

15
28800

25200

cladding
oxidation peak

-800
-1200

10

H2, H2O, Air and total flow rate (mg/s)

Gas sampling

1200

total flow rate
Air

-1600

5

-2000
-2400

0
Time (s)Fig. 2Thermal-hydraulic sequence of the VERDON-2 test
The transport of Ru can be analysed through thedeposits along the thermal gradient tubes. It canbe seen in Figure 4, that 103Ru did not deposit intube 1, which is consistent with a release thatstarted with the air injection. The profiles oftubes 2 and 3 are similar with a large
accumulation in the mid-plane area of the tubes.
The quantity of deposits in tube 3 are greaterthan those in tube 2, which is normal considering
its longer period of use. Tube 4 shows a rather
homogeneous profile of low amplitude. The sumof the profiles for tubes 2 and 4 is similar to tube
3, which shows there was no significant
revaporisation of ruthenium deposits at the endof the test.

3,0E+13

Tube 1

Ru 103

Tube 2

2,5E+13

Tube 3
Tube 4

Atoms number

2,0E+13

1,5E+13

1,0E+13

5,0E+12

0,0E+00
0

100

200

300

400

500

600

700

Distance from tube entry (mm)Fig. 4. Distribution of 103Ru along the TGTM
tubes.
To further study the transport of Ru, the filter
located downstream the condenser was analysed.
This filter was totally free from Ru, indicatingthat no gaseous RuO4 was transported so far.
II.D LOWER Ru RELEASE FROM MOX FUELIN VERDON-3 TESTThe VERDON-3 test was performed with thesame MOX fuel as VERDON-2, but under puresteam conditions up to 2300°C. Only a few
percent of Ru was released from this test, which
is very low in comparison with the Ru release136

NEA/CSNI/R(2016)5
measured in similar conditions with UO2 fuel, forinstance on VERCORS HT2 (65%) andVERCORS RT6 (28%) [17]. This result confirms
the lower Ru release in case of MOX fuelcompared to UO2 fuel, probably linked to adifference in the oxidation of the fuel matrix.

soluble behaviour both in the hot and cold legs.
There are no data available on the speciation.
Overall, taking also into account results ofseparate-effect tests such as VERCORS (CEA),
experimental data show that, contrary to otherlow volatile elements, under certain thermal
hydraulic oxidising/reducing conditions, afraction of Ru can be transported welldownstream of the fuel into the coldest regionsof the primary circuit, as noted for example in[19]. Concerning physical form, Ru wasobserved to be transported almost exclusively as
an aerosol.

Considering the observed behavior of MOX fuelrelative to the Ru release, it has been proposed tocomplement the VERDON data base with a new
VERDON-5 test, which will be similar to the
VERDON-2 test, but using a high burn-up UO2
fuel at 70 GWd/t (same fuel as VERDON-1). In
addition, boron will be injected during the initial
phase under steam. This supplementary objective
will address the impact of B4C rods degradationon the fission products chemistry in the primarycircuit, resulting in a potentially higher gaseousiodine fraction entering the containment. This
test will be performed by the CEA in 2015.

III.B SMALL-SCALE FACILITIESThe transport of ruthenium in a primary circuit
has been studied in VTT [4, 5], MTA EK´s
RUSET [6, 7] and in START [8, 9] separateeffects programmes. The START program is apart of the Source Term Evaluation and
Mitigation (STEM) project hosted by OECD andoperated by IRSN. The corresponding
experimental facilities are presented in Figure 5.
Both horizontal and vertical tubular flow
furnaces have been used. In VTT experiments,
the furnace tube was made of alumina (Al2O3)
and the thermal gradient tube (TGT) was ofstainless steel. MTA EK and IRSN used quartz
tubing which withstands the relevant thermal andchemical loads and allows observing andanalyzing deposits along the tube. In VTT
experiments, the Ru deposit profile was analysedwith the help of 103Ru gammatracer. The innerdiameter of the tubing was roughly 6 to 8 times
higher in VTT and START experiments than in
RUSET tests. However, the flow rate was alsosignificantly lower in RUSET tests, 0.171 Nl/min
compared to 5 Nl/min and 1.84 Nl/min of VTTand START tests, respectively. Thus, all facilities
were operated in a laminar flow regime.

III. Ru TRANSPORT IN PRIMARYCIRCUIT
III.A PHÉBUS FP - TRANSPORTThe overall transmission of ruthenium to thecontainment in the Phébus FP tests was only afew percent of the initial bundle inventory,
whereas the corresponding transmission e.g. forthe volatile elements Cs and I was roughly up to
20 and 30 times higher respectively [18].
Ruthenium was deposited significantly andprogressively from the exit of the bundle to theentrance to the containment. Distribution of Ru
deposits along the circuit was mainly
concentrated where the temperatures of the walland fluid decrease strongly, i.e. just above thebundle where the fluid cools from ~2000 °C to~700 °C and, in the hot leg of the steam
generator, from ~700 °C to ~150 °C. In thevertical line, simultaneously developing flows
and changes in geometry (successive reductions
in tube diameter) enhance deposition. The mainprocesses in the circuit are: chemicaltransformation of vapours, vapour condensation
onto structures as well as nucleation to form
aerosols, or onto aerosols, aerosol agglomeration(primarily by diffusion), and thermophoretic
deposition. Low steam injection rate increased
the deposition of Ru. Solubility measurements on
the Ru circuit deposits showed consistent non-The source of Ru was either anhydrous RuO2
powder, gaseous RuO4 or metallic Ru powderwhich was oxidized at high temperatures in aircontaining atmospheres (air and/or steam as
carrier gas). The oxidation temperature in VTT
experiments was varied in a range from 827 °C
to 1427 °C (outlet at 20 °C), whereas RUSET
and START tests were conducted at 1100 °C(outlet at 100 °C) and 1200 °C, respectively. The
temperature of the tube downstream the reaction

137

NEA/CSNI/R(2016)5
furnace was controlled down to 150/250 °C in
START tests. The following Ru speciation andtransport through a TGT to the containment
conditions were investigated. The influences ofseveral other FP components and different
surfaces (quartz, stainless steel (EN 1.4301),
zirconium/niobium binary alloy (E110), alumina,oxidised metal, and surfaces with RuO2, Mo or
Cs deposits) on the decomposition andrevaporisation of ruthenium oxides along atemperature gradient zone have also been
investigated, specifically in RUSET tests. The
transported aerosol particles were collected either
on filter and/or trapped in an alkaline solution
together with the gaseous Ru. The aerosol
number/mass size distributions and theircorresponding number/mass concentrations werealso analysed online in VTT Ru transport
experiments.

VTT and RUSET tests). This is several orders ofmagnitude higher than expected fromthermodynamic equilibrium calculations and thus
the surface-catalysed decomposition process of
RuO4 to RuO2 was not fast enough to follow
entirely the thermal equilibrium. This clearly
indicates the need to consider the effect oftemperature gradients in the RCS and residence
time in the circuit on the fraction of gaseous Ru.
This is currently being investigated in START
program [9].
In tests below the oxidation temperature of 1100
°C, less than a tenth of the released Ru wastransported as particles in VTT tests. However,no solid Ru was detected at the circuit outlet in
RUSET tests. The transported aerosol fraction (ofthe released Ru) increased and was two to fourtimes higher when the oxidation temperature
increased from ca. 1200 °C to 1400 °C. Gaseous
Ru seemed to react on the surface of the SS tube,
on top of which RuO2 particles had been
deposited. Thus Ru transport through the modelprimary circuit took place almost entirely as
particles when Ru was oxidized at temperatureshigher than 1200 °C. However, the partialpressure of the transported RuO4 remained in therange of 10-6 to 10-8 bar.

In general, the Ru vaporization kinetics wasobserved to be linear in steam/air mixtures and it
was dependent on the steam content. The highest
release rate of ruthenium was detected in dry air.
Ru release was very low when oxidised at ca.
800 °C. At temperatures between 1000 to 1100°C
ruthenium was primarily transported in gaseousform. The partial pressure of gaseous RuO4 at thecircuit outlet, corresponding to containment
conditions, was in the range of 10-6 to 10-7 bar (ina)b)

Quartz tube

Furnace
1200°C max
RuO2(c)Argon
SG

150°C/250°C
Aerosol
Filter

1.20m/0.70m

Furnace
150°C

c)

Injection lines :Steam + air

Liquid traps
NaOH 1M

Fig. 5. Experimental facilities for Ru transport studies (operated close to 1 atm): a) VTT Ru transport, b)
RUSET and c) START138

NEA/CSNI/R(2016)5
The presence of steam in the gas flow enhanced
the transport of gaseous ruthenium. In the long
duration tests of START programme, theexistence of transient phenomena on Ru transport
was observed in the short term. The transport ofRuO2

revaporised from the TGT surfaces at notably
lower temperature than the furnace set-point of1227 or 1427 °C at beginning of the TGT, thefraction of Ru transported in gaseous form wasas high as 65% in VTT experiments. This
indicated that the surfaces of RCS may act as asource of volatile ruthenium species even in thelate phase of an accident. The steam content of
the gas flow seemed to enhance the transport ofgaseous Ru again. The revaporisation behavior ofruthenium is being investigated in detail in
START program [9].

aerosol was decreased after the 1st hour of tests
and thus the gaseous fraction increased from 50% to 93 %. The fraction of the Ru released fromthe RuO2 crucible that was deposited inside thefacility ranged from 75 % to 97 %. This tendencyof the Ru to deposit was enhanced at low
oxidation temperature. It is very likely that intests at 1027 °C ruthenium was primarilytransported in gaseous form. The typical Ru
deposition
profilestogetherwith
thecomputational results of the tests are presented in
Figure 6. The main phenomena governing thetransport of pure Ru in the circuit were explainedwith CFD computations and they are presented in[5].

In RUSET tests, the presence of other FPs in thehigh temperature oxidation zone decreased thepartial pressure of RuO4 in the outlet air over theSS surface in the temperature gradient zone
compared with the pure Ru oxidation. For all
types of surfaces investigated, the maximum
concentration of RuO4 in the outlet flow
appeared with a time delay if other FPs were
present. Furthermore, a stainless steel surface
which was pre-coatedRevaporisation of the deposited RuO2 particles
was found to be a very significant source ofgaseous ruthenium oxides. After Ru was139

NEA/CSNI/R(2016)5

b) 90

1200

(a) - SS, Ru (37)
Temp.

80

1000
70

Ru
Fe
Cr
Ni
Mn
O

60
50

800

600

40
30

400

20
200
10
0

0
0

20

40

60

80

100

120

140

160

180

L (mm)

c) 90

1200

(b) - SS+Cs, Ru (42)
Temp.

80

1000
70
60

800
Ru
Cs
Mo
Fe
Cr
Ni
Mn
O

50
40
30
20

600

400

Temperature (°C)

Quantity of the elements (mass %)

d)

200
10
0

0
0

20

40

60

80

100

120

140

160

180

L (mm)

Figure 6. The typical Ru deposition profiles in experiments: a) VTT Ru transport, b) RUSET; distribution
on steel surface after pure Ru oxidation, c) RUSET; distribution on Cs coated steel surface after pure Ru
oxidation, and d) START.
with caesium trapped Ru efficiently between
about 485 and 640 °C, see Figure 6c. Also asample of CsI powder at ca. 100-150 °C trapped
gaseous Ru efficiently in VTT tests. The
formation of Cs2RuO4 was proposed in bothcases.

flow of Ru oxides (in humid air) was significant
both on the transport of Ru through the facility
and on the speciation of the transported
ruthenium. Transport of gaseous RuO4 wasincreased significantly, whereas at the same time
the amount of aerosols reaching the filter was
decreased. However, the release of Ru from the
crucible was similar as in an experiment without
NO2. This indicates that the molar ratio of
RuO3/RuO4 in the gas flow had changed. A
proposed explanation is based on the oxidation of
RuO3 to RuO4 by NO2. On the other hand, when
only pure Ag particles (diameter 0.5-1.0 μm)
were fed to the humid air flow, the transport of
RuO4 decreased significantly. Most likely,
gaseous RuO4 had reactively condensed on thesurface of Ag particles as RuO2. Addition of bothIn the newly initiated NKS-ATR activity [10] by
VTT and Chalmers the aim is to study the effectof aerosols and air radiolysis products on thetransport of gaseous and particulate ruthenium
species through a model RCS. All experiments
done so far were conducted with VTT´s Rutransport facility at 1227 °C with slightly humid
air (saturated at 20 °C) as the main carrier gas.
The preliminary results show that the impact ofadditional NO2 gas feed (75 ppm volume) to the140

Temperature (°C)

Quantity of the elements (mass %)a)

NEA/CSNI/R(2016)5
Ag particles and NOx in a form of AgNO3droplets (which decomposed to Ag and NO3
when heated) to the flow of Ru oxides (in humidair) enhanced the transport of gaseous RuO4 as
well, but not as much as in case of NO2 feed. In
these experiments (with NO2 or AgNO3), theobserved transport of gaseous Ru seemed to be
several orders of magnitude higher than in theprevious experiments with only pure Ru oxides
in the air atmosphere [4, 5]. These results
indicate that the composition of gaseousatmosphere in the primary circuit has asignificant effect on the amount and chemical
form of ruthenium transported to the containment
during a SA.

hydration of RuO2 was higher than on metal
samples. The crystal structure analysis of thedeposits by EXAFS supported also the formationof hydrated form of RuO2. Since the deeper
layers of the deposits could be analysed with thismethod, the inner structure was identified to beof RuO2. This verified that only the surface of
RuO2 deposits was strongly hydrolyzed.
The affinity of RuO4 towards Zn, Cu and Al wasstudied with the help of 103Ru. In general, theaffinity of RuO4 towards these metals wasincreased when the humidity of the atmosphere
was increased, see Table I. At humidity of ca.
3%, the deposited amount of ruthenium was up
to 3 times higher in case of aluminium than on
copper or zinc surfaces. However, the highest
affinity of RuO4 towards aluminium wasobserved at dry conditions.

IV. Ru BEHAVIOUR IN THE
CONTAINMENT
An extensive study on the behavior of Ru in
containment conditions has recently been
conducted by Chalmers University ofTechnology (2007-2014), [11, 12]. In theseexperiments, the interaction of RuO4 withdifferent containment surface materials and thefollowing chemical speciation of the formed
ruthenium rich layer were investigated.
Additionally, the possible revaporisation ofruthenium from these deposits on surfaces when
exposed to gamma radiation was studied. The
other part of experiments was focused on the
distribution ratio of ruthenium tetroxide between
gas and liquid phases in the facility simulating acontainment building.

TABLE I
Affinity of RuO4 towards metals in humid anddry atmospheres

Humid

Dry

Gaseous RuO4 was generated prior to all
experiments due to the unstable nature of thiscompound. After the interaction of RuO4 vapor
with the Al, Cu, Zn and epoxy paint samples attemperature of 25 °C and at relative humidity of
99%, a black colored ruthenium rich layer wasobserved on the samples. XPS analysis of theruthenium deposits on the containment surface
samples revealed that the surface of deposits wascomposed of ruthenium dioxide in its hydrated
form. The formed compound was rutheniumhydroxo-oxide, in which ruthenium was in theoxidation state +IV. This was probably due to thehygroscopic nature of RuO2 and high humidity of
atmosphere during the interaction of RuO4 withthe studied surfaces. The chemical speciation of
the deposits was the same regardless the surface
material. However, in case of epoxy paint the

Ru/sample(μg/cm2)
Ru/sample(μg/cm2)
Ru/sample(μg/cm2)
Ru/sample(μg/cm2)
Ru/sample(μg/cm2)
Ru/sample(μg/cm2)

Zinc

CopperAluminium

15.3 ± 0.3

18.0 ± 0.3

31.6 ± 0.6

13.2 ± 0.2

12.1 ± 0.2

24.8 ± 0.5

18.5 ± 0.3

19.2 ± 0.4

38.9 ± 0.7

2.73 ± 0.5

9.42 ± 0.2

103.29 ± 0.7

0.76 ±
0.02

0.39 ± 0.1

86.78 ± 0.6

2.23 ± 0.4

2.45 ± 0.5

98.35 ± 0.7

The revaporisation experiments were focused on
the release of Ru from deposits on epoxy paint(deposit speciation described above) when
exposed to gamma radiation. It was proposed,that Ru deposits would be oxidized back to Ru+VIII, and thus volatile RuO4 would be formed,by the generated oxidizing air radiolysis
products. The dose rate of a gamma source(Gammacell 220, 60Co) was ca. 14 kGy∙h-1. The
effect of cumulative radiation dose up to 264
kGy was investigated. Temperature in theirradiation chamber was 44 °C. A beaker withruthenium deposit on epoxy paint sample wasplaced into Erlenmeyer flask. The bank wasafterwards placed into the gamma field. In
addition, the impact of humidity on therevaporised fraction of Ru was examined. In
141

NEA/CSNI/R(2016)5
those experiments, 30ml of 1M KOH solution
was injected into the Erlenmeyer flask. The
solution acted both as a source of humidity andas an effective trap for volatile ruthenium
released from the samples. As a result, a strong
impact of humidity on the fraction of Ru
revaporised was noticed, see Figure 7, and thefraction seemed to increase linearly as a functionof the received radiation dose. In case of dry
atmosphere, virtually no revaporisation of Ru
was detected regardless of the received dose.

concluded by IRSN [13] that radiolysis promotes
the formation of ruthenium tetroxide in a higher
amount than could be expected by pure ozone
action (about one order of magnitude). The
enhancement of Ru revaporisation phenomenon
under radiation was explained by the presence ofO· and/or OH· radicals in a higher extent becauseof additional production by direct radiolytic
reactions in air/steam atmospheres.
The distribution of RuO4 between gas and liquid
phases and especially the corresponding partition
coefficient (kD) were determined in a temperature
range from 20 to 50 °C. The experiments were
performed using an experimental set-up
simulating a BWR containment building, shownin Figure 8. All surfaces of the set-up were madeof glass. In the experiments gas phase was a
mixture of nitrogen and oxygen (O2 fraction lessthan 5%) and liquid phase was composed ofMilli-Q water. Gaseous RuO4 (spiked with 103Ru)
was introduced to the system by sublimation ofcondensed RuO4 from the surface of a glass vial.
The distribution of Ru between gas and liquid
phases was continuously measured with two
NaI(Tl) detectors. When Ru was introduced tothe system at 20 °C, it was immediatelytransported into the liquid phase. After fiveminutes, the concentration of Ru in the gas phase
had decreased below a detection limit. A similar
phenomenon was also seen at higher
temperatures. The concentration of ruthenium inthe liquid phase was decreased to ca. 25% after
two hours. This behavior was explained by as a
reduction of RuO4 to solid RuO2 that was
deposited and accumulated on the glass surfacesof the set-up. The visual observation of black
deposit layers on the inner glass surfaces and theanalysis with HPGe detector confirmed thedeposition of Ru.

a)b)Fig. 7. Revaporised fraction of ruthenium
induced by gamma radiation in (a) dry and (b)humid atmospheres.
The observed results were explained by therole of hydroxyl radical produced in humid
conditions. Hydroxyl radical with the reduction
pair ∙OH/H2O (standard oxidation potential forthe couple Eo=1,4V) is a strong oxidizing agent
with an ability to revaporise ruthenium from thedeposit. When the results were compared withthe previous results of IRSN [13], it was noticed
that a lower fraction of revaporised ruthenium
per unit of received dose was obtained in thecurrent study. As temperature in the experiments
was significantly lower than in the previous
experiments (44 °C vs 90 °C), it was proposedthat temperature can affect strongly the kinetics
of ruthenium re-oxidation process. It was also142

NEA/CSNI/R(2016)5
Results on the irradiation tests of perruthenate
solutions clearly showed that the revaporisationof Ru from the sump is possible. A notable Ru
fraction was detected both in liquid trap and on
inner surfaces of the glass ampoules despite thepH of Ru solutions and the radiation dose. The
measured fraction of revaporised Ru ranged
between 5 and 13%. Furthermore, the presenceof Ru precipitates was observed in the liquid trap
(mainly) and on glass surfaces at the end of every
experiment. Therefore, the extent of Ru
revaporisation may appear quite surprising, when
taking into account that the observed precipitates
may result from the formation of reducing agents
under radiation. However, these experiments
were exploratory at this stage, and somequestions remain open, for example, the role of
pH and the effect of radiation dose on theformation of RuO4. Further studies concerning
Ru behavior in aqueous phase under radiolysis
will be needed to improve the knowledge of Ru
aqueous chemistry in SA conditions. In
particular, the uncontrolled formation of Ru
precipitates needs to be understood beforehand.

Fig. 8. Experimental set-up for the RuO4
distribution experiments.
In experiments without the liquid phase, it took
more than 200 minutes until the concentration of
Ru in the gas phase had decreased to thedetection limit of NaI(Tl) detector. This indicatesthat RuO4 can remain in the gas phase of dry
system for a long time when compared to asystem with a water phase as well. Based on theexperiments deposition velocities were also
calculated [11].

V. Ru TRANSPORT MODELLING

Some exploratory irradiation tests of liquid
phases containing ruthenium species have been
carried out by IRSN [13] in order to check thepotential oxidation reaction of Ru aqueous
species up to the +VIII oxidation state [RuO4(g)].
The objective was limited to qualitatively
evidence whether or not the sump has the ability
to be a source of volatile Ru under radiation.
Concerning the preparation of aqueous Ru
solutions, the concentration of ruthenium was set
to 10-4 mo/l according to the expectedconcentration in the sump in SA conditions [20].
Two pH values, 4 and 10, were studied. KRuO4
powder was dissolved into boric acid solution of
0.2 mol/l with respect to boron. The alkaline
solution was a mixture of KRuO4 powder
dissolved into potassium hydroxide solution(KOH) of 0.05 mol/l with a boric acid of 0.2
mol/l with respect to boron. Finally, pH values
were adjusted by addition of pure HNO3 or LiOH
solution of 5 ppm (wt). The aqueous Ru sample
was then exposed to gamma radiation (60Co) in a
glass ampoule and the released Ru was trappedin KOH solution (0.2 mol/l) located in the othercompartment of the ampoule.

It must be emphasized that VTT and RUSET
series of ruthenium transport tests clearly
demonstrated that the decomposition of RuO4 to
RuO2 was not complete and did not follow
thermal equilibrium. The experimentallyobserved high fraction of gaseous Ru transported
through a model primary circuit to thecontainment should also be considered in severe
accident simulation codes. Ru deposition patterns
calculated by VTT with the Computational Fluid
Dynamics (CFD) software package FLUENT
were very close to the experimental results when
the hypothesis was to investigate diffusionlimited RuO2 and RuO3 transport and deposition
to the TGT surfaces [21], see Figure 6a. The
kinetics of ruthenium conversion to RuO4 wasassumed to be so slow that the conversion was ofsecondary importance during the relatively briefcooling phase. Simulation studies verified thatthe deposition occurred mainly as diffusionlimited reactive condensation of RuO3(g) toRuO2(c), when the temperature decreased below
approx. 800 °C: 2 RuO3 → 2 RuO2 + O2.
Condensation of RuO2, deposition of RuO2
particles and reaction of RuO4 with surfaces
143

NEA/CSNI/R(2016)5(circuit and particles) were other minor
deposition processes. The roles of buoyancy andthermophoresis on the deposition of RuO2
particles, which were nucleated in the gas phaseof the circuit, became important when thetemperature gradient was high, e.g. at the outletof the furnace.

1227 °C. When the oxidation took place at arather moderate temperature, 1000–1100 °C, a
significant fraction of Ru was transported asgaseous RuO4. The oxidizing air radiolysis
products can further increase the fraction of
gaseous Ru. Ruthenium deposited on the circuitsurfaces in the form of RuO2. Simulation studies
verified that the deposition occurred mainly as
diffusion-limited reactive condensation ofRuO3(g) to RuO2(c), when the temperature
decreased below approx. 800 °C. Other minor
deposition processes were also identified. On theother hand, Ru was also revaporised from thedeposits and the gaseous fraction of thetransported Ru was high.

VI. CONCLUSION
In experiments on the effect of atmosphere on therelease of Ru from the fuel, a significant releaseof Ru from the irradiated MOX fuel wasobserved in air ingress conditions (VERDON-2).
The cladding of fuel pellet samples was oxidized
prior to reaching the high temperature conditionsin the course of test. When the test was repeatedin steam conditions (VERDON-3), only a few
percent of Ru was released. That is very low incomparison with the Ru release from UO2 fuel insimilar conditions in VERCORS tests. It was
concluded that probably the lower release of Ru
from MOX fuel than from UO2 fuel is linked to adifference in the oxidation of the fuel matrix. In
large-scale integral Phébus FP tests ruthenium
release from the irradiated UO2 fuel under steam
conditions was fairly limited as is typical for a
low-volatile element. In addition, a significant
fraction of it was deposited on the upper parts ofthe fuel bundle instead of being transported tothe model primary circuit. Ruthenium deposited
significantly and progressively from the exit of
the bundle to the entrance to the containment.
The deposits were mainly located in places
where the temperatures of the wall and the fluid
decrease strongly, such as the area above the fuelbundle and the hot leg of the steam generator.
Concerning the physical form of Ru transported
to the coldest regions of the circuit, Ru wasobserved to be almost exclusively as an aerosol.
In VERDON-2 no deposition of Ru was observe
to take place in the TGT before the air ingressphase, which is consistent with the Ru releaseobservations. Afterwards, an accumulation of Ru
on tube surfaces was noticed in the course of thetest. Ruthenium was not observed downstream
the condenser (coupled with a May-Pack filter inthe upstream) at the circuit outlet, thus indicatingthat no gaseous Ru was transported so far.

After RuO4 had being interacted with metals (Al,Zn, Cu) and epoxy paint surfaces at room
temperature with high relative humidity, theformation of black colored Ru deposits wasobserved. A detailed analysis indicated that thesurface of deposits was in a hydrated form of
RuO2, whereas the deeper structure was still of
RuO2. The affinity of RuO4 towards metals wasincreased by humid conditions. The highest
affinity was towards aluminium, although it
seemed to trap RuO4 even better in dry
conditions. When the epoxy paint surface with
Ru deposit was exposed to gamma radiation at
ca. 50 °C, Ru release was observed but only inhumid conditions. That was explained by the roleof strong oxidizing hydroxyl radical formed inhumid conditions due to radiation. Also, theamount of gaseous RuO4 remaining in the gas
phase was strongly dependent on the watercontent of the system in a temperature range
from 20 to 50 °C. When RuO4 was fed to the gas
phase, it remained there for a long time in case of
a dry system in comparison to a system with awater pool and a gaseous volume above it –simulating a containment building. RuO4 was
immediately transported to the water pool, in
which it was also reduced to solid RuO2 anddeposited on available surfaces. The release ofRu from a perruthenate aqueous solution due togamma radiation was qualitatively examined. Incase of alkaline or acidic solution, simulating acontainment sump, a notable fraction of Ru was
released in gaseous form. The presence of Ru
precipitates in liquid trap and on the surfaces ofthe set-up was observed in every test.

In the separate-effect tests ruthenium transported
through the circuit mainly as aerosol particles(>99%) when the Ru source was oxidized above
144

NEA/CSNI/R(2016)5
Data obtained from the above-mentioned performed and ongoing - separate-effect, semiintegral and fully-integral programs will
constitute a support to the model developments
and contribute to the validation effort. The
upcoming VERDON-5 tests is of specific interest
as it examines the effect of air ingress on FPs
release from UO2 fuel, and the following
transport through a TGT, and thus it supplements
the existing database of integral experiments.
ACKNOWLEDGMENTS
This work was conducted in the framework of
EC SARNET and SARNET2, ISTP/VERDON,NEA STEM/START and NKS-R (Nordic
Nuclear Safety Research – area of Reactor
Safety) projects.
The Phébus FP programme was initiated in 1988
by IRSN, in cooperation with the Commission of
the European Communities (EC), using the
Phébus facility operated by CEA (France), with
contributions from EdF (France), USNRC(USA), CANDU Owners Group (Canada), JNES
and JAEA (Japan), KAERI (Korea), HSK (now
ENSI) and PSI (Switzerland).
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THOUGHTS ON RUTHENIUM IN NUCLEAR ACCIDENTS
Michael Salay*, Richard Lee, and Dana Powers
U. S. Nuclear Regulatory Commission,
Rockville, Maryland, USA*Corresponding author, tel: (+1) 301-415-2408, Fax: (+1) 301-415-6671, Email:Michael.Salay@nrc.gov

Abstract – Ruthenium is considered to be released in accidents involving exposure of hot fuel to air whichis considered to occur in air ingress accidents or spent fuel pool drain-down or boil-off scenarios. In thepast several years considerable research has been conducted regarding the transport of ruthenium upon
release. Ruthenium release analyses have also been conducted using pre-oxidized cladding. Aspects ofruthenium release that have received less attention, including gas transport behavior, the impact of gas
consumption, and clad behavior, are discussed.

147

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I. INTRODUCTION

can be released under accident conditions alongwith ruthenium. Certainly releases of otherinorganic species of variable oxidation states canbe expected including releases of large amountsof tin, chromium, manganese, and iron as well asother fission products such as iodine. Organic
species including ketones and alcohols can be
expected to be present in the reactor containment
because of the steam oxidation and pyrolysis oforganic materials including paint, lubricants, andcable insulation. Finally, there is the additional
complication of intense radiation fields in both
the gas phase and the condensed phase.
Radiolytic processes can be expected to produce
both oxidants and reductants that will affect thechemical behavior of ruthenium.

Radioactive isotopes of ruthenium can havesignificant radiological effects if released
extensively from irradiated reactor fuel under
accident conditions. Alpert et al. [1986]
estimated that for equal release fraction of 10%of core inventory, the release of ruthenium during
a severe reactor accident would have about threetimes the effect of radioactive iodine release onlung dose and an effect equivalent to that ofcesium for latent cancers. That is, ruthenium
release could affect both prompt and latent
fatalities if the release from the degrading reactor
core was similar on a fractional basis to thereleases of cesium and iodine. This finding andsimilar findings from other considerations of theradiological effects of ruthenium [Leggett, 2012]
have prompted some interest in the behavior ofruthenium under reactor accident conditions.
Most of these studies presuppose that ruthenium
has been released from degrading reactor fueland focus on the behavior of the radionuclide in
the reactor coolant system and in the reactor
containment.

Unraveling the potentially complicated behaviorof ruthenium under reactor accident conditions
sufficiently well to support predictive modelingappears to be a formidable chore. It could benecessary to undertake this effort if it can beestablished that ruthenium releases for significant
classes of reactor accidents can be comparable to
the releases of more familiar radionuclides suchas iodine and cesium. The balance of this paper
provides some thoughts on the types ofconsiderations that will need to be made in orderto establish that significant ruthenium releases
from degrading reactor fuel are possible and that
there is an attendant need to understand well thedetails of ruthenium behavior in the reactor
coolant system and in reactor containments.

Once released, ruthenium behavior can becomplicated. The complication stems from themany oxidation states the element can adopt
depending on the chemical environment. Volatile
ruthenium tetroxide, RuO4, has a formaloxidation state of +8. Much of the study ofruthenium in aqueous solution has focused on theoxidation states of +2 to +6 [Rard, 1985].
Ruthenium-bearing aerosols are expected to haveruthenium in the +4 valence state, usually as
ruthenium dioxide, RuO2, or its various hydrates.
This complexity of the oxidation states ofruthenium is reminiscent of the complexities ofoxidation states of iodine under accident
conditions.

II. RUTHENIUM IN REACTOR FUEL
Ultimately, ruthenium is generated by the fission
process in reactor fuel. It is, however, not an
important primary product of fission. Most is
produced by the decay of molybdenum throughtechnetium. These elements are, then, always
closely associated in the fuel. They are also of
low solubility in the fluorite lattice ofhyperstoichiometric fuel typical of thermal
reactors [Kleykamp, 1993]. Somewhat higher
solubilities especially of ruthenium are possible
in hypostoichiometric fuel of fast reactors[Olander, 1982]. Because of the low solubility,
once a threshold of burnup is surpassed,
ruthenium can be detected in the so-called ‘whitephases’ of fuel [Kleycamp et al., 1985]. These are

The wide range of ruthenium chemistry becomes
accessible under accident conditions because of
the range of possible chemical conditions in thereactor coolant system and in the reactor
containment depending on the details of reactor
accident conditions. Furthermore, conditions oftemperature and pressure may be altered by
accident management measures of varying levelsof effectiveness. Further complications arise
because of the vast range of chemical species that
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NEA/CSNI/R(2016)5
nodules of alloys of Mo, Ru, Rh, Tc, and Pd.
Other elements may be present including
zirconium and barium to modest levels. A‘typical’ composition for light water reactor fuelsuggested by Sasa et al. [1980] in terms of mole
fraction is xMo = 0.429, xTc = 0.131, xRu = 0.177,
xRh = 0.049, xPd = 0.026, xBa = 0.063 and xZr =0.041. Compositions are variable depending on
burnup [Kleycamp et al., 1985] and the radial
thermal gradient in fuel pellets during normal
operations [Zhou, 2010]. It is also typical todetect two populations of alloy nodules. Larger
nodules (1-5 µm) are found in pores and on grainboundaries. Very much smaller nodules can beseen within the fuel grains themselves. The
relative amounts of these two types of nodules
depend on burnup and linear heating rate of the
fuel.

normal operations. Even under the steamhydrogen atmospheres characteristic of reactor
accidents, the partial pressures are too low todrive risk-significant releases of ruthenium fromthe fuel. Indeed, significant releases of ruthenium
were not detected during the accident at Three
Mile Island. Significant ruthenium releases havenot been observed in tests of reactor fueldegradation including the PBF tests conductedby the Idaho National Engineering Laboratory[Hobbins et al., 1993], the HI/VI tests conductedby the Oak Ridge National Laboratory [Osborne
et al., 1985], or the recent Phébus-FP tests
conducted in France. Except at the very highest
temperatures and steam partial pressures
expected to arise in reactor accidents,
atmospheres of steam and hydrogen cannot
produce sufficiently high oxygen potentials todrive significant releases of ruthenium.

The vapor pressure of pure ruthenium is not higheven at temperatures sufficient to cause extensive
degradation of reactor fuel: ~10-4 bar at 2833 K[Hultgren et al., 1973]. Alloying of theruthenium in the nodules will further reduce thevapor pressure of ruthenium metal by dilution
and by chemical interactions among the alloy
constituents[Rand and Potter, 1981].
Consequently, ruthenium release by direct
vaporization of the metal is thought to be
insignificant for reactor accidents. This does notpreclude some transport of metal vapors within
the fuel from hotter to cooler regions. Vapors
encountering metals such as the zirconium
cladding will alloy with these metals.

To get significant releases of ruthenium from
irradiated reactor fuel it is necessary to havehigher oxygen potentials than can be produced
typically by atmospheres of steam and hydrogenat plausible pressures and temperatures.
Sufficiently high oxygen potentials can beproduced if even small amounts of air are mixed
with steam. High releases of ruthenium from
irradiated reactor fuel in air were first
demonstrated in early experiments at Oak Ridge
National Laboratory [Lorenz et al., 1980]. A
more dramatic demonstration was reported by
Cox et al. [1991]. They exposed heated CANDU
fuel to air. Once the rather thin zirconium alloy
cladding on the CANDU fuel was oxidized,
nearly complete release of ruthenium wasobserved at even modest temperatures relative tothose expected to arise in fuel during severereactor accidents. In both the experimentreported by Cox et al. and the experiments
reported by Lorenz et al., ruthenium release wasaccompanied by releases of copious amounts ofuranium vaporized by oxidation of the fuel to thevolatile hexavalent state – UO3.

III. VAPORIZATION AS RUTHENIUM
OXIDERuthenium can form a variety of oxide vapor
species [Garisto, 1988] with partial pressure wellin excess of the vapor pressure of pureruthenium: RuO, RuO2, RuO3, and RuO4. At a
fixed oxygen potential, the dominant oxide vapor
species is the monoxide at the highest
temperatures expected in reactor accidents.
Higher oxides become dominant at progressively
lower temperatures. The tetroxide is thedominant vapor species at temperatures below
about 800 K.

The demonstration experiments by Cox et al. andthose reported by Lorenz et al. show that it ispossible to get extensive ruthenium release. As
issue becomes the types of reactor accidents thatcan lead to air being present in the steam
atmosphere during core degradation. The reactor
accident at Three Mile Island demonstrated that

The partial pressures of these ruthenium oxide
species are quite low within the fuel during
149NEA/CSNI/R(2016)5
during severe reactor accidents substantial coredegradation and relocation to the lower plenumof the reactor vessel could occur along thecenterline of the reactor core while fuel about the
periphery of the core remained largely intact.
Should the relocated core debris penetrate thereactor vessel, it would be possible for air fromthe reactor containment to enter the reactor
vessel by natural circulation and expose theresidual fuel in the vessel to high oxygen
potentials.

steam. In detail, there are differences [Steinbrück
et al., 2004]. For the conditions of interest, thedetailed chemical kinetics are of less interest
since reaction of air with the cladding will belimited most likely by gas-phase mass transport.
Until the cladding is oxidized, it will ‘getter’ airfrom the gas phase and prevent high oxygen
potentials developing in the vicinity of ruthenium
alloy nodules within the fuel. The barrier toextensive ruthenium releases posed by zirconium
reaction with the air is especially significant forboiling water reactors. Boiling water reactors use
zirconium alloys for both cladding the fuel andfor structures in the core region. Typically,boiling water reactors have about three times asmuch zirconium in the core region as dopressurized water reactors. It is noteworthy that
there have not yet been reports of importantreleases of ruthenium from fuel in the Fukushima
Dai-ichi reactors even though air may haveentered the cores of the reactors during theaccidents at this site.

High ruthenium releases, then, are possible
during the most severe reactor accidents that
progress to the point that the reactor vessel ispenetrated by core debris. This, of course, is asubset of reactor accidents involving reactor fueldegradation. The accident at Three Mile Island as
well as risk analyses [NRC, 1990] show thatimportant fractions of reactor accidents involving
fuel melting are arrested prior to reactor vessel
breach.
Other accidents that have been hypothesized to
involve exposure of inadequately cooled,irradiated, reactor fuel to air include refueling
accidents involving the loss of coolant feed,
spent fuel pool accidents involving loss of
coolant inventory, and rupture of casks for dry
storage of spent fuel.

The reaction of air with zirconium is about 70%
more exothermic than is the reaction of steamwith zirconium. It is possible, then, thattemperatures in the cladding will be driven to themelting point of the cladding alloy more quickly
in steam-air mixtures than in steam-hydrogenmixtures. Melting of the cladding below the zoneof reaction with the oxidizing gas could lead toliquefaction of the fuel. There would be asignificant loss of surface area for release ofradionuclides and even relocation of the fuelfrom the high temperature zone of the reactor
core. On the other hand, melting of theunderlying metal could lead to the rapid drainingof cladding and directly expose the irradiatedreactor fuel to an atmosphere with a high oxygen
potential. Powers et al. [1994] considered thesetwo possibilities for clad behavior when exposed
to air and concluded that there was insufficientphenomenological understanding to predict
confidently the course of accident progression.
They suggested that to develop reliable models
integral tests involving degradation of many fuelrods in air would be needed.

IV. MASS TRANSPORT AND
COMPETITIVE REACTIONS
The availability of high oxygen potentials in thegas phase is clearly a necessary condition forextensive ruthenium release from irradiatedreactor fuel. It is not a sufficient condition. For
extensive vaporization of ruthenium from alloy
nodules in the fuel to occur, the high oxygen
potentials must exist in the vicinity of thenodules. There are significant mass transport
limitations and competitive reactions that are
barriers to the establishment of high oxygen
potentials adjacent to the alloy nodules
containing ruthenium.
The most obvious of these barriers is thezirconium alloy cladding on the fuel. Zirconium
will react with oxygen and nitrogen in theatmosphere. The oxidation of zirconium by air isgrossly similar to the oxidation of zirconium by

Exposure of reactor fuel to air-steam mixtures isalso not sufficient to lead to extensive ruthenium
release. A simple examination of Ellingham
diagrams [Ewart et al., 1976] shows that many
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NEA/CSNI/R(2016)5
things in reactor fuel will react with and deplete
air from the gas phase before ruthenium can beoxidized. The fuel itself will have to be oxidized
to a hyperstoichiometric state consistent with theoxygen potentials needed to produce high partialpressures of ruthenium-bearing vapors:
UO2 + x/2 O2 → UO2+x
At the temperatures of interest, the diffusion ofoxygen in the fluorite lattice of reactor fuel issufficiently high that an oxidation front does notdevelop [Lay, 1970]. Rather, the whole body offuel must be oxidized before the necessary
condition for ruthenium vaporization exists.
Competitive with the process of developing theelevated oxygen potentials in the vicinity of theruthenium nodules is the slumping and relocationof reactor fuel at high temperature. Slumping of
the fuel reduces the surface area available forfission product vaporization to the atmosphere of
the reactor vessel and coolant system. It also
brings molten fuel into contact with metals that
can alloy with ruthenium. Relocation can remove
core debris from the high temperature regions of
the reactor core. At the very least, slumping andrelocation of core debris can limit the time
available for extensive ruthenium release.
V. CONCLUSIONSFor ruthenium release to have a significant
effect on the consequences of reactor accidents,
the release must be extensive – comparable toanticipated releases of iodine and cesium. It isevident from available information that
prolonged exposure of reactor fuel to unlimited
amounts of air can lead to extensive release ofradioactive ruthenium. There is, however,insufficient information available to judge thelikelihood of extensive ruthenium release fromreactor fuel during accidents. It is evident that
there are multiple barriers to the development ofnecessary and sufficient conditions for extensive
ruthenium release. There are important
limitations on the amount of fuel exposed toconditions conducive to extensive ruthenium
release and the amount of time that high rates ofruthenium release could persist. It is, then,difficult to formulate a risk-informed justification
for the expenditure of resources that will beneeded to understand adequately for predictivemodeling the potentially complicated behavior ofruthenium once it has been released from reactor
fuel.
REFERENCES
1. D.J. Alpert, D.I. Chanin, and L.T. Richie,
Relative Importance of Individual Elements toReactor Accident Consequences Assuming Equal
Release Fractions, NUREG/CR-4467, SAND852575,
SandiaNational
Laboratories,
Albuquerque, NM, March 1986.
2. D.S. Cox, C.E.L. Hunt, Z. Liu, N.A. Keller, R.D.
Barrand, R.F. O’Connor, and F.C. Iglesias,
Fission product releases from UO2 in air andinert conditions at 1700-2350 K: Analysis of theMCE-1 experiment, AECL-10438, Atomic
Energy of Canada Limited report, Pinawa,
Manitoba, 1991.
3. F.T. Ewart, R.G. Taylor, J.M Horspool and G.
James, “The Chemical Effects of Composition
Changes in Irradiated Oxide Fuel Materials II –Fission Product Segregation and Chemical
Equilibria”, J. Nucl. Mater., 61; 254-270, 1976.
4. F. Garisto, Thermodynamic behavior ofruthenium at high temperatures, AECL-9552,
Atomic Energy of Canada Limited, Pinawa,
Manitoba, 1988.
5. R.R. Hobbins, D.A. Petti, and D.L. Hagrman,“Fission Product Release From Fuel Under
SevereAccident
Conditions”,
NuclearTechnology, 101; 270, 1993.
6. R. Hultgren, P.D. Desai, D.T. Hawkins, M.
Gleiser, and K.K. Kelly, Selected Values of the
Thermodynamic Properties of the Elements,
American Society for Metals, Ann Arbor,
Michigan, 1973.
7. H. Kleykamp, “The solubility of selected fission
products in UO2 and (U,Pu)O2”, J. Nucl. Mater.,
206;82-86, 1993.
8. H. Kleykamp, J.O. Paschoal, R. Pejsa, and F.
Thommler, “Composition and Structure of
Fission Product Precipitates in Irradiated Oxide
Fuel: Correlation with Phase Studies in the MoRu-Rh-Pd and BaO-UO2-ZrO2-MoO2 Systems”,
J. Nucl. Mater., 130;426-433, 1985.

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9. K.W. Lay, “Oxygen Chemical Diffusion
Coefficient of Uranium Dioxide”, J. Amer.
Ceram. Soc., 53; 369-373, 1970.

During Severe Reactor Accidents, NUREG/CR6218,
SandiaNational
Laboratories,
Albuquerque, NM, 1994.

10. R.W. Leggett, “The biokinetics of ruthenium inthe human body”, Radiation Protection andDosimetry, 148;4,389-402, 2012.

16. M.H. Rand and P.E. Potter, “Thermodynamics
and Phase Diagrams of Mo-Pd-Ru and Related
Systems”, Physica, 103B; 21-30, 1981.

11. R.A. Lorenz, J.L. Collins, A.P. Malinauskis, O.L.
Kirkland, and R.L. Towns, Fission ProductRelease from Highly Irradiated LWR Fuel,
NUREG/CR-0722 R1, Oak Ridge National
Laboratory, Oak Ridge, Tennessee, 1980.

17. J.A. Rard, “Chemistry and Thermodynamics ofRuthenium and Some of Its Inorganic
Compounds and Aqueous Species”, Chemical
Reviews, 85;1-39, 1985.

12. NRC, Severe Accident Risks: An Assessment for
Five U.S. Nuclear Power Plants, NUREG-1150,
U.S.
Nuclear
Regulatory
Commission,
Washington, DC, 1990.
13. D.R. Olander, “A Mechanistic Analysis of
Ruthenium Transport in UO2”, Nuclear Scienceand Engineering, 82; 190-205, 1982.
14. M.F. Osborne, J.L. Collins, R.A. Lorenz,
K.S.Norwood, J.R. Travis, and C.S. Webster,
Data Summary Report for Fission ProductRelease Test HI-5, NUREG/CR-4037, Oak Ridge
National Laboratory, Oak Ridge, Tennessee,
1985.

18. P. Sasa, A.W. Cronenberg, M.G. Stevenson, “On
the Volatilization Potential of Metallic Inclusions
Found in Irradiated UO2 Fuel During
Overheating Events”, Nuclear Technology, 48;
233-250.
19. M. Steinbrück, A. Meier, U. Stegmaier, and L.
Steinback, Experiments on the Oxidation of
Boron Carbide at High Temperature, FZKA6979. Forschungszentrum Karlsruhe GmbH,
Karlsruhe, Germany, 2004.
20. S.Y. Zhou, The Redistribution of Ruthenium in
UO2 in a Temperature Gradient, LBL-12193,
Lawrence Berkeley National Laboratory,
Berkeley, California, 2010.

15. D.A. Powers, L.N. Kmetyk, and R.C. Schmidt, A
Review of the Technical Issues of Air Ingression

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THE MAIN OUTCOMES OF THE OECD BEHAVIOUR OF IODINE PROJECT
G.A. Glowa*, C.J. Moore, D. Boulianne
Canadian Nuclear Laboratories, Chalk River, Canada*Corresponding author, tel: (+1) 613-584-3311(ext. 46052), Glenn.Glowa@cnl.ca

Abstract – The second phase of the OECD Behaviour of Iodine Project (BIP) ended in September 2014.
The primary objective of this project was to investigate the interactions between iodine and containment
paint, including iodine sorption behaviour and the subsequent formation of organic iodides.
In the first phase of BIP, BIP-1 focused on the interaction between iodine and epoxy paints. Epoxy paints
are commonly used surface coatings within nuclear reactor containment buildings throughout the world.
In particular, experiments investigating the role of water on the deposition velocities and formation of
methyl iodide from irradiated paint showed that adsorbed water strongly affected both the sorption andorganic iodide formation processes.
BIP-2 investigated the same processes, with emphasis on developing a more mechanistic understanding oforganic iodide production during irradiation of iodine-loaded paint. Epoxy paint is a complex mixture ofpolymeric compounds and it is difficult to know where and how iodine is interacting and how methyl iodide
is formed. To isolate the influence of various functional groups present in epoxy paint, polymer samples
having simple, but representative, structures (such as amide groups) were prepared. The samples were
exposed to I2(g) and irradiated to determine the rate of methyl iodide formation. In addition to the polymer
tests, tests were performed with epoxy paint at 80°C (for comparison to EPICUR tests) with different
iodine loading schemes (to help our understanding of the effect of water) and with paint that was preexposed to Cl2 or NO2 (to simulate the effect of reactive species expected within containment).
Several spectroscopic techniques were utilized in BIP-2 to investigate iodine-paint surface interactions.
Scanning electron microscopy was used to determine coupon surface morphology, energy dispersive x-ray
analysis measured the depth of penetration of the iodine into the paint layer, and gas chromatographymass spectroscopy was used to study solvent release from 20-year-old epoxy paint.
This paper will discuss the main outcomes of BIP. The results from the BIP experimental program can beused to guide the development of computer models and provide data for model validation.

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I. INTRODUCTION

functional groups including ethylene, carbonyl,
phenol, epoxide and amine groups.

An Organization for Economic Cooperation andDevelopment (OECD) status report on iodine
behaviour published in February 2007 concluded
that, although the understanding of iodine
behaviour in containment had advanced
considerably over the past several decades, there
were still some areas where

Results from BIP-1 show that relative humidity
is an important factor determining the rate ofdeposition of iodine on epoxy paint. The results
also show that there is some variability in thedeposition velocity on various types of epoxy
paint. After exposure to iodine, much of theiodine is strongly retained on painted surfaces
and is presumed to be covalently bonded. In BIP2, the site specificity of iodine bonding wasevaluated. Polymeric model compounds withchemical structures characteristic of epoxy paintwere chosen for study. Knowing how the iodine
is bound may provide mechanistic insights into
how to model the process and how organic
iodides are formed when iodine-loaded paint isirradiated.

further investigation was warranted [1]. There
have recently been several international
initiatives (or programs) that have addressed this
topic, including the Phébus International Source
Term
Project,
the
OECD
THAI
(Thermalhydraulics Hydrogen Aerosol Iodine)
Project, and the OECD STEM (Source Term
Evaluation and Mitigation) Project.
The Behaviour of Iodine Project (BIP) was
initiated to investigate aspects of the iodine-paintinteraction. Although a few other containment
surfaces are tested, BIP focuses on epoxy paint,
which is a commonly used coating withincontainment buildings throughout the world.
Epoxy paint is a complex mixture of polymericcompounds, and the nature of its interaction withiodine is not known precisely. In BIP, several
types of epoxy paint were tested, which should
help determine if similarities exist to the extent
that a common modelling methodology can be
adopted.

(b) Organic iodide formation
The mechanism of organic iodide production
from irradiated iodine-loaded paint is not fully
understood. Possibilities include a radiationinduced reaction of iodine associated with aparticular functional group, or a radiationinduced reaction taking place between iodine andresidual paint solvents.
In BIP-1, progress was achieved quantifying therates of formation for methyl iodide from
irradiated paint. It was found that the methyl
iodide concentration, determined by a balance
between a fast formation and destruction rate,changed as a function of time during irradiation.
It was also observed that the presence of residual
water affected the methyl iodide production rate.
Tests using coupons loaded with iodine bycontact with aqueous solution had higher organic
iodide formation rates than gas-loaded coupons.
Water may be required to mobilize paint solvents
trapped within the paint matrix, or may berequired for an aqueous-phase process within thepaint pores.

The main technical objectives for BIP-2 were toobtain a more detailed and mechanistic
understanding of (a) iodine adsorption onto, anddesorption from, containment surfaces and (b)organic iodide formation.
(a) Iodine adsorption
Painted surfaces are an important iodine sink in
the post-accident containment building. The
adsorption of iodine on paint is often simplifiedas:
I2 + painted surface → I(ads)BIP-2 studies were designed to provide insight
into how organic iodides are formed by studying
organic iodide production on simple polymericmodel compounds. In addition, some tests withpainted samples included an irradiation phase at

The I(ads) refers to the adsorbed phase of iodine,and often includes several chemical forms,
including iodine that is covalently bound,physically bound, and involved with chargetransfer complexes. In the aqueous phase, iodinehydrolyzes to compounds such as HOI [2], whichhave been implicated as a reacting species [3].
Iodine is known to be reactive with various
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NEA/CSNI/R(2016)5
80°C to allow comparison to tests carried out inthe EPICUR(4) facility.

A or G to identify if the coupon was exposed toiodine in the aqueous (A) or gas (G) phase, and asequential number to identify replicate tests. For
example RAD-EPX-A-1, identifies irradiation
test 1, using epoxy resin loaded by contact withan aqueous solution containing I2.

The results of BIP-2 include qualitative insights,
which can be used to guide model design, andquantitative data (such as deposition velocities)
that are necessary input for iodine behaviourmodels. A mechanistic understanding of these
phenomena improves our ability to explain
results to our regulatory agencies, and supports
the application of these results to conditions
outside of the tested range (e.g., extrapolation toa wider selection of epoxy coatings and to largerscale experiments and containment conditions).

TABLE 1: Coupon Types
Name
GW

Coupon Material
Epoxy paint (Gehopon EW10)
Epoxy paint (Hydrocentrifugon Ripolin
EPICUR
901)
ORG
Epoxy paint (Amerlock 400)
NYLON Nylon 6/6
POLY
Polypropylene
EPX
DOW DER 661 epoxy resin
Amine-cured epoxy resin (DOW DER
EPX2
332 cured with triethylenetetramine)
EPX3
DOW DER 667 epoxy resin

The BIP-2 project officially started in April 2011and ended in September 2014. The member
nations included Belgium, Canada, Finland,
France, Germany, Japan, Korea, Spain, Sweden,
the United Kingdom and the United States. The
total budget of the project was about 900k€, halfof which was contributed by Canada. Good
progresswasachieved
regardingtheunderstanding of the nature of iodine bonding on
paint, and on the formation of organic iodides.

For typical irradiation experiments, a glass vessel
containing pre-loaded coupons (Fig. 1) andlaboratory air was irradiated at ambient
temperature in a Gammacell 220 60Co
irradiator(5). Gas samples were removed
periodically for methyl iodide analysis by Gas
Chromatography(GC).
Sampling
wasaccomplished using a gas-tight syringe via a gassampling septum located at the top of theirradiation vessel. At the end of the irradiation,
the gas-phase iodine was analyzed using a gasphase iodine speciation procedure.

II. TESTS PERFORMED
Two main types of tests were performed under
BIP-2: adsorption tests (AD), where the sorption
behaviour of iodine on various surfaces wasmeasured, and irradiation tests (RAD), whereiodine-loaded coupons were exposed to 60Co
gamma radiation to investigate the formation oforganic iodides.

The vessel used for the 80°C irradiation tests
included a thermo-well to accommodate athermocouple for temperature control. For these
special tests, a small amount (200 μL) of water
was added to the vessel, prior to the heatingphase, in order to raise the relative humidity.
Humidity was not measured during irradiation.
However, water loss during the heat-up phase
was quantified during commissioning tests andthe water additions were adjusted to yield 60%
RH.

For both types of test, iodine was adsorbed onto
the coupon surfaces by immersing the samples inaqueous solutions containing I2, or by exposing
the samples to an airstream containing I2 using a
specially-designed adsorption apparatus. Gasphase iodine loading was typically performed at
70°C and 70% relative humidity.
Test names generally include the type of test
(AD or RAD), the coupon type (see Table 1), an(4)

EPICUR (Experimental Project on Iodine
Chemistry Under Irradiation) experiments are
currently performed in France within the OECDSTEM Project. These experiments study organic
iodide formation using a complementary
experimental methodology.

(5)

155

The Gammacell 220 60Co
distributed by MDS Nordion.

irradiator

wasNEA/CSNI/R(2016)5
Amine-cured epoxy polymer: Coupons wereprepared from a polymer made by mixing DOW
DER 332 (a low molecular weight liquid
DGEBA) with TETA (triethylenetetramine). This
material was studied in order to investigate theimportance of the amine functional group.
Polypropylene: This material is simply a long
chain aliphatic polymer formed from propene.
Polypropylene does not contain any special
functional groups, but has many methyl groups.
It was chosen to represent any aliphatic portionsof epoxy paint.
IV. RESULTS OF AQUEOUS IODINEADSORPTION TESTS
Fig. 1. Open Irradiation Vessel with Coupons.

Adsorption/desorption measurements on theselected polymers were carried out using
aqueous I2 solutions to evaluate the influence ofthe various functional groups on iodine
adsorption and desorption in the absence ofradiation. An overview of the aqueous adsorption
and desorption measurements is shown in Fig. 3and compared to epoxy paint results from BIP-1.
Coupons were exposed to saturated I2 solutions(131I trace labelled) containing 0.01 M KH2PO4,
adjusted to pH 6. All polymers and paints tested
adsorb iodine as I2 from the aqueous phase.
Untreated Gehopon epoxy paint had similar
adsorption behaviour to the Amerlock and
Ripolin epoxy paints. It should be noted,
however, that iodine adsorbed faster onto
untreated Gehopon as compared to heat-treatedsamples (coupons were heat treated at 160°C for
21 hours to simulate 15 years of ageing).

Full experimental details are available in the BIP
data reports and in the BIP-1 Final Summary
Report [4].
III. POLYMER COUPONS
Epoxy paint is a complex mixture of polymericcompounds. In order to isolate the influence ofvarious functional groups present in epoxy paint,
coupons were prepared from various polymers.
Epoxy Resin: Epoxy resin material is a major
component of epoxy paint. DOW DER 661, a
low molecular weight Di-Glycidyl Ether ofBisphenol-A (DGEBA), was chosen as
representative of the epoxy resins found in epoxy
paint (Fig. 2). A higher melting point epoxy resin(DER 667) was used for the gas-phase iodine
adsorption tests performed at 70°C.

AD-GW-A-1 (un-treated)

Nylon 6/6: Nylon 6/6 is a polyamide consistingof repeating units containing 12 carbon atoms
and 2 amide groups. This material was chosenbecause structurally similar polyamides are acomponent of Amerlock 400 epoxy paint.

Adsorbed Iodine (mol/dm 2)

Fig. 2. Chemical Representation of an Epoxy
Resin Formed from Bisphenol-A andEpicholorhydrin

BIP-1 Test A-3 ( Amerlock)

AD-NYL-A-2
AD-NYL-A-1

AD-EPX2-A-1 (DOW DER 332/TETA)

AD-EPX-A-1 (DOW DER 661)

AD-POLY-A-2

AD-EPX-A-2 (DOW DER 661)
AD-POLY-A-1

0

50

100

150

200

250

300

350

400

Time (h)Fig. 3. Aqueous I2 adsorption and desorption on
polymers and epoxy paint (red arrows indicate
the initiation of the desorption phase).

DER is a registered trademark of the DOW
Chemical Company.

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NEA/CSNI/R(2016)5
The overall adsorption capacity for iodine
adsorbed from a saturated I2 solution was
greatest for epoxy paints with somewhat lessadsorption by nylon, reduced adsorption by theamine-cured epoxy resin (DOW DER
332/TETA) and significantly less by the DOW
DER 661 epoxy resin and polypropylene (Fig. 3).
This result suggests that the nitrogen-containing
functional groups are important for iodine
adsorption on epoxy paint.

BIP 1 Test G-4 (Riplolin)

Mass (g)

BIP 1 Test G-12 (Amerlock)

AD-NYL-G-1

In some cases it appears that the surface iodine
concentration decreases slightly during theadsorption phase. This observation can be caused
by a depletion of the concentration of the reactant(I2) or number of active sites on the coupon
surface, while desorption is occurringconcurrently. For paints, the maximum loadingthat could be achieved was limited by theavailable I2 in solution.

AD-EPX-G-1 AD-POLY-G-1

0

20

40

AD-EPX3-G-1
60

80

100

Time (h)Fig. 4. Iodine Adsorption at 70°C and 70% RH
for Polymers and Epoxy Paint.
The remaining tests in this series studied theeffect of humidity on gas-phase adsorption.
Experiments showed that the effect of humidity
on iodine retention for amine-cured epoxy was
small. The same was the case for polypropylene
and the uncured high molecular weight epoxy
resin (DOW DER 667). Only Nylon 6/6
exhibited a clear effect of humidity during gasphase iodine exposure. The deposition rateincreases with an increase in relative humidity.
Although the iodine adsorption rate wassignificantly less than for paint (~1/10), the effectof water was comparable to that observed forAmerlock (Fig. 5).

V. RESULTS OF GASEOUS IODINEADSORPTION TESTS
The adsorption of iodine onto the various
polymers at the standard 70°C and 70% RH
conditions are shown in Fig. 4, in comparison
with Amerlock and Ripolin paint (BIP-1 G-4, G12). Although the adsorption rate was only ~ 1/5of that observed for paint, Nylon 6/6 was the
only polymer tested with significant gas-phase
iodine adsorption. In fact, the adsorption onto
polypropylene and the epoxy resins was so lowthat it was not possible to achieve the target
loading for the RAD series tests.

BIP-1 Test G-5 (Amerlock)

60% RH

Mass (g)Experiments performed under low temperature,
low humidity conditions (30°C and 30% RH)
yielded extremely low or undetectable adsorption
rates. Iodine adsorption was similar forpolypropylene and Nylon 6/6, and undetectablefor DOW DER 661 epoxy.

40% RH

AD-NYL-G-3 (Nylon)
15% RH
40% RH

15% RH
0

20

40

60

80

60% RH100

Time (h)Fig. 5. Effect of Humidity on I2 Adsorption at
70°C and 15-40-60% RH for Nylon andAmerlock Paint.

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NEA/CSNI/R(2016)5
VI. RESULTS OF IRRADIATION TESTSPolymer Irradiation Tests
The polymer irradiation tests were performed togain insight into which functional groups within
epoxy paint could be important for CH3I
production. Fig. 6 shows CH3I production during
the polymer tests. Because the surface loading
was variable, a plot of the same data normalized
to the initial iodine loading is also useful (Fig. 7).
These plots can be used to help identify which
structures within the paint are important fororganic iodide production.
RAD-NYL-A-1

RAD-NYL-A-2
RAD-NYL-A-3
RAD-EPX-A-1

Concentration of CH 3I (M)RAD-EPX-A-2
RAD-POLY-A-1

RAD-POLY-A-2
RAD-POLY-G-1
RAD-POLY-G-2
RAD-EPX2-A-1
RAD-EPX2-A-2
RAD-NYL-G-1

RAD-NYL-G-2
RAD-NYL-G-4
RAD-EPX3-G-1

0

50

100

150
Time (h)

200

250

300

Fig. 6. CH3I Concentration as a Function of Time
(Polymer tests).
RAD-NYL-A-1

Concentration of CH 3I (M)/Initial Loading (mol/dm 2)

RAD-NYL-A-2
RAD-NYL-A-3
RAD-EPX-A-1
RAD-EPX-A-2
RAD-POLY-A-1
RAD-POLY-A-2
RAD-POLY-G-1
RAD-POLY-G-2
RAD-EPX2-A-1
RAD-EPX2-A-2
RAD-NYL-G-1
RAD-NYL-G-2
RAD-NYL-G-4

RAD-EPX3-G-1

0

50

100

150

200

250

Time (h) Nylon (poly-amide) behaves most like paintin terms of quantity and evolution of CH3I.
The nylon tests show the initial sharp peakoften observed for aqueous-loaded painted
coupons,
and
the
low
steady-state
concentration typical of gas-loaded painted
coupons.
 For aqueous-phase exposure, polypropylene
coupons took much longer to load than othercoupons. This loading behaviour can be
attributed to the chemical structure ofpolypropylene, which is a long chain aliphatic
polymer with no functional groups (but many
methylgroups).
Theaqueous-loaded
polypropylene coupons were among thehighest CH3I producers. In the gas phase, thetarget iodine surface concentration was neverachieved, even after ~70 hours of iodine
exposure. However, when normalized to theinitial iodine surface concentration, the gasloaded polypropylene coupons were thehighest CH3I producer (Fig. 7).
 Epoxy resin is the backbone of the epoxy
paint structure. It contains aromatic rings,
ether linkages, alcohol groups and epoxides atthe chain ends. Aqueous-loaded epoxy
coupons did not show the initial sharp peak in
CH3I evolution characteristic of painted
coupons, and the quantity produced was low.
Gas- loaded epoxy had the lowest CH3I
formation rate of all the polymers, mainly due
to the very low iodine surface concentrationachieved during the loading stage.
 The amine-cured epoxy produced more CH3I
than the epoxy resin, but when normalized tothe iodine surface loading, the CH3I
production is about the same. The aminecured epoxy also showed an early peak in the
CH3I production, although less sharp thanobserved for nylon. Amine-cured epoxy wasformed by mixing liquid epoxy resin with anamine hardener. Therefore, the polymer
contains the functional groups of the epoxy
resin plus amine groups.
The polymer tests suggest that all parts of thecomplex paint structure have the ability toparticipate in both iodine adsorption and CH3I
production. However, some functional groups
may be more important because they more
readily adsorb iodine or because they have a

Fig. 7. CH3I Concentration (Normalized to theInitial Iodine Surface Loading) as a Function ofTime (Polymer tests).
Several observations can be made from Figures 6
and 7.

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NEA/CSNI/R(2016)5
structure that is more favourable for CH3I
formation.

RAD-GW-A-1
RAD-GW-A-2 (Pre-irradiated Coupons)
BIP1 ORG37-A (Amerlock, 25°C)
RAD-EPICUR-A-1 (Ripolin)

CH3I (M)

Perhaps the biggest surprise was the relatively
high CH3I formation from polypropylene. Verylittle adsorption was expected (as there is nofunctional groups with the ability to react withiodine, other than double bonds at the chainends). Polypropylene, however, contains anabundance of methyl groups that may enhance
CH3I production during irradiation.
The results suggest that the initial peak observedwith aqueous-loaded paint is associated with theamide, and perhaps the amine, portions of thepaint polymer.

0

20

40

60

80
Time (h)

100

120

140

Fig. 8. CH3I Production during Tests Performed
with Gehopon Paint in Comparison to Amerlock
and Ripolin (Logarithmic scale).

Paint Irradiation Tests
In BIP-2, several tests on painted coupons were
performed to augment the results of BIP-1. There
were five tasks:

Qualitatively, Gehopon paint behaves similarly
to other epoxy paints, supporting the concept of acommon epoxy-iodine paint model. However,
the high organic iodide formation suggests thatfactors such as paint age, heat treatment and preirradiation may need to be considered within themodel.

1. Investigate the behaviour of Gehopon paintrelative to the other epoxy paints tested in
BIP-1.
RAD-GW-A-1 and RAD-GW-A-2 were
performed with painted coupons prepared inGermany with Gehopon paint. The irradiations
were performed at 80°C and about 60% relative
humidity. RAD-GW-A-1 showed the early peaknormally associated with aqueous-loaded
coupons, with a similar CH3I formation rate asother paints (Fig. 8). RAD-GW-A-2 used preirradiated coupons, and had very high CH3I
production. High CH3I formation was also
observed for pre-irradiated nylon (RAD-NYL-G4), Ripolin (RAD-EPICUR-A-3) and Amerlock(BIP-1) coupons (further discussed in Section
VII).

2. Perform Ripolin irradiations at 80°C toinvestigate the effect of temperature.
The irradiation vessel was modified to allow
irradiations at 80°C, in order to determine if theirradiation temperature affects the CH3I
production. The tests facilitate the comparison of
the BIP tests to the EPICUR (Experimental
Program on Iodine Chemistry Under Irradiation)
tests performed in France. Several tests
performed with Ripolin at 80°C are comparedwith tests performed at 25°C in Fig. 9. The
results do not conclusively show a large effect oftemperature. The results suggest a small increasein CH3I production at the higher temperature andan enhanced CH3I production from the preirradiated coupons.
Although the temperature is expected to affectthe mass transport and diffusion aspects oforganic iodide formation, the results suggest thatit does not affect the fundamental chemistry. The
radiation dose to the paint, which is not affectedby the temperature, is the primary driver ofmethyl iodide production.

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NEA/CSNI/R(2016)5

RAD-EPICUR-A-1
RAD-EPICUR-A-2
RAD-EPICUR-A-3 (pre-irradiated)
RAD-EPICUR-G-1
BIP1 ORG30-A
BIP1 ORG14-A
BIP1 ORG39-A

coupons. Iodine compounds were not detected in
the headspace of any of the vessels containing
the iodine-soaked coupons.

Irradiated at 80°C

Irradiated at 25°C

As it has been shown to greatly affect organic
iodide production, the influence of water may
need to be considered within post-accident iodine
behaviour models because, within containment,
epoxy paint will be submerged or exposed tocondensation and water sprays.

Concentration of CH3I (M)All Ripolin Paint

Aqueous load
Dry in dessicator (140 h)

0

40

80

120

160

Stop Irradiation
Aqueous Reload

Restart Irradiation(Clean Vessel)

200

Time (h)Fig. 9. CH3I Formation during Tests Performed
with Ripolin Coupons at 25 and 80°C
(Logarithmic scale).

ORG44-A (Dry after intial aqueous load)Concentration of CH3I (M)

ORG43-A (Dry after initial aqueous Load)

3. Perform additional tests to understand theeffect of water on CH3I production.
Test ORG43-A included coupons loaded in the
aqueous phase and subsequently dried in a
desiccator. The resulting CH3I production issimilar to that of a gas-loaded coupon (i.e., low
production and no initial peak) and indicates thatwater must be available during irradiation inorder to form the high initial peak, typical of
aqueous-loaded paints (Fig. 10). Test ORG44-A
was similar, but received a second aqueous
loading at 150 h, and was not dried. The resulting
CH3I peak at 150 h is further confirmation that
residual water greatly affects CH3I production
and that the opportunity for a later ‘peak’ is not
influenced by previous drying or irradiation.

0

50

100

150
Time (h)200

250

Fig. 10. CH3I Formation during Painted Coupon
Tests.
The enhanced detection of paint solvents fromwater-soaked coupons offers an explanation andpossible modelling strategy for increased organic
iodide production in aqueous-loaded RAD tests.
However, it must be noted that increased iodine
production was also observed during the nylon
radiation tests. It is not known if residual
solvents or other chemicals are trapped within
the nylon polymer. Future tests performed withsolvent-free paint (pre-leached or thermallytreated) may help determine if residual solvents
are important.

In addition to these two irradiation tests, a shortseries of experiments was performed on somesurplus ~20-year-old Ripolin coupons originallyprepared for Phébus Radioiodine Test Facility
tests. Headspace GCMS (Gas Chromatography
Mass Spectroscopy) analysis of the coupons
heated to 50°C showed two or three large peaks
and many very small peaks. The main peaks inthe chromatogram had mass spectra consistent
with ortho and para-xylene, while other peaks
indicate the presence of toluene, methyl isobutyl
ketone, butyl acetate and other compounds
related to xylene (e.g., mesitylene, ethyl
benzene). Much more release was observed forwater-soaked coupons than dry coupons(irradiated or un-irradiated). The release oftrapped paint solvents may be one reason for theenhanced CH3I production rate in water-soaked

4. Tests using oxidized forms of iodine on paint.
Oxidized forms of iodine will be present incontainment as a product of iodine oxidation inboth the liquid and gas phases. The reaction
between I2(g) and O3(g) is known to produce
aerosols consisting of compounds like I4O9(s)and I2O5(s), which can settle on painted surfaces
within containment. In contact with water, thesolids will dissolve and hydrolyze into
compounds including HIO3.
Experiments RAD-IOX-1 and RAD-IOX-2
explored the possibility that, in contact with160

NEA/CSNI/R(2016)5
paint, these compounds contribute to theformation of organic iodides. For test RAD-IOX1, painted coupons were sprinkled with I2O5(s),while for test RAD-IOX-2, coupons were loaded
by immersion in 10-3 M HIO3. Although low,
CH3I production was observed in both cases,
indicating that I2 need not be the adsorbed iodine
species in order to form methyl iodide. The HIO3
and I2O5 may be reacting directly with the paint,
or may be undergoing conversion to morereactive species during irradiation or byinteraction with residual water.

on CH3I production. For this test, Amerlock
coupons were exposed to 5 ppm NO2(g) in air at
70°C and 70% RH prior to the usual iodine
loading and irradiation. Fig. 11 shows that thisconcentration of NO2 had a negligible effect on
CH3I production at this concentration. No
additional products were detected in the GC
samples taken during the irradiation.
The results of these scoping tests show that a 1 h
pre-exposure to 5 ppm NO2 or Cl2 does not affectthe subsequent iodine paint interactions. These
concentrations were chosen to be approximately
representative of that expected in containment;
however, further study may be required todetermine if these concentrations represent
conservative values.

5. Tests with coupons pre-exposed to Cl2 & NO2.

Concentration of CH3I (M)RAD-CHLOR-G-S was a scoping test toinvestigate how chlorine gas, thought to be
produced during a reactor accident by thedegradation of cable insulation, might affect theinteractions between iodine and paint. For thistest, Amerlock coupons were exposed to 5 ppm
Cl2(g) in air at 70°C and 70% RH for one hourprior to the usual iodine loading and irradiation.
The concentration (1.8×10-7 M Cl2) is an order ofmagnitude larger than the usual iodine-loading
concentration of 10-8 M. Fig. 11 shows that theCH3I production from this test is within thenormal range for gas-loaded coupons. The
chlorine did not have any observable effect on
the coupon or on CH3I production. Methyl
chloride, a potential product of radiolysis
following Cl2 exposure, was not detected by GC
analysis during the test.

Iodine Compound Irradiation Tests
Small quantities of pure, commercially-available,
iodine-containing organic compounds were
irradiated in air to gain insight into what types ofstructures can produce methyl iodide upon
irradiation. Compounds included iodophenol,
iodobenzyl
alcohol,iodoethanoic
acid,
iodoproprionic acid, 1-iodobutane, 2-iodobutane,
2-iodo-2-methylpropane,
iodoacetamide,
iodoethane,
iodoethanol,and(iodomethyl)trimethylammonium iodide.
A small amount of each compound was placed ina small, glass vial equipped with a septum.
During irradiation, gas samples were removed
periodically to determine the methyl iodide
concentration by GC.
It was found that methyl iodide was produced
from all iodine compounds irradiated. The
production rate was generally higher for volatilecompounds, which may be related to the simple
chemical structure of the volatile compounds.
For example, it is easy to rationalize that fewer
steps would be required to make methyl iodide
from an aliphatic compound than an aromaticcompound. These results suggest that gas-phaseradiolysis reactions may contribute to CH3I
production.

Normal Range
for Gas-Loaded
Coupons

RAD-CHLOR-G-S
RAD-NOX-G-S
ORG18-G (Fresh Amerlock)
ORG1-G (Amerlock)
0

50

100

150

200

Time (h)VII. PRE-IRRADIATION

Fig. 11. CH3I production from Amerlock
coupons in the presence and absence of Cl2 or
NO2.

In several tests throughout BIP-1 and BIP-2,
coupons were pre-irradiated prior to iodine
loading. In all cases, the coupons tended to have

RAD-NOX-G-S was a scoping test thatinvestigated the effect of air radiolysis products
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NEA/CSNI/R(2016)5
higher CH3I production, in comparison to tests
performed without pre-irradiation.
 During BIP-1, pre-irradiated Amerlock paintproduced higher than average steady-state
CH3I concentrations.
 AD-GW-A-2 had higher CH3I production
than RAD-GW-A-1.
 RAD-EPICUR-A-3
hadhigher
CH3I
production than RAD-EPICUR-A-1 or RADEPICUR-A-2.
 Pre-irradiated nylon (RAD-NYL-G-4) had
higher CH3I production than samples without
pre-irradiation.
The accumulation of radiation damage to thepolymer enhances the formation rate of CH3I and
may be one reason that the CH3I concentration
increases as dose to the coupon accumulates
during the test.

Fig. 12. SEM Image of the Cross Section of a
Ripolin Paint Layer.
 Thickness of the paint layer is variable overthe surface of the coupon.
 Two-layer compositions can be observed
within the paint (Fig. 12). These layers arethought to correspond to the primer andtopcoat.
 Several metals are detected at low
concentrations, with some present only in theprimer layer (e.g., Mg, Pb).
 Iodine is not simply trapped on the paintsurface, but penetrates more than 100 µm(Fig. 13). Iodine concentration (after exposure
to an I2 solution) decreases as a function of
depth and seems to penetrate only the topcoat.
This depth of penetration may help explain
why a large fraction of iodine does not desorb
during irradiation.

VIII. SEM/EDX Analysis
As part of the work for BIP-2, some experiments
were performed on aged Ripolin coupons (~20years old) that were originally prepared for the
Phébus FP project for tests performed in theRadioiodine Test Facility. Coupons were
exposed to various combinations of water, iodine
and irradiation. Some of the coupons were
examined by Scanning Electron Microscopy(SEM) and Energy Dispersive X-ray (EDX)
spectrometry. Samples were cut using a slowspeed diamond saw (for metallurgical sample
preparation) to expose the cross-sectional area ofthe paint so that the penetration depth of iodine
could be measured. Various observations were
made:Paint Surface

 Paint
appears
porousunderhighmagnification (Fig. 12).
 Elemental EDX data show that paint is mainly
C, O, Si, Ti (nitrogen is difficult to detect).

Topcoat/Primer Boundary

Paint/Steel Boundary

1

Normalized Concentrtion

0.8

0.6

0.4

0.2

0
Depth (um)Fig. 13. Iodine penetration into epoxy paint.

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IX. CONCLUSIONS

Coupon Irradiation Tests
 CH3I production is not limited to a single
bonding site or functional group within thepaint structure. The mechanism for theradiolytic production of CH3I appears to becomplex and cannot be unambiguously
ascribed to any specific functional group in
the polymer structures.
 CH3I production has been observed duringexperiments performed with several different
epoxy paints (Amerlock, Ripolin, Epigrip,
and Gehopon).
 CH3I production has been observed duringexperiments performed with several model
polymers, even when the initial iodine loading
concentration is very low.
 The initial CH3I peak associated withaqueous-loaded paint coupons is clearlyobserved for nylon (a polyamide polymer).
Many epoxy paints contain polyamide
constituents.
 The shape of the methyl iodide evolution
curve for both aqueous and gas-loaded nylon
resembles that observed for epoxy paints.
 The initial CH3I peak associated withaqueous-loaded paint coupons was notobserved during aqueous-loaded epoxy resin
coupon tests. Although all epoxy paints
contain a form of epoxy resin, it appears that
this component is not responsible for theobserved CH3I peak.
 Amine-cured epoxy yields more methyl
iodide than the uncured epoxy resin.
 Residual water following submersion isrequired for the CH3I peak associated withaqueous loading. Drying the coupons after
aqueous loading negates the CH3I peak.
 Soaking promotes the release of paintsolvents (as observed in the GCMS study ofaged Ripolin coupons). This phenomenon is apossible explanation for the high initial
production rate of CH3I observed during
irradiation for aqueous loaded paints.
 Methyl iodide production is slightly larger
when Ripolin coupons are irradiated at 80°C
(relative to room temperature). The
significance of this small increase is notknown.
 Pre-irradiating coupons prior to iodine
loading generally increases the CH3I

The results from the final phase of BIP-2 buildon the observations from BIP-1. Someconclusions derived from BIP-2 (organized bytest series) include:
Gas Phase Adsorption
 Iodine does adsorb onto the selected
polymers, although at slower rates than is
typical for epoxy paints.
 At 70°C and 70% RH, the adsorption rate ofiodine on pure epoxy resin (DOW DER 667)
is similar to polypropylene, while adsorption
on nylon and amine-cured epoxy is
significantly faster.
 Only nylon has similar behaviour to paintwith respect to deposition velocity
dependence on relative humidity (i.e.,
deposition rate increases with increasing
relative humidity).
From this information, we can conclude that,while iodine seems to preferentially associatewith the nitrogen-containing functional groups(i.e., amine, amide), reaction with many parts ofthe complex paint structure are possible. This canhelp explain the differences in sorption
behaviour between various epoxy paints, andmay help explain the observed methyl iodide
formation rates.
Aqueous Phase Adsorption
 All polymers and paints tested adsorb iodine
as I2 from the aqueous phase.
 Iodine adsorption on nylon is fast, but slower
than that on Amerlock paint.
 Amine-cured epoxy polymer adsorbs iodine
more rapidly than the unmodified epoxy resin.
 The iodine adsorption rate for epoxy andpolypropylene are lower than the amine-cured
epoxy polymer and significantly slower thanfor nylon or epoxy paint.
Similar to gas phase adsorption, we can conclude
that, while aqueous iodine seems to preferentiallyassociate with the nitrogen-containing functionalgroups, reaction with many parts of the complex
paint structure are possible. This can help explain
the differences in sorption behaviour between
various epoxy paints.

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NEA/CSNI/R(2016)5
production (observed for Amerlock, Ripolin,
Gehopon and Nylon coupons).
 Gehopon epoxy paint behaves similar to otherepoxy paints but has higher CH3I production.
This observation may be associated with theheat treatment and pre-irradiation.
 Exposure to Cl2 and NO2 at 5 ppm (in air)does not seem to affect organic iodide
production from irradiated coupons.
 CH3I is detected when Amerlock coupons areloaded with oxidized forms of iodine such as
HIO3 and I2O5. These chemical species may
be converted to more reactive forms of iodine
during the irradiation.

be proposed as part of the potential BIP3/STEM-2 project.
ACKNOWLEDGMENTS
We would like to thank the BIP members fortheir financial and intellectual contributions over
the years.
NOMENCLATURE
BIP = Behaviour of Iodine Project
DGEBA = Di-Glycidyl Ether of Bisphenol-A
EDX = Energy Dispersive X-Rays
EPICUR = Experimental Project on Iodine
Chemistry Under Irradiation
GC = Gas Chromatography
GCMS
=
Gas
Chromatography Mass
SpectroscopyOECD = Organisation for Economic Cooperation and Development
SEM = Scanning Electron Microscopy
STEM = Source Term Evaluation and Mitigation
TETA = Triethylenetetramine

These observations suggest that while iodine
may react with various chemical compounds on
and within paint, the resulting organic iodides
can break down under irradiation to form methyl
iodide. The methyl iodide formation does notappear to be a specific reaction associated with aparticular bonding site or functional group, butrather seems to be a common degradation
product during a complex free radical
degradation process.
The results from the coupon irradiation tests canbe used to guide the development of computermodels and provide data for their validation.

THAI = Thermalhydraulics Hydrogen Aerosol
IodineThe data shows that there are similarities in theiodine-paint interaction between the epoxy paints
studied, which allows some optimism for thedevelopment of a generic iodine-paint model.
However, the results also suggest that thetreatment of the paint (e.g., pre-irradiation), anddifferences in the chemical makeup of the paint(i.e., type of hardener) can affect its behaviour
with respect to iodine.

REFERENCES
1. B. Clément, L. Cantrel, G. Ducros, F. Funke,
L. Herranz A. Rydl, G. Weber and J.C. Wren,“State of the Art Report on Iodine
Chemistry”, Nuclear Energy Agency Report,
NEA/CSNI/R(2007)1, (2007).
2. J.C. Wren, J.M. Ball, G.A. Glowa, “TheChemistry of Iodine in Containment”, Nucl.
Tech., 129, 297 (2000).

While the BIP-2 work has progressed ourunderstanding of the iodine-paint interactions,some areas for further study have been identified
including expanding our knowledge of:

3. J.C. Wren, J. Paquette, D.J. Wren, G.G
Sanipelli, “The Formation and Volatility ofOrganic Iodides”, Proc. of the Specialists’Workshop on Iodine Chemistry in Reactor
Safety, UK Atomic Energy Authority,
Harwell, England, p. 333 (1985).

 Various paint “ageing” mechanisms;
 Reactants that compete for active sites on thepaint;
 Formation and destruction of methyl iodide in
the gas phase.

4. G.A. Glowa and C.J. Moore, “NEA
Behaviour of Iodine Project Final Summary
Report” Nuclear Energy Agency Report
NEA/CNSNI/R(2011)11(2012).

These areas will be discussed in a separate paper(Yakabuskie et al., in these proceedings) and will

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NEA/CSNI/R(2016)5
THE BEHAVIOUR OF IODINE PROJECT: A PROPOSAL FOR BIP-3
P.A. Yakabuskie*, C.J. Moore, G.A. Glowa
Canadian Nuclear Laboratories, Chalk River, Canada*Corresponding author, tel: (+1) 613-584-3311 (ext. 43146), Pam.Yakabuskie@cnl.ca
Abstract – Continued improvement of existing computer models for predicting post-accident fission
product behaviour in containment relies on a comprehensive understanding of all relevant phenomena andhigh-confidence data. Unexpected iodine behaviour observed during the international Phébus FP program
highlighted the need for a greater understanding of iodine behaviour. Recently, there have been several
NEA initiatives to study the behaviour of iodine in containment: the THAI (Thermalhydraulics, Hydrogen,Aerosol, Iodine) project, the STEM (Source Term Evaluation and Mitigation) project and the Behaviour of
Iodine Projects (BIP-1 and BIP-2). Despite considerable advancements in our understanding of this topic
over the past several decades, there are still outstanding gaps in our knowledge that warrant further
investigation.
Several topics of interest for a potential third phase of the NEA Behaviour of Iodine Project (BIP-3) wererecently assembled and a poll of the BIP-2 Programme Review Group was conducted to gauge the relativeinterest in each area. The poll results provided a prioritized ranking of research topics that has helped
inform the proposed BIP-3 research program. While the preliminary topics were not limited to studies ofiodine behaviour, the proposed research areas for BIP-3 are complementary to the completed BIP-1 andBIP-2 programs, and focus mainly on remaining questions surrounding the iodine-paint interaction.
In particular, we intend to further explore the effects of paint ageing and pre-irradiation, total dose anddose rate, humidity and wetting of painted surfaces and the presence of competing species (e.g., NO x, Cl2)
on adsorption and desorption processes. Radiolytic studies relating to CH3I production and degradation in
the gas phase are also recommended. An outline for the proposed BIP-3 program will be discussed indetail.

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I. INTRODUCTION

and unresolved topics, such as the stability ofiodine-containing aerosols, are better handledwithin the OECD-STEM project.

The NEA Behaviour of Iodine Project (BIP) was
created to investigate the interactions between
iodine and paint. The adsorption of iodine onto
epoxy paint and the subsequent radiolytic
production and release of organic iodides frompainted containment surfaces were of particularinterest. A database of iodine deposition data was
accumulated in BIP-1 and expanded in BIP-2.
Additionally, BIP-2 probed the mechanisms of
iodine-paint interactions, and improved ourunderstanding of the nature of iodine bonding on
paint. Key results from the first two phases of the
BIP are available [1, 2]. (See paper by Glowa etal., in these proceedings [1]).

II. PROPOSED TOPICS
Paint is an important iodine sink in containment;
therefore, kinetic data that describes the iodinepaint interaction is needed to simulate postaccident iodine behaviour. Thus far, BIP has
provided deposition velocities for a variety of
epoxy paints under various conditions. The
topics suggested for BIP-3 are related to theiodine-paint interaction, specifically sorption
behaviour and the formation of organic iodides.
Closely related to these topics are the effects ofpaint ageing, residual water and the competitiveeffect of reactive species such as Cl2 and NOx.

In addition to BIP, several other NEA initiatives
have contributed to our understanding of iodine
behaviour in containment, including the THAI
(Thermalhydraulics, Hydrogen, Aerosol, Iodine)and the STEM (Source Term Evaluation and
Mitigation) projects. The development of acomputational model that can universally
reproduce the experimental observations fromeach of these projects is the primary long-termgoal. However, many key questions surrounding
iodine behaviour remain unresolved, andadditional data is required to improve ourmodeling capabilities [3].

This section will introduce the proposed study
topics that received the highest ranking from the
BIP-2 program review group, and their rationale.
Iodine Desorption Behaviour
In the absence of irradiation, gas phase iodine
desorption from epoxy paint has been observed
under the conditions of both BIP and STEM tests(25 to 80 °C).

Canadian Nuclear Laboratories (CNL) is
proposing a 3-year project (BIP-3) to build upon
the progress made to date in the Behaviour of
Iodine Projects. The size and funding structure
will be similar to BIP-2, which was completed in
September 2014. The project scope will address
outstanding modeling issues and provide an
opportunity to build upon work initiated in BIP2.

A typical gas phase adsorption and desorption
profile (I2 loading concentration of 10-8 M) is
shown in Fig. 1. It is also clear from STEM tests
that the release of both inorganic and organic
forms of iodine is promoted by irradiation.
In BIP-3, we propose that the relationshipbetween iodine loading concentration andfraction desorbed (without irradiation) be
systematically determined. This data is requiredin order to properly quantify the radiolytic
release process and improve the interpretation of
results from BIP and STEM tests.

At the conclusion of the BIP-2 programme, aselection of topics for inclusion in the BIP-3
program was prioritized by the group members.
The proposed topics of study are required tofurther improve the understanding of postaccident iodine behaviour in nuclear reactor
containment buildings with a particular focus on
iodine adsorption, desorption and organic iodine
generation from interaction with paints.
Topics for BIP-3 have been chosen to makeeffective use of our existing facilities andexpertise. It should be noted that other important
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2.0E-033.0E-05

2.0E-05
Mass (g)Mass (g)1.5E-03

1.0E-03

1.0E-05

Adsorption Phase

5.0E-04

Adsorption Phase

Desorption Phase

Desorption Phase

0.0E+00
0

5

10

15

20

25

30

35

40

0.0E+0045

020Time (h)

40

60

80

100

120

140

160

180

Time (h)Fig.1. Iodine adsorption and desorption on aged
Amerlock paint at 30 °C and 80% RH (BIP-1
Test G-19).

Fig. 2. Iodine adsorption and desorption on
fresh Amerlock paint at 70 °C and 70% RH(BIP-1 Test G-12).

Adsorption Capacity

Iodine Retention during Long Irradiations

Although saturation of paint with iodine is not
anticipated on post-accident containment
surfaces, it is useful to know the adsorption
capacity of paint as an input for modeling. The
adsorption capacity is a fundamental parameterthat defines the number of available active sites.
Adsorption measurements can be performed
using the iodine adsorption apparatus (gasphase) and using repetitive aqueous loadings.
Iodine will be adsorbed for various lengths oftime (some long term), followed by desorption.
By altering the temperature, humidity and gas
concentration (both higher and lower thanprevious tests) the capacity (and initialdeposition velocity) can be measured over awide range of conditions. An example of along-term gas phase exposure test (~100 hours)
is shown in Fig. 2 (I2 loading concentration of
10-8 M).

For modelling purposes, it is useful to determine
how much iodine is retained on paint at longirradiation times. This fraction could affect thedesign of an iodine model; any iodine that ispermanently fixed will affect simulation results,
especially for accident scenarios lasting manyweeks. Long-duration irradiations will be
performed on iodine-loaded painted coupons todetermine if some iodide is permanently retained.
It is not known if the rate of iodine loss from anirradiated paint surface is linear, exponential or if
a fraction is permanently bound. This information
can affect the simulated gas phase organic iodide
concentration and the design of any model tosimulate this phenomenon. Thus far, BIP tests
have been limited to ~150-200 h (at 1-4 kGy/h).
We propose increasing the total dose to the extent
practical.
Paint Ageing
Most members of the BIP-2 programme review
group are interested in studying the effects ofpaint ageing on sorption and organic iodide
production. The effects of ageing, both in-service
ageing and ageing processes that occur during
reactor accidents, can significantly impact theinteractions of iodine with paint [3]. Thermal,
chemical and radiation- induced changes to thepaint can influence:
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NEA/CSNI/R(2016)5
 the quantity and accessibility of residual
solvents for reaction,
 the number of available active sites for surface
chemical reactions, oxidation of some functional groups on theouter surfaces, and mechanical paint degradation (chalking,checking, cracking, flaking, blistering or
delamination).

work that the iodine adsorption rate decreases forpre-irradiated specimens at doses approaching 1
MGy [6]. To date, our results do not suggest that
adsorption of iodine on paint will decrease withdose. However, measurements are required atdoses higher than in previous BIP studies todetermine if adsorption is reduced above somecritical dose.
Paint Ageing – Effect on Residual Solvents

To date many iodine experiments have been
performed with relatively fresh paint (e.g.,
Radioiodine Test Facility (RTF), BIP and STEM
experiments). Some experiments have utilized
thermal-curing processes to accelerate ageing,
but it is not definitively known how well thissimulates the ageing of in-service paint. The
chemistry of thermally-cured paint may bedifferent from that of more naturally aged paint.
However, it should be noted that, in the initialstages of some accident cases, the containment
paint may be subjected to high temperatures.

Residual solvents trapped in the paint matrix
have been implicated as a source of reactants forthe production of organic iodides during paintirradiations. Curing time (ageing) influences theavailability of solvents that can be leached fromthe paint surface during exposure to liquid water.
For vinyl-painted coupons exposed to water,
Wren et al. [7] found that the release of MIBK
(methyl isobutyl ketone), a major paint thinnerconstituent, was reduced as a result of ageing(Fig. 3). Proposed tests will compare the amountof releasable solvents from new, aged andpre-irradiated epoxy paint to determine the effectof these treatments on the release of residual
solvents. Solvent release as a function ofsubmersion time can be investigated.

Paint Ageing – Effect on Sorption Behaviour andCH3I Production

4
3.5

-4

Conc. x10 (molarity)

Paint ageing standards [4, 5] may contain usefulprocedures for simulating the mechanical aspectsof paint ageing, but may not accurately simulate
the chemical changes to paint during routine
operation. Alternatively, artificial ageing
techniques using thermal treatment arecommonly used to accelerate the degradation
processes associated with long-term service.
Previous BIP studies investigating the effects ofpaint ageing have used painted coupons that had
simply aged for extended periods in a laboratory
setting.

3
2.5
2
1.5
1
0.5
0
10

100

1000

10000

Days

We propose tests with new and aged paints togain a further understanding of the effects ofageing on iodine adsorption and CH3I production.
The effect of thermal and laboratory ageing
techniques will be compared.

Fig. 3. The final steady-state concentration of
MIBK released from vinyl-painted coupons as
a function of age [7].
CH3I Radiolysis

Paint Ageing – Effect of Pre-irradiation on
Sorption Behaviour

When organic iodides such as CH3I are released
from irradiated paint, they are subject todegradation by reaction with air radiolysis
products. This degradation strongly influences
the accumulation of organic iodides in the gas

We propose that the effect of irradiation on
iodine adsorption behaviour be determined athigh doses. There are some indications in earlier
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phase, and must, therefore, be consideredwithin iodine behaviour models. Kinetic data
(i.e., radiolytic destruction rate) measured in
BIP-1 agrees with other literature values under
the conditions tested. Fig. 4 shows typicalresults for a multiple injection test to quantify
the rate of gas phase methyl iodide degradation(BIP-1, Test ORG32-M).

In BIP-3 we propose to investigate the processby irradiating dilute mixtures of I2 and CH4 in
humid air. The detection of small
concentrations (~10-12 M) of methyl iodide ispossible using our gas chromatograph system.

It has been hypothesized that water vapor canchange the concentrations of air radiolysis
products such that the degradation rate of gas
phase CH3I is decreased. Because accident
scenarios generally involve the release of
steam, we propose that the effect of humidity(and perhaps other added impurities) be further
studied in BIP-3. The quantification of thisprocess under various conditions will help
extract organic iodide formation rates fromearlier BIP and RTF experiments.

In BIP-2, exploratory tests were performed to
study how reactive containment species such as
Cl2 (from cable pyrolysis), and NO2 (from air
radiolysis) affects the iodine-paint interaction.
In these tests, Amerlock coupons were exposed
to 5 ppm Cl2(g) or NO2(g) in air at 70 °C and
70% RH for one hour prior to the usual iodine
loading and irradiation. The results showed nosignificant change to iodine sorption or CH3I
formation.

1.5E-08

Competition with Reactive Containment Gas
Species (Cl2, NOx)

In BIP-3, we propose tests with higher
concentrations of competing species (up to
1000 times the concentration of iodine). Postaccident containment could contain iodine at amuch smaller concentration than other gas
phase impurities. These competing species may
occupy available sites on the paint surface,
reducing the sites available for iodine
adsorption. The influence of competing species
is critical to the modeling of iodine behaviour
under realistic accident conditions.

Re-injection of methyl iodide

Concentration (M)1.0E-08

5.0E-09

Effect of Wet/Dry Cycling
0.0E+00
0

50

100

150

200

250

300

350

400

Time (minutes)

Fig 4. CH3I concentration as a function of time
during irradiation.
CH3I Formation in the Gas Phase
The possibility that organic iodides are formedin the gas phase is generally dismissed due tothe large concentration of O2 relative to I2 (i.e.,
the O2 outcompetes the I2 for any organic
radicals formed). However, work by Bartoníček
and Habersbergerová [8, 9] shows that methyl
iodide formation is detected in irradiatedmixtures of air, methane and I2.

BIP-1 and -2 showed that the method used forloading iodine on paint affected the formationof methyl iodide during irradiation. Painted
coupons loaded by submersion in iodine
solutions had higher initial CH3I production
compared to coupons loaded by exposure to gas
phase I2.
Further tests in BIP-2 showed that the CH3I
production arising from coupons loaded byexposure to an aqueous iodine solution, but
subsequently dried, more closely resembled thatof coupons loaded by exposure to gas phase I2.
The CH3I concentration profiles during BIP-2
tests ORG43-A and ORG44-A are similar tothat observed for gas-loaded coupons (i.e., low
production and no initial peak), Fig. 5.

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NEA/CSNI/R(2016)5
NOMENCLATURE

However, after a second aqueous phase iodine
loading in BIP-2 Test ORG-44, the high initial
peak in CH3I concentration, typical of aqueousloaded paints, was observed. This indicates thatwater must be available during irradiation inorder to form the peak.

BIP = Behaviour of Iodine Project
CNL = Canadian Nuclear Laboratories
FP = Fission Product
MIBK = Methyl Isobutyl Ketone

Aqueous load
Dry in dessicator (140 h)

Stop Irradiation
Aqueous Reload

Restart Irradiation(Clean Vessel)

NEA = Nuclear Energy Agency
OECD = Organisation for Economic Cooperation and Development

ORG44-A (Dry after intial aqueous load)

RTF = Radioiodine Test Facility

Concentration of CH3I (M)

ORG43-A (Dry after initial aqueous Load)

STEM = Source Term Evaluation and Mitigation
THAI = Thermalhydraulics, Hydrogen, Aerosol,
IodineREFERENCES
0

50

100

150
Time (h)200

1. G.A. Glowa and C.J. Moore, “NEA
Behaviour of Iodine Project Final Summary
Report,” Nuclear Energy Agency Report,
NEA/CNSNI/R(2011)11 (2012).

250

Fig. 5. CH3I formation during painted coupon
tests.

2. G.A. Glowa, C.J. Moore and D. Boulianne,“The Main Outcomes of the OECD
Behaviour of Iodine Project”, This
Conference, Paper 1.8 (2015).

To complete the series, we propose to wet
coupons that were loaded by exposure to gas
phase I2. This case is intended to be
representative of containment walls that arewetted by sprays or condensation.

3. L. Bosland, S. Dickinson, G.A. Glowa, L.E.
Herranz, H.C. Kim, D.A. Powers, M. Salay
and S. Tietze, “Iodine-Paint Interactions
during Nuclear Reactor Severe Accidents,”
Annals of Nuclear Energy, 74, 184 (2014).

III. CONCLUSIONS

4. AFNOR, “Peintures pour l’industrienucléaire: essai de tenue à des conditions
accidentelles de reference (réacteur à eau
sous pression) et de répartabilité”.
Association Française de Normalisation,
AFNOR NF T30–900, ISSN 0335–3931,(1996).

A new 3-year NEA project (BIP-3) is being
proposed to build upon the progress made inprevious Behaviour of Iodine Projects. The
proposed topics for study address important andunresolved topics related to iodine-paintinteraction as prioritized by the BIP-2 program
review group. It should be noted, however, thatthe program is not intended to be static; the test
matrix will evolve based on test results. A portionof the test matrix will be unpopulated to allow
new ideas to be added during the project life.

5. ASTM, “Standard test method for evaluating
coatings used in light-water nuclear power
plants at simulated design basis accident(DBA) conditions”. American Society for
Testing and Materials International, ASTMD3911-08, West Conshohocken, PA. (2008).

ACKNOWLEDGMENTS
Thanks to BIP members for their support andguidance during BIP-1 and BIP-2 and for theirranking and comments regarding the proposed
BIP-3 program.

6. A.B. Aleksandrov, N.I. Ampelogova, M.A.
Karaseva, V.G. Kritskii and N.G. Petrik,“The effect of physicochemical conditions on
the absorption of molecular iodine by170

NEA/CSNI/R(2016)5
protective varnish and paint coatings during
accidents at a nuclear power station with a
VVER-640 reactor,” Therm. Eng., 42, 12,991-996 (1995).

8. B. Bartoníček and A. Habersbergerová,“Investigation of the Formation Possibilities
of Alkyl Iodides in Nuclear Power Plants,”
Radiat. Phys. Chem., 28, 5, 591 (1986).

7. J.C. Wren, D.J. Jobe, G.G. Sanipelli, andJ.M. Ball, “Dissolution of Organic
Compounds from Vinyl, Epoxy andPolyurethane Coated Surfaces,” Nucl.
Technol., 125, 991-996 (1999).

9. B. Bartoníček and A. Habersbergerová,“Formation of Methyl Iodide by Ionizing
Radiation,” Proc. of the 4th Working Meetingon Radiation Interactions, p. 49, Leipzig,
Germany (1987).

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OECD/STEM PROJECT AND ITS FOLLOW-UP STEM2
Christian MUN(1)*, Loïc BOSLAND(1),Laurent CANTREL(1), Juliette COLOMBANI(1), Olivia LEROY(1),Marie-Noelle OHNET(1), Thierry ALBIOL(1)(1) Institut de Radioprotection et de Sûreté Nucléaire (IRSN), PSN-RES, St Paul lez Durance, 13115, France
Corresponding author, Tel: (+33)4.42.19.96.24, christian.mun@irsn.fr
Abstract –The STEM (Source Term Evaluation and Mitigation) OECD project operated by IRSN, hasbeen launched mid-2011 in order to gain confidence into the Source Term (ST) evaluation and to reduce
uncertainties on specific phenomena dealing with the chemistry of two major fission products: iodine andruthenium, in the event of a severe accident (SA) on a nuclear power plant. These data are also useful todesign mitigation means like FCVS or others and also to evaluate the gain of potential deployed mitigationmeans. More precisely, the STEM project consists in providing additional knowledge and improvements ofcalculation tools in order to allow a more robust diagnosis and prognosis of the progression of a SA, and abetter evaluation of potential releases of radioactive materials. Initially, two phases were defined at theexpert meeting in October 2010: the first one from mid-2011 to mid-2015 and the second one (STEM2) up
to mid-2018. The first phase addressed three main issues: i/ middle-term iodine releases with specificattention to the chemical stability of iodine aerosol particles under radiation (transformation into gaseousiodine species), ii/ short and medium-term iodine-paint interactions under irradiation, iii/ rutheniumtransport chemistry in order to determine the speciation of Ru, in particular the partition between gaseousand condensed forms, during its transport through the Reactor Cooling System (RCS). This paper presents
the main outcomes of the STEM first phase. Together with recent results of other R&D programs in thefield, notably the OECD/BIP2, STEM results have helped to define more precisely major remaining issues
that are proposed to be investigated during the second phase of the project. The paper provides a
description of the updated proposal for the second phase of the project that will be conducted from mid2015 to mid-2019.

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I. INTRODUCTION

affecting ST evaluations. It should be noted thatsome of the identified issues, such as for example
the effect of pre-irradiation of paints on iodine
paint adsorption behaviour, the CH3I formationin the gaseous phase, or the competition withreactive containment gaseous species (Cl2, NOx),
are proposed to be studied in the OECD-BIP3
proposal [2], which is complementary to the
STEM2 proposal.

The STEM (Source Term Evaluation and
Mitigation) OECD project [1] operated by the“Institut de Radioprotection et de Sûreté
Nucléaire” (IRSN), has been launched mid-2011
in order to improve the evaluation of Source
Term (ST) for a severe accident (SA) on a
nuclear power plant and to reduce uncertainties
on specific phenomena dealing with thechemistry of two major fission products: iodine
and ruthenium. More precisely, the STEM
project consists in providing additional
knowledge and improvements for calculation
tools in order to allow a more robust diagnosis
and prognosis of radioactive releases in a SA.
Initially, two phases were defined at the expert
meeting held in October 2010: the first one frommid-2011 till mid-2015 and the second one(STEM2) till mid-2018. The first phaseaddressed three main issues: i/ medium-term
iodine releases with specific attention to thechemical stability of iodine aerosol particles
under radiation (radiation induced decomposition
producing gaseous iodine species), ii/ short andmedium-term iodine-paint interactions underirradiation, iii/ ruthenium transport chemistry inorder to determine the speciation of Ru, inparticular the partition between gaseous andcondensed forms, during its transport through theReactor Cooling System (RCS). In the past four
years, kinetics of iodine release from paints andaerosol decomposition kinetics have been
quantified. New phenomena have also been
identified whose influence on iodine volatility
could be significant. Concerning the ruthenium
part, the effect of the oxygen potential on thedeposits formation has been assessed, as well as
the kinetics of Ru transport after vaporizationand kinetics of revaporization from the deposits
in a thermal gradient tube. More precisely, after
presenting the main outcomes of the STEM firstphase, this paper gives insights on the main
issues that are proposed to be investigated during
the updated second phase, i.e. from mid-2015 tomid-2019.

Let us note that in addition to STEM, several
other NEA initiatives have contributed recently
to the understanding of iodine behaviour in thecontainment building, including the THAI2(Thermalhydraulics, Hydrogen, Aerosol, Iodine)and the BIP/BIP2 (Behaviour of Iodine Project)
projects [19] [20]. The duration and the cost of
STEM2 are similar to that of STEM1. A merging
of the STEM2 and BIP3 proposals in order toprovide in a single project offering a more
complete coverage of ST remaining issues isunder discussion. If a single project is finallyproposed, two operating agents, IRSN and CNL,
would operate the project.
II. MAIN OUTCOMES OF STEM/IODINEPARTThe STEM/IODINE experiments, carried out inthe EPICUR facility, were focused on thereleases of volatile molecular iodine (I2) andorganic iodides (RI) from representative painted
coupons loaded with molecular iodine [3] oriodine aerosol species (CsI/IOx) and placed in
the gaseous phase of an irradiation vessel for ≥30 hours to evaluate medium term releases. Six
tests were performed to study the releases ofiodine from epoxy painted coupons loaded withI2. Seven tests were performed to study thereleases of iodine from CsI aerosols deposited on
epoxy painted coupons, but also deposited onquartz and stainless steel coupons to make easierthe data interpretation by preventing any
interaction with paint. One test was performed
with IOx deposited on quartz and three more arestill to be performed. For CsI, the effect of theparameters of main interest (temperature, relative
humidity, initial concentration of iodine
deposited on the sample surface) on the releasesof I2 and RI (and on the global volatilization,
defined as the difference between the initialquantity deposited on the coupon before
irradiation and the quantity of iodine remaining

Based on recent R&D results in the ST area,
notably those of the OECD/BIP2 and STEM
programs, topics for STEM2 have been
identified, proposed and ranked by the STEM
partners to fill main remaining knowledge gaps
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NEA/CSNI/R(2016)5on the coupon after irradiation) are presented anddiscussed below. Further details are given in [4].

The trend towards an increase in the RI fraction
released at lower initial iodine surface
concentration on the coupon would suggest that alarger fraction of iodine is bounded to thechemical sites responsible for RI release when
the surface concentration is lower. This in turn
indicates a preferential interaction of iodine withthese sites, rather than with reversible sites as
discussed above. Saturation of these preferredsites could be reached after a certain amount of
iodine has deposited on the coupon. Further
adsorption would then occur on other chemicalsites identified above, leading to a relative
decrease of the RI release. When the relative
humidity increased from 20 to 60%, the organic
iodide release and the global volatilization,
mainly due to I2 release, decreased moderately.
This may be explained as follows: steam maypolarize iodine and ease its interaction with polargroups of paint (chemisorption) which could
therefore lead to lower releases. When thetemperature increased from 80°C to 120°C, theglobal volatilization and the molecular iodine
release decreased significantly; whereas theorganic iodide release was not significantly
affected. A possible explanation is that iodineinteraction with paint is enhanced with a higher
temperature, favouring chemical adsorption
reactions and leading to a decrease in themolecular iodine release at higher temperature[5].

II.1. Releases from painted coupons loaded with
I2
Figure 1 shows the general trend of the releasekinetics under radiation of I2 (blue curve) and RI(red curve) from epoxy painted coupons preloaded with I2. Two phases are clearly identified:
one at short term for which fast release kinetics isobserved and one at mid-term (> 10 hours) withslower kinetics.

Fig. 1: General trend of the RI and I2 releases
under radiolysis.
The study of initial iodine surface concentration
onto the coupon on the releases showed adifferent impact for I2 and RI releases. Indeed,
the organic iodides release was rather low (<5 %)with small variations (close to a factor 2,
depending on the initial concentration), whereas
the percentage of molecular iodine released andthe global volatilization increased significantly
when the initial iodine surface concentration on
the coupon increased [3]. These results may be
explained taking into account the moleculariodine interaction with the Epoxy paint during
the loading phase. Different chemical sites are
suspected for iodine-paint interaction [5]:II.2 Releases from coupons loaded with CsI
under radiation
It is recalled that the importance of this process
was suspected following analysis of the
PHEBUS-FP tests but was not quantified up tonow; that is why tests investigating effects ofirradiation on deposited CsI were performed in
STEM. The results showed that the irradiation ofaerosols of CsI deposited on a quartz or astainless steel coupon led to an almost rapid andtotal volatilization of iodine species whereas theirradiation of aerosols of CsI deposited on apainted coupon (epoxy paint) led to a partial
volatilization. This difference of behavior may be
due to different interaction processes between
CsI aerosols and the surface of coupons ofdifferent nature (epoxy paint or quartz or
stainless steel). The general trend of the I2 and RI- chemical sites responsible for the I2 releaseduring the irradiation phase (the reactions orinteractions between iodine and paint are
reversible),- chemical sites responsible for the RI releaseduring the irradiation phase (the reactions orinteractions between iodine and paint are notreversible, and lead to the formation of RI).

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releases showed only one release kinetics foreach species whatever the nature of the coupon.
The study of the impact of the CsI concentration
loaded on the epoxy painted coupon showed thata decrease of the CsI concentration by a factor 24
and/or of the CsI particles size on the epoxy
painted coupon leads to a decrease of the releasesof iodine species by a factor of 3. An important
decrease of the fraction of molecular iodine
volatilized and a more moderate decrease of thefraction of organic iodides volatilized wereobserved. This decrease of the volatilization ofiodine species could be due to the interactions
between CsI aerosols and epoxy paint and/or by
the adsorption of gaseous I2 on the painted
coupon once formed near the surface by CsI
decomposition. The increase of relative humidityfrom 20% to 60% promoted molecular iodine
release from CsI deposited on a quartz coupon
whereas the increase of the temperature from
80°C to 120°C did not affect significantly thefraction of molecular iodine released from astainless steel coupon; whatever the studied
boundary conditions, CsI is totally oxidized intoI2(g) in a few hours.

[8], a series of vaporization and re-vaporization
experiments have been performed in the START
test facility (see Fig. 2). Ru is vaporized from a
crucible in a furnace at 1200°C, and thentransported through a thermal gradient quartz
tube, leading to an outlet temperature of 150°C to
250°C depending on the tests. The main studied
parameters were the carrier gas which was asteam-air mixture at different ratios, the type ofthermal gradient (“abrupt” or “smooth”) in thetube, and finally the tube material.
Quartz tube

Furnace
1200°C max
RuO2(c)Argon
SG

150°C/250°C
Aerosol
Filter

1.20m/0.70m

Furnace
150°C

Injection lines :Steam + air

Liquid traps
NaOH 1M

Fig. 2: Scheme of the START test facility.
After a detailed characterization of the heatingphase with argon flow, the Ru vaporization
kinetics in the furnace with oxidizing conditions
was characterized for different carrier gas
mixtures (H2O/air) and can be considered linear
and steam content dependent. The dry air
condition is the most favourable condition for thevaporization of RuO2 from the crucible at
1200°C. A good reproducibility was obtained forthe vaporization test with steam/air gaseous
mixture and for one hour duration.

II.3 Releases from coupons loaded with IOx
Air radiolysis leads to the formation of air
radiolytic products like NO2, O3 or HNO3 [6] aswell as shorter-lived radical intermediates thatcan oxidize iodine and lead to iodine oxides(IOx) particles formation, that sediment onto
surfaces [7]. IOx are small aerosol particles,
whose composition and chemical behavior,particularly under irradiation; are not wellknown. A specific device to generate this kind ofaerosols has been designed by IRSN and thenexperiments could be performed in the EPICUR
facility. The results have shown that IOx
particles partly decompose under irradiation.
Further investigations are thus necessary to better
understand this potential additional source ofvolatile iodine species as their speciation andbehaviour might be modified by the conditions(temperature, humidity and dose rate) over theirradiation time.

During the vaporization study with an “abrupt”
thermal profile, the long duration tests (7 hours)
have shown the existence of transient phenomenaduring the 1st hour, confirming thus the necessity
to study the ruthenium chemistry for several
hours to catch the precise phenomenology.
Whatever the gaseous mixture, most of thevaporized Ru is deposited in the quartz tube. The
total Ru transported downstream of this tube (gasand aerosols, including Ru deposited on thedipping tube located between the tube and theliquid traps) represents a few percent of the Ru
mass vaporized for long duration tests, and ismainly under gaseous form, which confirms thepotential important impact of this process on Ru
release. Concerning the vaporization tests, nosignificant effect of steam could be evidenced on
the transported Ru fractions to the liquid traps;
however, the total Ru amount transported islarger in the presence of steam. Moreover, thisIII. MAIN OUTCOMES OF STEM/RU PARTIn the frame of the STEM/RUTHENIUM
program aimed at ruthenium transport studies
175NEA/CSNI/R(2016)5
study allowed the measurement of the deposit
profiles depending on the wall temperature anddid not reveal any effect of the gas mixture (withor without steam) on the Ru deposit profile in thequartz tube.
Two-thirds of the tests have been performed witha re-vaporization phase, consisting of sweeping
the steam-air mixture into the tube containing Ru
deposits (without generating ruthenium from the
crucible). Contrary to the vaporization, notransient phenomenon was observed during thisre-vaporization phase. Whatever the gaseous
mixture, the Ru amount deposited in the quartz
tube after re-vaporization tests represents always
the major part of the initial inventory. Contrary
to the vaporization tests, two peaks of Ru deposit
were observed. The first peak at high temperature
was detected as in the vaporization tests (~850°C) and the second peak was observed in thelower range of temperature ~ 450°C. In addition,
this study highlighted the importance of thegaseous mixture composition during the Ru
deposit formation and consequently on the revaporization phase. The amount of Ru
transported during the re-vaporization phase islower than in the direct vaporization phase;nevertheless, the speciation is a crucial issue as
in this last case the ruthenium is only transported
under gaseous form, and potentially during aquite long period (which remains to be defined
more precisely). Contrary to the vaporization
tests, there is a gaseous composition effect on the
Ru transported during the re-vaporization tests: it
is favoured by air rich mixtures. The Ru
experimental mass balances for all the START
tests are quite well estimated (about 80%).
Finally, the total Ru transported (gas andaerosols) at the tube outlet after vaporization andre-vaporization in long duration experiments
amounted to several percent of the initial
inventory, in the conditions studied during thisfirst STEM phase.

and radiolytic decomposition of gaseous anddeposited iodine oxides aerosols, iii/. Radiolytic
conversion of gaseous I2 into CH3I, iv/.
Interaction of I2 with steel and aerosols. Despite
these recent significant progresses obtained orvalidated with the results of the first phase of the
STEM program, knowledge of some processes is
not sufficient to quantify their impact on iodine
ST estimations; that is why in the frame of the
STEM follow-up project it is proposed toinvestigate further the specific issues describedhereafter.
-

Paint ageing
For paints, natural ageing in normal operating
conditions as well as their change during a
nuclear accident (under irradiation, hightemperature and humidity rate) might modify the
paint capacity to trap iodine and/or produce RI.
This could lead to a modified iodine volatility.
The study of Aleksandrov [11] indicates that preirradiating anEpoxy paint between 0.1kGy and 1
MGy before adsorbing iodine leads to an I2
adsorption kinetics that is decreased by one orderof magnitude. Except this study, no quantitative
information is available in the literature although
the BIP and BIP2 programs [19] [20] havementioned the same tendency for iodine
adsorption. One can also assume a potential
effect on the release kinetics as suggested by
BIP2 program [20]: CH3I release was found to besignificantly increased in the short term whereas
no data was published for I2. That is why IRSN
proposed to perform some experiments with theobjective to verify if high doses (> 100 – 1000kGy) might lead to significant chemical
modifications in the paint, and to assess to what
extent the iodine releases kinetics (I2 and CH3I)
could be modified by the dose received by the
paint before and all along the accident.

-

Iodine oxides radiolytic decomposition
As it has been shown that IOx particles partly
decompose under irradiation and might lead togaseous molecular iodine, a more complete
understanding of their radiolytic stability isnecessary. Moreover, due to the lack ofknowledge on the precise composition andevolution of IxOy species (which mightsignificantly impact their behaviour), a more
analytical
study,
basedonsurface

IV. PROPOSED TOPICS FOR STEM2
IV.1. STEM2/IODINE topics
As clearly indicated in [9], the modeling of key
phenomena concerning iodine chemistry has
been recently updated or added in the ASTEC
code [10], i.e. mainly: i/. Interaction of I2 andCH3I with paint under irradiation, ii/. Formation
176

NEA/CSNI/R(2016)5
characterizations analysis, is also needed in order
to better estimate the size and composition ofsuch aerosols under different conditions. IRSN
has thus proposed to study this decomposition
phenomenon with a mixed approach:volatility before settling or once deposited onto
surfaces. After having evidenced during the firstphase of STEM the release kinetics of iodine
from CsI decomposition and having gained first
insights concerning IOx decomposition, it isnecessary to carry out experiments with thesemore representative iodine aerosols, in order tocomplete the current modeling for iodine ST
estimations. That is why IRSN proposes to study
the multi-components iodine aerosols, or at least
other metallic iodide aerosols than CsI. From asafety point of view, it is crucial to evaluate howmuch volatile iodine could be released underirradiation from these deposited mixed iodine
solubleaerosols
and/ormixed
iodine{insoluble+soluble} aerosols on dry/wet
surfaces.

- irradiation tests in the EPICUR facility bymeasuring on-line the release kinetics of I2 andCH3I with the objective to study the influence ofthe dose, temperature and specifically higher
humidity rates in order to reach and/or exceed thedeliquescence point.
- surface characterizations analysis to determine
the speciation of the adsorbed species.
-

Iodine oxides
monoxide

decomposition

bycarbonVarious authors have reported the I2O5
decomposition by carbon monoxide [12] [13](and other gaseous organic compounds like
propane, butane, benzene…) as being complete
at ambient temperature:

IV.2. STEM2/Ru topics
As already demonstrated by experimental data
[14][15][16][17], ruthenium species released
from the core can reach the containment andform some volatile ruthenium tetroxide. The
transport in the RCS is one of the key
phenomena impacting ST calculations. The firstphase of STEM/RUTHENIUM generated
information on the key parameters impacting thetransport kinetics, the partition between RuO4
and particles of RuO2 and the extent of therevaporization process of deposited species in thethermal gradient quartz tube. A preliminary Ru
transport model through the RCS has been
recently elaborated [18] ; nevertheless data are
not complete and not representative enough to be
confident for the extrapolation to reactor casesituations. As detailed in [18], a first attempt wasmade to integrate possible Ru source-term in
PSA-2 tools and it was concluded that ruthenium
radiological consequences could be significant.
That is why in the frame of STEM2/Ru, IRSN
proposes some complementary tests, mainly
based on the revaporization processes, andfocused on:

I 2O5  5CO  I 2  5CO2This reaction might play a significant role on
iodine volatility as carbon monoxide can bepresent in significant amount (several orders ofmagnitude over the iodine volatile concentration)in the containment by: i/. the B4C absorbents
degradation (for some PWRs and BWRs) and, ii/.
the molten corium-concrete interactions in thecontainment. However, the precise speciation of
IxOy into the containment building remains
unclear (pure I2O5, I4O9, and/or a mix between
hydrated forms and non-hydrated forms…), as
well as the extent of the interaction of these nonI2O5 species with CO. It has to be evaluated if, in
the conditions of a severe accident (temperature,humidity, CO concentration in the gaseous phaseand IOx composition and concentration on thesurfaces), this reaction might lead to significant
volatile iodine release or not.
-Radiolytic oxidation of multi-components iodine
aerosols- a higher representativity of the deposition
surface, i.e. stainless steel.

According to PHEBUS-FP tests mainly, multicomponents iodine aerosols are formed and/ortransported in the Reactor Coolant System (RCS)
to the containment building during the accidental
sequence. Their radiolytic stability has neverbeen checked and they might contribute to iodine- the use of stronger oxidizing conditions, likethose induced by air radiolysis products (i.e. O3,
NOx,…) in order to increase the oxygen potential
and thus be closer to the radiolytic conditions
occuring in the RCS. Indeed, it is important to177

NEA/CSNI/R(2016)5
keep in mind that in the presence of passive
autocatalytic recombiners operating, the molar
excess of O2 could be quite limited during thetime window where they operate, therefore themain re-vaporization processes of Ru deposits
could be linked to the oxidative action of OH
radicals resulting from steam radiolysis, insteadof an oxidative action of oxygen directly.

(Germany), the Korea Atomic Energy ResearchInstitute, the Korea Institute for Nuclear Safety,
The US Nuclear Regulatory Commission.
REFERENCES
1.

B. Clement and B. Simondi-Teisseire,“STEM: An IRSN project on source term
evaluation and mitigation”, Transactions of
the American Nuclear Society - 2010 ANS
Annual Meeting; Las Vegas, NV, U.S.;
November 7-11th, 103, 475-476, (2010).

2.

P.A. Yakabuskie, C.J. Moore, G.A. Glowa,“The Behaviour of Iodine Project: A
proposal for BIP3”, Proc. of the Int. OECDNEA/NUGENIA-SARNET Workshop on the
Prog. in Iodine Behaviour for NPP Acc.
Anal. and Manag. - March 30, April 1 Marseille (France), (2015).

3.

J. Colombani et al., “Experimental study oforganic iodide volatilization from painted
surfaces present in the containment during asevere accident”, Proceedings of the 6th
European Review Meeting on Severe
Accident Research, Avignon, 2-4 October,France (2013).

4.

J. Colombani, A.C. Grégoire, S. Morin,“Main findings of the IRSN experimental
programs performed on iodine chemistry insevere accident conditions”, Workshop on
the Prog. in Iodine Behaviour for NPP Acc.
Anal. and Manag. - March 30, April 1 Marseille (France), (2015).

5.

L. Bosland et al., “Towards a mechanistic
interpretation of the iodine – paintinteractions”, Proceedings of the 4th
European Review Meeting on Severe
Accident Research, Bologna, 11-12 May,
Italia (2010).

6.

F. Funke, S. Gupta, and al., “Interaction of
gaseous I2 with painted surfaces andaerosols in large-scale THAI tests”, Proc. of
the Int. OECD-NEA/NUGENIA-SARNET
Workshop on the Prog. in Iodine Behaviour
for NPP Acc. Anal. and Manag. - March 30,
April 1 - Marseille (France), (2015).

7.

L. Bosland, F. Funke, and al., “PARIS
project: Radiolytic oxidation of moleculariodine in containment during a nuclear- the use of representative “gaseous pollutants”(i.e. seed particles, silver aerosols…) that could
significantly impact the RuO4(g) behaviour.
V. CONCLUSIONS
Following STEM, the second phase of the
project, named STEM2 is being proposed in the
NEA framework. This 4-year follow-up project
should be focussed on medium- and long-term
releases and with three main items as prioritizedby the STEM Program Review Group: i/ effectof the ageing of paints on iodine behaviour: i.e.
irradiation tests on paints aged in representative
reactor conditions and loaded with iodine, toconfirm iodine releases processes observed with
un-aged paints (including adsorption/desorption
kinetics) , ii/ iodine aerosol radiolytic
decomposition (IOx species as well as multicomponents iodine aerosols, and by extension
also the iodine oxides decomposition by carbon
monoxide), iii/ ruthenium revaporization
processes occurring with RCS stainless steel
surfaces and highly oxidizing conditions. It has
also to be noted that an analytical working groupshould be set up to promote a final
comprehensive analysis of the results for reactor
case ST evaluation including mitigation aspects.
Finally, conditions of experimental investigations
shouldbeconsolidated
followingtherecommendations of the International OECDNEA/NUGENIA-SARNET Workshop on the
Progress in Iodine Behaviour for NPP Accident
Analysis and Management.
ACKNOWLEDGMENTS
Authors are grateful to the OECD. Authors
acknowledge for their support and guidance all
the STEM project partners: the Electricité De
France, the Canadian Nuclear Laboratories, theTeknologian tutkimuskeskus VTT (Finland), The
Nuclear Research Institute (Czech Republic), theGesellschaft für Anlagen – und Reaktorsicherheit
178

NEA/CSNI/R(2016)5
14. Y. Pontillon, G. Ducros, “Behaviour offission products under severe PWR accident
conditions. The VERCORS experimental
programme—Part 3: Release of low-volatile
fission products and actinides”, Nucl. Eng.
Des. 240(7), pp. 1867-1881 (2010).

reactor severe accident: Part 2: Formationand destruction of iodine oxides compounds
under irradiation – experimental resultsmodelling”, Nucl. Eng. Des., 241, (9), p.
4026-4044, (2011).
8.

9.

O. Leroy, M.N. Ohnet, S. Planteur-Kieffer,“Study of the Ruthenium transport underprevailing conditions in the primary circuitin case of a PWR severe accident”,
ERMSAR 2015 conference, Marseille(France).

15. N. Vér, L. Matus, M. Kunstár, J. Osán, Z.
Hózer, A. Pintér, “Influence of fission
products on ruthenium oxidation andtransport in air ingress nuclear accidents”,
Journal of Nuclear Materials., 396, pp. 208217 (2010).

L. Bosland, L. Cantrel “Iodine behaviour inthe circuit and containment: Modeling
improvements in the last decade andremaining uncertainties”, Proc. of the Int.
OECD-NEA/NUGENIA-SARNET Workshop
on the Prog. in Iodine Behaviour for NPP
Acc. Anal. andManag. - March 30, April 1 Marseille (France), (2015).

16. C. Mun, L. Cantrel, C. Madic, “Radiolyticoxidation of ruthenium oxide deposits”,
Nuclear Technology 164, pp. 245-254(2008).
17. T. Kärkelä, Ruthenium databook – version
1.0, SARNET2-ST-D8.7 report (EU Severe
Accident Research Network of Excellence 2)(2013).

10. L. Cantrel, F. Cousin, and al., “ASTECV2
severe accident integral code: Fission
product modelling and validation", Nucl.
Eng. & Des., 272, p. 195-206, (2014).

18. F. Miradji, F. Cousin, S. Souvi, V. Vallet, J.
Denis, V. Tanchoux, L. Cantrel. “Modellingof Ru behaviour in oxidative accident
conditions
and
first
sourceterm
assessments”, ERMSAR 2015 conference,
Marseille (France).

11. A. B. Aleksandrov, N. I. Ampelogova, andal., “The effect of physicochemical
conditions on the absorption of moleculariodine by protective varnish and paintcoatings during accidents at a nuclear power
station with a VVER-640 reactor”, Therm.
Eng., 42, (12), p. 991-996, (1995).

19. G. Glowa, C. J. Moore, and al., “The main
outcomes of the OECD Behaviour of Iodine
Project”, Proc. of the Int. OECDNEA/NUGENIA-SARNET Workshop on the
Prog. in Iodine Behaviour for NPP Acc.
Anal. and Manag. - March 30, April 1 Marseille (France), (2015).

12. C. M. Stevens and L. Krout, “Method forthe determination of the concentration andof the carbon and oxygen isotopic
compositionofatmosphericcarbonmonoxide”, Int. J. Mass Spect. & Ion Phys.,8,p. 265-275, (1972).

20. G. Glowa, C. J. Moore, and al., “The main
outcomes of the OECD Behaviour of Iodine(BIP) Project”, Annals of Nuclear Energy,
61, p. 179-189, (2013).

13. H. J. Kavanaugh, J. W. Dahlby, and al.,“The gravimetric determination of carbon in
uranium-plutonium carbide materials”, LA7981 - Los Alamos Scientific Laboratory,(1980).

179

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180

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Session 2: iodine transfer to aqueous phase, aqueous-phase chemistry181

NEA/CSNI/R(2016)5
IODINE AND SILVER WASH-DOWN MODELLING IN COCOSYS-AIM BY USE OF THAIRESULTS
G. Weber(1)*, F. Funke(2), W. Klein-Hessling(3), S. Gupta(4)(1)

Gesellschaft für Anlagen- und Reaktorsicherheit (GRS) gGmbH, Garching, Germany,
AREVA GmbH, Erlangen, Germany, (3)Gesellschaft für Anlagen- und Reaktorsicherheit (GRS) gGmbH,
Cologne, Germany,(4)
Becker Technologies GmbH, Eschborn, Germany(2)*Corresponding author, Tel: (+49)89 32004 506, Fax: (+49)89 32004 300, Email: gunter.weber@grs.de
Abstract – An accurate prediction of the iodine source term requires not only an adequate simulation of
all iodine reactions in the containment but also a precise calculation of thermal hydraulics and aerosol
behavior and their interaction with iodine chemistry [1]. Wet deposition of iodine species on containment
surfaces, their reaction with the decontamination paint and the wash-down by draining condensate areimportant interaction processes.
In COCOSYS-AIM the models for wet iodine deposition onto paint, for iodine and silver wash-down andfor the iodine/silver reaction have been revised using findings and data from several large-scale THAI
tests. In this paper three models are highlighted.
First, a new water layer model for I2 deposition onto paint by steam condensation is described. It considers
all relevant chemical reactions in the water film as well as iodine adsorption onto paint covered by water
film and rivulets. The model was first checked on the THAI tests Iod-24 with two painted cooling elements
making different wall condensation rates possible at the same time.
Second, the new model AULA for the wash-down of insoluble aerosols like silver particles has beendeveloped. It is based on a model for sediment transport applied in geology. Particles erode and are
washed down when a critical flow velocity is exceeded. Two flow types are considered: rivulets and films.
For a first validation a THAI AW-3 laboratory test was analyzed with AULA.
Third, the reactive surface of silver particles in a sump has been implemented in the model for theiodine/silver reaction. The reactive surface is reduced when Ag particles are deposited on the sumpbottom.
The revision of the iodine and silver wash-down modelling is expected to reduce significantly theuncertainty in the iodine source term prediction with COCOSYS-AIM.

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I. INTRODUCTION

considered and the wash-down of insoluble
aerosols is modelled empirically.

During a severe accident in a LWR iodine andother fission products are released into thecontainment together with large amounts ofsteam and non-condensable gases. The steam
condenses on the cold structures and thecondensate flows down the walls and over floors
and finally reaches the main sump at thecontainment bottom. Together with thecondensing steam gaseous molecular iodine (I2),
particulate iodine (CsI, IOx) and other aerosols
are transferred into the wall condensate andtransported downwards. Since the surface of themain sumps is rather small compared to all
deposition areas in a containment (about 3 % in a
German Konvoi PWR) most of the nuclear
material, i.e. fission products, control rod andstructure materials, is settled on the surfaces in
the containment at first and finally washed down
into the main sump.

Therefore a new wash-down concept has beendeveloped comprising three models which havebeen verified on several tests:- Iodine water layer model (THAI Iod-21 andIod-24)- Wash-down model for insoluble aerosols
AULA (German: “Abwaschen unlöslicher
Aerosole” (THAI AW-2, first part of AW-3
including laboratory tests)- Model for the reactive surface of silver
particles in sumps (PHEBUS FPT1 andsecond part of AW-3)
The iodine water layer model describes thedeposition of gaseous iodine onto paint by wallcondensation, all relevant iodine reactions withina water film and with the paint and the iodinetransport with the draining condensate.
AULA simulates the erosion of insoluble
aerosols by flowing condensate. Ag and
Ag/AgOx aerosols which react with iodine are of
special interest.

The wash-down process governs the distributionof the fission products between structures,
elevated water pools and the main sump. Fission
products in the water phase produces steam dueto the decay heat while decay heat released fromdry structures causes a dry heating of thecontainment. Furthermore distribution andrelocation of radionuclides is important to assessradiation damages of local components likeelectronic devices, seals, etc. The wash-down
processes interact with several other thermal
hydraulic and aerosol processes in thecontainment. Therefore an accurate modelling of
the wash-down is relevant for a reliable
simulation of the iodine behaviour in thecontainment and the source term.

In an extension of the iodine/silver model the
reduction of the reactive surface of particles
settled on the sump bottom is described.
II. WATER FILM AND RIVULETS
The major part of the surfaces in LWR
containments is coated with decontamination
paint based on Epoxy resins. With steam
condensation the hydrophobic surface of thepaint promotes the formation of droplet
condensation. If a cluster of droplets becomes
too heavy it moves downwards forming a local
rivulet which gathers other droplets on its way.
The classical closed water film is only
occasionally created, e.g. with high condensation
rates. The two different flow types have different
wash-down efficiencies for soluble and insoluble
aerosol particles, which are considered in the
new models.

At wall condensation two flow patterns canoccur: droplet condensation with rivulet
formation and water films. Both are treated in the
new models described in this paper.
In the containment code COCOSYS including
the iodine module AIM-3 wash-down processes
are currently modelled only in a simplified way[2,3]. For instance the iodine deposition onto wet
paint is calculated by a constant rate, chemical
reactions in the condensate layer are not

Rivulet formation is the prevailing condensation
type in a LWR. Rivulets on vertical walls occur
stochastically at different locations. In the longrun a large part of the wall surface will becovered by the individual rivulet tracks. Rivulets

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NEA/CSNI/R(2016)5
on slightly inclined surfaces are broader, theyflow slower and their tracks are more stable.

III. I2/PAINT REACTION AT WALL
CONDENSATION

Fig. 1 a) and b) show droplet condensation andrivulet tracks on a vertical wall coated withGerman GEHOPON paint at two different wallcondensation rates in the THAI test Iod-24 [4]. Inb) with the higher condensation rate, droplets are
smaller. The photographs c) and d) show rivulets
on slightly inclined steel and painted plates withpreviously deposited CsI aerosol. The plates are
1.2 m long and about 0.3 m wide. On both plates
two stable rivulets were formed. The time after
the start of wash-down is given in min:sec [5].

The current AIM model for the I2 reaction with
wet paint during wall condensation is describedby the reaction scheme depicted in Fig.2. E.g. theI2 deposition onto wet paint is given by reaction
rate (RR) 4, desorption by RR 62. One fractionof the I2(g) deposited into the water film iswashed down into the water pool (RR 44)
another fraction is converted to I- before it moves
into the pool. In the model the I2 fraction isconstant and washed off instantaneously.
This model has two main restrictions:- The reactions do not depend on the wallcondensation rate.
- Chemical reactions within the water film are
not considered. An exception is the simplified
modelled I2(w)  I-(w) reaction.
paint4, fraction (1 - F444)

I2(g)
62

I2(dep,p,g)0.13 g/(m2s)fraction F444
Ichs(dep,p,g)

44
9

I-(w)

Fig. 1 Droplet condensation and rivulet
formation on a vertical painted wall at different
wall condensation rates (a, b) and rivulets onslightly inclined plates covered with CsI aerosol(c, d).

I2(w)

water

Fig.2 Current I2/wet paint interaction and washdown model in AIM-3
In an ASTEC calculation on PHEBUS test FPT1
the water film along the painted wet condenser
was explicitly modelled for the first time [6]. The
main objective was to demonstrate the large
amount of gaseous I2 produced by radiolyticoxidation of I- from CsI dissolved in the water
film. The higher dose rate from the depositediodine and other fission products in the water
film was considered. This simple water filmmodel was not coupled to the containment
thermal hydraulics and the build-up of the water
film as well as the deposition and the wash-downof fission products were not calculated.
Nevertheless, the good results with this earlymodel are a motivation for the new water layer
model in COCOSYS.

In Tab.1 characteristics of rivulets and water
films under severe accident conditions arecompiled. The values are calculated assuming awall condensation rate of 5.E-4 kg/(m²s).
Rivulets are about 10 times faster than water
films, but their tracks are narrow.
Tab.1 Characteristics of condensate flow types ina LWR containment (approximate values)
Vertical wall Floor (2% inclined)
Water
Water
Flow type Rivulet
Rivulet
film
film
Width
3 mm
large
3 cm
large
Thickness 1 mm 0.1 mm 2 mm 0.3 mm
Mean
1 m/s 0.1 m/s 0.3 m/s 0.03m/s
velocity184

NEA/CSNI/R(2016)5
III.A New iodine water layer model

PAINT

In the new water layer model droplets and films
are simulated uniformly by a thin water layer.
The area of each layer complies with the
COCOSYS nodalisation. It can be 10 m² toseveral 100 m² in a large LWR containment. The
thickness of each layer is calculated dynamicallytaking into account its orientation (wall or floor)
and the wall condensation rate. Surplus
condensate is drained through drain junctions
into the layer below.

WATER LAYERSTEAM/AIR ATMOSPHERE

Inflow

Steam condensation
Diffusiophorese

I2, I-, HOI,
IO3-, CH3I,
Ag, AgI, etc.

I2 physisorption
I2, I- chemisorpt.

Gaseous I2
and aerosols:
CsI, IOx, Ag,
etc.

pH,
dose rate

I2; CH3I

Hydrolysis
Radiolysis
Iodine/Ag
etc.

Diffusion

Outflow

Fig.3 Drawing illustrating the new water layer
model

Assuming homogeneous steam condensation athigh vertical containment walls the thickness andthe flow velocity of the rivulets respectively thewater film are increasing from top to bottom.
This situation can be handled by defining several
water layers one above the other. The mean
thickness of each layer and the condensate flow
rates between the layers are calculated by thethermal hydraulic module of COCOSYS. In caseof rivulets in the model the water is assumed tobe distributed over the wall segment.

The iodine species in each individual water layer
are treated like in the main sump. In the waterlayer iodine reactions, i.e. hydrolysis, radiolysis,
iodine/silver reaction and others, as well as the I2
and CH3I mass transfer to the gas phase aremodelled. At present the same reaction rate
constants as for the main sump are used unless
new water layer specific measurements areavailable.
Moreover, a possible dry out of the water layer isconsidered. At dry out gaseous iodine species (I2,
CH3I) are released completely to the atmosphere.
Non-volatile substances like CsI, Ag and AgI
become immobile and all the chemical waterphase reactions are suppressed until the layer isfilled by new condensate again.
The I2(g) transport is a combination of iodine
diffusion and the transport with the condensing
steam (Stefan-flow). The I2 diffusion velocity inthe atmospheric boundary layer close to thewater layer is identical with the gas side masstransfer coefficient. The combined adsorption
velocity becomes [7]:The iodine species concentration in each waterlayer is balanced from sources and sinks causedby the condensate flows, transfer from theatmosphere and adsorption onto the immersed
paint (Fig.3). Each layer is treated as if it were
well mixed. If the volatile iodine concentration
exceeds the equilibrium with the gas phasegaseous iodine is transferred into the atmosphere.
Local iodine concentration differences in thecontainment maybe reduced by the wash down
and transfer processes. Decisive for the
homogeneous chemical reactions in the waterlayer respectively the reactions with the paint are
the local aqueous species concentrations.

𝑅

𝑘𝑎𝑑 = 1−𝑒 −𝑅 𝑘𝑔 (1)with𝑣

𝑅 = 𝑆𝑡𝑒𝑓and 𝑣𝑆𝑡𝑒𝑓 =
𝑘
𝑔

kad

𝐺𝑐𝑜𝑛𝑑
𝜌𝑠𝑡

I2 adsorption velocity into the water layer
under condensing conditions, m/s
Gcond Condensation steam flux to wall, kg/(m²s)
kg
Gas side mass transfer coefficient for I2,
m/s (AIM default: 1.4E-3 m/s)
vStef Stefan velocity of condensing steam, m/sρstSteam density, kg/m³
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NEA/CSNI/R(2016)5
kg is identical with the I2 adsorption velocity bydiffusion at dry conditions. Fig. 4 shows the I2
adsorption rate into the water layer upon thesurfaces (paint, steel, etc.) as function of thecondensate rate according to Eq. 1. For low
condensation rates kad equals kdif. Although thetwo wall condensation rates used in Iod-24,
differ by a factor 9.2 the kad ratio is only 1.7, i.e.
the I2 concentration in the condensate is higher
with the low wall condensation rate.
2,0E-02

At stable condensing conditions 0.55 g ofgaseous I2 traced with some radioactive I-123
was injected (time t = 0). With the condensing
steam I2 was transported to the painted coolingelements. One fraction was adsorbed on the paintthe other fraction drained with the condensate
and was diverted outside the vessel andmeasured. The iodine on the elements wasmeasured online by gamma-detectors throughglass windows in the vessel.
After 14 hours, the cooling of the painted
surfaces and steam injection were stopped andthe painted surfaces dried out in order to measurepossible iodine desorption
For the COCOSYS-AIM calculation of the test a9-compartment nodalisation of the THAI vessel
was worked

I2 adsorption velocity as function of thewall condensation rate

I2 adsorption velocity, m/s

Kad

kdif

1,5E-02

Iod-24, low rate
Iod-24, high rate
1,0E-02

5,0E-03

0,0E+00
1,00E-05

1,00E-04

1,00E-03

1,00E-02

Wall condensation rate, kg/(m² s)

Fig. 4 Iodine adsorption by diffusion andcondensation
In a COCOSYS-AIM nodalisation the waterlayers are relatively large and the condensate is
assumed to be homogeneously distributed. Thusin the model droplets and rivulets are simulated
by a water layer with parameters (thickness, flow
velocity) different from water films.
III.B Validation
Fig. 5 Vessel configuration with two painted
cooling elements in test Iod-24

In the THAI test Iod-24 the deposition of
gaseous I2 onto paint under condensing condition
was measured. Two cooling elements with a total
surface area of 3.7 m² each were coated withGerman GEHOPON paint and installed in thevessel. The paint was thermally aged to 15 years.
The gas atmosphere was superheated and therelative humidity was maintained constantthroughout the test at 90% by continuous
injection of steam at a constant mass flow rate.
Steam was released into the vessel (Fig. 5). The
surface of the elements was kept on different
temperatures below the saturation point
achieving nearly constant condensates rates of
0.13 g/m²s and 1.2 g/m²s. More details on thistest and a description of the similar Iod-21 can befound in [4].

out. In two zones heat conducting structures
represented the two cooling elements. The paint,
the condensate layer
and the surrounding gas space were simulatedwith the new water layer model. The water layer
thickness was given with 6.5E-5 m according tomeasurements by Becker Technologies. For the
pH of the condensate layer the measured value of
pH = 4.5 – 5 was used. Additionally a release of
reductive substances from the GEHOPON paint
was assumed. The substances had been measured
under comparable conditions in earlier tests [8].
They can reduce I2(w) quickly to the non-volatile
I-(w).
The iodine mass deposited onto the paint is wellcalculated by the new water layer model for the186

NEA/CSNI/R(2016)5
high and the low wall condensation rates (Fig. 6
and Fig. 7). It is remarkable that the depositediodine mass with a low condensation rate is 56 %higher than that with a high rate. The reason isthat the iodine concentration in the condensate
with the low rate is higher than that with the highrate, as the wet I2 adsorption velocity does not
rise linearly with the condensation rate (s. Fig.
4). This unexpected behavior is well calculatedwith the new model. Only for the highcondensation rate the mass is about 10 %underestimated.
Iod-24 further shows that adsorbed iodine is notwashed-off by pure condensing steam. The
iodine physisorbed on paint is rapidly
chemisorbed and fixed.

Fig. 7 Iod-24: Total iodine mass on the paintwith low condensation rate.
(condensation rate, temperature), flow type(rivulet, film), nature of surface (smooth, rough),
etc.

IV.WASH-DOWN OF AEROSOLS
During a reactor accident the condensate flowing
down washes off the nuclear aerosols, transportthem into lower compartments and finally into
the reactor sump. The efficiency of the washdown process depends on the particle properties(size, density, and solubility), surface orientation(wall, floor), thermal hydraulic conditions

The wash-down processes for soluble andinsoluble aerosols are rather different. Soluble(hygroscopic) material, like CsI, is quickly
dissolved and transported with the condensate. In
COCOSYS-AIM the dilution process is
simplified treated assuming an instantaneous andcomplete dissolution of soaked particles. Small
puddles and larger pools might retard thecondensate and the dissolved material on their
way down.
Under reactor conditions the wash-down issignificantly more efficient for soluble than for
insoluble aerosols as shown by THAI tests AW1, AW-2 and AW-3 [9]. The model in
COCOSYS-AIM for soluble aerosols was
successfully validated on AW-1 with CsI and
AW-2 with the aerosol mixture CsI + SnO2 [10].
The wash-down of insoluble aerosols like Ag
and AgOx particles is more complex because theerosion process depends on both, thehydrodynamic of the condensate and the particle
properties.

Fig. 6 Iod-24: Total iodine mass on the paintwith high condensation rate

The current wash-down model for insoluble
aerosols in COCOSYS-AIM describes theprocess in a rather simplified way. Particle
erosion rate and wash-down efficiency have to begiven by the user.

187

NEA/CSNI/R(2016)5
Guo [13] is applied. With the dimensionlessauxiliary parameter𝑹∗ =𝒅𝒑 √𝟎,𝟏(𝒔−𝟏)𝒈𝒅𝒑𝒗(2)

and the density ratios = ρp / ρw
the critical dimensionless shear stress, known as
Shields parameter, becomes
𝟎.𝟏

𝝉∗,𝒄 = (𝑹 )𝟐/𝟑 + 𝟎. 𝟎𝟓𝟒 [𝟏 − 𝐞𝐱𝐩 (−∗(𝑹∗ )𝟎.𝟓𝟐)]
𝟏𝟎ρpeffective density of particles, kg/m³
ρw
density of water, kg/m³
R∗
Dimensionlessparameter,
RouseReynolds-number𝜏∗,𝑐 Dimensionless critical shear stress (Shields
parameter)

Fig. 8 Forces acting on a settled particle in awater flow (FA buoyancy, FS hydrodynamic
force, FG gravity, FR friction and FK cohesion).
The new wash-down model AULA [11] is basedon an approach used in geology to describe thetransport of sediments in water flows. A particle
erodes when the hydrodynamic force and thebuoyancy exceed the forces of gravity, friction
and cohesion (Fig. 8). This type of problem was
first solved by A. Shields by formulating acriterion [12]. This Shields criterion says that theerosion of a settled particle starts when the flow
velocity directly above the particle bed exceeds a
critical value. This criterion is valid for a large
range of particle sizes and flow velocities

Finally the critical shear velocity is given by𝑢∗,𝑐 = √𝜏∗,𝑐 (𝑠 − 1)𝑔 𝑑𝑃(4)

For particles between 1 and 10 µm the critical
shear velocity depends only slightly on theparticle size, i.e. all particles will start to erode
nearly at the same flow velocity. The density of
the aerosol material has a significant impact on
the critical shear velocity. While 10 µm particles
with a density of 2000 kg/m³ erode at a critical
shear velocity of 0.8 cm/s particles with 10000kg/m³ (e.g. Ag) need a shear velocity of 2.2 cm/s
(Fig. 9).

IV.A The AULA model
According to the Shields criterion the erosion ofa particle from a structure or from a bed ofparticles takes place if the shear velocity at theparticle bed exceeds the critical shear velocity.
𝑢∗,𝑐 (𝑑𝑃 ) is calculated with the Shields equation
for the critical shear stress.
𝑢∗ > 𝑢∗,𝑐 (𝑑𝑃 ) (Particles erode)
𝑢∗
𝑢∗,𝑐
m/s
dP

Shear velocity at the particle bed, m/s
Critical shear velocity at the particle bed,
Mean particle diameter, m

The Shields criterion is valid for particles
between 1 µm and about 1 cm. If the criterion is
met for a cluster of equal particles, all particles
will erode at the same time.
The Shields equation cannot be solved
analytically. Several approximationsareavailable. Here the one of Shields, Rouse andFig. 9 Critical shear velocity for different aerosol
particles.

188

NEA/CSNI/R(2016)5
For cohesive particles the erosion process lasts
significantly longer. Most particles < 50 µm arecohesive (attraction between particles) and/or
adhesive (attraction between particles and wall).
Behaviour and modelling of the cohesive
particles are described below.

and
kE
kE,0

3 𝑣 𝑚̇

3

𝑤

δ
𝑚𝑤
̇
W
𝜈
g

(5)

𝒅𝒄𝑨𝒆,𝒅𝒆𝒑
𝒅𝒕

With the condensate thickness the mean flow
velocity is calculated𝑢=

The deduction of the shear velocity depends on
the type of flow. Because of the slight roughnessof the decontamination paint surface, smooth
turbulent flows can be expected. The shear
velocity for hydraulically smooth flows is
calculated by use of a parabolic flow velocity
profile.
𝑢∗ = 𝑢𝜅𝑢 𝛿𝑣𝑙𝑛( ∗ )+5.2

At Becker Technologies, laboratory wash-down
tests with an insoluble silver aerosol were
performed [15]. The aerosol was deposited onsteel plates, most of which were coated withGEHOPON decontamination paint. At the upper
end of the differently inclined plates, water was
released homogeneously by use of a special
distributor. The Ag aerosol washed down wasmeasured at the bottom end of the plate. In 14
tests the particle size, the inclination of the plate,the surface material (paint, steel) and the waterflow rate were varied. The wash-down always
occurred with rivulets and in no case a closed
water film was observed. Because of the stablerivulets a part of the plate surface covered with
the Ag aerosol remained dry.

(7)

Small nuclear particles deviate from the ideal
Shields conditions, because of the cohesive andadhesive forces which are not considered. To
overcome this problem in AULA the masserosion rate kE based on Shields’ parameter isused. This rate was introduced by Ariathurai(cited in [14]) for cohesive particles.
(𝑢∗ 2 −𝑢∗,𝑐 2 )𝑢∗,𝑐 2(9)

IV.B First validation

𝜅 is the Karman constant, 𝜅 = 0.41 and 5.2 is an
integration constant. Since u* appears on bothsides of the equation, it has to be solved by
iteration.

𝑘𝐸 = 𝑘𝐸,0= −𝒌𝑬 𝒄𝑨𝒆,𝒅𝒆𝒑 + 𝑸

cAe,dep Concentration of deposited aerosol (kg/m²)
Q
Sources due to deposition processes (resettling, etc.), kg/(m² s)

(6)

3𝑣

Aerosol mass erosion rate, s-1
Erosion constant, s-1

In each COCOSYS calculation time step theerosion rates for wall and floor areas arecalculated with AULA. The time derivation of
the deposited aerosol mass in each control
volume isThickness of condensate flow, mCondensate mass flow, kg/s
Flow width, mKinematic viscosity of water, m²/s
Acceleration due to gravity, m/s2

𝛿 2 𝑔 𝑐𝑜𝑠𝛽

for 𝑢∗ ≤ 𝑢∗,𝑐

By use of the mass erosion rate the erosion ofcohesive particles is initialized by the Shields
criterion and goes on until the total aerosol mass
has eroded. The erosion constant kE,0 is the time
constant for the decreasing erosion process.
Formally kE,0 is the mass erosion rate at the shear
velocity √2-times of 𝑢∗,𝑐 . With a small kE,0 theerosion process lasts long and with a large KE,0
the process approaches the ideal Shields “puffrelease”. The erosion constant, i.e. thecharacteristic time of the erosion process, has tobe measured under reactor typical conditions.

The shear velocity of the condensate flows(rivulets and water films) close to the structure
can be calculated by use of the mean flow
velocity. The thickness of the condensate flow isgiven by the continuity equation which can beapplied for both, rivulets and films by using
different flow widths. β is the angle between thewall surface and the vertical direction.
𝑤
𝛿 = √𝑔 𝑊 𝜌 𝑐𝑜𝑠𝛽𝑘𝐸 = 0

The results showed the highest erosion rate at the
beginning of the wash-down, which thendecreasing exponentially with time. Small
amounts of silver could be detected until the endof the tests at t = 15 min. As an example thefor 𝑢∗ > 𝑢∗,𝑐 (8)189NEA/CSNI/R(2016)5
measured results of test no. 4 are given in [15].
The Ag aerosol loading on the paint was 12.5
g/m², the MMD of the particles was about 2 µm.
The inclination of the plate was 20° and thecondensate flow was 11 g/s. About 10 stablerivulets developed covering approximately 45 %of the plate surface.

AULA calculation on THAI AW-3 LAB Test 4
Ag wash off per sample, g

0,5In the first 130 s 23.2 % of the Ag from the area
soaked by the rivulets was washed off. In thewhole test which lasted 15 min 36.8 % were
washed off.

AULA

0,4

Test

0,3

0,2
0,1
0
1

10

20

30

40

50

60

70

80

90 100 110 120 130

Time, sWith AULA the erosion process due to therivulets was simulated. Ten rivulets with a mean
width of 1.5 cm were assumed. By use of the
thickness (Eq. 2) and flow velocity (Eq. 3) of therivulets the shear velocity was calculated with
Eq. 4. The critical shear velocity was calculatedby the approach of Shields, Rouse and Guo (Eq.
2 to 4).

Fig.10 Measured and calculated erosion rate andrivulet flow pattern (right) in the AW-3 LAB test4.
A systematic evaluation of the remaining 13 testsof the AW-3 LAB series will be possible when
the integration of AULA into COCOSYS has
been completed.

𝑢∗ = 1.64E-2 m/s𝑢∗,𝑐 = 9.8E-3 m/s

V) AQUEOUS IODINE/SILVER REACTION

Since 𝑢∗ > 𝑢∗,𝑐 the silver particles erode in test
4. The erosion rate is according to Eq. 5

The specific surface of the silver particles in thesump was one of the important uncertain
parameters determined in the uncertainty andsensitivity study on a COCOSYS-AIM
calculation on PHEBUS test FPT1 [16]. In thisCOCOSYS version as in most other iodine codes
the specific Ag surface is considered to beconstant. The iodine /silver reaction takes placeat the particle surface, i.e. the kinetics of thereaction depends linearly on the specific surfaceof the particles.

kE = kE,0 · 1.8 1/s
The erosion constant kE,0 ≈ 0.0205 1/s wasestimated by use of some detailed results of test 4
concerning the time behaviour. Finally the masserosion rate in the AW-3 LAB test becomeskE ≈ 3.7E-2 1/s
In Fig.10 the eroded Ag masses calculated are
depicted. The history of the erosion rate is in a
fairly good agreement with the measurement.
The erosion process lasts several minutes. For t >60 s the wash-down is underestimated.

𝑘𝐼2

𝐼2 + 2𝐴𝑔 → 2 𝐴𝑔𝐼𝑘𝐼−

𝐼 − + 𝐴𝑔𝑂𝑥 + 2𝐻 + →𝐴𝑔𝐼 + 𝐻2 𝑂

The specific surface is the surface of all particles
divided by their total mass (m²/g). For monodisperse spherical 3 µm Ag particles it is 0.42m²/g.
Ag particles deposited on the bottom of the sump
are less efficient for the reaction with iodine thansuspended particles because of the reduction of
their reactive surface. When particles settle to thesump bottom the iodine/silver reaction is slowed
down rapidly. During the evaluation andinterpretation of the PHEBUS test FPT1 this190

NEA/CSNI/R(2016)5
problem appeared already [6]. To overcome thisproblem the following approach is suggested.

Ag aerosol settling in a well-mixed sump
Water level 2 m

The settling velocity of Ag particles in the sump
water is calculated by the Stokes equation.

100

vset =
vset
dAg
ρ
η
g

d2Ag g(ρAg −ρw )18ηParticles suspended, %V.A Reactive silver surface

120

(10)

Settling velocity, m/s
Particle diameter, mDensity; w water, Ag silver, kg/m³
Dynamic viscosity, Pa.s
Acceleration due to gravity, m/s²

SAG
dP
ρP

𝑃∙

20

5

10

15

20

25

30

Time, hFig. 11 Ag aerosol settling in a well-mixed
reactor sump
In COCOSYS-AIM the kinetics of the I2/Ag
reaction is modelled by𝑑 𝐼2(𝑤)
1
=− 1
1
𝑑𝑡
+

· 𝐼2(𝑤) ·𝑘𝑤 𝑘28 ∙𝐴𝐺𝑠𝑢𝑠

AAg,tot
I2(w)
AGsus
kw
k28
V

𝐴𝐴𝑔,𝑡𝑜𝑡(12)

𝑉Total reactive surface of Ag particles, m²
Aqueous concentration of I2, mol/lConcentration of suspended Ag, mol/l
Water side I2 mass transfer coefficient,
m/s
I2/Ag
reactionrate
constant,= 0.2 m∙l∙mol-1∙s-1
Sump volume, m³

In the new extended model the total reactive
surface is composed of the surfaces of thesuspended and of the deposited particles.

The specific surface of spherical aerosol particles
is
6

40

0

In reactor containments only a small fraction of
the Ag aerosol settles directly into the sump,
since its surface is small compared to the total
floor area. In a German Konvoi-PWR only about
3 % of the total Ag containment inventory aresettled directly into the sump. Most particles are
first deposited onto structures, before they are
washed down into the sump. On their way theygrow due to agglomeration. It is expected that theAg particles in the reactor sump are significantly
larger than in the airborne state. Appropriate
measurements are not available.

𝑃

10 µm particles
60

0With the density of compact Ag (1.E4 kg³) for 3µm particles, the settling velocity in the water is
1.35E-4 m/s. 1 µm particles have a velocity of
1.49E-5 m/s and 10 µm particles 1.49E-3 m/s.

𝑆𝐴𝑔 = 𝑑 ∙𝜌 1𝐸3

3 µm particles

80

(11)

AAg,tot = AAg,sus + AAg,dep

Specific surface of Ag particles, m²/g
Particle diameter, mParticle density, kg/m³

(13)

with
𝐴𝐴𝑔,𝑠𝑢𝑠 = 𝐴𝐺𝑠𝑢𝑠 · 𝑆𝐴𝑔 · 1000 · 𝑀𝐴𝐺 · 𝑉

For 3 µm Ag particles with a density of 1E4
kg/m³ the specific surface is 0.2 m²/g and for 1µm particles 0.6 m²/g. For fluffy particles the
specific surface is larger.

AAg,sus Surface of suspended Ag particles, m²
AAg,dep Reactive surface of deposited Ag
particles, m²
MAG Molecular mass of Ag, 107.87 g/mol
1000 Dimension factor in l∙m-3

In a well-mixed and non-boiling reactor sumpwith a typical water level of 2 m an Ag aerosol of
3 µm particles settle completely to the sumpbottom within 1 day (Fig. 11). 10 µm particles
deposit within 2½ hours.

The reactive Ag surface of all deposited particles
is the fraction FAg of the total surface of theseparticles, i.e. it is the part of the particle surface
facing upwards. The reactive surface of allparticles deposited to the bottom is
𝐴𝐴𝑔,𝑠𝑏 = 𝐴𝐺𝑑𝑒𝑝 · 𝐴𝑠𝑏 ∙ 𝑀𝐴𝐺 ∙ 𝑆𝐴𝑔 ∙ 𝐹𝐴𝑔 (14)191

NEA/CSNI/R(2016)5
sump water recirculation started for short time
period during I2 injection. At = 5.5 h, thebeginning of phase 3.2, the recirculation of thesump water was activated causing an efficientresuspension of the Ag particles from the bottom.
Additionally 8.2 g of fresh Ag aerosol was
released to the water. At the high Ag
concentration of 0.076 g/l the remaining I2 massof 0.08 g reacted rapidly and the I2 concentration
dropped below the detection limit while themeasured AgI mass reached its highest value
with 0.24 g.

𝐹𝐴𝑔 = 0.5 (𝐷𝑒𝑓𝑎𝑢𝑙𝑡)
AAg,sb

Total (not limited) surface area of Ag
particles deposited on sump bottom, m²
Sump bottom area, m²
Ag concentration on sump bottom,

ASb
AGdep
mol/m²FAg
Ag surface fraction, i.e. the fraction of
the reactive surface on the total surface
of deposited Ag particles
In AIM the reactive surface is assumed to be halfof the total Ag surface. Due to the package
density of the particle bed and the shape of theAg particles FAg may vary.

Phase 3.2:
recirculation, Ag suspended

Phase 3.1:
no recirculation
S-050-B: I2
S-200-B: I2
S-350-B: I2
COCOSYS-AIM: I20,60,5
Iodine mass / g

If the bottom is not completely covered byparticles the reactive silver surface is AAg,sb. If
the bottom is covered with one or severalmonolayers of particles the reactive surface
equals the bottom area. i.e. the total reactive
silver area of all deposited particles is limited tothe sump bottom area.

THAI AW-3: Iodine and suspended AgI particles in sump water
0,7
S-050-B: I-minus
S-200-B: I-minus
S-350-B: I-minus
COCOSYS-AIM: AGI

S-050-B: AgI based on filtration
S-200-B: AgI based on filtration
S-350-B: AgI based on filtration
COCOSYS-AIM: I-

0,4

0,3
I2 quickly depletes upon
injection of fresh Ag
suspension: k = 2.3E-3 /s

0,2
0,1

AAg,sb < Asb: 𝐴𝐴𝑔,𝑑𝑒𝑝 = 𝐴𝐴𝑔,𝑠𝑏(15)
AAg,sb ≥ Asb: 𝐴𝐴𝑔,𝑑𝑒𝑝 = 𝐴𝑠𝑏

I2 detection limit approx. 0.005 g

0,00

The sum of AAg,dep and AAg,sus is the total reactive
Ag area used in the kinetic equation for the I2/Ag
reaction (Eq. 12). The procedure can also be
applied to the reaction of iodide (I-) with AgOx
particles. These are Ag particles with a AgOx
shell.

2

4

6

8

Time after start of I2 injection / h

Fig.12 THAI AW-3: Measured and calculated
iodine species masses in the stratified and wellmixed sump
The COCOSYS-AIM calculation on AW-3 was
performed by use of the variable reactive silver
surface described. The suspended Ag in phase
3.1 was taken into account.

V.B Validation of the concept
In the second part of THAI test AW-3 theinfluence of the reactive Ag surface on the I2/Ag
reaction in the sump was investigated. At the
beginning of the stagnant phase 30 g of Ag
aerosol were carefully deposited on the dry sumpbottom covering 1.3 m². Afterwards the sump
was filled with 500 l of water. . A pH 2 wasestablished using sulfuric acid to suppress I2hydrolysis. Then 0.68 g of I2 was released to thewater pool. The measured I2/Ag reactions in thestagnant phase 3.1 and the well-mixed phase 3.2
are shown in Fig.12.

In phase 3.2 when the maximum of silver wasavailable, the I2 mass decreased fast and the AgI
mass rose accordingly. The reaction rate constantfor the I2/Ag reaction, which is essentially thewater side mass transfer coefficient kw = 1.E-5
m/s, is probably underestimated under test
conditions. Additionally the AgI concentration
was overestimated because the settling of the AgI
particles was not considered.
Nevertheless the AW-3 test confirms clearly the
reduction of the reactive Ag surface when all or apart of the particles deposit on the sump bottom.
In reactor accident calculations it is necessary totake this effect into account. Without
consideration the I2 concentration in the waterWithin the first two hours of phase 3.1 the total
iodine mass in the sump decreased and the AgI
mass increased because approximately 0.5 g of
Ag has been released uncontrolled, probably due
to resuspension of fine Ag particles which
occurred either during injection process or due to192

10

NEA/CSNI/R(2016)5
and gas phases as well as the iodine source term
can be underestimated considerably.

an accurate wash-down simulation for insoluble
aerosols like silver which are source term
relevant has to be of major concern in accident
analyses.

VI) CONCLUSIONSThe COCOSYS-AIM-3 model for the washdown of iodine and aerosols was further
developed in three steps and validated on THAI
tests:- water layer model;- AULA model;- iodine/silver model (reactive Ag surface).

Finally the iodine/Ag model was improved byintroducing the “reactive Ag particle surface”. It
describes the loss of reactive surface area ofparticles settled on the sump bottom. Under
reactor conditions the Ag particles settle within a
few hours and consequently the iodine/silver
reaction is slowed down. This effect was clearlymeasured in the THAI test AW-3. It was
qualitatively well calculated with COCOSYSAIM, but the I2 concentration was overestimatedin the first hours of both test phases. The I2/Ag
reaction rate constant used might have been too
small. This discrepancy will be investigatedfurther.

The new water layer model for condensing
conditions calculates the transfer of gaseous I2
into the water layer, the iodine chemistry within
the layer and the reaction with the surface
material, e.g. paint. Iodine diffusion andtransport with the condensing steam areconsidered. The model is suitable for film anddroplet condensation.

In current COCOSYS calculations the sump is
treated to be well-mixed. In a stagnant sump I2,
Ag particles and other material can be distributed
inhomogeneously in the water. With the new
COCOSYS module CoPool it is possible tosimulate the temperature distribution and local
flows in a reactor sump. It is planned to extend
the module for the transport of dissolved andsuspended material. Then the simulation of
inhomogeneously distributed iodine species andlocally occurring reactions will be possible.

One outcome of the model application is that theiodine adsorption onto paint can be higher with alow condensation rate than with a high one. Thiswas clearly confirmed in test Iod-24.
Another test outcome is that iodine once
adsorbed by the paint will not be washed off
again, i.e. only little physisorbed I2 remains on
the paint. This behavior is well reproduced by the
new water layer model.
As seen in an earlier interpretation of PHEBUS
test FTP1 the water layer can be a considerable
source of radiolytically produced I2. This andother reactions, like the iodine/silver reaction, areregarded in the new water layer model.

In a nuclear aerosol soluble and insolublecomponents are well mixed forming compact
particles. Leaching of soluble material
distributed in the compact particles takes
considerably longer than the dissolution of purematerial. Moreover the dissolved material
released between the particles can weaken thecohesive forces and support erosion. Therefore
further wash-down tests are required with arealistic reactor typical aerosol of compact
particles consisting of soluble and insoluble
materials.

The newly developed model AULA describes
the erosion of insoluble particles by downflowing condensate under reactor conditions. The
erosion rate is calculated from aerosol properties
and hydrodynamic parameters. Contrary to large
particles (> 50 µm) the erosion process for small,cohesive particles takes significantly longer anddrops with time. The mass erosion constant,which describes the time behaviour of theprocess, can be obtained from tests. The THAIAW tests delivered a suitable data base. On
containment floors with slight inclination thewash-off is not very efficient. Rivulet flow
velocities may be too slow to initiate erosion andthe rivulets leave parts of the floor dry. Therefore

The good results with the improved wash-down
model in COCOSYS-AIM highlight theimportance of an adequate simulation of theinteraction of iodine chemistry with thermal
hydraulic and aerosol physics.

193

NEA/CSNI/R(2016)5
ACKNOWLEDGMENTS

7. R. B. Bird. W. E. Stewart, E. N. Lightfoot,“Transport Phenomena”, J. Wiley Sons, NewYork (1960)

The work was funded by the German Federal
Ministry for Economic Affairs and Energy(BMWi) in the frame of the German reactor
safety research projects RS 1508, 1514 and 1532.

8. S. Hellmann, F. Funke, G.-U. Greger, A.
Bleier, W. Morell, "The reaction between
iodine and organic coatings under severe
PWR accident conditions an experimental
parameter study", OECD Workshop on theChemistry of Iodine in Reactor Safety, PSI,
Würenlingen, Switzerland, June 10 12, 1996

NOMENCLATURE
AIM = Advanced iodine model
AULA = Abwaschen unlöslicher Aerosole
COCOSYS = Containment Code System
GEHOPON = Trademark of a German
decontamination paint
THAI = Thermal Hydraulics Hydrogen Aerosol
Iodine9. S. Gupta, F. Funke, G.Langrock, G. Weber,
B. von Laufenberg, E. Schmidt, M. Freitag,
G. Poss, ”THAI experiments on volatility,
distribution and transport behaviour of iodine
and fission products in the containment”,
OECD-NEA/NUGENIA-SARNET Workshop
on the Progress in Iodine Behaviour for NPP
Accident
AnalysisandManagement,
Marseille (France), March 30, April 1, 2015

REFERENCES
1. B. Clément, L. Cantrel, G. Ducros, F. Funke,
L. Herranz, A. Rydl, G. Weber and J.C.
Wren, “State of the Art Report on Iodine
Chemistry”, Nuclear Energy Agency Report,
NEA/CSNI/R(2007)1, (2007)

10. M. Hoehne, G. Weber,” Interpretation of theOECD THAI CsI Aerosol Wash Down Test
AW by COCOSYS analyses”, OECD THAI
Seminar, Paris, 6 and 7 Oct., 2010

2. W. Klein-Heßling, S. Arndt, G. Weber,
H.Nowack, C. Spengler, S. Schwarz"COCOSYS V 2.4 user's manual", GRS P3/1, March 2015

11. G. Weber, ”Ein neues Abwaschmodell für
unlösliche Aerosole (AULA)”, GRS-TNWEG-01/2011, Aug. 2011

3. G. Weber and F. Funke, “Description of theiodine model AIM-3 in COCOSYS”, GRS
report A-3508, November 200912. Shields,
A.,"AnwendungderÄhnlichkeitsmechanik
undderTurbulenzforschungauf
dieGeschiebebewegung“,
Mitt.
Preuss.
Versuchsanst. Wasserbau u. Schiffbau,
Berlin, 26, 26 (1936)

4. F. Funke, S. Gupta, G. Weber, G. Langrock,
G. Poss, “Interaction of gaseous I2 withpainted surfaces and aerosols in large-scale
THAI
tests”,
OECD-NEA/NUGENIASARNET Workshop on the Progress in Iodine
Behaviour for NPP Accident Analysis andManagement, Marseille (France), March 30,
April 1, 2015

13. Guo, J., “Hunter Rouse and Shields diagram”,
Advances in Hydraulics and Water
Engineering,
Proc.
13th
IAHR-APD
Congress, Vol. 2, Singapure (2002)

5. Gupta, S., G. Langer, “Aerosol Wash-down
Test (AW), Technical Report Becker
Technologies 1501326-AW-QLR, Dec. 2009

14. “Erosion and Sedimentation Manual”, U.S.
Department of the Interior, Bureau of
Reclamation, Denver, Colorado (Nov. 2006)

6. L. Bosland, G. Weber, W. Klein-Hessling, N.
Girault, and B. Clement, “Modeling andinterpretation of iodine behavior in PHEBUS
FPT-1 containment with ASTEC and
COCOSYS codes”, Nuclear Technology, Vol.
177, pp 36-62, Jan. 201215. B. v. Laufenberg, M. Colombet, M. Freitag.,“Wash-down Test of Silver Aerosol from
Stainless Steel and Painted surfaces“,
NUTHOS Conf., Okinawa (Japan), 14 – 19
Dec. 2014
16. G. Weber, B. Krzykacz-Hausmann, F. Funke,
W. Klein-Hessling, “Uncertainty andSensitivity Analysis on the Iodine Model in

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theContainment
Code
COCOSYS”,
NUTHOS-10 Conference, Okinawa, Japan,
14-18 Dec. 2014

195

NEA/CSNI/R(2016)5
Effects of constituents of seawater on formation of volatile iodine by aqueous phase radiation
chemistry
Kuniki Hata (1)*, Kentaro Kido (1), Yutaka Nishiyama (1), Yu Maruyama (1)(1)

Japan Atomic Energy Agency, Tokai-mura, Ibaraki, Japan*Corresponding author, tel: (+81) 292826778, Fax: (+81) 292825406, Email:hata.kuniki@jaea.go.jp

Abstract – Model calculations of radiolysis of seawater with iodide were carried out to predict effects ofseawater constituents on iodine chemistry at the time of a severe accident. Through the calculations basedon gamma-radiolysis of solutions at ambient temperature, it was found that the production of moleculariodine (I2) was promoted by the addition of seawater constituents. Especially, Br- and HCO3- had a large
impact on its production. The production yields of radiolytic species were affected by the addition of I - aswell. It was also shown that the pH of seawater is an important parameter to determine radiolytic
conditions of iodine. These results imply the potential that the injection of seawater has significant impact
on iodine source term.

196

NEA/CSNI/R(2016)5of the OH behavior in water radiolysis, iodine
chemistry may be also affected if seawater is
injected in reactors right after an accident occurs.
However, there is little information about iodine
chemistry in irradiated seawater. In this study,
the authors applied a reaction set of I− on thecurrently developed radiolysis model6-8 and some
calculations were carried out to predict effects ofthe constituents of seawater on oxidation of I− inwater radiolysis.

I. INTRODUCTIONRadioiodine is a hazardous fission product which
could be released from fuels of nuclear reactors
when a loss-of-coolant accident (LOCA) occurs.
Because it could form highly-volatile species,
radioiodine has been a big concern of the severe
accident management. It is known that the
production of volatile iodine is mainly initiatedby radiolysis of coolant water. Water molecules
are ionized by energy deposition of radiation,
which results in the production of reactive
species, such as hydroxyl radical (OH) andhydrated electron (eaq−).
H2O -^^^-> OH, eaq-, H,   .

II. CALCULATIONS
To build up the model of seawater radiolysis, 0.5
mol dm-1 Cl−, 8.0  10-3 mol dm-1 Br− and 2.3 
10-3 mol dm-1 bicarbonate ion (HCO3−) weretaken into account. This seawater-simulated
solution is called “AQ1” in this paper. The
authors have confirmed that AQ1 was equivalent
to artificial seawater in terms of waterradiolysis.7 The other constituents, such as borate
ion and sulfate ion, were ignored because of their
low reactivity with radiolytic species. Theinteractions of microorganisms and organic
compounds were also neglected in thiscalculation. As effects of cations contained inseawater on the radiolysis is considered to be notimportant, only sodium cation (Na+) was
included in this model to maintain chargebalance of the solutions. It is known that
dissolved oxygen (DO) suppresses the
production of I2 from irradiated water.9 However,the concentration of DO generally changes withchanging of the concentration of solutes in water.
The concentration of DO in seawater, forexample, is known to be about 5ppm, which islower than that of fresh water, 8ppm. In thisstudy, to focus on the effects of each seawater
constituents on the iodine behavior, allcalculations have carried out in deaerated
conditions. The reaction sets used in thiscalculation were taken from the report by Elliot
10
for reactions of water radiolysis and the reportby Kelm 2 for the reactions of the solute ions. To
simulate the reactions of iodine, the reaction set
applied in an iodine chemistry simulation tool,
Kiche, was used.11 Interaction of radical anions
derived from halide ions, Cl2− and Br2−,reported by Ershov et al. were also taken into
account.12,13 Some important reactions for the
production of I2 are summarized in Fig. 1 and

(R1)

−

In the coolant water, iodide (I ), which exists as aresult of dissociation of associated chemical
species such as cesium iodide (CsI), is easily
oxidized by OH, and iodine atom (I) could be
produced. Because of low stability of I, it reacts
with I− or another I, and eventually volatile
molecular iodine (I2) is produced. Thus, it isquite important to understand the behavior of
OH in radiolysis processes to estimate
production
yieldsofvolatile
iodineappropriately.
In March 2011, the severe accident occurred at
the Fukushima Daiichi Nuclear Power Station(NPS) as a result of the station blackout. As an
accident management measure, lots of seawater
was injected into the cores and the fuel pools tocool the fuels. The concentration of chloride(Cl−) in the contaminated water of Unit 2 wasreported to be 16,000 ppm (0.45 mol dm-1) in
July 2011.1 Reactions of ions in seawater, such as
Cl− and bromide (Br-), with OH are generally
known to be very high.2-5 Because of the highconcentration of Cl−in the reactors, it is presumed
that the radiolytic conditions in reactors at the
Fukushima Daiichi NPS were affected by thereactions of these ions. The authors havedeveloped a seawater radiolysis model by
combining the reaction sets of seawater
constituents and have provided some calculation
results which show that the production ofradiolytic products, such as hydrogen molecules(H2), are promoted by the addition of theseseawater constituents as a result of the reactionsof these ions with OH.6-8 Because of the change197

NEA/CSNI/R(2016)5
their rate constants are shown in TABLE I. It is
known that halide ions are generally effective
OH scavengers. However, the reactivity of Cl− isrelatively lower than that of Br− at neutral pH
conditions because of the high reaction rate of
the back reaction of R2 as well as the low
reaction rate of R3 which is dependent on pH.14,15
All the calculations were performed using
FACSIMILE.16 Computer calculations were
designed for the “homogeneous” stage ofradiolysis, which starts 10−7 s after the energy
deposition of ionizing radiation to watermolecules. The primary chemical yields ofradiolytic species at this point were importantparameters for performing the calculations. In
this study, the primary yields measured by
Pucheault et al. were used for concentrated Clsolutions.17 In their report, there is a small
difference between oxidative species (i.e.,dichloride radical anion (Cl2−), OH, andhydrogen peroxide (H2O2)) and reductive species(i.e., eaq−, hydrogen atom (H), and H2). The
authors considered this difference to be the yieldof ClOH−. For calculations of solutions which
are free of Cl-, the primary yields of pure water
reported by Elliot was applied.10 The primaryyields utilized for the calculations are listed in
TABLE II. If concentrations of Cl− are high,these rate constants vary due to the ionic strengthof the solution. The effect of ionic strength wasprogrammed using the extended Debye-Hückel
law. All solutions in this study were assumed tobe irradiated with gamma-ray at ambient
temperature. The dose rate was 1.0 Gy s-1.

Fig. 1. Chemical reactions of ions in waterradiolysis.

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NEA/CSNI/R(2016)5
TABLE I. Rate constants of the chemical
reactions of halide ions in water radiolysis. 2,11-13

by diluting these ions by less than 20 %. The pHof the solutions increased with diluting seawater
constituents. This result indicates that theoxidation of I- is suppressed by the pH change or
promoted by the addition of Cl-, Br-, or HCO3-.
According to Fig. 2 (a), I2 concentration
gradually reached steady-state during irradiation;
however, I2 is a highly volatile species. If thissystem faces the gas phase, the production of I2
may become progress continuously.
To identify the mechanism of I2 production in
irradiated seawater, calculations for the solutions
which did not contain these ions were carriedout. The time-dependent change of theconcentration of I2 produced in the solutions
containing 1.0  10−5 mol dm−1 of I− is shown in
Fig. 3(a). The concentration of I2 in the solution
without Cl− was similar to that of AQ1. Becauseof the low reactivity of Cl− with OH, it may notaffect the oxidation of I−. On the other hand, theconcentration of I2 significantly decreased for thesolutions without Br− or HCO3−. According to theauthors’ previous calculations, HCO3−was lessimportant for radiolysis of seawater.6 However,the calculation result in Fig. 3(a) indicates thatthe presence of HCO3− significantly affects theoxidation of I−. Fig. 3(b) shows the timedependent change of pH of these solutions. The
pH of the solution without HCO3− gradually
increased and exceeded 8. The increase in pH isknown to suppress the oxidation of I−.18 It isconsidered that HCO3- plays an important role tocontrol the pH of AQ1. On the other hand, Braffects the I2 production without such a
noticeable pH change. Bromide ion is considered
to be one of the most important species forseawater radiolysis because of its high reaction
rate with OH.6 As OH is the most reactive
reactant for I- to produce I2, it is presumed that
OH scavenging property of Br- affects the iodine
behavior. Because of lower reactivity of I− thanthat of Br− in the calculation condition, most of
OH is thought to be scavenged by Br−, anddibromide radical anion (Br2−) is produced. It isreported that this radical easily oxidizes I−
instead of OH (reaction R24).13 As a result, a lotof I2 is produced. If Br− is removed from AQ1, it
is assumed that OH can oxidize I− directly or
become a water molecule through recombination
reactions because of the low concentration of I−,

TABLE II. Primary yields of concentrated Clsolutions (molecules/100 eV). 16

III.
EFFECTOFSEAWATERCONSTITUENTS ON THE PRODUCTION OF
I2
To predict effects of the constituents of seawateron oxidation of I− in water radiolysis,
calculations of AQ1 containing I− wereconducted. Fig. 2 shows the time-dependentchange of the concentration of I2 and pH in waterof diluted AQ1 containing 1.0  10−5 mol dm-3 I−
up to 100 s. The production of I2 was suppressed
199

NEA/CSNI/R(2016)5
which results in the suppression of I2 production.
To confirm this assumption, calculations of thesolutions containing 1.0  10−3 mol dm−1 of I−
were carried out. In this condition, OH
scavenging property of I− is equivalent to that of
Br−. The calculation results showed that the timedependent changes of I2 concentration in AQ1
containing Br− and AQ1 without Br− werecompletely the same, indicating that I− isoxidized by Br2− which is produced from thereaction of Br− and OH or by OH directly
before OH returns to a water molecule. These
results imply that Br− plays an important role in
the oxidation of I− when the concentration of I− isvery low.

It is known that the constituents of seawater
affect the behavior of radiolytic products, such as
H2.6-8 To predict the effects of iodide on the
behavior of the radiolytic products, radiolysis
calculations of deaerated AQ1 containing 1.0 
10-6 - 1.0  10-4 mol dm-3 I− were started at pH 7.
Fig. 4 shows the time-dependent change of H2
concentration during irradiation up to 2  104 s.
Hydrogen molecule for AQ1 in the absence of I−
was produced continuously and reached its
saturating concentration (~ 8 mol dm-3) in 2  104
s. The continuous production of H2 from
irradiated seawater was reported by gammaradiolysis experiments.19 It has been consideredthat this H2 production is due to the OH
scavenging reactions of Cl− and Br− because OH
is an effective H2 scavenger. According to thecalculation result, the production of H2 was
suppressed by the addition of I−. In the reaction
set prepared for this study, H2 can be also
scavenged by Cl2− and I in the presence of theions (reaction R7 and R18 in Fig. 1). The
reaction between H2 and Cl2− was introduced by
Kelm et al. to fit their calculation results to the
gamma-radiolysis experimental results using
autoclaves;20 however, they withdrew thisreaction from their reaction set later.21 In either
case, the concentration of Cl2− during irradiation
was so low (< 1.0  10-13 mol dm-3) that thereaction of Cl2− cannot be an important reaction
in radiolysis of AQ1. On the other hand, theconcentration of I, which is another possible H2
scavenger, was much higher (~1.0  10-9 mol dm3) than that of Cl2− during irradiation. The
authors presumed that the suppression of H2
production in the presence of I− was mainly due
to the H2 scavenging reaction of I. To verify theeffect of this reaction on the suppression of H2
production, radiolysis calculation of AQ1
containing I− excluding the reaction of H2 and I
was carried out. The result showed that the
production rate of H2 was the same as that of
AQ1 in the absence of I−, which indicates that thereaction of I with H2 is quite important toestimate the yield of H2 production. The rate
constant of this reaction was determined to
explain the experimental results of gammaradiolysis.10 Direct measurement of this reactionis thought to be important to prepare a proper

Fig. 2 Time-dependent change of (a) theconcentration of I2 and (b) pH in diluted AQ1 orwater containing 1.0  10−5 mol dm-3 I−. The
percentage means the dilution rate of AQ1.

Fig. 3. Time-dependent change of (a) theconcentration of I2 and (b) pH in AQ1 and thesolutions which does not contain each ion
containing 1.0  10−5 mol dm−1 of I−.

IV. EFFECT OF IODIDE ON SEAWATERRADIOLYSIS

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NEA/CSNI/R(2016)5
reaction set of radiolysis model for severe
accidents.

steady-state pH during irradiation. There is little
experimental evidence about pH change ofseawater by irradiation. Some experiments arenecessary to show the validity of this calculation
result.

Fig. 5. Time-dependent change of (a) theconcentration of I2 and (b) pH in AQ1 containing
1.0  10−5 mol dm−1 of I−. The initial pH of thesolution was changed from 6 to 9.

Fig. 4. Time-dependent change of theconcentration of H2 produced in AQ1 containing() 0, (  ) 1.0  10-6, (   ) 1.0  10-5,and () 1.0  10-4 mol dm-3 I− during
irradiation.

VI. CONCLUSION
To estimate effects of constituents of seawater on
iodine chemistry during a severe accident,
calculations using the model of seawaterradiolysis were carried out. It was found that the
production of I2 was promoted by the addition ofseawater constituents. Especially, Br- and HCO3had a large impact on it. On the other hand, theproduction yields of radiolytic species wereaffected by the addition of I-. It was also shownthat the pH of seawater is an important parameterto determine radiolytic conditions of iodine. The
calculation results in this study provided an
important aspect of iodine chemistry in thepresence of seawater. However, at the time of an
accident, the other factors, such as concentrationsof organic compounds and dissolved oxygen andtemperature, have to be taken into account. In
future, by considering effects of these factors aswell as the effects of ions in seawater, the mostimportant parameters which control the iodine
behavior in irradiated seawater will be discussed
to predict I2 production from a coolant which is
injected seawater.

According to the H2 concentrations in Fig. 4, theproduction rate of H2 increased with increasing I−
concentration. This might be due to the reactionof I− with I, which results in the release of H2from scavenging reaction by I.
V. EFFECT OF INITIAL pH CONDITIONS
It is known that pH is one of the most importantparameters for the oxidation process of I−. Toestimate the effect of initial pH on the oxidationof I−, radiolysis calculations of AQ1 containing
1.0  10−5 mol dm−1 I− at different initial pH
conditions were conducted. Fig. 5 (a) shows theI2 concentration during irradiation. It was foundthat the production rate of I2 was decreased
significantly when the initial pH was adjusted tobe 9. The time-dependent change of pH is also
shown in Fig. 5 (b). The pH changed quickly
right after starting irradiation. The pH of thesolutions adjusted its initial pH from 6 to 8
became 7.1 – 7.2. On the other hand, the pH of
the solution adjusted its initial pH to 9 wereslightly higher. The difference of the I2
production between the AQ1 at different initial
pH is thought to be caused by the difference of

ACKNOWLEDGMENTS
The authors would like to express their gratitude
to Professor Katsumura and Dr. Yamashita at the
University of Tokyo and Dr. Muroya at Osaka201

NEA/CSNI/R(2016)5
University for their support in coding thesimulation model of water radiolysis.

Hwa Nuclear Society, Taichung, Taiwan(2013).
8. K. Hata, S. Hanawa, S. Kasahara, T.
Motooka, T. Tsukada, Y. Muroya, S.
Yamashita, and Y. Katsumura, “Effects ofdissolved species on radiolysis of diluted
seawater,” Proc. of Nuclear Plant Chemistry
Conference (NPC2014), Atomic Energy
Society of Japan, Sapporo, Japan (2014).

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9. K. Moriyama, S. Tashiro, N. Chiba, F.
Hirayama, Y. Maruyama, H. Nakamura, andA. Watanabe, “Experiments on the Releaseof Gaseous Iodine from Gamma-IrradiatedAqueous CsI Solution and Influence of
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2. M. Kelm and E. Bohnert, “A kinetic modelfor the radiolysis of chloride brine, Itssensitivity against model parameters and acomparison with experiments,” FZKA 6977,(2004).

10. A. J. Elliot and D. M. Bartels, “The reaction
set, rate constants and g-values for thesimulation of radiolysis of light water over
the range 20 to 350C based on information
available in 2008”, AECL EACL 153127160-450-001 (2009).

3. S. Sunder and H. Christensen, “Gamma
radiolysis of water solutions relevant to the
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Nuclear Technology, 104, 403-417 (1993).
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aqueous bromide ions by hydroxyl radicals.
A pulse radiolytic Investigation,” The
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11. K. Moriyama, Y. Maruyama, and H.
Nakamura, “Kiche: A simulation tool for
kinetics of iodine chemistry in ther
containment of light water reactors under
severe accident conditions,” JAEAData/Code 2010-034 (2011).

5. Z. Cai, X. Li, Y. Katsumura, and O. Urabe,“Radiolysis of bicarbonate and carbonate
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12. B. G. Ershov, E. Janata, and A. V. Gordeev,“Pulse radiolysis study of I− oxidation withradical anions Cl2− in an aqueous solution,”
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13. B. G. Ershov, E. Janata, and A. V. Gordeev,“Pulse radiolysis study of the oxidation of
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Formation and release of molecular iodine in aqueous phasechemistry during severe accident with seawater injection(1)

Kentaro Kido(1)*, Kuniki Hata(1), Yu Maruyama(1), Yutaka Nishiyama(1) and Harutaka Hoshi(2)Japan Atomic Energy Agency, Tokai-mura, Ibaraki, Japan, (2) Nuclear Regulation Authority, Minato-ku,
Tokyo, Japan*Corresponding author, tel: (+81) 29-282-6159, Fax: (+81) 29-282-6147, Email:kido.kentaro@jaea.go.jp

Abstract – Seawater injection into the degraded core is one of the measures of accident management as it
has been performed at Fukushima Daiichi nuclear power plant. The constituents in seawater deeply relates
to the iodine chemistry in the water pool of the suppression chamber, which indicates that it is important toassess their effect on the source term in a severe accident. In the present study, by employing a fourcomponent seawater (SW) model we try to simulate the time evolution of I2 molecules yielded in aqueous
solution based on several datasets on chemical reaction kinetics and to evaluate its fraction for the initial
inventory released from aqueous solution to gas phase.
The amount of I2 molecules in gas phase was in proportion to the mixing ratio of the SW model. The
combination of bromide and hydrogen-carbonate anions considerably contributes to the behavior of thetime evolution of I2 fraction. The oxygen molecules in aqueous solution drastically reduced yielding I2
molecules by catalytically consuming hydroxyl radicals, while I2 molecules increased by the carbon
dioxide gas involved in gas phase. The effects of both SW constituents and carbon dioxide gas arerecommended to be considered in the quantitative discussion on the I2 molecules released from aqueous
solution

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TABLE IThe gas-liquid partition constants and the mass transfer coefficients for volatile molecules used in the present study. The
latter values are given in cm/s.
10-3 mass transfer coeff.

partition const.
molecule
all systems

system I

N2

0.0228

3.55

46.6

H2

0.0279

4.37

46.6

O2

0.0473

4.02

69.8

I2

71.3

2.49

11.7

CO2

0.85

3.43

66.1

Cl2

1.52

2.89

43.6

I. INTRODUCTION

system II, III and IV

Atomic Energy Agency (JAEA) [7-9]. The
results implied that the constituents of seawater
had a non-trivial impact on the I2 yielding inaqueous solution. Also, by using chemical
kinetics simulation software, KICHE [10], theiodine chemistry under radiation dose in purewater has been investigated. However, limited
knowledge is currently available for the effect of
seawater constituents on the iodine chemistry.

Radioactive iodine is a representative source
term in a severe accident of nuclear power station
due to the high volatility of its chemical species
such as I2 molecule and organic iodine (typicallymethyl iodide). The chemical species are yieldedin aqueous solution, e.g., the water pool of the
suppression chamber, by reacting radioiodine
compounds released from the fuels with various
radicals yielded by water radiolysis. Especially,hydroxyl radicals play an important role in thechemical reactions. The amount of the iodine
species strongly depends on not only the energy
deposition of radiation but also the other solute
molecules in the aqueous solution. For example,
chloride and bromide anions, which are involved
in seawater, are well-known to the scavengers ofhydroxyl radical [1-4] and can not be negligible
to quantify the release of iodine species.

In the present study, based on several datasets on
the kinetic information about chemical reactions
among seawater constituents, we try to simulate
the time profiles of their concentrations by
employing a four-component seawater (SW)model aqueous solution [8]. The aqueous
solution involves sodium cation, chloride,hydrogen-carbonate and bromide anions. We
also try to evaluate the amount of I2 molecules
released from the aqueous solution to gas phaseas the function of time. In addition, the effects ofoxygen and carbon dioxide gases areinvestigated. To perform these simulations, we
utilize KICHE software by updating the chemical
reaction dataset and by adding datasets to treat
chemical species involved in the SW model. It isnoted that, different from FACSIMILE, gas
phase can be treated by the interactions withaqueous solution through the gas-liquid interface
in the KICHE framework. The transfer ofvolatile molecules yielded in aqueous solution is

Seawater injection into the degraded core is ameasure of severe accident management in
nuclear power plant. In practice, in the accident
at the Fukushima Daiichi nuclear power station,
seawater was injected in the degraded core to
mitigate the accident consequences [5]. It is notclear that how a variety of constituents inseawater have an influence on yielding the iodine
species. In order to study the effect of seawater
constituents on yielding the iodine species, byusing FACSIMILE software [6] a series ofparametric analysis has been performed at Japan
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NEA/CSNI/R(2016)5
described by both gas-liquid equilibrium andinterfacial mass transfer.

is 298.15 K. The gamma-ray dose rate is set to
3.6 kGy/h (=1.00 Gy/s). The radiation iscontinued for five hours from 0h. The initial pH
in these three systems is set to 7.0 and totalpressure is 1.0 atm.
B.

In order to study the effect of seawater injection
on the I2 molecule yielding, we employ aseawater model which consists of sodium
chloride (0.500 mol/L), sodium hydrogencarbonate (2.30×10-3 mol/L) and sodium bromide(8.00×10-4 mol/L). In this article, the aqueous
solution is called “SW”. Various mixtures of SW
and pure water are used in system II, III and IV.

Fig. 1. The schematic pictures of system I, II, III
and IV.
II. Method and Models
A.

Models for chemical processes

Systems

In this study, four-type systems consisting ofgas phase and aqueous solution (system I, II, III
and IV) are investigated as shown in Figure 1.
The gas phase of system I (0.286 L) is filled witha realistic air (N2: 80%, O2: 20% and CO2:
0.032%, total pressure is 1.0 atm) and is swept
by the same gas at a constant flow rate (1.00
L/min = 0.0167 L/s). The aqueous solution(0.100L) contains 1.00×10-4 mol/L of cesium
iodide and 2.67×10-4 mol/L of oxygen molecules.
It is buffered by 1.00×10-4 mol/L of boric acidsodium hydroxide. The interfacial area amonggas phase and aqueous solution is 56.7 cm2. The
temperature is 298.5 K. Although thetemperature fluctuates from 288.0K to 298.5K inthe experimental condition, in this simulation wekeep the temperature constant because thereaction rates in the present dataset are obtained
at 298K [1, 12-15]. The gamma-ray dose rate is
set to 7.2 kGy/h (= 2.0 Gy/s) for 1.0×10-3 mol/Lof Cl- concentration whereas 6.1 kGy/h (=1.69
Gy/s) for 1.0×10-2 mol/L and 1.0×10-4 mol/L of
Cl- concentrations. The radiation is carried out
for two hours from 0h. The initial pH is set to7.0. The conditions are based on the experiments
previously performed at JAEA [11].

To simulate radiolysis occurring in aqueous
solutions of the systems I, II, III and IV, thechemical reaction list published by Elliot et al.
[12] was employed to express the waterradiolysis. The reaction datasets were taken from
Kelm et al. [1] to treat the interactions between
chemical species yielded by water radiolysis andanions in the SW model aqueous solution. The
parameters were used for primary yields inconcentrated Cl- solutions reported by Pucheault
et al. [13]. To take the chemical reactions ofiodine species into account, the reaction dataset
implemented in the original KICHE software
was employed. The interactions among theradical species yielded from chloride, bromide
and iodide anions were simulated by thechemical reactions reported by Ershov et al.
[14,15] It is quite critical to incorporate thesereactions because both chloride and bromide
anions can strongly scavenge hydroxyl radical
which deeply relates the I2 molecule yielding.
Since the effect of cations on the radiolysis isnegligibly small, the sodium cations are treatedto satisfy the charge neutrality of solution. In thepresent study, the ionic strength is not taken into
account.

The volume of gas phase and aqueous solution insystems II, III and IV is 1.00 L with the 100 cm2
interfacial area. 1.00×10-5 mol of sodium iodide
is added to each aqueous solution. In system II,
the gas phase is filled by only N2 gas. In contrast,
the gas phases in systems III and IV involve 20%of oxygen gas. Only system IV is swept by air
with a flow rate of 1.00 L/min. The temperature

To realize this simulation, we utilize KICHE
software by incorporating chemical reaction
datasets mentioned above. Although the original
KICHE can treat chloride and hydrogencarbonate anions, the chemical reactions are very
simplified. In the KICHE framework, the volatile
molecules in aqueous solution can transfer to gas
phase in terms of gas-liquid partition and206

NEA/CSNI/R(2016)5
interfacial mass transfer [17-19]. The partition
constants and mass-transfer coefficients used in
the present study are summarized in Table 1. In
the KICHE manual [10], readers can obtain moredetailed information for their assumption and themethod to calculate them.

performed at JAEA as a joint research with theformerJapan
Nuclear
EnergySafetyOrganization (JNES) to investigate the influenceof chloride anion on yielding I2 molecules inaqueous solution. Since the aqueous solution
involves no bromide anion, the datasets ofchloride and hydrogen-carbonate anions arechecked.
Figure 2 shows the simulated andexperimental time evolutions of the fraction of I2
molecule for the initial inventory transferredfrom aqueous solution in system I. As mentioned
above, the radiation is cut at 2.0h. The former
results are represented by the blue dotted(1.00×10-4mol/Lof
chlorideanion
concentration), solid (1.00×10-3 mol/L) anddashed (1.00×10-2 mol/L) lines. The latter one isrepresented by the open squares (1.00×10-4
mol/L), triangles (1.00×10-3 mol/L) anddiamonds (1.00×10-2 mol/L). First, let uscompare the simulated time evolutions with theexperimental ones. In the simulation results, thefraction is linear proportional to time andbecomes small by increasing the Clconcentration. The trend does not appear in theexperimental results. In 1.00×10-3 mol/L case(open triangles), there is a shoulder in the time
profile not reproduced in the simulated one. In
the other two cases (open squares and diamonds),
although the fraction is in linear proportion totime, the difference does not become small fromthe corresponding simulation result as time
evolves. Next, we discuss the final fraction at
2.5h summarized in Table 2. The simulation
seems to show good agreement with theexperiment except for 1.00×10-4 mol/L of Clconcentration in spite of the difference of time
evolution due to the short radiation time (twohours). In the 1.00×10-4 mol/L case, thesimulation rather overestimates the final fraction.
These discrepancies among the simulation andexperiment remain unclear and need further
experimental information to improve the model.

Fig. 2. Time profiles of the fraction of I2
molecules released from system I. Blue lines
correspond to the simulation results. The dotted,
solid and dashed lines represent 1.00×10-4 mol/L,
1.00×10-3 mol/L and 1.00×10-2 mol/L of Clconcentration, respectively. The experimental
results are plotted by the open squares (1.00×10-4
mol/L), triangles (1.00×10-3 mol/L) anddiamonds (1.00×10-2 mol/L).
TABLE II
The comparison of final fractions of gaseous I2
molecules in system I. The Cl- concentrations are
given in mol/L.
Cl- conc.

this work

exptl.

1.0×10

-4

0.112

0.077

1.0×10

-3

0.107

0.110

1.0×10-2

0.081

0.092

Although not shown in Figure 2, we also
compare the simulation results with those by theoriginal KICHE reaction dataset. The original
dataset overestimates the time evolutions for any
Cl- concentration than those by the present one.
This is partially explained as follows; by theIII. Results and Discussion
A.

Benchmark test

At first, using system I, we examine the validityof chemical reactions newly added to the original
KICHE dataset by comparing with the smallscale fundamental experiments previously
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NEA/CSNI/R(2016)5
dataset modification, a few reactions involvinghydroxyl radical are newly added, for example,

volume fractions (100% (pure SW): blue solidline, 75%: blue dotted, 50%: blue short-dashed,
25%: blue dotted-dashed, 10%: blue longdashed, 0% (pure water): black solid). The blackline seems to be nearly flat except the low peakvery near 0h, which shows that in pure water theI2 yielding reaches the equilibrium around 1h. In
contrast, the blue solid line (100% SW) has thehigh peaks in the vicinity of 0.13h. But after the
peaks, the I2 fraction exponentially decays andbecomes smaller than the black line near 2h,which presents that the I2 molecules in gas phasetransfer into aqueous solution by the gas-liquid
partitioning due to the decrease of I2 molecules inaqueous solution. These curvatures are similar
for all SW volume fractions. In addition, the I2
fraction is in proportion to the SW mixing ratio,
which is consistent with the results reported by
Hata et al. [20]. So, in the below let us discuss
the curvatures of 100% SW.

 OH  HClO  ClO  H2O(rate const. = 9.0×109 L mol-1 s-1). (1)This reaction constructs a new pathway toscavenge hydroxyl radicals by associating withthe following reactions; Cl  Cl  Cl 2
(rate const. = 8.8×107 L mol-1 s-1) (2)OH-  Cl2  HClO  Cl(rate const. = 6.0×108 L mol-1 s-1). (3)Since the hydroxyl radical is needed to yield I2
molecules, the pathway contributes to reducing
it.

Fig. 4. The fractions of I2 molecules in gas
phase of system II as the function of time. The
solid, dotted, dashed and dotted-dashed lines
correspond to pure SW, case A, case B and caseC, respectively. The small figure is an enlarged
view near 0h.

Fig. 3. The time evolutions of the fraction of I2
molecules in gas phase of system II. The blackline represents the aqueous solution not mixed
with SW (0%, pure water and sodium iodide).
The blue solid, dotted, short-dashed, dotteddashed, long-dashed lines correspond to 100%,75%, 50%, 25% and 10% of SW volume
fraction, respectively. The small figure is anenlarged view near 0h.
B.

The effects of mixing SW

Figure 3 plots the time evolutions of thefraction of I2 molecules in gas phase of system II
for the initial inventory and for various SW
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NEA/CSNI/R(2016)5
phase consists of air in system III, oxygen
molecules dissolve into aqueous solution withthe saturated concentration (2.67×10-4 mol/L) at
the initial condition. Figure 6 plots the time
evolutions of the I2 fraction in gas phase of
system III for the initial inventory (blue andblack solid lines). It is noted that the vertical axis
in this figure is scaled by 1/10 of that in Figure 3,
which shows that the I2 yielding decreases byoxygen molecules in aqueous solution. In thewater radiolysis, the following two chemical
reactions proceed much faster than system II
because of oxygen molecules in aqueous
solution.

O2  H  HO2
Fig. 5. The time profiles of the fraction of I2
molecules in gas phase of system II. The solid,
dotted, dashed and dotted-dashed lines
correspond to pure SW, case D, case E and caseF, respectively. The small figure is an enlarged
view near 0h.

(rate const. = 2.00×1010 L mol-1 s-1) (4)O2  e  O2
(rate const. = 1.94×1010 L mol-1 s-1) (5)The HO2 and O2 anion radicals consumehydroxyl radicals;

In order to clarify the contribution of eachanion involved in the SW model, we simulate
three cases by removing two kinds of anion fromthe SW model; only chloride anion (case A),
only hydrogen-carbonate anion (case B) and only
bromide anion (case C). The results are shown in
Figure 4. In any case, the solid line (pure SWcase) is not reproduced, which presents that thecurvature of pure SW model do not result fromthe contribution of the single anion. Furthermore,
we investigate the three types of combination;
bromide and hydrogen-carbonate anions (caseD), chloride and bromide anions (case E) and
chloride and hydrogen-carbonate anions (case F)
as shown in Figure 5. The dotted line (case D) is
fairly similar to the solid one (pure SW) except
around the peak, which reveals that the
combination of bromide and hydrogen-carbonate
anions is significant for the time evolution of I2
fraction, which is consistent with the analysis by
FACSIMILE reported by Hata et al. [20].
C.

 HO2  OH  O2  H2O(rate const. = 7.10×109 L mol-1 s-1) (6) O2  OH  O2  OH(rate const. = 9.96×1010 L mol-1 s-1). (7)As shown in Eqs. (4)-(7), the oxygen molecules
catalytically consume hydroxyl radicals.
Consequently, the chemical reaction of hydroxyl
radicals with iodide anions to yield iodide
radicals is strongly inhibited by the oxygen
molecules. The black solid line (0% SW) is far
below the blue solid one (pure SW). In the SW
model aqueous solution, the chemical species
yielded from Cl-, Br- and HCO-3 anions reactwith hydroxyl, HO2 and O2 anion radicals, which
weakens the role of oxygen molecules as theinhibitor of yielding I2 molecules. These results
are parallel with the discussion on the pure watercase.

The effect of oxygen gas

In pure water, the effects of oxygen and carbon
dioxide gas on the I2 yielding have been welldiscussed. In the present study, we investigate
the effects in the SW model aqueous solution. In
this section, the oxygen effect is discussed by thecomparison of system II and III. Since the gas
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NEA/CSNI/R(2016)5
into the gas phase of system III. The result isshown in Figure 6 with the blue dotted line. In
contrast to the oxygen effect, the carbon dioxide
makes the I2 fraction increase. This is because of
the pH decrease by CO2 gas dissolving into theaqueous solution. In practice, as shown in Figure
7, the pH decreases by about 0.5 units for twohours from 0h (blue dotted line) while it
increases in the condition without CO2 gas (bluesolid line). In a low pH condition, the chemical
reaction to consume I2 molecules in aqueous
solutionI2  OH-  I2OH
(rate const. = 1.00×1010 L mol-1 s-1) (8)becomes slower than a high pH condition [16].
Similar to the oxygen effect, the CO2 effect also
is parallel with the investigations in pure watercase.

Fig. 6. The time profiles of the fraction of I2
molecules in gas phase of system III. The blacksolid and dotted lines correspond to 0% SW
without and with carbon dioxide, respectively.
The blue solid and dotted lines represent pureSW without and with carbon dioxide,
respectively. The small figure is an enlarged
view near 0h.

By using system IV, the amount of I2
molecules released from the system by the sweep
is investigated. Except for the sweep, the otherconditions are the same as system III. Figure 8
shows the time evolutions of the fraction of I2
molecules released from the system. Similar tosystem III, the highest fraction is found in thecase of pure SW and with CO2 gas (blue dottedline). The fractions at 5h are 5.38×10-5 (0% SW
and without CO2 gas), 8.51×10-4 (0% SW andwith CO2 gas), 1.97×10-3 (pure SW and without
CO2 gas) and 1.59×10-2 (pure SW and with CO2gas), respectively. This result indicates that it iscritical for the quantitative evaluation of I2 gas
released from aqueous solution to incorporate theeffects of both seawater constituents and carbon
dioxide in gas phase.
X. SUMMARY
In this study, the I2 molecules yielded in andreleased from aqueous solution was investigatedwith a four-component seawater model tosimulate an iodine chemistry in the water pool ofsuppression chamber when seawater was injected
as a measure of accident management, based on
the several kinetic datasets of chemical reactions
for water radiolysis, chloride anion, bromide
anion and hydrogen-carbonate anion. The
software to simulate iodine chemistry in aqueous
solution, KICHE, was utilized to perform thecomputation. Four-type systems (I, II, III and IV)

Fig. 7. The time profiles of pH of aqueous
solution in system III. The black solid and dottedlines correspond to 0% SW without and withcarbon dioxide, respectively. The blue solid anddotted lines represent pure SW without and withcarbon dioxide, respectively.
D.

The effect of carbon dioxide gas

Finally, we examine the influence of carbon
dioxide on the I2 yielding in aqueous solution.
0.032% of the CO2 gas for total pressure is added210

NEA/CSNI/R(2016)5
were prepared to study the effects of seawater
constituents, oxygen gas and carbon dioxide gas.
In order to examine the chemical reaction
datasets newly added for chloride and hydrogencarbonate anions to the original KICHE dataset,the time evolutions of the I2 fraction for the
initial inventory was simulated for three chloride
anion concentrations. Although the result wascompared with the experiments, the simulation
did not show an agreement with the measured
time evolution. The error sources to cause thediscrepancy are not clear at the present.
Using system II, we investigated the effect of
the constituents involved in the SW model. The
I2 fraction in gas phase was in proportion to the
SW mixing ratio. The time evolution for pureSW model aqueous solution had sharp and highpeaks in the short range of time but became
smaller than the SW 0% case by the rapidly
decay after the peaks. We found that the
combination of bromide and hydrogen-carbonate
anions considerably contributed to its curvature.
In system III, the effects of oxygen molecules
dissolving into the aqueous solution werediscussed, compared with the result obtained bysystem II. The oxygen molecules drasticallyreduced the I2 yielding by catalyticallyconsuming hydroxyl radicals. On the other hand,by the carbon dioxide gas involved in air with arealistic partial pressure, the I2 molecules
increased to transfer from aqueous solution. Thiswas because of the pH decrease by CO2
dissolving into the aqueous solution. The similar
trend was provided in system IV with the sweepof gas phase by air. These results indicated thatthe effects of the SW mixing and the dissolutionof carbon dioxide gas are recommended to beconsidered in the quantitative discussion on the I2
molecules to transfer from aqueous solution.

Fig. 8. The fractions of I2 molecules released
from system IV as the function of time. The
black solid and dotted lines correspond to 0%
SW without and with carbon dioxide,
respectively. The blue solid and dotted lines
represent pure SW without and with carbon
dioxide, respectively. The small figure is anenlarged view near 0h.
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Tsukada, Y. Muroya, and Y. Katsumura,“Radiolysis
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9. K. Hata, S. Hanawa, S. Kasahara, T. Motooka, T.
Tsukada, Y. Muroya, S. Yamashita, and Y.
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JAEA-Data/Code, 2010-034 (2011).
11. K. Moriyama, S. Tashiro, N. Chiba, F. Hirayama,
Y. Maruyama, H. Nakamura, and A. Watanabe,“Experiments on the Release of Gaseous Iodine
from Gamma-Irradiated Aqueous CsI Solution
and Influence of Oxygen and Methyl Isobutyl
Ketone (MIBK),” Journal of Nuclear Scienceand Technology, 47, 229 (2010).
12. A. J. Elliot and D. M. Bartels, “The reaction set,
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15. B. G. Ershov, E. Janata, and A. V. Gordeev,“Pulse radiolysis study of the oxidation of the I−
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20. K. Hata, K. Kido, Y. Nishiyama, Y. Maruyama,“Effects of constituents of seawater on formationof volatile iodine by aqueous phase radiation
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13. J. Pucheault, C. Ferradini, R. Julien, A. Deysine,
L. Gilles, and M. Moreau, “Radiolysis ofconcentrated solutions. 1. Pulse and g radiolysis
studies of direct and indirect effects in LiCl212

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Session 3: organic iodide formation and partitioning, chemistry and interactions with aerosols,
droplets and surfaces in the gas phase213

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Chalmers - Project on volatile organic iodine species behavior and retention under severe
nuclear accident conditions in LWRs
S. Tietze¹*, M. R.StJ. Forema¹ and C. Ekberg¹(1)

Nuclear Chemistry, Chalmers University of Technology, Goeteborg (SE)*Corresponding author, Email:Tietze-Sabrina@gmx.de

Abstract – At Chalmers University of Technology extensive experimental studies on the volatile iodine
source term, particularly with a focus on the chemistry of organic iodides have been performed between
2009 and 2015. The formation, partitioning behavior between gaseous and aqueous phase, the gaseous
and aqueous phase radiolysis, the hydrolysis, the sorption behavior on painted surfaces, as well as
possibilities to remove volatile iodine species in dry, solid filter beds and wet-filter systems have been
investigated under conditions relevant during severe nuclear accidents in light water reactors.

214

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I. INTRODUCTION

traditionally used filter systems such as charcoal
filter and the wet-scrubber filter systems
containing a 0.015 M alkaline sodium thiosulfate
solution. However, the retention of organic
iodides is assumed and in some case also
determined to be significant lower for methyl
iodide. The knowledge obtained about theradiolysis and hydrolysis behavior of alsoorganic iodides other than methyl iodide wasused to investigate suitable filter materials toretain organic iodides in solid-filter beds and inwet-scrubber filter systems.

Within in a 5 year research project the impact oforganic iodides on the volatile iodine source term
was studied and compared to other volatile
iodine species at the Nuclear Chemistry
Department at Chalmers University ofTechnology.
The formation of organic iodides from different
sources of organic materials such as cable
plastics, paint films and pump oil wasinvestigated both qualitatively and quantitatively
under a series of severe nuclear accident relevant
conditions. Different organic iodides wereidentified to be released in significant highamounts and were furthermore investigatedtowards their partitioning behavior in thecontainment, as well as their interactions withcontainment surface materials. The different
chemical pathways resulted in the formation of
new iodine species that were found to be as wellpartially volatile or re-volatilizable and therefore
be able to (re)enter the volatile iodine cycle.
Varying containment conditions in both gaseous
and aqueous phase (pH, T, impurities, dose anddose rate), pyrolysis products, and radiolysis
products as well as other fission products such as
Cs and Ru, and the use of sea water for coolingwere taken into consideration. Attempts havebeen made to model the pathways of organic
iodides in the containment (e.g. OIPHA-1 andOIPHA-2 model) to predict the impact of organic
iodides on the volatile iodine source term.
Different currently used solid absorber and wetscrubber filter systems have been tested for theirability to retain the identified organic iodides andiodine model substances, and alternative
approaches
have
been
investigated.
A main focus of the project was on the effects of
epoxy paint films and their ageing behavior on
the iodine speciation, interactions and retention.
The obtained results were used for thedevelopment of modified paint formulations forpaint coatings that could act as passive iodine
traps inside the containment by permanently
retaining significant amounts of gaseous oraqueous iodine species. The potential release ofiodine into the environment is a remaining
concern in severe nuclear accident management.
Elemental iodine can be well retained in the

II. RESULTS AND DISCUSSION
This publication contains an overview of theresults obtained. Some of the results have been
already published in scientific journals, otherdata are still in progress to be published.
Particularly quantitative results will be reported
in the corresponding journal publications.
FORMATION OF ORGANIC IODIDESThe formation of organic iodides wasinvestigated from mainly epoxy paint films, butalso for cable plastics and pump oil that could
leak into the containment in case of a pipe break.
Therefore, a large material study of these organic
materials was performed [1]. The composition, as
well as the thermal and radiolytic degradation
behavior of common materials used in Swedish
nuclear power plants such as Teknopox Aqua
VA epoxy paint and chlorine rich CSPE and
PVC cable plastics was determined using
analytical methods like TGA, (pyro)-GC-MS andresidual solvent analysis. The release of the main
solvents and plasticizer were studied andinvestigated further. To determine thecomponents and the groups that form organic
iodides the cured paint, the single identified
components and synthesized surrogates of theresin were mixed with elemental iodine andexposed to gamma irradiation. It was found that
not only methyl iodide was formed but also a
range of other, longer chained organic iodides. It
was found that the bisphenol-A based resin itself
formed only very low amounts of methyl iodide
and at doses higher than 1 MGy some aromaticorganic iodides were formed (Fig. 1). The paintsolvents such as texanol ester alcohol and benzyl
215

NEA/CSNI/R(2016)5
alcohol, however, caused the formation of avariety of organic iodides [2]. The mostdominating ones are shown in Fig. 2. With
increasing dose the yield of methyl iodide
formation was found to decrease, whereas thefraction of longer chained organic iodides
increased. The same experiments were repeatedwith alpha radiation using tributyl borate as alpharadiation source. The neutron activation was
performed at Berkeley University. The higher
LET radiation caused the formation of higher
yields of longer chained organic iodides.

A

B
Fig. 3. A) Bisphenol-Z based resin B) Vegetableoil based resin
Fig. 1. MeI formation from the two methylgroups associated with the aromatic rings of thebisphenol-A unit in the bisphenol-A based resinof Teknopox Aqua VA epoxy paint.

Another way to reduce the formation of organic
iodides in the containment that was investigated
was the addition of an additive that can reactwith elemental iodine but does not rerelease it
under accident conditions. Aromatic additives
that were linked with an amine that canfurthermore react with the resin before the paintis cured with the hardener were found to besuitable. After 1 month heating at 100 °C approx.
50 % more elemental iodine were retained byadding 1 w % of the anthracene containing
additive (Fig. 4) [3].

Fig. 2. Variety of organic iodides formed from
Teknopox Aqua VA epoxy paint and elemental
iodine in presence of gamma irradiation.
The yield of organic iodide formation dropped
significantly when the paint films were thermallyaged for short periods (less than a week) at
temperatures above 100 °C. From the studies it
was concluded that the yield of organic iodide
formation can be reduced by a change of thesolvent profile to more radiation resistantcomponents such as aromatic components withless side chains or components with little
branching. Different types of alternative resins
were synthesized and it was found that abisphenol-Z or a vegetable oil based resin
resulted in a significantly lower methyl iodide
yield (Fig. 3) [3, 4].

Fig. 4. Retention of elemental iodine in ananthracene modified bisphenol-A resin216

NEA/CSNI/R(2016)5
PARTITIONING BEHAVIOUR OF
ORGANIC IODIDES

hydrophobic character determines the rate of thehydrolysis. Particularly, branched organic
iodides that preferably undergo SN1 reactions
other than linear, non-branched organic iodides
such as methyl iodide, and therefore hydrolyses
muchquicker
(Fig.
5)
[3,
8].

k1

3.0E-04

Isopropyl
iodide2.0E-04

Methyl iodidec [H+]The partitioning behavior of the organic
iodides that were identified in the irradiation
studies of the paint films with iodine was studied
using I-131 labelled species. Synthesis methods
were developed to generate impurity free I-131
labelled organic iodides [5]. The mass transfer
behavior of gaseous I-131 labeled organic
iodides was studied using the FOMICAG facility
which is a self-build small scale apparatus
simulating a Swedish BWR containment [6]. The
experimental data are evaluated with a newly
developed mathematical model (OIPHA) thatallows to determine the partitioning coefficientfor organic iodides for every gas/liquid ratio andknown interface area. The model andexperimental method are described elsewhere [6,7].

1.0E-04

0.0E+00-10000Ethyl iodide
10000

30000

Butyl iodide

t [s]Fig. 5. Hydrolysis of organic iodides in distilled
water

kh

RI ( g )  RI (aq)  ROH  I   H  (1)k2

The high ionic strength and the higher activity
coefficient in sea water or chlorine release fromcable plastics was found to accelerate thehydrolysis [3,9].

𝐾𝐷 =2𝑅𝐼𝑔,0 𝑘1 +𝑅𝐼𝑎𝑞,0 (𝑘1 −𝑘2 −𝑘ℎ )+𝑅𝐼𝑎𝑞,0 √−4𝑘1 𝑘ℎ +(𝑘1 +𝑘2 +𝑘ℎ )2

2𝑅𝐼𝑎𝑞,0 𝑘2 +𝑅𝐼𝑔,0 (−𝑘1 +𝑘2 +𝑘ℎ )+𝑅𝐼𝑔,0 √−4𝑘1 𝑘ℎ +(𝑘1 +𝑘2 +𝑘ℎ )2(2)

8

The main result found was that longer chainedorganic iodides and aromatic organic iodides
persist longer in gaseous phase than methyl
iodide which is important for the filtration ofcontainment air [3]. At low pH and for increasing
temperatures the mass transfer rates were foundto increase for all studied organic iodides.

pH

6
4

Dest. water2

Sea water

00RADIOLYSIS BEHAVIOUR OF
ORGANIC IODIDES IN AQUEOUS ANDGASEOUS PHASE

20000

40000

t [°C]
Fig. 6. Hydrolysis of EtI in sea water anddistilled water (40 °C)

Radiolysis studies of organic iodides were
performed in both gaseous and aqueous phase. In
aqueous phase it was found that in presence of
irradiation immediately elemental iodine wasformed in aqueous phase that partitions back into
the gaseous phase independent of the pH andcomposition of the aqueous phase. It was also
found that the conversion rate into elemental
iodine is dependent on the hydrolysis rate of theorganic iodide. Hydrolysis studies showed that
not only the chain length and therefore the

The replacement of distilled water with sea water
was found to increase the rate of elemental iodine
formation from all the organic iodine compounds
tested (Fig. 7, Table 1) [3]. In distilled waterhydroxyl radicals remove a hydrogen from sp3
hybridized organic iodides. In chlorine richmedia large amounts of dichloride radicals arealso formed, which chloride ions reduce iodide
ions to form elemental iodine [8].
217

25

100
90
80
70
60
50
40
30
20
10
0

20

Conversion in I2 [%]Converted elemental I2 [%]NEA/CSNI/R(2016)5

Methyl iodide

15

Dest. Water
Sea water

Ethyl iodide

10

Isopropyl iodide

5Butyl iodide

0

Benzyl iodide
0

200 400 600 800 1000

Dose [kGY]0500

Fig. 8. Gamma radiolysis of organic iodides at 10
kGy/h at circa 43 - 45 °C

1000

D [kGy]Fig. 7. Radiolytic conversion of methyl iodide
into elemental iodine (10 kGy/h, 21 °C)

25

Table 1: Radiolytic conversion of aqueous iodine
species (%) into elemental iodine(dose rate 10 kGy/h, 43 °C)

20
0 kGy/h

I2 [%]

15

MediumDistilled
water
Sea
water
Dose [kGy]
50
200
500
50
200
500
Iodide0.5 ± 0.1 3.7 ± 0.4 8.5 ± 0.1 0.9 ± 0.3 28.6 ± 0.6 75.1 ± 1.4
Methyl iodide
16.5 ± 0.8 28.6 ± 0.5 35.1 ± 0.9 22.9 ± 0.1 35.7 ± 0.4 77.0 ± 9.1
Ethyl iodide
9.2 ± 0.3 19.8 ± 0.4 21.2 ± 0.8 15.3 ±0.1 24.3 ± 0.2 32.7 ± 0.8
Isopropyl iodide 22.4 ± 0.8 37.3 ±0.2 77.1 ± 1.7 40.2 ± 0.3 42.5 ±0.8 94.5 ± 5.2
Benzyl iodide
34.1 ± 0.4 54.6 ± 0.1 64.2 ± 2.1 45.2 ± 0.6 57.8 ± 1.3 77.3 ± 4.7
Butyl iodide
16.9 ± 0.8 27.0 ± 0.1 32.0 ± 0.4 37.5 ± 0.4 47.8 ± 1.3 54.3 ± 2.1
Allyl iodide
23.4 ± 0.4 42.2 ± 0.9 46.3 ± 0.5 93.7 ± 7.2 96.2 ± 6.1 99.3 ± 3.5
Chloroidomethane 32.5 ± 0.7 81.7 ±1.2 97.7 ± 7.1 61.9 ± 0.7 85.7 ± 4.7 99.5 ±6.4
Diiodomethane 8.6 ± 0.3 21.1 ± 1.0 37.2 ± 0.7 15.9 ± 1.9 22.7 ± 0.7 51.0 ±2.4
Bromoiodomethane 12.5 ± 0.1 22.6 ± 1.0 31.7 ± 0.3 19.9 ± 2.3 28.4 ± 0.6 78.8 ± 2.8
Iodo-anisole
27.3 ± 0.4 41.73 ± 0.3 66.1 ± 0.2 31.0± 1.0 47.7 ± 0.7 71.2 ± 3.3
HI16.3 ± 0.3 22.1 ± 0.2 74.8 ± 0.3 18.1 ± 0.4 25.8 ± 0.5 91.3 ± 4.6

3 kGy/h10

5 kGy/h5

10 kGy/h

0
0

500

1000

Dose [kGy]Fig. 9. Gamma radiolysis of methyl iodide atdifferent dose rates at circa 43 - 45 °C
The data obtained for the speciation change oforganic iodides in both aqueous and gaseous
phase by hydrolysis and radiolysis are
implemented in the OIPHA model to obtain moreaccurate partitioning coefficients for organic
iodides to determine their gaseous fraction in thecontainment that could possibly enter the filter
systems.

The radiolysis in gaseous phase showed that theyield of elemental iodine formation is highest
from methyl iodide, whereas longer chainedorganic iodides form with an increasing dose anincreasingly larger fraction of other organic
iodides (Fig. 8) [3]. The formation of elemental
iodine was also found to be proportional to thedose and dose rate (Fig. 9) [3].

SORPTION BEHAVIOUR OF ORGANICIODIDES ON PAINT FILMS IN GASEOUS
AND AQUEOUS PHASE
13

C-NMR studies revealed that methyl iodide andother alkyl iodides are able to be retained in
Teknopox Aqua VA TM epoxy paint by thereaction of the methyl group (alkyl group) with218

NEA/CSNI/R(2016)5
alcohol groups (-OH) forming esters, Eq. (3).

with the aromatic carbon rings [11]. As observedin the hydrolysis studies for different groups oforganic iodides the selectivity of the filters canvary between different organic iodides. An initialstudy of the selectivity of a 10 % loaded DABCO
charcoal confirmed that more than 50 % lessisopropyl iodide was retained than methyl iodide(Table 3) [3]. Pre-irradiation of the absorber with
1 MGy additionally revealed that the DABCO
lost its ability to retain the organic iodides after
irradiation, whereas the non-impregnatedcharcoal showed the same ability as before
irradiation (Table 3) [3]. It is assumed that due tothe gamma irradiation some of the DABCO
evaporated out of the pores of the charcoal. The
aromatic compound benzyl iodide could not beretained on any of the studied absorber withmore than 1 %.

(3)
As found for the formation of organic iodides therentention of organic iodides was significantly
reduced in a paint film that was thermally aged
(1 month at 100 °C reduced by ca. 38 %). It wasproven to be due to the loss of solvents (Table 2)[3,4]. A comparison made between the pure polybisphenol-A epichlorohydrin resin and a cured
but non-aged Teknopox Aqua VATM paintconfirmed that the presence of solvents increases
the reactivity of paint with methyl iodide.
Table 2. MeI exposed resin and paintA (I-131) [%]
Resin
Paint
After exposure
20.3 ± 0.3
78. 8 ± 3.9
20 h, 150 °C
43.1 ± 0.6
71.7 ± 6.6

Table 3. Retention of organic iodides on
DABCO impregnated charcoal[%]
MeI
Isopropyl-I
Charcoal non(irr)5.1 ±1.2
2.0 ± 0.3
10 % DABCO
14.3 ± 0.4 10.4 ± 0.3
Irr: 10 % DABCO 8.0 ± 2.4
5.2 ± 3.1

In the thermally aged paint films a greater
fraction of the original adsorbed organic iodide
persisted in the matrix. These paint films also
showed a higher re-vaporisation rate which was
identified to be due to the higher fraction ofphysisorption of the organic iodides [3,4]. The
same phenomena was observed in aqueous phaseor on wet surfaces. The sorption rate was much
lower into the paint film and the leaching ratefrom the paint film was found to be significantly
higher [3,4]. The leaching rate andreavaporisation rates were also accelerate inpresence of either aqueous or gaseous chlorine
species[3,4].

Experiments with a silver-loaded zeolite and asilver loaded SAMMS (silica) revealed that thesilica had a circa 30 % higher ability to retain
isopropyl iodide but a 60 % higher retainabilityof methyl iodide than ordinary charcoal [3]. The
retention of both organic iodides was similar(Table 4). Both of them showed to be moreradiation stable than the DABCO loaded
charcoal and therefore to be more suitable for theorganic iodide retention. However, the reactivityof the silver loaded SAMMS reduced when being
exposed to gamma irradiation due to the mixed
inorganic-organic composition whereas thezeolite is purely inorganic and therefore more
irradiation resistant.

The effects of surface sorption, revaporisationand leaching on paint films is to be implemented
in the OIPHA-III model.
RETENTION OF ORGANIC IODIDES IN
SOLID FILTER BEDS

Table 4. Retention of organic iodides on
silver-loaded solid absorber%
MeI
Isopropyl I
Ag-loaded zeolite
65.7 ± 0.1 34.3 ± 0.1
Ag loaded SAMMS 66.5 ± 0.3 33.3 ± 0.3
Pre-irradiated zeolite 64.3 ± 0.1 33.9± 0.1
Pre-irradiated SAMMS 60.2 ± 4.2 29.5 ± 3.9

Solid absorbers have been mainly investigated
for their ability to retain particulate matter,elemental iodine and methyl iodide [10]. The
retention of organic iodides on activated charcoal
filters requires that a nucleophilic substitution
occurs, while elemental iodine can also be
reliably retained in non-impregnated charcoalsby the formation of charge transfer complexes219

NEA/CSNI/R(2016)5
A retention higher than 65 % could not be
obtained for any of the organic iodides andbenzyl iodide was not retained with more than
1 % [3]. Chlorine and iodine bearing species
such as chloroiodomethane were retained with asimilar efficiency as isopropyl iodide [3]. The
retention on solid absorber materials is often
dependent on many parameters (humidity,
temperature, flow, loading, impurities) that are
difficult to be controlled in case of an accident
scenario. Therefore it was concluded that themost reliable retention could be obtained with awet-scrubber filter system.

100

I-131 [%]

80
60
40
20
0
0

100

200

300

400

t [s]
Fig. 10. Removal of gaseous methyl iodide (2.102mol) from air in the FOMICAG facility by atributyl nphosphine added at 260 seconds.

RETENTION OF ORGANIC IODIDES IN
WET-FILTER SYSTEMS

It has been calculated that after a scram in theend of a life-cycle of a Swedish BWR up to 20 gof organic iodine and 620 g of elemental iodine
could enter the containment [3, 12]. Cesium
iodide and other inorganic iodine species
entering the scrubber can be ignored as they will
decompose in aqueous phase. If the assumption
is made that both the elemental iodine and theorganic iodides pass unchanged through thecontainment into the scrubber-tank then the small
amount of organic iodine (expressed as methyl
iodide, 20 g (0.14 mol)) produced in the reactor
will only require 34.5 ml of tributyl phosphine. If
the scrubber was to be operated without thethiosulfate solution a small amount of additional
tributyl phosphine (605 ml) would be required toconvert the elemental iodine into inorganic
iodide ions. Depending on the scrubber design it
might be difficult to obtain the required mixing
with such a small amount of the phosphine. The
optimum organic to aqueous ratio and otherparameters need to be investigated for the
scrubber design specific conditions.

The wet-scrubber filter system in current use in
Sweden is of today to be operated with a 0.015
M sodium thiosulfate solution. It has been shownin both real scale tests and in lab scale
experiments that gaseous organic iodides likemethyl iodide are only slowly removed. In the
FOMICAG facility several hours were required
until all gaseous methyl iodide was removed(21.5 ± 0.5 °C) from the nearly steady gaseous
phase. The polar nature of the aqueous medium
and the short residence time of the incoming gas
stream containing the volatile organic iodine
species in the industrial scale scrubber wereidentified as reasons for the app. 20 times lower
retention of methyl iodide with the current
scrubber design [3,12]. Altering the inorganic
scrubber solution with a few µl of tributyl,
trihexyl or trioctyl phosphine let to an instant
reduction of the gaseous organic iodide activity,
even in the not-well mixed FOMICAG facility.
The measurement interval in the FOMICAG
facility was 10 seconds. Within less than 1
minute the gaseous activity of all studied organic
iodides was removed (Fig. 10) [3,12]. In shaking
experiments and flow through studies simulating
the conditions in a Venturi scrubber all studiedorganic iodides were instantly removed [12,3].
The approach has been patented in collaboration
with Westinghouse (Sweden) [13] and iscurrently studied on industrial scale byWestinghouse in Sweden.

As the iodine is not chemically bonded to theorganic additive, the system, similarly to thecurrent system, will still depend on the aqueousphase pH. However, possible revolatilisation ofelemental iodine as a result of an acidification
will be prevented as long as enough organic
iodide or thiosulfate if added is available. The
chemical reactivity of the trialkyl phosphines
was proven to be independent on the pH of theaqueous solution or impurities in aqueous
solution [3,12]. The main disadvantage of the220

NEA/CSNI/R(2016)5
scrubbing approach is the oxygen sensitivity oftrialkyl phsophines [3,12]. It is therefore
proposed to use this solution for inert systems or
systems with predictable oxygen content.

inventory in each containment and the
additionally greatly varying conditions indifferent accident scenarios make a generalprediction of the gaseous organic iodide fractiondifficult.

OTHER PROJECTS ON THE VOLATILEIODINE SOURCE TERM

The sides in the molecules causing the formationof organic iodides were identified. As a result
proposals are made to modify currently-usedpaint products to either reduce the formation ofaliphatic organic iodides or to increase theirreversible retention of elemental iodine in thepaint film by adding an accident-conditionsresistant additive. The chemical behaviour oforganic iodides in the containment was identified
to be also very dependent on the structure of theorganic iodide and not only on the conditions in
the containment. This affects greatly the choiceof filter materials to retain organic iodides innuclear power plants. Chlorine species wereidentified to be important for decomposition oforganic iodides and the volatile elemental iodine
yield. Addition of trialkyl or triaryl phosphines
was identified to be an instant, efficient andcheap approach to retain any alkyl iodides inwet-scrubber filter systems independent ofradiation, impurities, other fission products, pH
and temperature in the filter solution.

The sorption and revaporisation behavior of
iodine oxide aerosols and caesium iodide andtheir impact on the volatile iodine source term
were studied in collaboration with VTT (Finland)[14, 15].
The effects of the sorption of ruthenium
tetroxide on surface materials on the volatile
iodine source term are studied in collaboration
with Mr. Ivan Kajan at Chalmers University ofTechnology.
The effect of other impurities on thesorption behaviour of iodine species but also theeffect of the leaching of organics out of paintfilms on the nitric acid formation have been
investigated.
The behaviour of Gehopon epoxy paint usedin the THAI facility is compared to the behaviorof the paint films studied at Chalmers within acollaboration with GRS (Germany).

The release of hydrochloric acid and the use of
sea water for emergency cooling were proven toaffect both the formation and retention of organic
iodides and to favour the formation of gaseous
elemental iodine from organic iodides. The
majority of the chlorine inventory of cable
plastics was proven to be sealed in acidscavengers and will not enter the containment
unless the scavenger comes into contact withwater. Irradiation of cables showed to causehydrochloric acid releases to start at lower
temperatures (< 300 °C) and in higher amounts.
In order to simplify the iodine chemistry halogen
containing materials should be exchanged for
halogen free, highly crosslinked and irradiation
stable plastics.

III. SUMMARY OF SELECTED RESULTS
AND CONCLUSIONS
It was found that a range of organic iodides canbe formed from the organic materials present in
the containment. Paint solvents and plasticizers
were identified to determine the yield and varietyof organic iodide formation from modern
bisphenol-A based paints. The thermal age of thepaints was identified to be the dominating
parameter determining organic iodide formation.
Aged paint films showed a greatly reduced
ability to form organic iodides but after 20 yearsof ageing under real reactor conditions still
relevant amounts of paint solvents that generatedorganic iodides were detected on paint disks
from the Barsebäck nuclear power plant in
Sweden. Due to the great variety of paintproducts, non-regulated cycles of paint film
exchanges in nuclear plant, the varying organic

REFERENCES

1. S. Tietze, M.R.StJ. Foreman, C.H. Ekberg,
B.E. van Dongen, Identification of thechemical inventory of different paint types

221

NEA/CSNI/R(2016)5
applied in nuclear facilities, Journal ofRadioanalytical and Nuclear Chemistry, 3,
pp. 1981-1999 (2013).

materials on the iodine chemistry during
severe nuclear accidents in LWRs,
Manuscript (2015).

2. S. Tietze, M.R.StJ. Foreman, C.H. Ekberg,
Formation of organic iodides fromcontainment paint ingredients caused by
gamma irradiation, Journal of Nuclear
Science and Technology, 50(7), pp. 689-694(2013).

10. IAEA, IAEA Safety Standards Series,
Design of Reactor Containment Systems for
Nuclear Power Plants, Safety Guide No. NSG-1.10, 2004.
11. S. Kothai, Charge Transfer Complexes of
Iodine with Aromatic Hydrocarbons, Journalof Applied Chemical Research, 11, pp. 24-33(2009).

3. S.Tietze, The chemistry of organic iodides
under severe nuclear accident conditions in
LWRs, Dissertation, Chalmers University ofTechnology (SE), ISBN 978-7597-158-2(2015).

12. S. Tietze, M.R.StJ. Foreman, C.H. Ekberg,
The use of trialkyl phosphines as novel wetscrubber filter additives to enhance thedecontamination factor for organic iodides
from vented containment gas streams,
Manuscript (2015).

4. S. Tietze, M.R.StJ. Foreman, C.H. Ekberg, F.
Davis, P. Wormald, Degradation studies ofbisphenol-A based epoxy paints under severe
nuclear accident conditions, Manuscript(2015).

13. Patent on a novel wet-scrubber solution to
reduce the release of organic iodides in caseof a containment venting, Registration
number: 1451118-2 (2014, Sweden)
Inventors: S. Tietze, M.R.StJ. Foreman,
C. Ekberg(Chalmers
UniversityofTechnology) Patented with: Westinghouse
Electric Sweden.
14. S. Tietze, T. Kärkelä, C. Ekberg,
A. Auvinen, U. Tapper, J. Jokiniemi,
Adsorption and revaporisation studies of thin
iodine oxide and CsI aerosol deposits fromcontainment surface materials in LWRs,
NKS-R, NKS-285, ISBN 978-87-7893-360-7(2013).

5. S. Tietze, M.R.StJ. Foreman, C.H. Ekberg,
Synthesis of I-131 labeled iodine species
relevant during severe nuclear accidents inlight water reactors, Radiochimica Acta,
101(10), pp. 675–680 (2013).
6. H. Glänneskog, J.-O. Liljenzin, L. Sihver, G.
Skarnemark,A.
Ödegaard-Jensen,
Interactions of I2 and CH3I with reactive
metalsunder
BWRsevere-accident
conditions, Nuclear Engineering and Design,227,
pp.
323-329(2004).
7. S. Tietze, M. Lindgren, S. Holgersson, C.
Ekberg, OIPHA-I and II Model –Mathematical models to describe thepartitioning and hydrolysis behavior oforganic
iodides
betweengaseous and aqueous phases, Iodine
Workshop, Marseille (2015).
8.

15. S. Tietze, T. Kärkelä, M.R.StJ. Foreman,
C. Ekberg, A. Auvinen, U. Tapper,
S. Lamminmäki, J. Jokiniemi, Adsorption
and revaporisation studies on iodine oxideaerosols deposited on containment surface
materials in LWRs, NKS-R, NKS- 272,
ISBN 978-87-7893-345-4 (2012).

R.T. Morrison, R.N. Boyd, Organic
Chemistry, 5th edition, Allyn and Bacon
Inc., 1987.

9. S. Tietze, M.R.StJ. Foreman, B.E. van
Dongen, C.H. Ekberg, The impact of cable

222

NEA/CSNI/R(2016)5
OIPHA-I AND II MODEL - MATHEMATICAL MODELS TO DESCRIBE THE
PARTITIONING AND HYDROLYSIS BEHAVIOR OF ORGANIC IODIDES BETWEENGASEOUS AND AQUEOUS PHASES
S. Tietze¹*, M. Lindgren², S.Holgersson¹ and C. Ekberg¹(1) Nuclear Chemistry, Chalmers University of Technology, Goeteborg (SE)(2) Energy and Environment, Chalmers University of Technology, Goeteborg (SE)*Corresponding author, Email:Tietze-Sabrina@gmx.de

Abstract – Models to determine the gaseous fraction of organic iodides under severe nuclear accident
conditions are under development. The Organic iodide partitioning and hydrolysis analysis models(OIPHA) are based on experimental studies that investigate the partitioning behavior of organic iodides
between gas and aqueous phase considering the effects of temperature, pH, impurities (e.g. chlorine
species), radiolysis, hydrolysis and sorption phenomena on paint films for any LWR containment.

223

NEA/CSNI/R(2016)5
OIPHA-I studies

I. INTRODUCTION

The majority of the experimental data used forthe description of the mass transfer of organic
iodides from gaseous phase to aqueous phase andtheir hydrolysis in absence of irradiation were
performed in the FOMICAG facility (Fig. 1) [1].
Organic iodides have a high vapor pressure andcan therefore easily be converted from theirliquid form into their gaseous form by placing
the desired liquid amount into a round bottom
flask. The flask was then sealed with a septum.
After about 1 h at room temperature (21.5 ±0.5 °C) the liquid partitioned into the gaseous
phase and could be transferred using a gas-tightglass syringe. The gas saturated with the organic
iodide was then injected through another septum
into the gas phase of the FOMICAG facility (Fig.
1). The change of the gaseous I-131 activity ingaseous and aqueous phase was determined with
two sodium iodide detectors that were placed in
the gaseous and aqueous phase of the facility.
The effects of temperature (21.5 ± 0.5 °C to
250 °C (gas phase)), pH (2 – 12) and impurities
on the mass transfer between gaseous andaqueous phase were studied.

Due to a limit amount of data describing the
behavior of organic iodides under severe nuclear
accident conditions in LWR`s in safety codes
like MAAP, mathematical models weredeveloped to determine the partitioning
coefficient of gaseous organic iodides such as
methyl iodide. The OIPHA-I and -II Models(Organic iodide partitioning and hydrolysis
analysis models) are based on experimental
studies of the partitioning and the hydrolysis
behavior of organic iodides for any liquid-gas
phase systems with known gas phase volume,
aqueous phase volume and interface area. The
OIPHA-I model allows to determine the kinetic
parameters of the partitioning and hydrolysis
reactions of organic iodides which are required
for the determination of the partitioning
coefficient for varying pH and temperature. The
effects of gamma irradiation on the iodine
speciation and on the partitioning coefficient
have been taken into consideration in the
OIPHA-IImodel.
The models are verified with experimental data
determined with the FOMICAG (Facility for
Online Measurements of I-131 Concentrations
between an Aqueous and Gaseous phase) facility[1], a small-scale BWR containment set-up, at
Chalmers University of Technology, as well as insolvent extraction experiments and ion selective
electrode measurements. I-131 labelled organic
iodide model substances (methyl-, ethyl-,
isopropyl-, butyl-, benzyl-, allyl iodide,chloroiodomethane and iodo-anisol) weresynthesized for the studies.
II. EXPERIMENTAL

Fig.1. Diagram of the FOMICAG facility
illustrating the injection of gaseous methyl iodide

Paragraph text here. I-131 labelled organic
iodides (methyl-, ethyl-, isopropyl-, butyl-,
benzyl-, allyl iodide, chloroiodomethane andiodo-anisol) in their liquid form for the studies
were synthesized as described in [2].

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NEA/CSNI/R(2016)5
OIPHA-II studies
Due to the measurement with sodium iodide
detectors in the FOMICAG facility the change ofthe speciation of the organic iodides could not be
determined. Therefore, further batch studies
using e.g. solvent extraction experiments wererequired to separate the aqueous species formed
as a result of the hydrolysis. In these studies also
the effect of irradiation and paint could bedetermined.
A majority of the studies was performed in glass
flasks as small as 20 ml with varying gas/liquidratio. For these studies typically less than 50 µlof the I-131 labelled organic iodide were addedin liquid form. For studies involving gamma
irradiation the flasks were irradiated in a gamma
irradiator with a dose rate between 20 kGy/h and3 kGy/h at Chalmers University of Technology.
The speciation of the radioactive iodine in thebulk was determined with a two-step solvent
extraction procedure (Fig. 2). 1 ml water samples
were extracted withoctane (1 ml) in a 3.5 ml glass shaking vial. The
extraction was performed with a shaking
machine (IKA Vibrex VSR basic) to which acustom made sample holder was attached using ashaking time of 1 minutes at 1500 rpm. It was
assumed that all organic iodine compounds (RI)
and elemental iodine (I2) were partitioned into
the octane layer, while all ionic inorganic iodine
compounds remained in the aqueous layer (I-).
Samples of the organic layer bearing the organic
iodides and the aqueous phase bearing the ionicinorganic iodine species were subject to liquid
scintillation. The octane layer (0.5 ml) was
subjected to a second extraction with a 1 M KI
solution (0.5 ml) which converted all elemental
iodine present in the organic phase (visible as apink colour of the organic layer) into triiodide
ions (I3-) leaving the organic iodides in theorganic phase. Aliquots of each phase were again
evaluated by LSC.

Fig. 2. Schematic of the solvent extraction
procedure to separate aqueous iodine species
The hydrolysis of organic iodides was studied
also with non-radioactive methods using online
measurements with an iodide selective and pH
electrode (Titrando, Methrohm).
III. MODEL DEVELOPMENTOIPHA-I
The ratio of a chemical substance between twophases at equilibrium can be described by thepartitioning coefficient KD (Eq. 1). The
partitioning coefficient helps to make an estimateof the fraction of organic iodides that will remain
in gaseous phase and that could possibly bereleased into the environment. The model isbased on two key reactions of gaseous organic
iodides:
1) the partitioning behavior/mass transfer
between the containment gas phase and aqueous
phase
(Eq.
2)
and
2) the hydrolysis of the organic iodides in the
water
pool
(Eq.
3).
The effects of radiolysis and sorption effects on
surface materials have been neglected at thisstage.
The partitioning coefficient of methyl iodide(MI) (Eq. 1) and similar structured organic
225

NEA/CSNI/R(2016)5
iodides (RI) between a gaseous and aqueous
phase can be described by the ratio of the amountin moles of methyl iodide in aqueous phase(MIaq) relative to the amount of methyl iodide
available in gaseous phase (MIg), (Eq.1). It is
assumed that an equilibrium saturation of the gas
phase in the aqueous phase is obtained prior.
MIaq

K 𝐷 = MIginterface area can be obtained, Eq. (8)-(10).
RIaq,o , RIg,o and Iaq,o are start values at a start time
t = 0. These equations were used for the analysisof experimental data which were obtained inexperimental
studies.

𝑅𝐼𝑔 (t) = RIg,0 𝑒 −k1 𝑡 +(1)+

RIg,0 k1 ((−X+Z)𝑒 −k1 𝑡 +𝑒 −Z𝑡 (X−k1 )+𝑒 −X𝑡 (−Z+k1 ))k2

k1

(X−Z)(Z−k1 )(−X+k1 )MI ( g )  MI (aq) (2)
k2

kh



(8)

𝑅𝐼𝑎𝑞 (𝑡) =RIaq,0 (X𝑒 −X𝑡 − Z𝑒 −Z𝑡 ) + RIg,0 (−𝑒 −X𝑡 + 𝑒 −Z𝑡 )k1

In aqueous phase dissolved methyl iodide (MIaq)
undergoes furthermore hydrolysis (Eq. 3) leading
to the formation of iodide ions (I-) which aredefined as the main inorganic, ionic,
immobilized iodine species for the model.

− (t)
I𝑎𝑞

+RIaq,0 (−𝑒 −X𝑡 + 𝑒 −X𝑡 )k1
X−Z(9)

RIaq,0 (−𝑒 −X𝑡 + 𝑒 −Z𝑡 )k h
X−Z(−X + Z − Z𝑒 −X𝑡 + X𝑒 −Z𝑡 )(RIg,0 k1 k h + RIaq,0 k1 k h )−X(X − Z)Z(10)−
= 𝐼𝑎𝑞,𝑜+

MI (aq)  MOH  I  H (3)k1, k2 are temperature dependent kinetic rate
constants and kh is the hydrolysis rate constant.
MOH is the amount of methanol in moles which
is formed from the hydrolysis of methyl iodide
and the remaining proton (H+) is contributing toan acidification of the water pools. The
formation of elemental iodine becomes relevant
for acidic conditions (pH ˂ 4) and has been
neglected for the studied pH range of 4 < pH <10.
Based on the chemical reactions representing the
behavior of gaseous organic iodides RI(g) in thecontainment the differential equations for thereaction rates of the involved iodine species (Eq.
4) can be formulated (Eq. 5) – (Eq.7).
k1

RIaq,0 (−𝑒 −X𝑡 + 𝑒 −Z𝑡 )k 2
X−Z

𝑋=
1
(k1 + k 2 + k h ) −2
1
4√k1 k 2 − k1 (k 2 + k h ) + ( k1 + k 2 + k h )2 (11)𝑍=1
(k1 + k 2 + k h ) +2
1√k1 k 2 − k1 (k 2 + k h ) + (k1 + k 2 + k h )2(12)4kh

RI ( g )  RI (aq)  ROH  I   H  (4)The set off differential equations Eq. (5)-(7) wastransformed and solved in Laplace space.
Equations(8)-(10)
were
obtainedbytransforming the solutions to real space.
From the equations Eq. (8) and Eq. (9) an
expression
for 𝑡 → ∞, 𝐾𝐷 (13) could be obtained using Eq.
(1).

k2

dRIg
= k 2 RIaq − k1 RIg(5)dt𝑑𝑅𝐼𝑎𝑞= 𝑘1 𝑅𝐼𝑔 − 𝑘ℎ 𝑅𝐼𝑎𝑞 − 𝑘2 𝑅𝐼𝑎𝑞 (6)
𝑑𝑡
−
𝑑𝐼𝑎𝑞
= 𝑘ℎ 𝑅𝐼𝑎𝑞 (7)𝑑𝑡

By solving the set of differential equations Eq.
(5)-(7) expressions for the time (t) dependent−change of the amounts of RIg, RIaq and 𝐼𝑎𝑞
inmoles for a aqueous-gas phase system withknown aqueous and gaseous phase volume and226

NEA/CSNI/R(2016)5𝐾𝐷 =2𝑅𝐼𝑔,0 𝑘1 +𝑅𝐼𝑎𝑞,0 (𝑘1 −𝑘2 −𝑘ℎ )+𝑅𝐼𝑎𝑞,0 √−4𝑘1 𝑘ℎ +(𝑘1 +𝑘2 +𝑘ℎ )2

10000

(13)

8000
A [Bq]

2𝑅𝐼𝑎𝑞,0 𝑘2 +𝑅𝐼𝑔,0 (−𝑘1 +𝑘2 +𝑘ℎ )+𝑅𝐼𝑔,0 √−4𝑘1 𝑘ℎ +(𝑘1 +𝑘2 +𝑘ℎ )2

The constants k1, k2 and kh that are required tosatisfy equation Eq. (13) can be obtained fromthe fitting of experimental data of at least one of
the three iodine species involved in the model
Eq. (8, 9, 10). The fitting was performed using
the fit function in Wolfram Mathematica 10.1
which is using the method of least square.

MeI

6000

EtI

4000
2000

ButI

0
0

1000
t [s]

OIPHA-II

2000

BenzyliodideFig.3. Partitioning behavior of organic iodides fromgaseous phase (air) into aqueous phase (21 ± 0.5 °C)

For acidic pH’s (Eq. 14) and under the presenceof gamma irradiation (Eq. 15-16) the formationof elemental iodine was observed, see Eq. (17),and is currently to be implemented in the model.

The mass transfer rate increased with increasing
temperature for all the studied organic iodides (Figure
4).

4𝐻 + + 4 𝐼 − + 𝑂2 → 2 𝐻2 𝑂 + 2𝐼2(14)𝐻𝑂 ∙ +𝐼 − → 𝑂𝐻 − + 𝐼 ∙(15)
2 𝐼 ∙ → 𝐼2 (16)
kh

k2

k4

2 RI ( g )  2 RI (aq)  2 H 2O (aq)  2 ROH (aq)  2 I  (aq)  2 H  (aq) 
k1

k3

k6

I 2 (aq)  2 H  (aq)  2 ROH (aq)  I 2 ( g )  2 H  (aq)  2 ROH (aq)
k5

(17)

III. RESULTS AND DISCUSSION
This section contains only selected data. Some ofthe experimental data are published in Ref. 3.
The quantitative assessment of the experimental
data is still in progress and will be published by
the authors in 2015/16 in scientific journals.

Fig. 4. Partitioning behavior of methyl iodide fromgaseous phase (air) into aqueous phase
1 (21 ± 0.5 °C), 2 (40 ± 0.5 °C), 3 (70 ± 0.5 °C).

The partitioning coefficient for the organic
iodides was determined in dependence of pH andtemperature. Fig. 5 shows the comparison of KD
for methyl iodide determined with the OIPHA-I
model in comparison to temperature dependent
correlations reported in the literature [4]. For
other organic iodides no such data are reported.

OIPHA-I
The studies in the FOMICAG facility revealed
that organic iodides have a different fast masstransfer behaviour to the aqueous phase. The
studies showed that longer chained and aromaticorganic iodides persist longer in gaseous phasethan shorter chained ones (Fig. 3).
2270.00035

4
3.5
3
2.5
2
1.5
1
0.5
0

Chalmersc [H+]KD

NEA/CSNI/R(2016)5

Borkowski

0.00025

Isopropyl iodide

0.00015

Methyl iodide
EtI

5E-05

ButI

Hasty
-5E-05 0
0 10 20 30 40 50 60 70 80

20000

Benzyl It [s]T / °C
Fig. 6. Hydrolysis of organic iodides in distilled water

Fig. 5. KD for MeI obtained with OIPHA-I in
comparison to correlations reported in the literature[4]

It can be concluded that the secondary andtertiary alkyl iodides will be much easier washeddown (removed from air) by containment sprays
and trapped in aqueous scrubber solutions thaneither primary alkyl iodides or aryl iodides.
Hydrolysis studies of ethyl iodide in salt waterand distilled water showed an acceleratedhydrolysis behavior of it in salt water (Fig. 7). As
a pH change was observed a simple halogen
exchange reaction between iodide ions bond tothe organic iodide and chloride ions in the waterphase can be excluded. The high ionic strength ofsea water due to the chloride ions is assumed toaccelerate the hydrolysis.

OIPHA-II
The hydrolysis studies showed that in agreementwith the mass transfer between the gaseous andaqueous phase the kind of organic iodide matters.
The hydrolysis of the different organic iodides
was found to be not only governed by thesolubility and the chain length but also by the
structure of the organic molecules, the presenceof chlorine impurities, as well as pH, temperatureand irradiation.

8

In isopropyl iodide the iodine is bond to abranched alkyl chain. Isopropyl iodide despite
the fact that it is more hydrophobic than methyl
iodide (and thus less soluble in water) was shownto hydrolyze more quickly in the aqueous phasethan methyl iodide (Fig. 6). Branching at the
carbon bearing the iodine causes a sterichindrance of the SN2 reaction [5]. This branchingfavors SN1 reaction which is only dependent on
the isopropyl iodide concentration. Benzyl iodide
was expected to hydrolyse much quicker than theother organic iodides due to the aromatic ring.
However, in this case the low solubility of thearomatic iodide in the aqueous solution is thedominating factor.

pH

6
Dest. water4

Sea water

2
0
0

20000
t [°C]

40000

Fig. 7. Hydrolysis of EtI in sea water anddistilled water (40 °C)
When gamma irradiation was present during thehydrolysis a rapid formation of elemental iodine
was observed independent of the pH of thesolution and the presence of chlorine.
The dissolution of the organic methyl iodide
showed to be the rate controlling step andappeared therefore to increase instead to decrease(Fig. 8). As the aqueous methyl iodide (MeI(aq))
decreased it can be concluded that the conversion
into inorganic iodide is a faster reaction. The
228

NEA/CSNI/R(2016)5
kinetics of the radiolytic oxidation of the iodide
ions was found to be a faster step than thehydrolysis as the elemental iodine showed to
rapidly increase whereas the iodide ion
concentration remained low and nearly stable. As
this was observed it was concluded that inaqueous phase the organic iodides are notdirectly converted into elemental iodine. It is
assumed that OH. radicals which are formed in
the radiolysis of water attack the organic iodide
and form organic iodide radicals that
decomposes into a iodine radical and a aldehyde.
The aldehyde can be readily reduced to a primaryalcohol, whereas the iodine radicals canrecombinetoform
elemental
iodine.

type of cable present in different containments
will vary a lot as will the conditions the cables
are exposed to. Salt water has been considered as
worst case scenario when all water would be
replaced with sea water. It was found that in thechloride rich water the conversion rate frommethyl iodide into elemental iodine wasincreased (Fig. 9). Similar results were observed
when iodide ions were the iodine source present
in aqueous phase. This suggests that theradiolysis of organic iodides is altered bychloride ions. One possibility is the formation ofelemental iodine by the reaction of elemental
chlorine with iodide anions (Eq. 18). Anotherphenomenon which could cause the higher
conversion of methyl iodide into elemental
iodine is a halogen exchange reaction(Finkelstein reaction) of chlorine radicals withthe organic iodide which will eject an iodine
radical that can recombine to form elemental
iodine, (Eq. 19).

1.20E-02

c [mol/l]

1.00E-02
8.00E-03

I-

2𝐼 − + 𝐶𝑙2 → 𝐼2 + 2 𝐶𝑙 − (18)6.00E-03

I2(aq)

𝑅𝐼 + 𝐶𝑙 . → 𝑅𝐶𝑙 + 𝐼 . (19)I2(g)

4.00E-03

MeI(aq)
2.00E-03

MeI(org=g)

0.00E+00
0

100

200

Dose [kGy]Fig. 8. Radiolysis of methyl iodide
in distilled water (10-2 mol/l MeI,
10 kGy/h)

The chlorine content of the water pools showed
to have an additional impact on the iodine
chemistry when gamma irradiation was present(Fig. 9). It is not easily possible to predict how
large a chlorine release into the containment
water pools will occur from chlorinated cable
plastics. The majority of the chlorine release is to
be expected when a total decomposition of theorganic coatings of cable rich in chlorine such as
PVC or CSPE’s occurs and the inorganic
chlorides are exposed to water. The length and229

NEA/CSNI/R(2016)5

100
90
80
70
60
50
40
30
20
10
0

20
Conversion in I2 [%]Converted elemental I2 [%]25

Methyl iodide

15

Dest. Water
Sea water

Ethyl iodide

10

Isopropyl iodide

5Butyl iodide
Benzyl iodide

0
0
0

500
D [kGy]

200

400

600

800 1000

Dose [kGY]1000

Fig. 10. Radiolytic conversion of organic iodides
into elemental iodine (dose rate 10
kGy/h)

Fig. 9. Radiolysis of methyl iodide in distilled
and sea water (10-2 mol/l MeI, 10 kGy/h,
43 °C) into elemental iodine

REFERENCES

1. H. Glänneskog, J.-O. Liljenzin, L. Sihver, G.
Skarnemark,A.
Ödegaard-Jensen,
Interactions of I2 and CH3I with reactive
metalsunder
BWRsevere-accident
conditions, Nuclear Engineering and Design,
227, pp. 323-329 (2004).

IV. FUTURE WORK
Experimental data are currently evaluated todetermine the required kinetic parameters to beable to more reliable estimate the volatile iodine
source term. The OIPHA-III model will discuss
furthermore the effects of radiolysis in gaseous
phase and the sorption behavior of iodine species
on painted surfaces. The radiolysis studies ingaseous phase showed that longer chainedorganic iodides form a lower yield of elemental
iodine than methyl iodide but an with chain
length increasing yield of other organic iodides
and other inorganic iodine species such as iodineoxide aerosols (Fig. 10).

2. S. Tietze, M.R.StJ. Foreman, C.H. Ekberg,
Synthesis of I-131 labeled iodine species
relevant during severe nuclear accidents inlight water reactors, Radiochimica Acta,
101(10), pp. 675–680 (2013).
3. S.Tietze, The chemistry of organic iodides
under severe nuclear accident conditions in
LWRs, Dissertation, Chalmers University ofTechnology (SE), ISBN 978-7597-158-2(2015).
4. R.
Borkowski,
Untersuchungenzum
Chemischen Verhalten des Methyliodides bei
schweren Störfällen in Druckwasserreaktoren,
Dissertation, KfK 3969 (1985).
5. R.T. Morrison, R.N. Boyd, Organic
Chemistry, 5th edition, Allyn and Bacon
Inc., (1987).

230

NEA/CSNI/R(2016)5

THE RADIOLYSIS OF GASEOUS METHYL IODIDE IN AIR
G M N Baston(1), S R Bowskill(2), S Dickinson(3)*, H E Sims(2)AMEC, Harwell, UK (2) National Nuclear Laboratory, Harwell, UK.
*Corresponding author, tel: (+44)7894 598707, Email: shirley.dickinson@nnl.co.uk(1)

Abstract – Organic iodides, exemplified by CH3I, are among the most volatile forms of iodine andtherefore important when considering the behaviour of radioiodine in reactor faults. In many small- andlarge-scale tests, low-molecular weight organic iodide has been determined to be the dominant form ofgaseous iodine, particularly when measures have been taken to minimise the concentrations of volatile
inorganic iodine, for example by the use of elevated pH in aqueous solutions. In order to predict the
behaviour of CH3I in PWR containment under fault conditions it is necessary to know both its formationand destruction rates.
The mechanism of CH3I formation is uncertain, but it probably occurs on containment surfaces and in the
aqueous phase. The formation rates at surfaces and in solution have been measured by a number ofworkers, generating extensive data for use in model development. Decomposition can occur via several
mechanisms, however in a PWR containment the majority of CH3I is expected to be found in the gaseous
phase due to its low partition coefficient and the large gas to liquid volume ratio. Gas-phase radiolysis
therefore needs to be considered as one of the potentially important decomposition routes.
This paper describes experimental measurements of the radiolytic destruction of CH3I in moist air whichwere carried out as part of the EC 5th Framework Project on Iodine Chemistry and Mitigation Mechanisms(ICHEMM) and have not previously been published in the open literature. In the only previous reported
study, the experimental conditions were limited to radiolysis in dry air and low temperature. The main
objective of our work was to extend the measurements to more accident-relevant conditions of hightemperature and steam atmospheres, and to study other possible sensitivities.
Experimental and analytical techniques were developed to measure radiolytic decomposition as a functionof concentration of CH3I, concentration of O2, surface area to gas volume ratio, temperature, liquid water
volume, dose-rate and surface area to gas volume ratio. Experiments were carried out with C 2H5I tocompare decomposition with CH3I.
The decomposition rates measured in this study are of a similar magnitude to those in previously reported
work conducted at very low water vapour concentrations. No significant effect of temperature wasobserved, but increasing the concentration of O2, the surface area to gas volume and the water vapour
concentration all decreased the decomposition rate. The decomposition showed a square-root dependence
on dose-rate. The rate of decomposition of C2H5I is found to be similar to that of CH3I.
Overall, no significant sensitivities have been observed and the rates measured can be applied tocontainment models and the prediction of organic iodide behaviour in reactor faults.

231

NEA/CSNI/R(2016)5(Iodine Chemistry and Mitigation Mechanisms).
In this work, the decomposition of CH3I in moist
air has been measured as a function ofconcentration, temperature, gas composition,
moisture content and surface area to gas volume
ratio in order to obtain data under a range of
relevant conditions.

I. INTRODUCTION
Organic iodine, in particular methyl iodide(CH3I), is one of the most volatile forms of
iodine and so is important when considering the
behaviour of radioiodine in reactor faults. In
many small- and large-scale tests it has beenfound to be the dominant form of gaseous iodine,particularly when measures have been taken tominimise the volatile inorganic iodine
concentration, for example by the use of elevatedpH in aqueous solutions. The formation route of
CH3I is uncertain but is thought to occur oncontainment surfaces and in the aqueous phase.
Its presence as the major gaseous form is
probably partly associated with its lower
reactivity with surfaces.

II. METHODS
Irradiation Vessel Design
To examine the effect of varying conditions on
the radiolytic destruction of CH3I, an irradiation
vessel and sampling system were developed togive reproducible and reliable results.
The basic vessel design is shown in Fig. 8. A
reusable glass irradiation cell was attached to acapillary tube. A septum on the end of thecapillary tube, which was as remote as possible
from the high dose-rate region of the irradiation
cell, was used to inject samples of CH3I into theFormation rates of organic iodine have beenmeasured from surfaces and from solution by
many workers, although no widely acceptedmechanism is available. There are several
potential decomposition routes. In aqueous
solution CH3I can be hydrolysed by thermal
reactions and/or radiolytically decomposed byreaction with the hydrated electron or hydrogenatom. However, because of its low partition
coefficient and the large gas to liquid ratio in a
PWR containment, the majority of CH3I will bein the gas phase where in the presence of a
relatively high dose-rate from fission product Xeand Kr, radiolysis may be an importantdecomposition route for CH3I.

Thermocouple
pocketSeptumCapillarytube

In the only previous reported study of theradiolytic decomposition of CH3I in air, Tangand Castleman1 measured the gas phasedecomposition of CH3I as a function of its
concentration in a dry air atmosphere at room
temperature. Other data from the same authors inmoist air are included in a review by Postma2.
Containment conditions would be rather different
with temperatures of up to approximately 100°C
and saturated steam, so it is important toascertain the decomposition rate of CH3I underthese conditions.

vessel and to collect samples for analysis.

Fig. 8. Vessel 2, upon which the other vessel
designs are based.

Various modifications to the basic vessel
design were made to allow for different
vessel surface areas and volumes. In total,
four designs were used in this work, seeThis paper describes an experimental study of theradiolysis of gaseous CH3I, conducted as a partof the EC 5th Framework project ICHEMM

232

NEA/CSNI/R(2016)5
Table III. Vessel 1 was used only for low
require a thermocouple pocket.
temperature (~30°C) experiments and did not
TABLE III
Irradiation Vessel Design Parameters
DesignVolume (cm3)Surface Area
(cm2)

S:V ratio(cm-1)

Vessel 1

100

150

1.5

Vessel 2

100

150

1.5

Vessel 3

100

380

3.8

Vessel 4

650

455

0.7

 Initial CH3I concentration(6x10-7 to 3x10-9 mol dm-3)
 Liquid water injected (5 μl and 500 μl)
 Temperature (~30°C and 80°C)
 Atmosphere (air, O2, 1% O2 in N2, N2 andN2O) Vessel surface area and volume (rate of thermal decomposition or leakage fromthe vessel.

Radiolysis Experiments
Vessel irradiations were performed at Harwell
Laboratory using a Co-60 gamma source tomeasure the effects of a range of variables on theradiolysis of CH3I: TABLE III) Dose-rate (5.5, 71, 560 Gy hr-1)
 Addition of C2H5I instead of CH3I

For tests conducted under “dry” conditions theirradiation vessels were dried overnight at 80C
and then fitted with the septum. The irradiation
vessels were then purged via syringe needles
inserted through the septa with air that was driedby flowing through a tube filled with silica gel.
In order to inject a relatively dry sample of CH3I,
liquid CH3I was equilibrated with the gas volume
in a glass vial with a septum and an aliquot ofgas withdrawn into a syringe. This gas sample
was diluted in a similar manner before 100 l
was added to the irradiation vessel.

For experiments conducted at low temperature
the temperature was uncontrolled and isestimated to be approximately 30 °C.
Filling the flasks
A dilute solution of CH3I (~0.01 mol dm-3) was
prepared by dissolution in H2O. The irradiation
vessel was heated if required, and when at thecorrect temperature, was sealed with the septum.
If an oxygen atmosphere was required, the vessel
was flushed with the gas, using two hypodermic
needles as inlet and outlet, inserted through theseptum. A 5 l or 500 l volume of CH3I
solution was injected into the vessel, which wasthen placed into the irradiation chamber. Samples
were removed periodically using a gas syringe
and analysed by gas chromatography. A few tests
were done without irradiation to determine the

SAMPLE ADDITION AND ANALYSISThe CH3I concentration was measured byinjecting a 100 l sample of CH3I/air into the gas
chromatograph. The GC used was a Pye Unicam
model 4550 with a SGE BP5-1.0 column and anelectron capture detector.
Surface sorption experiments
233

NEA/CSNI/R(2016)5
A single experiment was set up to measure theamount of iodine deposited onto the vessel walls
after decomposition of CH3I. 50 l of methyl
iodide solution was injected into the glass vessel,
to give 1.6×10-7 mol dm-3 methyl iodide in air.
The vessel was irradiated for 2.5 hours at ~30°C
at a dose rate of 496 kGy hr-1, by which time theconcentration of methyl iodide had fallen to 51%
of its original value. The vessel was then flushedwith nitrogen and the outlet gas passed through asolution of 10-3 molar NaOH, and the outlet pipe
was rinsed with 10-3 molar NaOH into the trap
solution. A sample of the gas from the vessel wasthen analysed to ensure that all oxygen andmethyl iodide had been removed, before washing
the walls of the irradiation vessel with 10-3 molar
sodium hydroxide. The two sodium hydroxide
solutions (purge gas trap and wall wash), plus a
blank solution, were analysed for iodine by ICPMS.

Fig. 9. Control tests without gamma irradiation
showed no evidence of thermal decomposition orleakage of methyl iodide from the vessels.
In the following sections data are normalized tothe percentage of CH3I remaining and plotted on
semilogarithmic coordinates as a function of total
dose (Gy) absorbed by the atmosphere unlessotherwise stated.

Dosimetry
Dosimetry was by the Fricke method3.
Correction was made to the doses to allow for theslightly lower electron density of the gas
compared with that of liquid water.
III. Results and discussion
The change in methyl iodide, CH3I,
concentration during γ irradiation has beenmeasured for a wide range of conditions,generating data that can be applied to the
behaviour of volatile iodine in reactor
containments. In all of the tests, with the
exception of those conducted in a nitrogen
atmosphere, the concentration of CH3I decreased
exponentially with increasing dose according to
the decay equation:
C = C0e-kD(1)where C and C0 are the concentration of CH3I at
a absorbed dose (D) and at zero dose
respectively.
For each experiment, the decomposition rate
constant, k, has been calculated. The
experimental rate constants from tests conductedunder an air atmosphere are summarised in
234

NEA/CSNI/R(2016)5

Fig. 9. Summary of the decomposition rate constants of methyl iodide, k, for experiments conducted in an
air atmosphere under a range of conditions. The irradiation conditions altered are temperature (°C), dose
rate (kGy hr-1), water added to the system (μl) and surface area to volume ratio (S:V). One set ofexperiments was conducted with ethyl iodide (C2H5I) instead of CH3I. Error bars give one standard
deviation between replicates.
Effect of liquid water at ~30°C

there is not a large effect of the addition of water
to the system.

Experiments were conducted in an air
atmosphere, and 5 μl or 500 μl of water was
introduced with the injected CH3I to examine theeffects of water on radiolysis. The saturationpressure of water at ~30°C is 4.2 kPa which
equates to about 3 mg of water in 100 ml.
Therefore a small amount of liquid water would
have been present even in the tests with 5 μl
injected. There was a large degree of scatter on
results from duplicate experiments, although theindividual data points for each experiment
generally lie close to an exponential trend-line(Fig. 3). The mean decomposition rates in thetests with 5 and 500 μl water were 0.45 and0.58 kGy-1 respectively, with relative standard
deviations of 60% and 30%. Within theuncertainties, therefore, no effect of liquid water
can be determined at ~30°C.

It can be seen from the exponential trend line
plotted in Fig. 3 that the results of these tests
agree reasonably well with those of Tang andCastleman2. In the current work, the water
concentration at ~30°C is ~10-3 moles dm-3, afactor of ca. 5 more than the maximum used byTang and Castleman for decomposition of CH3I
in air containing water vapour. They observed afactor of 2-3 decrease in decomposition rate
when the H2O concentration was decreased from
2.110-4 moles dm-3 to 3.710-7 moles dm-3.
However their result at the lowest water vapour
concentration is not consistent with their data for
dry CH3I, which gave a higher rate ofdecomposition than with water addition.

Two experiments were conducted under “dry”
conditions. These gave decomposition rates ofapproximately 0.85 kGy-1, at the upper end of
those measured in the presence of liquid water.
However, in view of the large scatter in the data
it is difficult to conclude on whether thedifference is significant; it is clear, however, that235

NEA/CSNI/R(2016)5
Effect of liquid water at 80°C
The effect of liquid water at higher temperature
is shown in Fig. 4. The saturated vapour pressureof water at 80°C is about 47 kPa, corresponding
to a steam concentration of 1.6x10-2 mol dm-3, soin the 500 μl tests about 30 mg of water would
have vaporised in the 100 cm3 vessel. In the 5 μl
tests, all the water would have evaporated givinga steam concentration of ~3x10-3 mol dm-3.
The results indicate that the decomposition ratewith 500 l of water in the vessel was about 50%
lower than with 5 l (0.45 compared with
0.79 kGy-1). The tests with the higher water
volume gave similar decomposition rates to thosemeasured at ~30°C, supporting the observation
made in the previous section that the presence ofliquid water may have more effect than thetemperature. However, it can again be seen thatthe effect is quite small.

Fig. 10.The decomposition of CH3I during γ
irradiation in air at ~30°C with the addition of
5 μl () or 500 μl of water, (). The averagerate of methyl iodide decomposition under ‘dry’
conditions is plotted as a solid line. The averagerate of change seen in the Tang and Castleman
experiments in moist air1 is plotted as a dottedline

Effect of temperature
All experiments were conducted at either ~30 or
80°C. The decomposition rate at 80°C with 5 μl
water initially added was 0.79 kGy-1, about twice
as high as in comparable tests at ~30°C.
Reproducibility was much better in the highertemperature tests; the relative standard deviation
was less than 10% even though the initialconcentrations of CH3I ranged from 10-8 to
5x10-7 mol dm-3.

Fig. 11. The decomposition of CH3I during γ
irradiation in air at 80°C with the addition of 5 μl() or 500 μl () of water. The trend lines show
the average decomposition rate for the two
experimental conditions.
236

NEA/CSNI/R(2016)5
studied (10-8 to 610-7 mol dm-3 CH3I). Theremay be a slight trend towards a faster rate at
higher initial concentration, but this result is not
significant within the uncertainties. In their workin dry conditions, Tang and Castleman2 found noeffect of initial CH3I concentration at levels
studied in the current work.

Fig. 12. The decomposition of CH3I during γ
irradiation in air at 80°C with the addition of 5 μl
water from different starting concentrations.
Exponential trend lines are fitted to eachindividual experiment. CH3I remaining is given
in mol.dm-3.
Fig. 13. The decomposition of CH3I during γ
irradiation in air at 80°C with the addition of 5μl
water in vessels of varying surface area tovolume ratios (cm-1). Trend lines are fitted to thedata from vessels S/V of 1.5cm-1 and 3.8cm-1.

It should be noted that in these 80°C tests, all the
injected water would have evaporated. It istherefore difficult to conclude whether theapparent rate increase is due to the higher
temperature or the lack of liquid water, since therate at 80°C is close to the “dry” rate measured at~30°C. However, it is clear that the decomposition rate is not strongly temperature dependent.

Effect of surface area to volume ratio
Two experiments were carried out using a vessel
with increased surface area at 80°C (Vessel 3,
Table I). In Fig. 6 these results are comparedwith those of the standard vessel (Vessel 2, TableI) under the same conditions. Increasing thesurface area of the glass vessel by a factor of 2.5
reduced the decomposition rate by about half.

Unfortunately it is not possible to study someeffects independently in this system, for example
increasing the temperature increases the water
vapour concentration because CH3I is injected asaqueous solution as discussed. Similarly, at
80C, introducing liquid water also increased thewater vapour concentration.

It was not possible to decrease the surface area tovolume (S/V) ratio in the high dose-rate positionof the irradiation facility, so experiments withlarger volume vessels were carried out in thelower dose position where more space wasavailable. For these experiments the vessel usedpreviously for high dose-rates was used along

Effect of initial methyl iodide concentration
The effect of the initial CH3I concentration onradiolysis was studied at 80°C as the results weremore reproducible at this temperature (Fig. 5).
The decomposition was first order over the range
237

NEA/CSNI/R(2016)5
with a larger volume vessel (Vessel 4, Table I).
At the lowest S/V ratio the decomposition rates
are, on average, the same as those for theintermediate S/V ratio although there isconsiderable scatter in the results. At the highest
S/V ratio there is again a decrease in thedecomposition rate by about a factor of 3 (seeFig. 6).

Effect of Dose-rate
The effect of reducing the dose-rate is shown in
Fig. 7. Results show that reducing the dose-rate
by a factor of 8 (from 560 to 71 Gy hr-1) only
decreases the decomposition rate (expressed as afunction of time) by a factor of approximately 3.
In terms of dose, therefore, the decomposition
rate increased from 0.58 to 1.54 kGy-1 when thedose rate was decreased from 560 to 71 Gy hr-1.

The effect of increasing surface area was todecrease decomposition at both high and low
dose-rates, indicating losses of reactants at
surfaces. Surfaces have been found to beimportant in other gas phase studies particularly
involving measurements of radiolysis products
from water vapour.

Such dose-rate effects are often observed inradiation chemical systems as a result of second
order reactions. Decreasing the dose rate
decreases the concentrations of radical species,
making them more likely to encounter otherreactants (e.g., CH3I) than each other.
Decreasing the dose rate again to 5.5 Gy hr-1
gave no further change in the rate ofdecomposition. It appears that there is a change
in the dose dependency of decomposition at very
low doses, which cannot be satisfactorily
explained.

Fig. 14. The decomposition of CH3I during γ
irradiation, at a high, low or very low dose rate,
in air at ~30°C. The high dose rate (560 kGy hr1) data is averaged over various adsorbed dosepoints. Error bars give the range ofthe results todemonstrate the large amount of scatter.
Exponential trend lines are fitted to eachexperimental condition.

Fig. 15. The decomposition of C2H5I ()compared with decomposition of CH3I ()during γ irradiation in air at ~30°C. Trend lines
are fitted to the data from C2H5I decompositions.

The presence of liquid water appears to have asimilar effect to that of increasing the surface
area, and this may be associated with dissolutionof reactants.
238

NEA/CSNI/R(2016)5
Decomposition of C2H5I
All work described so far has been carried out
with CH3I, however, the reactions of I withorganic materials found in containment could
produce a range of volatile organic iodides, suchas C2H5I. Two tests therefore were carried out toestablish whether the decomposition rate of
C2H5I was significantly different to that of CH3I.
Fig. 8 shows that the decomposition of C2H5I iswithin the experimental scatter of the CH3I
results. This suggests that the decomposition
mechanisms are similar in both cases and areprobably similar for all aliphatic organic iodides.
Post irradiation reactions
Fig. 16. The decomposition of CH3I during γ
irradiation at a dose rate of 560 Gy hr-1 in air at~30°C with the addition of 5 μl water before
irradiation was stopped at 3.5 hours.

To examine and detect any post-irradiation
reactions, a sample of CH3I was irradiated for 3.5hours; irradiation was then stopped and the CH3I
concentration was repeatedly sampled. After the
3.5 hours of irradiation, only ~10% of the initial
CH3I remained. No further decrease wasobserved suggesting that no post-irradiation
reactions occurred (Fig.9). This indicates thatstable air radiolysis products such as HNO3,
HNO2, NOx and O3 do not react significantlywith CH3I in the quantities produced at this dose.

Different Atmospheres
A number of experiments were conducted underatmospheres other than air to give information on
possible reaction mechanisms of CH3I
decomposition and to obtain data relevant toinerted atmospheres used in some reactor
containments. The decomposition rate constantsfor these experiments are summarised in Fig. 10.

The absence of any post-irradiation reactions iscontrary to a note in the paper by Tang andCastleman2. In this work the residual CH3I
concentration was quite low which could be an
important factor as could the relative dose-rate.
The dose-rate used by Tang was probably afactor of 10 greater than that used in this work.

Effect of pure oxygen atmosphereWith experiments at 20C, there was difficulty
obtaining reproducible results and no difference
between results in air and oxygen atmospheres
could be seen. The average rate constant was
0.86 kGy-1 compared with 0.45 kGy-1 forcomparable tests in air, but again, the relative
standard deviation was high (~40%). At 80C the
reproducibility of results was better (Fig. 11) andthe mean decomposition rate was 0.58 kGy-1
compared with 0.79 kGy-1 for the same
conditions in air.
Effect of N2O
One of the main initiating reactions for CH3I
decomposition is thought to involve the electron:
CH3I + e- → CH3 + I-

239

(2)

NEA/CSNI/R(2016)5
In two experiments N2O was used on the basisthat it also undergoes dissociative electron
capture and may compete with CH3I for e- andhelp confirm the mechanism.

moist 1% O2 was even faster with kave = 8.30
kGy-1. Results are shown in Fig. 11.
The increase in CH3I decomposition rate withdecreasing O2 is consistent with a mechanism inwhich the main reactant for CH3I is the electron.
Competition between O2 and CH3I for e- would
result in the observed behaviour.

However, this was not entirely successful since
the rate constant of N2O with e- is not sufficiently
high to compete effectively. Also, the radiation
chemistry of N2O is very different to N2/O2 socomparing the decomposition behaviour in thetwo gases is not straightforward. The
decomposition rate constant did decrease, theaverage value for two experiments was 0.56 kGy1
compared with dry air where kave = 0.85 kGy-1(see Fig. 10).

Fig. 17. Summary of the decomposition rate constants of methyl iodide, k, for experiments conducted in
O2, N2O, 1% O2 in N2 & N2 atmospheres in a range of conditions. The irradiation conditions
altered are temperature (°C), dose rate (high or low) and water added to the system (dry or moist
conditions). Error bars give one standard deviation

Effect of 1% O2
Effect of pure N2
There were two reasons for measuring the effectof low O2 concentrations: (i) BWR reactor
containments operate with a nitrogen inerted
atmosphere which by definition is low in O2 and(ii) to provide further information to help
understand the decomposition mechanisms

Tests in pure N2 completed the range of O2/N2
atmosphere experiments. The decomposition rate
was too fast to measure accurately at the highdose-rate and rather scattered and nonexponential at the low dose-rate. The calculated
rate constants are included in Fig. 10 forcompleteness. At the low dose-rate one

The decomposition rate constant, k, for a dry 1%
O2 atmosphere was 1.9 kGy-1. Decomposition in
240

NEA/CSNI/R(2016)5
experiment gave a decomposition rate constantof 7.20 kGy-1, consistent with the lack ofcompetition for e-.

Experimental ScatterIn this work there was considerable scatter as
mentioned several times above. Tang andCastleman1 reported data which appear to varyby ± 20%. In the text they noted that in low
concentration experiments the impurity level
often
interferedwiththeradiolytic
decomposition of CH3I. In this work described
here, no particularly stringent measures weretaken to exclude impurities; glassware waswashed with pure water but not annealed at
500°C and vacuum degassed as carried out in thework of Tang and Castleman2. It seems likelythat surface or airborne impurities are responsible
for the scatter. This scatter has restricted theamount of mechanistic information that is
possible to derive from the results but doessuggest that they are rather more applicable toreactor containment.
IV. CONCLUSION

Fig. 11. The decomposition of CH3I during γ
irradiation in air at 80°C with an oxygen (), air() or 1% O2 in N2 () atmosphere. Trendlines are fitted to each data set.

The radiolytic decomposition of CH3I has beenmeasured as a function of concentration of CH3I,
concentration of O2, surface area to gas volume
ratio, temperature, liquid water volume, dose-rateand surface area to gas volume ratio.
Experiments have also been carried out withC2H5I to compare with CH3I.

One reason for the scatter could be residual O2 inthe sample although the GC measurement
suggests this was not large.
Deposition of iodine
decomposition of CH3I

onto

surfaces

The decomposition rates measured here are of asimilar magnitude to those in the only previously
reported work at very low water vapour
concentrations. There is no significant effect oftemperature, but increasing the concentration ofO2, the surface area to gas volume and the water
vapourconcentration all
decrease thedecomposition rate. The decomposition showed asquare-root dependence on dose-rate. The rate ofdecomposition of C2H5I is similar to that of
CH3I. Overall, no significant sensitivities havebeen observed and rates measured here can beapplied to containment situations.

after

As noted above, a single test was carried out todetermine the fate of iodine after radiolytic
decomposition of methyl iodide gas and theiodine distribution after irradiation was
calculated. After irradiation 51% of the iodine
remained as CH3I, but only 19% found on thevessel walls. No iodine (<1%) was measured in
the NaOH trap, on the purge line from the vessel.
Although there is uncertainty in accounting forthe 30% of iodine not recovered, it is thought
most likely to be due to experimental errors
and/or losses during the experiment, rather than
the iodine being in a gaseous form that was nottrapped by the sodium hydroxide.

The results of these experiments have been used
to develop a mechanistic model of CH3I
decomposition and this is described in a separate
paper [4].

241NEA/CSNI/R(2016)5
Iodide in Air,” Journal of Physical
Chemistry, 74, 22, 3933-3939, (1970).

ACKNOWLEDGMENTS
This work was originally conducted as part of the
5th Framework ICHEMM project, which was
jointly financed by the European Commission.

2. I. N. Tang and A. W. Castleman BNL12854, Figure given and referenced in A. K.
Postma and R. W. Zavadoski, WASH-1233,
1972, but does not appear in the accessedcopy of BNL-12854.

NOMENCLATURE

3. Standard Test Method for Absorbed Gamma
Radiation Dose in the Fricke Dosimeter.
American Society of Testing and Materials,
Designation: D 1671 – 72, 682-685 (1972).

D = absorbed dose
GC = Gas Chromatograph

4. S. Dickinson, S. Bowskill, H. E. Sims, “TheIODAIR model for radiolysis of gaseousiodine species in air: data comparisons andpredictions”, This conference, Paper 3.4.

ICHEMM = Iodine Chemistry and Mitigation
Mechanisms
ICP-MS = Inductively Coupled Plasma Mass
Spectrometry
PWR = Pressurised Water Reactor
S:V = Surface Area to Volume Ratio

REFERENCES
1. I. N. Tang and A. W. Castleman, “Kineticsof γ-Induced Decomposition of Methyl242

NEA/CSNI/R(2016)5
THE IODAIR MODEL FOR RADIOLYSIS OF GASEOUS IODINE SPECIES IN AIR: DATACOMPARISONS AND PREDICTIONSS Dickinson*, S Bowskill, H E Sims
National Nuclear Laboratory, Harwell, Didcot, Oxon, OX11 0QT, UK*Corresponding author, tel: (+44) 7894 598707, Email:shirley.dickinson@nnl.co.uk(1)

Abstract – This paper describes the mechanistic IODAIR model which was developed to predict andinterpret the behaviour of gaseous iodine species in irradiated moist air. The model predictions arecompared with experimental measurements of I2 radiolysis, including the PARIS tests, and with data fromexperimental studies of CH3I radiolysis carried out at Harwell as well as later measurements from theOECD Behaviour of Iodine Programme (BIP). The Harwell experiments on CH3I radiolysis are describedin a companion paper.
The model predicts that the radiolytic destruction of both I2 and CH3I results in the formation of various
intermediate gaseous species, the most abundant being HI and IONO2 under the conditions of theexperiments modelled. Formation of the final solid oxide products occurs via reactions involving the minor
intermediate species IO, I2O2 and IO2, and there is considerable uncertainty in some of the rates andmechanisms of these reactions. The model gives good agreement with experimental measurements ofgaseous I2 decomposition during irradiation at relatively high starting concentrations. However, at lower
concentrations the extent of decomposition is overestimated, and it is possible that back-reactions areimportant.
The model also gives good agreement with measurements of the radiolytic destruction of CH3I in air, andreproduces the observed increase in rate at low O2 concentrations. Mechanisms are proposed to accountfor the experimentally-observed dose rate dependence, which changes from linear at ≥500 Gy hr -1 to asquare-root dependence at lower dose rates, and for the effect of humidity.
Model calculations to investigate the potential importance of CH3I formation by homogeneous gas-phasereactions of I2 and CH4 are also presented.

243

NEA/CSNI/R(2016)5𝛽,𝛾I. INTRODUCTION

𝑁2 → 2.12𝑁, 0.54𝑁 + , 2.15𝑁2+ , 0.29𝑁2∗ , 2.69𝑒 −Iodine has a complex chemistry which
determines its chemical form and, consequently,
its volatility in the containment. In reducing
conditions, iodine would be released from thefuel predominantly as CsI or other metal iodides,
which are involatile at containment temperatures
and would normally dissolve in blowdown or
reflood water. Radiolytic oxidation is the main
mechanism for oxidation of iodide in solution tovolatile forms such as I2. I2 can react withsurfaces in the containment, leading to its
retention but also to the potential formation ofvolatile organic forms [1]. Once these species are
released into the containment atmosphere they
will be subject to radiolytic reactions which
result in the conversion of the gaseous species
into involatile iodine oxides, which may deposit
on surfaces or re-dissolve in water pools. The
concentrations of volatile species in theatmosphere will therefore be determined by thebalance between their formation and destruction
reactions.

𝛽,𝛾𝑂2 → 1.32𝑂, 1.23𝑂+ , 2.07𝑂2+ , 1.9𝑂2∗ , 3.3𝑒 −
𝐻2 𝑂
𝛽,𝛾

→ 0.45𝑂, 4.07𝐻, 4.17𝑂𝐻, 0.67𝐻+ , 0.165𝐻2 ,2.56𝐻2 𝑂+ , 3.23𝑒 −The figures given above are the G-values for
production of the species in units ofmolec./100eV absorbed by the appropriate
gaseous component, taken from Busi et al. [6].
The interactions of these primary products resultin the accumulation of ozone, HNO2, HNO3 andsmaller amounts of N2O. Other N-O and O-H
species are also predicted to be formed with low,
constant
concentrations.
Experimental
measurements of the major products can be used
to validate the predictions of mechanistic modelsof air radiolysis; however, it is debatable howmuch of an insight such measurements can give
into the rate of oxidation of gaseous iodine
species if the dominant reactions mainly involve
short-lived intermediate species rather than thestable products.

The current paper describes the mechanistic
IODAIR model which was developed to predict
and interpret the behaviour of gaseous iodine
species in irradiated moist air. The model
predictions are compared with experimental
measurements of I2 radiolysis, and with datafrom experimental studies of CH3I radiolysis
carried out at Harwell [2] as well as latermeasurements from the OECD Behaviour of
Iodine Programme (BIP) [3].

Reactions of gaseous iodine species were added
to an air/steam radiolysis model to produce the
IODAIR model, which was used to simulate theexperiments discussed in Section III. These
calculations showed that I2 reacts initially withOH, O and H radicals to form intermediate
species:
I2 + OH  HOI + I
I2 + O  IO + I
I2 + H  HI + I

II. DESCRIPTION OF THE IODAIR MODEL

(1)
(2)
(3)

HOI and HI are oxidised by OH:

Kinetic models of the chemistry of iodine species
in moist air in a radiation field have beenassembled by Sagert [4] and Aubert [5], andthere are many similarities with the model
presented here. The basis of these models is a setof reactions describing the production andinteraction of ionic and radical species producedby the irradiation of N2, O2 and H2O gases. Theinteraction of radiation with the molecularspecies produces ions, electrons and excitedradical species. In the H2O-air system, theprimary reactions are:

HOI + OH  IO + H2O
HI + OH  I + H2O

(4)
(5)

HOI and I can also form the intermediate INO2
HOI + HNO2  INO2 + H2O
I + NO2  INO2

(6)
(7)

which re-forms I2:
INO2 + I  I2 + NO2
2 INO2  I2 + 2 NO2

244

(8)
(9)

NEA/CSNI/R(2016)5
CH3I + O  CH2I + OH
CH3I + O  IO + CH3
CH3I + OH  CH2I + H2O
CH3I + H  CH3 + HI
CH3I + e-  CH3 + I-

IO reacts with NO:
IO + NO  I + NO2(10)

and reversibly with NO2 to form an IONO2
intermediate
IO + NO2 ↔ IONO2(11)

The above reactions would result in the almost
complete conversion of I2 to IONO2. Iodineoxides (such as I2O5 or I4O9) are produced by arelatively minor route involving IO2, which is
produced by the reactions of I2 and IO with O3:
I2 + O3  IO + IO2
IO + O3  IO2 + O2

The reaction rates used in the model for thesewere taken from the NIST compilation [9] andthe electron reaction rate is the average of several
values tabulated by Burns et al. [10]. Modellingstudies indicate that electron reaction isdominant, and the decomposition rate observedin air can be reasonably well reproduced by thisreaction alone. The reaction leads to first-orderdecomposition in air at low CH3I concentration
because the majority of the electrons are
consumed by reaction with O2 (see equation (33)
below), and this is consistent with the higher
decomposition rate observed in 1% O2/N2
atmosphere compared with air. Aubert [5]suggested that the reaction of CH3I with N atoms
was at least as important as with electrons,
although the products of this reaction have notbeen identified. This mechanism would also giverise to first order kinetics at low CH3I
concentrations since atomic N also reacts rapidly
with O2.

(12)
(13)

However, the reaction rate for IO + O3 given by
Larin et al. [7] is quite low and IODAIR predicts
that the main pathway is as proposed by Bloss etal. [8]:
IO + IO  IO2 + I
2O2 + M

(14)
(15)

There are two possible isomers of the I2O2
intermediate, with different decompositionroutes:IOOI  I + I + O2
IOIO  IO2 + I

(16)
(17)

IO2 is assumed to dimerise to I2O4 and otheroxide aerosols:
IO 2

IO 2

Kinetic data have been reported [11] forCH3I + NO3  CH3O + I + NO2

O3

IO 2  IO 2  I 2 O 4  I 3 O 6  I 4 O 8  I 4 O 9(24)

However, including this in the model was foundto give very fast decomposition rates, possibly
because NO3 is overestimated. In the reported
work, NO3 was generated by reaction of NO with
CH3 radicals and it was concluded that theasymmetric form (O-N-O-O) would bedominant. In the radiolysis of air, the main
formation reaction for NO3 is

IO2 also reacts with IO:
IO + IO2  I2O3

(19)
(20)
(21)
(22)
(23)

(18)

and this presumably undergoes polymerisation
and/or oxidation to produce the final products.
The above scheme implies that the reaction of O3
with I2 does not have an important influence on
the rate of I2 removal, and in fact the calculated
rate of I2 destruction is not significantly changed
if this reaction is removed from the model. The
main role of ozone in this reaction scheme is in
the production of more highly oxidised products
than could be formed by polymerisation of IO
and IO2.

NO2 + O3 = NO3 + O2

(25)

which has been estimated to give only 3% of
NO3 in the asymmetric form [12]. As it is notclear that the reported rate constant for reaction(24) is applicable to this system, and its inclusion
leads to considerable overestimation of the CH3I
decomposition rate, this reaction has been
excluded from the model.

The decomposition of CH3I can be initiated byseveral reactions:

No reaction between CH3I and O3 was includedin the kinetic model as no reported rate constant
was found for such a reaction. It has been shown
245

NEA/CSNI/R(2016)5
that iodine oxide particulate is not formed when
CH3I and O3 are combined in the absence of UV
radiation, confirming that direct reaction withozone is not a significant mechanism for theformation of solid iodine oxides [13].
The I- ions produced by the reaction of CH3I withelectrons would be rapidly neutralised to I atoms
by positive ions such as N+. Reaction of two I
atoms to produce I2 is only predicted at high
CH3I concentrations, as observed by Tang andCastleman [14]; at lower concentrations the Iatoms are more likely to react with the productsof air radiolysis so the reaction mechanism andproducts will be similar to those for I2 oxidation.

Fig. 1. Comparison of IODAIR model with datafrom Funke et al. [15] for experiments at 25°C
with an initial I2 concentration of 4×10-6 mol dm3. The points are experimental data and the lines
are IODAIR calculations.

III. COMPARISON WITH
EXPERIMENTAL DATA ON I2
DECOMPOSITION(A) Funke tests.
Funke et al. [15] reported measurements of I2
destruction by air radiolysis in air and steam/air
atmospheres, at dose rates of 1.9 and 20 kGy hr-1
and temperatures of 25, 80 and 130°C. The I2
remaining after irradiation was measured bywashing the irradiation vessel with an organic
solvent
which
was
then
analysed
photometrically.
The predictions of the IODAIR model arecompared with these results in the following
figures. Fig. 1 shows the effect of dose rate andsteam concentration at 25°C. The “dry air” and“steam/air” calculations assumed H2O fractionsof 2×10-5 (~0.1% RH) and 0.02 (~80% RH)
respectively. The experimental results show that
there is no significant effect of dose rate or steam
concentration on the I2 decomposition rate, andthe calculations reproduce this behaviour and themeasured decomposition rates quite well.

(a) 80°C

Fig. 2 shows the effect of I2 concentration at
80°C and 130°C. The measured decomposition
rates increase with decreasing initial I2
concentration, and again this behaviour isreproduced by the model.

(b) 130°C
Fig. 2. Comparison of IODAIR model with datafrom Funke et al. [15] for experiments at
different I2 concentrations. The points areexperimental data and the lines are IODAIR
calculations.

246

NEA/CSNI/R(2016)5
The effect of temperature on the decomposition
rate was small, showing, if anything, a decreasein rate with increased temperature. The model
overestimates the decomposition somewhat at
130°C.

assumed to be oxidised iodine and unreacted I2
respectively. The results show that the extent ofradiolytic oxidation after ~ 1 kGy decreased withincreasing iodine concentration. For a given
initial I2 concentration, the degree of oxidation
was generally higher in O2/steam, and lower in
H2/steam, than in air/steam atmospheres.
Complete oxidation occurred in most tests after~15 kGy, except at the highest concentrations(>10-7 mol dm-3) and in some tests with H2 where
the results were rather inconsistent.

(B) I2 decomposition: PARIS tests.
The radiolytic oxidation of I2 at lower initialconcentrations was studied in the PARIS
programme [16,17]. In these tests, gas mixtures(air, O2 or H2 + 30% steam) containing between
2.5x10-9 and 7.5x10-7 mol dm-3 I2 were irradiatedat 80°C and about 1 kGy hr-1 for ~1 hour or ~15
hours. CCl4 and water were injected into thevessel at the end of the test to extract the waterand organic-soluble iodine fractions, which wereThe IODAIR model reproduced the observed
trends in these tests, but generally overpredicted
the extent of I2 destruction occurring after a doseof ~1 kGy as shown in Table I.

TABLE I
Comparison of IODAIR model with PARIS test results
Test refGas[I2]init
mol/dm3

Fraction remaining after ~1 kGy
I2 (exp)

I2 (calc)

Iother (calc)

0%
0%
31%
31%
0%

32% IONO2
26% IONO2
48% IONO2
48% IONO2
30% HOI

0%16% HOI

0%17% HOI

9%80% HI

6V21
8V1†
4V25
4V24

air
air
air
air

3.1E-09
1.1E-08
6.1E-08
6.1E-08

44%
72%
87%
90%

11T13

O2

2.5E-09

7%

6V17

O2

7.2E-09

20%

6V16

O2

7.2E-09

6%

6V26

O2

9.8E-09

7%

3V18

O2

1.6E-08

70%

3V19

O2

1.9E-08

69%

4V13

H2

2.6E-08

93%

4V14

H2

2.6E-08

91%

4V15

H2

2.6E-08

89%†

The iodine mass balance was very low (35%) in this test so results are subject to considerable
uncertainty

247

NEA/CSNI/R(2016)5
For initial I2 concentrations below 2×10-8 mol
dm-3, the model predicts complete removal of I2
after less than 1 kGy dose in either steam/air or
steam/O2, in contrast to the test results. However,a significant fraction of the iodine is calculated toform IONO2 (in steam/air) or HOI (in steam/O2).
These products would be expected initially todissolve in the aqueous phase during theextraction procedure, but could then hydrolyse
and/or disproportionate to liberate I2:
IONO2 + H2O = HOI + HNO3

(26)

5HOI = HIO3 + 2I2 + 2H2O

(27)

It is possible, therefore, that the measured I2
fraction also includes some intermediate species.
In the H2 atmosphere, the model predicts that theI2 is rapidly converted to HI:
I2 + H = HI + I

Fig. 3. Comparison of IODAIR model withdata from PARIS tests in steam/air after 15
kGy irradiation.

(28)

which reacts with further H atoms to re-form I2:
HI + H = H2O + I

It can also be seen that the highest I2
concentrations in the PARIS tests, ~5×10-7 and7×10-7, are similar to the lowest concentration
used in Funke’s tests, in which 20% of the I2
remained after 15 kGy irradiation under similar
conditions (Fig. 2(b)). This could provide
support for the suggestion that other species areincluded in the I2 measurements.

(29)

I + I = I2 (30)This results in a steady state HI/I2 ratio of about
10 for the conditions studied here. I2 oxidation by
OH radicals (reaction (2)) produced fromradiolysis of steam is largely suppressed by thereaction
OH + H2 = H2O + H

IV. COMPARISON WITH
EXPERIMENTAL DATA ON CH3I
DECOMPOSITION(31)

so oxidation occurs more slowly by O atoms(reactions (2) and (14)). The large difference
between the model and experimental resultscannot be readily accounted for by possible
changes in the product speciation during theextraction procedure.

(A) Harwell tests.
Experimental measurements of CH3I radiolysis
are reported in a companion paper [2]. Thesestudies looked at the effect of a number ofvariables, including CH3I concentration,
atmosphere, dose rate and humidity. The
predictions of the model are compared with theexperimental results in the following sections. In
the tests discussed below, the irradiation vessel
contained a small amount of liquid water, unlessstated otherwise, so the bulk gas would havebeen saturated in H2O.

After ~15 kGy irradiation, practically no I2
remained in the tests in O2 atmosphere, whereasin H2 the remaining fraction ranged from 9 –90% with no apparent correlation with the initialconcentration. The model predicts complete
oxidation at this dose in both cases. In air, theamount of I2 remaining appears to correlateroughly with the initial concentration above 10-8
mol dm-3, as shown in Fig. 3. This also shows thecalculated concentrations of I2 and (I2 + IONO2),
which show a similar trend.

Effect of initial methyl iodide concentration
The experiments showed that the decompositionof CH3I was first order over the range of initialconcentrations studied (~10-8 – 5×10-7 mol dm-3).
The calculations reproduce the observed
behaviour as shown in Fig. 4.
248

NEA/CSNI/R(2016)5
so the decomposition would no longer be firstorder.
Effect of atmosphere
Tests were conducted in air, O2 and 1% O2/N2
and the results are compared with the IODAIR
calculations in Fig. 5.

Fig. 4. Comparison of IODAIR model withdata from CH3I radiolysis tests at 80°C:
effect of initial CH3I concentration. The
points are experimental data and the lines
are IODAIR calculations.
Fig. 5. Comparison of IODAIR model with datafrom CH3I radiolysis tests at 80°C: effect of
atmosphere. The points are experimental data
and the lines are IODAIR calculations.

The first order behaviour is consistent with thedominant removal reaction in air being
CH3I + e-  CH3 + I-

(23)

This is in competition with the reaction between
electrons and O2:
O2 + e- + M  O2- + M

The experiments at 80°C show a slight decreasein the decomposition rate in O2 compared withair, whereas the model predicts about a factor of
2 increase. The dominant removal reaction in O2
is with OH radicals (reaction (21) as theelectrons react mainly with oxygen. The
discrepancy suggests that there may be otherreactions of OH that are missing from the model;
however, these conditions are of limited interest
here so this was not pursued. At low O2
concentration the CH3I decomposition rateincreased, and this is consistent with the
behaviour discussed above (equation (33)).

(32)

where M is a third body in the termolecular
reaction. Although reaction (23) is much fasterthan (32), the concentration of CH3I is much
lower than that of O2 so most of the electrons are
removed by (32). The removal rate of CH3I byreaction (23) is therefore given by:
𝑑[𝐶𝐻3 𝐼]𝑘 [𝐶𝐻3 𝐼]= 𝑘 23[𝑂 ][𝑀]𝐷𝐺𝑒 − = 𝑘𝑒𝑓𝑓 [𝐶𝐻3 𝐼] (33)𝑑𝑡
32 2Where 𝑘23 and 𝑘32 are the rate constants ofreactions (23) and (32), 𝐷 is the dose rate, 𝐺𝑒 − isthe electron production rate and 𝑘𝑒𝑓𝑓 is aneffective first-order decomposition rate which
depends on [O2] (and on the identity of the third
body M as discussed below). Equation (33) only
applies if 𝑘32 [𝑂2 ][𝑀] ≫ 𝑘23 [𝐶𝐻3 𝐼]; at higher[CH3I] or lower [O2] it becomes𝑑[𝐶𝐻3 𝐼]
𝑘23 [𝐶𝐻3 𝐼]= 𝑘 [𝑂 ][𝑀]+𝑘
𝐷𝐺𝑒 −
𝑑𝑡
32 2
23 [𝐶𝐻3 𝐼]

Effect of dose rate
Experiments at 560, 71 and 5 Gy hr-1 showed that
the CH3I decomposition rate is proportional to
the square root of the dose rate. This is notconsistent with the behaviour reported by Tangand Castleman, who found that thedecomposition rate was directly proportional tothe dose rate over the range 0.35 to 12 kGy hr-1.
This apparent change in the dose rate dependenceof the decomposition rate suggests that there

(34)

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NEA/CSNI/R(2016)5
could be two main reactants for CH3I whose
concentrations change differently with dose rate.

The model overestimates the decomposition rate
at the very low dose rate, which may indicatethat the value of k36 is too high or that othereffects (e.g. with surfaces or impurities) become
more important under these conditions.

Primary radiolysis products such as OH, O and eare produced at a rate that is directly proportional
to the dose rate. The main reactions of O and eare with bulk gaseous components:
O2 + e- + M  O2- + M(32)

O2 + O + M  O3 + M(35)

Effect of temperature and humidity
In most of the experiments described in [2], CH3I
was introduced as an aqueous solution. The
volume of solution was either 5 or 500 µl, whichmeant that at 25°C liquid water was present in all
the tests. At 80°C, 5 µl of water would havecompletely evaporated giving about 7% H2O in
the gas phase; with 500 µl, the saturated vapour
pressure corresponds to about 47% H2O andsome liquid water would have remained.

so the steady-state concentrations, and hence therate of reaction with CH3I, will also be
proportional to the dose rate. OH radicals react
mainly with N-O species that accumulate with
dose, so the reaction of CH3I with OH could giverise to the observed dose rate effect; however,this reaction only makes a very minor
contribution to the overall decomposition at
25°C.

There are a number of ways in which the water
concentration can affect the CH3I decomposition
rate. Firstly, decreasing [H2O] decreases theconcentration of OH radicals, but this has a small
effect as reaction (21) is only a minor pathway
for decomposition.

It has not been possible to unequivocally identify
the reactant(s) responsible for the dose rate
effect, but one possibility is the NO3- ion. A
number of possible mechanisms have been
considered, and the one that appears the mostthermodynamically favourable isCH3I + NO3- = I- + CH3ONO2

At relatively high [H2O], there is a third bodyeffect on the O2 + e- reaction (32); the rate
constants for this reaction with M = O2, N2 andH2O are 7.2×1011, 3.1×1010 and 5×1012 mol-2 dm6
s-1 respectively [18]. The overall rate in dry air istherefore 1.7×1011 mol-2 dm6 s-1 so a few percentof H2O would increase the reaction rate andtherefore decrease the CH3I decomposition rateaccording to equation (33). However, for H2O
fractions below about 0.5% the effect on thisreaction will be insignificant.

(36)

A rate constant of 108 mol-1 dm3 s-1 for thisreaction produces the dose rate effect shown in
Fig. 6.

Another mechanism is therefore needed toaccount for the effect of humidity at much lower
concentrations. Tang and Castleman observed afactor of 2-3 decrease in the CH3I decomposition
rate when [H2O] was decreased from 2.110-7
moles cm-3 to 3.710-10 moles cm-3 [13]. Thissuggests that H2O competes with CH3I for
another reactive species. One possibility is theO2- ion formed in reaction (32). This could reactwith CH3I:
CH3I + O2- = CH3O2 + I-

Fig. 6. Comparison of IODAIR model with datafrom CH3I radiolysis tests at 20°C: effect of dose
rate. The points are experimental data and thelines are IODAIR calculations.

(37)

No kinetic data were found for reaction (37) but
the rate constants for the analogous reactions ofCH3Cl and CH3Br are 4.5×1011 and 7.8×1011
mol-1 dm3 s-1 respectively [19] so it is reasonable
to estimate a value of 1012 for reaction 37.
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NEA/CSNI/R(2016)5
At high H2O concentrations, most of the O2- isremoved by clustering reactions with H2O:
O2- + H2O + M = O2-H2O + M(38)
O2-H2O + H2O + M = O2-(H2O)2 + M(39)
O2-(H2O)2 + NO2 = NO2-H2O + H2O + O2
(40)
NO2-H2O + M = NO2- + H2O + M(41)
At low [H2O], these reactions will be lessimportant so more O2- will be available to reactwith CH3I by reaction (37).
The experimental data show that decomposition
at 80°C is faster at lower [H2O], and this isconsistent with the mechanisms discussed above.
The model gives the same qualitative behaviour
but the effect is less pronounced than observed,
as shown in Fig. 7.

Fig. 8. Comparison of IODAIR model with datafrom CH3I radiolysis tests at 20°C: effect ofhumidity. The points are experimental data andthe lines are IODAIR calculations.
Although the model somewhat under-predicts theeffect of changing [H2O] at 80°C, it does reflect
the experimental observation that humidity has arelatively minor effect on CH3I decomposition.
There may be other effects associated with thepresence of liquid water that are not taken into
account in the model.
The model predicts that reaction (37) makes anincreasing
contribution
to
the
CH3I
decomposition rate at H2O concentrations below
about 0.1%, as shown in Fig. 8.
Comparison of Fig. 7 and 8 also shows thatchanging the temperature from 25 to 80°C has asmall effect on the measured decomposition rate,
although the large scatter in the low temperature
tests makes this difficult to quantify. This low
sensitivity to the temperature is also apparent in
the model results.

Fig. 7. Comparison of IODAIR model with datafrom CH3I radiolysis tests at 80°C: effect ofhumidity. The points are experimental data andthe lines are IODAIR calculations.

(B) BIP tests.
The OECD Behaviour of Iodine Project (BIP)
investigated the adsorption of iodine adsorbed
onto painted surfaces and the subsequent
production of organic iodides under irradiation[3]. Because the irradiation tests were carried out
in a closed system over many hours, themeasured CH3I concentrations reflected thebalancebetweenthe
production
anddecomposition processes. A quantitative
understanding of the decomposition reactions is

A similar tendency towards increasing
decomposition rate at lower humidity is shownby the “dry” test at 25°C in which CH3I was
injected as a gas rather than in aqueous solution.
This is also reproduced by the model as shown in
Fig. 8.

251

NEA/CSNI/R(2016)5
therefore necessary to determine the long-term
release behaviour.
Some of the tests showed that the CH3I
concentration gradually increased at longirradiation times, suggesting either that the
production rate was increasing or thedecomposition rate decreasing. A possible
mechanism for the latter is the reaction between
e- and HNO3
HNO3 + e- = NO2- + OH

(42)
13

3

which has a rate constant of ~3×10 dm mol-1 s1[20]. As HNO3 builds up due to irradiation, thisreaction could compete with reaction (23) andthus decrease the CH3I decomposition rate. There
are only a limited number of reactants that would
have this effect since, as equation (34) shows, theproduct of the rate constant and reactant
concentration would have to be comparable to𝑘32 [𝑂2 ][𝑀] to significantly affect the CH3I
decomposition rate.

Fig. 9. Comparison of IODAIR model with datafrom BIP tests with multiple CH3I injections(Data are proprietary to the BIP partners so areshown without axis markings)
The decomposition rates measured in the BIP
tests were similar to those in the AEAT tests, andthe data showed much less scatter. This could be
due to the fact that the irradiation vessel was
baked at 500°C before the tests, which would
have minimised the amounts of impurities. Also,
the CH3I was introduced as a gas so there would
have been no liquid water present, though noparticular means were taken to dry the air within
the vessel.

The effect of irradiation time (or accumulateddose) on the CH3I decomposition rate wasinvestigated by the BIP team with tests similar to
those described above, except that once the initialcharge of CH3I was exhausted, additional
injections were made without opening the vessel.
The results showed that the destruction rate was
not significantly changed by the buildup ofradiolysis products. The IODAIR model results
are compared with the experimental behaviour in
Fig. 9. In this calculation, the model was tuned
slightly (by removing the CH3I + NO3- reaction(36) and setting the H2O concentration to 0.1%)
to give good agreement with the results from thefirst injection. It can be seen that the effect ofreaction (42) is small over the duration of thetest, but agreement is slightly better at longirradiation times if reaction (32) is not included.
The most likely explanation for this behaviour isthat HNO3 does not build up in the gas phase, asassumed by the model, but is lost to surfaces.
Another implication of this comparison is thatthe rate of reaction (36) is lower than suggested
in the data shown in Fig. 6, since this reaction
would lead to an increase in the decomposition
rate at long irradiation times which is notconsistent with the experimental observations.

V. COMPARISON WITH
EXPERIMENTAL DATA ON CH3I
FORMATION
Experimental measurements of CH3I formationfrom irradiated I2 - CH4 mixtures were reported
by Bartonicek et al. [21]. They found that whenmoist air containing 10-8 mol dm-3 I2 and 10-3 mol
dm-3 CH4 was irradiated at 0.05 Gy s-1, the CH3I
concentration increased to about 10-9 mol dm-3
(i.e. about 5% of the total iodine) after a dose of
20 kGy, after which the CH3I concentration
slowly decreased. Less CH3I was observed in Ar,
up to a maximum of ~1% of the iodine.
The IODAIR model is unable to duplicate thefindings of these tests, and predicts only a verysmall concentration of CH3I (~10-18 mol dm-3).
The model contains reactions leading to theformation of CH3I such as:

252

NEA/CSNI/R(2016)5
CH3 + I2 = CH3I + I(42)

CH3 + I = CH3I

(43)

degree of radiolytic oxidation of gaseous I2 atconcentrations below 10-6 mol dm-3. The reason
for this is not known, but one explanation could
be that other reaction products were included in
the analysis of I2 in these tests. Anotherpossibility is that decomposition of the oxide
product occurs, possibly due to hydrolysis
reactions on the vessel surfaces, resulting in a
low but constant I2 concentration at longirradiation times.

where the CH3 radicals are produced by reactions
such asCH4 + OH = CH3 + H2O(44)However, the dominant reaction of CH3 in air is:
CH3 + O2 = CH3O2(45)
9

which has a rate constant of about 7×10 dm3
mol-1 s-1 at 298 K. The rate constant of reaction(42) is similar, 3.5×109 dm3 mol-1 s-1, but the O2
concentration is much higher than that of I2(about 8×10-3 mol dm-3 at 298 K) so the fractionof CH3 radicals that would react with I2 in theconditions of Bartonicek’s experiments is about
6×10-7. The model predicts that the total CH3
formation rate is about 4×10-9 mol dm-3 s-1 at adose rate of 0.05 Gy s-1, so the formation rate of
CH3I by reaction (42) would be 2.5×10-15 mol
dm-3 s-1 giving a concentration of 2.5×10-12 mol
dm-3 after 20 kGy (neglecting any decomposition
reactions). The reported concentration of ~10-9 isa factor of 400 higher than this. The CH3I
concentration calculated by the model is evenlower than given by this simplified calculation,
because other effects such as CH3I
decomposition are also taken into account.

The model also gives reasonable agreement withexperimental
measurementsofCH3I
decomposition, and successfully reproduces theeffects of changing CH3I and oxygen
concentrations. Reactions have been postulated
that explain the observed dependences on dose
rate and humidity, but the rate constants used are
not based on independent kinetic measurements
and so there are large uncertainties associatedwith these parts of the model. In particular,
comparison with the BIP results suggests that therate constant used for the reaction of CH3I with
NO3-, which is postulated to explain the nonlinear effect of dose rate, is probably too high as
it would lead to a change in the decomposition
rate at long irradiation times which is notobserved experimentally.
One area where the model has not beensuccessful is in reproducing the results ofexperiments showing significant production of
CH3I from the gas-phase reaction of CH4 and I2
under irradiation. It is difficult to rationalise thisobservation with the expected reactions ofmethane in irradiated air; the same conclusion
was reached by Sagert in an earlier modellingstudy [4]. It is possible that surface reactions
between the radiolysis products play animportant role. Further experimental work would
be required to confirm and explain theseobservations.

A search of the NIST database has not revealed
any plausible mechanism by which CH3O2 would
react directly with I2 to form CH3I, or generate
any other species that would be expected to doso. The main CH3-containing product in themodel is methanol, CH3OH, although ultimately
complete oxidation to CO2 would be expected tooccur. It is therefore difficult to reconcile theexperimental observations with the expectedreactions in this system. A possible explanationis that the radiolysis products are able to react at
surfaces to produce methyl iodide, and any futurestudies of this system should be designed to takethis into account.

VII. CONCLUSIONThe IODAIR model has been shown to givereasonable agreements with experimental
measurements of CH3I and I2 radiolysis.
Although there are some discrepancies between
the test data and model predictions that remain
unexplained, the use of a mechanistic modelleads to a better understanding of the important
processes, and highlights the main uncertainties

VI. DISCUSSION
The IODAIR gives good agreement with the I2
removal rates measured by Funke et al. [15] atrelatively high I2 concentrations. However, it
does not predict the apparent shift towards firstorder kinetics at low concentrations shown by the
PARIS data [16,17], and so overestimates the253

NEA/CSNI/R(2016)5
that could be addressed in future experimental
programmes.

9. J. A. MANION et al., NIST Chemical
Kinetics Database, NIST Standard Reference
Database 17, Version 7.0 (Web Version),
Release 1.6.8, Data version 2013.03,
National Institute of Standards andTechnology, Gaithersburg, Maryland, 208998320.
Web
address:
http://kinetics.nist.gov/NIST.

ACKNOWLEDGMENTS
The IODAIR model was originally developed aspart of the 5th Framework ICHEMM project,which was jointly financed by the European
Commission.

10. S. J. BURNS, J. M. MATTHEWS and D. L.
MCFADDEN, “Rate coefficients fordissociativeelectron
attachment
byhalomethane compounds between 300 and
800 K”, J. Phys. Chem., 100, p. 19436,(1996).

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5. F. AUBERT, « Destruction par Radiolyse
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the Containment Atmosphere”, Iodine
Aspects of Severe Accident Management,
Vantaa,Finland,May
1999,
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6. F BUSI et al., “Radiation treatment of
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16. L. BOSLAND et al., “PARIS project:
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18. H. SHIMAMORI and Y. HATANO,“Thermal electron attachment to O2 in thepresence of various compounds as studied bya microwave cavity technique combined withpulse radiolysis”, Chem. Phys., 21, p. 187,
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21. B.
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255

NEA/CSNI/R(2016)5
INTERACTION OF GASEOUS I2 WITH PAINTED SURFACES AND AEROSOLS IN
LARGE-SCALE THAI TESTS
F. Funke(1)*, S. Gupta(2), G. Weber(3),G. Langrock(1), G. Poss(2)(1)

AREVA GmbH, Erlangen, Germany, (2) Becker Technologies GmbH, Eschborn, Germany,(3)
Gesellschaft für Anlagen- und Reaktorsicherheit mbH, Garching, Germany,..

*Corresponding author, Tel: (+49)90097681, Fax: (+49)90097312, Email: Friedhelm.Funke@areva.com
Abstract – Deposition of gaseous molecular iodine (I2) onto dry or wet surfaces in nuclear power plantcontainments during severe accidents represents an important sink to reduce iodine volatility.
Decontamination paint, covering large surface areas of walls and floors, is well-known to provide thenecessary high reactivity towards I2. Aerosols, with their high specific surface of small particles, cansignificantly increase the I2 deposition surface area. Models to predict the I2 depletion from atmospheresby interaction with painted surfaces are available. These are commonly based on results obtained fromsmall-scale tests including volumes of only up to about 0.3 m3. Up to now no models have been included insevere accident containment codes to quantify the I2 depletion by adsorption onto aerosols. The 60 m3
vessel of the THAI test facility is used to bridge the differing scales of laboratory tests and containment
dimensions. At the same time coupled effects such as I2 / aerosol interaction and iodine deposition inaerosol form have been studied in the large-scale THAI facility.
The reaction of gaseous I2 with painted surfaces was addressed in a number of THAI tests (Iod-15, Iod-17,
Iod-20, Iod-21, Iod-24, Iod-27A, Iod-28, and Iod-30). An empirical deposition / desorption model wasdeveloped and included in COCOSYS/AIM-3, but it still needs more refinements: (1) paint is only aGerman epoxy paint, and there is no connection to compare its behaviour versus iodine with the effects ofother paints, (2) ageing of paint is not explicitly scaled but always referring to 15 years, (3) ageing
procedure is different from other experimental programs and a deeper analysis of this issue including
connecting tests is necessary, (4) modeling of relative humidity and temperature effect on chemisorption
are to be developed, (5) the partitioning between physisorbed and chemisorbed iodine on paint is to bechecked and adapted to the high fraction of chemisorbed iodine measured. This issue could entrain thenecessity of further tests in the laboratory-scale.
The reaction of gaseous I2 with wet painted surfaces was studied with tests Iod-21 and Iod-24 and the
expected phenomena of I2 deposition and wash-off were quantified. A new water-film-based model, which
explicitly considers detailed condensation rate, and which considers the iodine chemistry in the water film,
was designed and preliminary adjusted to THAI test Iod-24. More analytical and validation work isnecessary for optimisation.
The two THAI tests on the I2 / aerosol reactions (Iod-25, Iod-26) are providing the effect of I2 removal fromcontainment atmospheres by interaction with containment aerosol, the reactive silver (Ag) aerosol
providing a high reactivity, and the inert tin oxide (SnO2) a low reactivity under the given boundary
conditions of the tests. The I2 chemisorption reaction with the Ag particle surface is as high as for thehigher range of I2 / paint reactions, as evident from comparison of large-scale THAI tests and associatedlaboratory-scale tests. Real mixed aerosol in the containment can be expected to exhibit I2 removal rates
between the bounding cases "reactive Ag" and "inert SnO2". If source term analyses reveal a significant
effect of the I2 / aerosol interaction in the containment, the actual surface reactivity of an aerosol definedby the thermochemical phases available to gaseous I2 lays between the bounding cases "reactive Ag" and"inert SnO2". Additional tests using a more realistic aerosol (e.g. from simulated core melts) could turn out
necessary, as well as the study of an effect of steam.

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I. INTRODUCTION

the surface temperaturecondensation, respectively.

Deposition of gaseous molecular iodine (I2) onto
dry or wet surfaces in nuclear power plantcontainments during severe accidents represents
an important sink to reduce iodine volatility.
Decontamination paint, covering large surface
areas of walls and floors, is well-known toprovide the necessary high reactivity towards I2.
Aerosols, with their high specific surface ofsmall particles, can significantly increase the I2
deposition surface area. Models to predict the I2
depletion from atmospheres by interaction withpainted surfaces are available. These arecommonly based on results obtained from smallscale tests including volumes of only up to about
0.3 m3. No models are included in severe
accident containment codes to quantify the I2
depletion by adsorption onto aerosols. The 60 m3
vessel of the THAI test facility bridges thediffering scales of laboratory tests andcontainment dimensions. At the same time
coupled effects such as I2 / aerosol interaction
and iodine deposition in aerosol form can bestudied in the large-scale THAI facility. This
paper presents test results and conclusions on theinteraction of I2 with dry or wet paint and withsuspended aerosol from a number of THAI tests
performed during different THAI projects fundedby the German Federal Ministry for Economic
Affairs and Energy or within the NEA THAI-2
project. Results on the I2 / aerosol interaction
from supporting laboratory tests are included.

or

force

wall

Iodine instrumentation
A sketch of a typical iodine and aerosol
instrumentation relevant to this paper is shown in
Fig. 1, as it was in detail relevant to the tests
Iod-25 and Iod-26. Numbers and positions of the
instrumentations are partly adapted in the othertests of this paper according to the specificobjectives.
The injection of the gaseous I2, radio-labelledwith I-123, into the THAI vessel atmosphere was
performed using carrier gas, usually air. The I2
injection amounts were between 0.3 and 2.4 g.
The preparation of the labelled I2 was always
using 1 GBq I-123. The resulting I2
concentrations are clearly higher than in a severe
accident containment but more reliably tomeasure, as (i) artefacts from conceivable
background reactions are suppressed and as (ii)supporting chemical measurements are possibleon the required trace levels.
Gaseous I2 concentrations were measured by six
gas scrubbers distributed over the whole vessel.
Additionally, in tests where different gas-borne
iodine species can occur, two Maypack filter
stations at different heights were discriminating
iodine in aerosol form, gaseous I2, and organic
iodide. In such tests, the interpretation of the gas
scrubbers results requires special attention, as gas
scrubbers also trap iodine in aerosol form andpotentially some organic iodide.

II. EXPERIMENTAL

The evolution of iodine surface depositions weremonitored by scintillation detectors focused on
deposition coupons mounted in front of glass
windows. The total accumulated iodine loadings
were additionally measured in some tests byrecovering a number of deposition coupons at theend of a test.

Test set-up
The THAI facility and the THAI vessel aredescribed in detail in paper 1.4 of this Workshop.
The painted surfaces relevant for the I2 / paintinteraction studies discussed in this paper were
provided as deposition coupons (about 10 20 m2), by mounting differently sized painted
plates and / or painted bodies into the THAI
vessel. Compartments are established in the
THAI vessel using steel internals (plates, innercylinder). Some painted surfaces were locallymonitored online for iodine uptake, some ofthem were recovered after the end of a test tomeasure the final iodine uptake. Some of thepainted bodies can be heated or cooled byseparate inner steam or water circuits to increase

An irradiation source has never been available in
THAI, so there is no relevant dose rate, and thethermally induced production of organic iodides
is only on a very small or negligible level.
Aerosol instrumentation and material
Aerosol mass concentrations were measured by 4different filter stations. Continuous monitoring ofaerosol mass concentrations was performed using

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a photometer measuring the extinction of laser
light. The particle size distribution was measured
with a low-pressure cascade impactor. The
aerosol particle deposition was measured
gravimetrically from recovered aerosol coupons.

Advantages of always performing this same
ageing procedure with the same paint material isthat (1) all THAI I2 / paint tests can directly becompared to each other, and that (2) the I2 / painttests from THAI can directly be compared toearlier laboratory-scale tests [Hellmann 1996]. It
was concluded in [Hellmann 1996] that the I2
deposition kinetics on paint did not depend on
ageing, whether ageing was for 20 weeks atambient conditions or artificially heated tosimulate 10 years (1000 hours at 80°C, 24 hours
at 125°C, using the above equation). However,this conclusion was based upon comparing only
two tests, and these two tests were run incondensing conditions at 160°C, so thatsimultaneous iodine wash-off by the condensate
flow was strongly interfering with persistentiodine deposition and could have masked aneffect of ageing on I2 deposition onto dry paint.

The Ag powder was provided by the company
CHEMPUR, Karlsruhe, Germany. According to
manufacturer, particle sizes were 0.7 - 1.2 µm
and the measured specific surface area was
2.5 m2/g. Sauter mean diameters and thecalculated specific surface area of the SnO2
aerosol particles were 0.62 -0.74 µm and1.3 m2/g, respectively, assuming sphericalparticles with a density of 6950 kg/m3. SEM
images were taken from a number of filter
loadings or deposition coupon loadings, partly
including an EDX analysis providing elemental
mass ratios.
Painted surfaces: material and ageing

After mounting into the THAI vessel, the painted
surfaces are washed together with the whole
vessel before a THAI iodine test by condensing
steam from subsequent steam injections forcleaning purposes.

The
epoxy-type
decontamination
paint
GEHOPON from GEHOLIT & WIEMER
GmbH, Graben-Neudorf, Germany, which wasused in several German reactor containments,
was also used in all THAI tests with painted
surfaces. The painted coupons were produced
before each test by painting one coat of metal
primer (GEHOPON E24) and typically two
finishing coats (GEHOPON EW 10, colour
RAL 1013), to obtain a total thickness of thecoating of 150 - 200 µm after drying in ambientconditions.

The above estimation of a simulated age only
considers thermal ageing of paint in dry air andthe does not consider the effect radiation or
humidity in containment, during normal
operation and during severe accidents, for which
no model is available.
The described pre-treatment of the paint appliedin THAI tests adds to the several different pretreatments applied in other relevant projects
world-wide, see e.g. [Bosland 2014]. As theiodine / paint reactions are also depending on the
pre-treatment procedure, a model to include theeffect of pre-treatment on iodine / paintinteraction would be desirable to enable
comparison of the different experimental projects
with differing pre-treatments.

Artificial ageing of the painted surfaces wasalways performed by heating the painted
coupons for 21 hours at 160 °C or 24 hours at
155°C. According to [DIN 2578] and Siemens
data underlying the I2 /paint tests in[Hellmann 1996], the Arrhenius-type expression
t1 / t2 = exp{8816·(1/T1 - 1/T2)} was used tocorrelate simulated ageing and artificial heating
temperatures (T1, T2 in Kelvin, T1=303 K) withartificial and simulated ageing times (t1, t2). For
the above THAI ageing conditions, a simulated
ageing at plant operation conditions of 15 years
is thus estimated. The norm [DIN 2578] isaiming at optical, mechanical and electricalproperties, but not on chemical properties, so thesimulated age can only be considered as an
estimate.

Test procedure
An iodine test in THAI is usually initiated byinjecting radio-labelled gaseous I2 into the dome
part of the THAI vessel within a few minutes. In
the single-compartment tests atmospheric
convection leads to homogeneous I2 mixing
within < 15 min. During a first test phase,typically lasting a couple of hours or even longer,iodine deposition on the walls and the mounted
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deposition coupons is taking place whilethermal-hydraulic boundary condition are kept
constant. In further test phases, one or moreboundary condition(s) are changed and the iodine
measurements are continued to measure theimpact of the different boundary conditions on
the iodine behaviour. Such changes typically
include e.g. increase / decrease of temperature,
increase / decrease of relative humidity,
switching on condensation of steam, andinjection of aerosol. In the multi-compartment
tests the atmospheric mixing conditions within
the vessel were changed additionally.

2 orders of magnitude within the first 4 - 5 hours.
The 116 m2 of stainless steel in this test alone
would decrease the gaseous I2 only by much lessthan one order of magnitude. The I2 / paintinteraction is therefore significantly influencing
the I2 behaviour in the steel vessel during test
Iod-17.
During the transition phase, where temperature
was increased from 75°C up to 120°C by heatingof the vessel walls, the decrease of the gaseous I2
concentrations stops (Fig. 2). Even a small,
temporary increase of gaseous I2 at 8-10 hours
could have occurred. Obviously, I2 desorption
from steel surfaces and deposition on painted
surfaces is taking place. Accordingly, a relatively
constant level of gaseous I2 is present in the gas
phase during the I2 relocation between steel andpainted surfaces.

Vessel pressures are typically between 1.2 and2 bar, mainly depending on steam injection, andthey usually change during the tests and thedifferent tests phases. Even in tests without
steam a slight over-pressure is nearly always
used for a convenient operation of the iodine
sampling instrumentation.

The evolution of iodine deposits on a painted
coupon is shown in Fig. 3. During the first
5 hours, I2 is accumulating quickly andapproaches a saturation level. During the 120°C
phase, there is a very slight increase of the iodine
loading, and this is explained by the I2 desorbed
from the heated steel surfaces. Obviously, I2 isrelocated from steel to paint.

Test matrix
Table 1 summarises all I2 / paint and I2 / aerosolinteraction tests in THAI.

III. REACTION OF GASEOUS I2 WITH
DRY PAINTED SURFACES

The online monitored painted coupon was
temporarily heated up twice, at 2.5 h and at
10.5 h up, to about 130°C. No desorption isobserved, and this indicates that the iodine on thepaint is mainly efficiently bound, i.e.
chemisorbed, in contrast to being weakly
physisorbed.

Experimental phenomena
The general I2 behaviour after injection is the
spreading and dilution of the initial, concentrated
I2 cloud by transport with the atmospheric flows
inside the vessel. Homogeneously mixed I2 istypically achieved within less than 15 min insingle-compartment tests. In parallel and during
the further test periods, the I2 is deposited onto
surfaces, which decreases the atmospheric I2
concentration. In the absence of painted surfaces,
the large steel wall surfaces alone are removing afraction of the gaseous I2, depending on relative
humidity and temperature. The persistentgaseous I2 concentration is due to theequilibration between I2 desorption from the steel
walls and the I2 chemisorption onto paint.

The specific iodine loading on recovered paintcoupons at the end of the test is almost the same
as for the non-recovered, online-monitored
deposition coupon (52 mg/m2, uncertainty offactor 2 possible based upon long-term
experience in THAI). A set of smaller painted
coupons had a specific iodine loading of
82 mg/m2 (std. dev. ~ 19 %), a set of larger
painted coupons was at 70 mg/m2 (std. dev.
~ 16 %). The loadings on neighbouring
recovered steel coupons were 1.7 (small
coupons) and 0.6 mg/m2 (large coupons), at std.
dev.'s ~ 36 and 26 %. Although local effects canintroduce significant uncertainties on the detailed
iodine loadings, the paint is more efficient in

The test Iod-17 is exemplarily discussed todemonstrate
theadditional
phenomena
introduced by painted surfaces (10.3 m2). Fig. 2
shows that the gaseous I2 is decreased by259

NEA/CSNI/R(2016)5
bounding I2 than steel by factors of 50-100 in theconditions of test Iod-17.

there is no radiation and as thermal organic
iodide production is relatively weak.

Gaseous organic iodide concentrations, obtained
from Maypack filters, are generally low. Thermal
production of volatile organic iodide on iodineloaded painted surfaces is only weak andgenerally does not influence the analyses andassessments of the iodine behaviour in the THAI
tests.

In the model of I2 / paint interaction
physisorption is depending on temperature but
not yet chemisorption. It does not depend on
relative humidity, unlike the I2 / steel model,
because no effect of relative humidity was
detected in the underlying THAI older tests
performed in a significant but narrow band of
relative humidity, and because more recent NEA
BIP results on I2 / paint deposition depending on
relative humidity as well as Iod-27A, in which
relative humidity was only < 1 %, did not yet
exist. The solid line in Fig. 2 represents theresults of an AIM-3 calculation including both,the I2 / steel and the I2 / paint deposition model.
Whereas the gaseous I2 concentrations from an
I2 / paint test without desorption phase, Iod-15(no Figure), where reproduced very well, themodel is not fully satisfactory during thetransition phase and hot phase in test Iod-17:
during the heat-up measured gaseous I2 seems tobe slightly underestimated, and during the hotphase I2 is overestimated. Reasons are probably
non-optimal rate constants for I2 desorption fromsteel and/or paint, and non-optimal rate constantsfor conversion of physisorbed I2 into
chemisorbed iodine on the paint.

Modelling
As the stainless steel walls of the THAI vessel(steel 1.4571, ~AISI 316Ti) represents an I2
deposition area in the range of 100 - 160 m2(depending on use of internal structures, 116 m2
in Iod-17), the I2/steel interaction is always
significantly influencing the I2 behaviour. An I2 /steel interaction model was therefore developed
based on a number of THAI tests with only steel
surfaces. The model enables to numericallydiscriminate the I2 / steel effect in tests to study
the I2 / paint or I2 / aerosol interaction. The I2 /steel model considers two steps, (i) physisorptionof I2, and (ii) conversion of physisorbed I2 into
chemisorbed iodine ("FeI2"), going back to aconcept described in [Wren 2001] and included
in AIM-3 [Weber 2012]. The chemisorption step
is strongly depending on the relative humidity, sothat it is essential in analyses of THAI tests withgaseous I2 to precisely know the relative
humidity, either from humidity sensors and / orfrom steam mass balances.

The later THAI test Iod-27A, a multicompartment test with I2 injection into the dome
compartment and the painted surfaces mounted
only in lower compartments, was performed in
air without any steam. The multi-compartment
issue is discussed in Gupta's paper 1.4 of thisWorkshop. The I2 / paint interaction in Iod-27A
is of relevance as the effect on iodine behaviour
was surprisingly small. Fig. 5 displays the
specific iodine loading on paint and steel
coupons in test Iod-27A, and it reveals
surprisingly low iodine loading on the paint.
Further THAI multi-compartment tests including
painted surfaces, high relative humidity, andaerosol, Iod-28 and Iod-30, have recently been
performed, also with a focus on the low effect ofpaint in the very dry Iod-27A.

An I2 / paint deposition / desorption model wasdeveloped according to the I2 / steel model andbased upon the single-compartment tests Iod-15,
Iod-17, and Iod-20 with generally similar I2
phenomena (Fig. 4). It consists of two steps,
according to the concept of the I2 / THAI steel
model in AIM-3. The first step is physisorptionof I2 onto dry paint. The second step is theconversion of deposited I2 on the paint into
chemisorbed iodine on paint. The release of I2
from paint back into the gas phase is only
possible through desorption of physisorbed I2.
The release of chemisorbed iodine from paintinto the gas phase is possible in AIM-3, but
requires the formation of volatile organic iodide,induced thermally or by radiation. Organic iodide
is almost of low importance in THAI tests, as

To establish a comparison of all I2 / paint tests
with dry paint, so-called distribution coefficients(KD) were derived from all relevant THAI tests.
KD is defined as the ratio of the specific iodine
loading on paint at the end of a test or the end of260

NEA/CSNI/R(2016)5
a representative test phase ("final loading"), andthe corresponding, representative I2 gas phase
concentration. The derived KD values (Fig. 6) lie
fully within the broad range of values from older
laboratory-scale tests for different paints anddifferent ageing, as compiled in [Sims 1997].
The KD values from THAI tests are increasing
with relative humidity, namely by a factor of
about 25 when increasing from nearly zero tonearly 80 % relative humidity. This goes alongwith the increase of the deposition rate constantfor I2 onto Ripolin epoxy paint as recently
measured in the NEA BIP project for other painttypes. It is noted, without specific figure, that KD
does not depend on temperature in the studied
range from 60 to 100°C.

IV. INTERACTION OF GASEOUS I2WITH WET PAINTED SURFACE
Experimental phenomena
THAI tests in condensing conditions andproducing wet wall surfaces showed that there isno closed water film on the paint, but that small
local and short-lived rivulets are formed atchanging locations. Upon flowing downwards
and integrated over a time period of hours, thewhole painted surface can be considered to be
washed off by the condensate. Besides gaseous I2
deposition on paint as in dry conditions, afraction of the I2 is penetrating into a water layeron the paint before interacting with the paint. The
downward flow of the condensate can transport
I2 in the water film directly into a sump without
I2 having reacted with the paint. I2, initiallydeposited on the paint surface, can be washed off
by the condensate flow. Reaction of I2 with paintcomponents and I2 hydrolysis can produce
iodide, which is also washed off and transported
into a sump.

The multi-compartment tests Iod-27A and Iod-28
included helium to support dissolution of
atmospheric stratification and as a tracer foratmospheric mixing in the tests, at concentrationlevels of 12 -13 % in the mixed test phases. It
was speculated whether helium would hinder I2
deposition by intrusion into the paint where it
could then block further I2 adsorption. Helium
was not used in the multi-compartment test
Iod-30, and Fig. 6 suggests that there is no such
helium effect.

THAI test Iod-24 was run to measure the effectof wet painted surfaces (7.4 m2) on the removalof gaseous I2 and on the iodine wash-off into asump. The painted surfaces were provided in theform of two large deposition bodies with 3.7 m2
each, which were cooled from the inside by aseparate cooling circuit to establish condensation
rates at the painted surface representative for
PWR containments. Cooling power for thepainted bodies, steam injection, and heating of
the vessel were operated such that all othersurfaces remained dry besides the paint. One
painted body was run at a low condensation rateof 0.13 g/(m2·s), and the other at a highcondensation 1.2 g/(m2·s). I2 was injected into
the phase of condensing conditions, and theiodine deposition and wash-off was measured for13.5 hours at constant condensation rates,
followed by a subsequent dry-out phase. The gas
temperature was 100°C, the relative humidity
90 %.

It will be necessary to include the dependency of
the I2 deposition onto paint on relative humidity
into the AIM-3 model. Additional laboratoryscale tests, including the parameters age and typeof ageing, type of paint, and temperature would
be helpful. Whether relative humidity or steam
concentration is a relevant parameter for the I2
adsorption on paint, is currently discussed but
not concluded in BIP.
Current GRS plant calculations with AIM-3
reveal surprisingly high fractions of physisorbed
I2 on paint and, as a consequence, desorption of
I2 is rather strong so that very high iodine
amounts in the containment atmosphere areresulting. With a modified model more analyses
will be necessary on this issue, also considering
the length of adsorption phase in BIP (up to
70 hours at constant I2 level) being much longer
than in THAI tests (significant but quickly
decreasing I2 concentrations only during the first
few hours). The longer adsorption phase shouldeven more favour the fraction of chemisorbed
iodine on the paint.

At the end of the test, 19 % of the injected I2 wasfound in the sump, from I2 wash-off from coolsteel tubes required for the cooling circuit of thepainted bodies, and from I2 mass transfer at thegas/liquid interface. Another 24 % was on thevessel steel surfaces. The non-washed iodine
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deposition on both paints was 25 %, and theiodine wash-off was 23 % from the paint with thehigher condensation rate, and 9 % from the paintwith the lower condensation rate.

thickness of the water film is provided by theuser. The iodine sump chemistry in AIM-3 isapplied to the water film, including the iodine
reactions with immersed painted surfaces.
Gaseous I2 depositing onto paint needs to firstpenetrate the water film by the known interfacial
mass transfer at the gas/liquid interface before
reacting with the immersed paint. The pH valueof the water film is required as an input.

Iodine concentrations in the condensate areshown in Fig. 7. With a small delay caused by
the condensate transport to the sampling station,
the decreasing iodine concentrations in thecondensates are qualitatively going along withthe decreasing iodine concentrations in theatmosphere(seeFig. 8).
The
iodineconcentration in the condensate with the higher
rate is a factor 0.25 lower than in the condensate
with the lower rate. Qualitatively, this isunderstood as a consequence of the higher iodine
dilution at the higher condensation rate. The
iodine wash-off rates and the integrated amountsof iodine wash-off are higher by factors of
about 2 at the higher condensation rate.

Fig. 7 compares model calculations of iodine
concentrations in the condensates using a water
film thickness of 65 µm. High (upper) and low(lower) condensation rate at the two painted
bodies are considered. Fig. 8 compares
equivalent I2 model concentrations withmeasured data in the gas phase. Additionalparameters variations, not shown here, indicate
the potential of the model for further
optimisation. The accumulated iodine loading on
the painted surfaces is also reasonably
reproduced by the model. Test Iod-21 with wet
painted surfaces in the 2nd test phase isproviding more data for further model
development, these analyses have not yet been
performed.

There was almost no I2 in the condensate, as
measured by UV/VIS. Obviously, conversion of
I2 into other iodine species took place, either
during the test itself or in the time between
samplings and their analyses. The pH of thecondensate was 4.5 to 5 during the whole test
Iod-24. Gaseous I2 decreased by two orders ofmagnitude within the first 6 hours.

V. REACTION OF GASEOUS I2 WITH
AEROSOLS

Modeling

The reaction of gaseous I2 with aerosols wasstudied in the THAI tests Iod-25 and Iod-26.
Both tests were run at about 70°C and in air
atmosphere without steam. These tests were
supported by laboratory-scale tests to study the I2
adsorption / desorption behaviour of various
materials in various boundary conditions.

The interaction of gaseous I2 with painted
surfaces in condensing conditions is modeled in
AIM-3. I2 physisorption and conversion into
chemisorbed iodine are similar to the dry case.
Iodine wash-off in the form of I2 or iodide are
additional phenomena. The model is based uponlaboratory-scale tests [Hellmann 1996] andrepresentative for experimental condensation
rates at 0.4 to 0.44 g/(m2·s) at T = 160°C. The
condensation rate is no explicit parameter in themodel. The I2 fraction washed off immediately
after deposition is fixed at 68 % (Iod-24 at low
condensation rate 38 %, at high condensation
rate 69 %).

In Iod-25, tin oxide (SnO2) aerosol was injected
to provide a non-reactive, inert aerosol, where
gaseous I2 should only be influenced by weakphysisorption onto the particles. In Iod-26, silver(Ag) aerosol was injected to provide a reactive
aerosol, where gaseous I2 should be chemisorbed,
i.e. bound as AgI, on the surface of the particles.
Both tests Iod-25 and Iod-26 were essentially run
identically: the first test phase was to inject I2
and to measure the small effect of I2 depletion by
the steel walls. In the second test phase, a puff
release of aerosol was performed to achieve ahigh and representative aerosol concentration,
which is depleting in the course of this test

Due to the short-comings of the AIM-3 model, amore detailed model was developed at GRS in
the course of the preparation of test Iod-24, using
a film-based approach. The water film in eachzone is simulated by a flat water pool, throughwhich the condensate is flowing. Iodine
chemistry is similar to that in the sump. The
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phase. In a third test phase, aerosol was injected
continuously, to keep the aerosol concentration
on a constant level. The aerosol massconcentrations showed similar evolutions, the Ag
aerosol is shown in Fig. 9.

needs to consider that the injected aerosol waspoly-disperse in both cases, SnO2 and Ag. The
log-normal size distribution had a geometricstandard deviation of 2.0 respectively 1.8. The
larger particles are settling faster than the smaller
ones, but the smaller, more persistent particles
are providing the larger surface area and thus alarger iodine adsorption. Consequently, theremoval of trapped iodine from the vessel
atmosphere by settling is somewhat slower thanthe overall aerosol settling.

Gas scrubbers 1, 4, 5, and 6 measured the total
airborne iodine, i.e. potentially gaseous I2 plus
aerosol-bound iodine. Gas scrubbers 2 and 3
were equipped with aerosol pre-filters, so they
were measuring always only I2.
Upon injecting SnO2 aerosol in Iod-25, anaccelerated iodine removal from the vessel
atmosphere takes place (Fig. 10). However, theinjection of Ag in Iod-26 is clearly more efficient
in removing gas-borne iodine (Fig. 11). The fastand complete decrease of the gaseous I2
concentrations in the Ag test is demonstrated by
the results of gas scrubbers 2 and 3.

Laboratory-scale tests were performed in supportof the THAI tests Iod-25 and Iod-26, to study theeffects of a limited number of conceivablechemical parameters. Temperatures and relative
humidity were also varied. Radio-labelled I2 waspassed over thin powder layers of several
materials, which circumvents the difficulty ofproviding suspended particles in these simple
chemistry tests. The accumulating iodine
adsorption on the powder was monitored for
typically 2 - 3 hours. Thereafter, the I2 waseliminated from the carrier gas flow anddesorption phases were thus performed for some
additional hours. The relative humidity at the
point of the powder was mostly at a percent
level, resulting purely from passing nitrogen as acarrier gas over an aqueous I2 source. The iodine
loading decreased from Ag powder (samepowder as in Iod-26), cesium iodide (CsI)
crystals and iron (Fe) chips down to SnO2
powder, iodine on SnO2 even remained below thedetection limit. No iodine desorption from Ag
and Fe was measured, indicating strong
chemisorption. Adsorption was fast in the
beginning and then slowed down to apparently
achieve saturation of the iodine loading. No
temperature dependency (80°C - 140°C) was
measured for the initial adsorption rate in the I2interaction with Ag. The specific iodine loadingon Ag particles increases with the I2
concentration.

Maypack measurements also show that Agaerosol was efficiently trapping the gaseous I2
within a few minutes, and that iodine was in anaerosol form for the rest of the test. In contrast,
during the whole SnO2 test, gaseous I2 remained
the predominant iodine species, and iodine in theaerosol form was only on a percent level.
The specific iodine loadings on the different
aerosol materials were visualised by monitoring
iodine deposition on local deposition coupons
covered by SnO2 powder or covered by an Ag
foil, although these material forms may not be
fully representative of the surface area of anaerosol. The I2 loading on the SnO2 coupon wasvery similar to the I2 loading on steel. The iodine
loading on Ag foil was higher than on steel bymore than two orders of magnitude at the end of
the test (Fig. 12).
Whereas the I2 removal from the vessel
atmosphere in the Ag test is governed by thesettling rate of the Ag/AgI particles, the I2
removal by SnO2 aerosol is strongly influenced
by I2 desorption from SnO2 surfaces. This also
includes desorption from I2-loaded SnO2 particles
that had already been deposited at the bottom.
The net difference of the I2 removal efficiency inthe two THAI tests is about a factor of 4, as
obtained from comparing Fig. 10 and Fig. 11.

Fig. 13 shows KD values obtained from thelaboratory tests, together with the range of KD
values from the THAI tests with paint (seeFig. 6). The Ag reactivity at 80 - 93°C is at theupper range of paint reactivity from THAI tests.
For the Ag reactivity, a small influence ofrelative humidity as well as on temperaturecannot be excluded. For the SnO2 powder, noA future detailed kinetic analysis of the iodine
decreases by deposition onto settling aerosols
263

NEA/CSNI/R(2016)5
iodine loading was measured, corresponding to
its expected inert character. Iron (Fe) powder isless reactive than Ag powder by at least oneorder of magnitude, and the iodine loading on Fe
clearly decreases with temperature. The CsIreactivity at 80°C is surprisingly high. At 140°C,
iodine loadings are generally lower than at thelower temperatures.

analytical work is necessary for optimisation,
including e.g. the wet phase of THAI test Iod-21.
The two THAI tests on the I2 / aerosol reactions(Iod-25, Iod-26) are providing the effect of I2
removal from containment atmospheres byinteraction with containment aerosol, the reactive
Ag aerosol providing the maximum reactivity,
and the inert SnO2 the minimum reactivity under
the given boundary conditions of the tests. The I2
reaction with the Ag particle surface is as high as
for the higher range of I2 / paint reactions, asevident from comparison of large-scale andlaboratory-scale tests in THAI. Real containment
aerosol mixed of different materials can be
expected to exhibit I2 removal rates between theinvestigated bounding cases "reactive Ag" and"inert SnO2". If source term analyses reveal asignificant effect of the I2 / aerosol interaction in
the containment, the actual surface reactivity of
an aerosol defined by the thermochemical phases
available to gaseous I2 needs to be interpolated
between the bounding cases "reactive Ag" and"inert SnO2". Additional tests using a more
realistic aerosol (e.g. from simulated core melts)
could turn out necessary, as well as the study ofan effect of steam.

VI. CONCLUSION
The reaction of gaseous I2 with painted surfaces
was addressed in a number of THAI tests(Iod-15, Iod-17, Iod-20, Iod-21, Iod-24, Iod-27A,
Iod-28, and Iod-30). The experimental work wasaccompanied by analytical work to specify thetests, to improve existing models, and to develop
new models.
For I2 reaction with dry surfaces in THAI, anempirical deposition / desorption model wasdeveloped, but it still needs more refinements:(1) paint used is a German epoxy paint, and there
is no connection to compare its behaviour versus
iodine with the effects of other paints, (2) ageingof paint is not explicitly scaled but always
referring to 15 years, (3) ageing procedure isdifferent from other experimental programs and a
deeper analysis of this issue including connecting
tests is necessary, (4) modeling of the effects ofrelative humidity and temperature on thechemisorption reaction is to be developed, (5) the
partitioning
betweenphysisorbed
andchemisorbed iodine on paint is adapted to
produce the correct high fraction of chemisorbed
iodine, this issue could entrain the necessity offurther tests in the laboratory-scale.

ACKNOWLEDGMENTS
The THAI tests reported in this document were
funded by the German Federal Ministry for
Economic Affairs and Energy (BMWi) in theframe of the reactor safety research projects
150 1272, 150 1325, 150 1361, 150 1420, and
150 1455.
The THAI tests Iod-25, Iod-26, and theassociated lab-scale tests were additionally
funded by the partners of the OECD THAI-2
project (2011-2014).

The on-going international development of amechanistic iodine/paint model could provide
answers to the above issues from the empiricalmodel. However, this work including also a
consistent release model for organic iodide is far
from being completed. Nevertheless, semiempirical models will still be required by
accident codes for plant applications.

NOMENCLATURE
AIM-3: Advanced Iodine Model [Weber 2009]
COCOSYS: Containment Code System

The reaction of gaseous I2 with wet painted
surfaces is qualitatively providing the expectedphenomena, but a new model was designed andpreliminary adjusted to THAI test Iod-24. More

[Klein-Heßling 2013]
THAI:
Thermal Hydraulics,
Aerosols, Iodine.

264

Hydrogen,NEA/CSNI/R(2016)5

R07

R0

9200
R11

9000
R15

REFERENCES

8400

Filter station I( = 135° and 225°)

I2 injection

GS 6

Maypack I( = 45°)7700[Bosland 2014]: L. BOSLAND et. al., "Iodinepaint interactions during nuclear reactor severe
accidents", Annals of Nuclear Energy, 74, 184–
199 (2014).

7000
GS 5

Online detectors( = 135°, 180° and
225°)

6300
5600

Photocell( = 45°)

GS 4

Laser diode( = 225°)[DIN 2578]: "Kunststoffe: Bestimmung derTemperatur-Zeit-Grenzen bei langanhaltender
Wärmeeinwirkung", European standard
EN ISO 2578 : 1998 corresponding to German
standard DIN EN ISO 2578, October 1998.

4900
GS 3 + filter

Impactors (2)( = 45°)4200

GS 2 + filter

3500
2800
2400

Filter station II( = 135° and 225°)

Maypack II( = 45°)

2100
SnO2 injection

1600
GS 1

[Hellmann 1996]: S. HELLMANN et al., "Thereaction between iodine and organic coatings
under severe PWR accident conditions. An
experimental parameter study", Proc. of the 4th
OECD/CSNI Workshop on the Chemistry of
Iodine in Reactor Safety, p. 345 - 365, PSI,
Würenlingen, Switzerland (1996)

700
0Fig. 1. Sketch of the iodine and aerosol
instrumentation in the THAI tests on I2 reaction
with paint and aerosols.

[Klein-Heßling 2013]: W. KLEIN-HESSLING etal., "COCOSYS V 2.4 user's manual", Revision0, GRS, April 2013

THAI Iod-17: Iodine concentrations in the gas phase
1.E-04Gaseous iodine concentrations / g/l[Sims 1997]: H. E. SIMS et al., “Iodine behavior
in containment - the reaction of iodine withsurfaces”, EPRI report ACEX TR-B-06, Draft(1997)[Weber 2009]: G. WEBER et al., "Description of
the iodine model AIM-3 in COCOSYS", GRS
report A-3508, November 2009

Transition
Phase 1phase
T: 75 Crh: 60-70 % (heat-up)

Phase 2
T: 120 C
rh: 20-15 %Cool-down

GW1
GW2
GW3
GW4
GW5
GW6
COCOSYS / AIM-3

1.E-05

1.E-06

1.E-07

1.E-080[Weber 2012]: G. WEBER et al., "Thermalhydraulic–iodine chemistry coupling: Insights
gained from the SARNET benchmark on the
THAI experiments Iod-11 and Iod-12", Nuclear
Engineering and Design, 265, 95–107 (2013).

5

10

15

20

25

30

Time after start of iodine injection / hFig. 2. THAI test Iod-17: Gaseousiodine concentrations in I2 tests with dry
painted surfaces.

[Wren 2001]: J. C. WREN et al., “Kinetics ofgaseous iodine uptake onto stainless steel duringiodine-assisted corrosion,” Nuclear Technology,
133, 33–49 (2001).

265

35

NEA/CSNI/R(2016)5I2 deposition onto dry paint in THAI tests:
Distribution coefficient Kd = I-surface(final) / I-gas(repres.)

THAI Iod-17: Iodine deposition on local painted surface
monitored by NaI scintillation detector in front of painted deposition coupon

0.10
0.09

Phase 1
T: 75 Crh: 60-70%

Transition Phase 2
phase
T: 120 C(heat-up) rh: 20-15 %1000

Cool-downSingle-compartment THAI tests
Iod-21

Multi-compartment THAI tests

Iod-20
Iod-17

10.07

2

2 x coupon heatup --> 130 C

0.06

Kd / dm

Iodine loading / g/m2

0.08

0.05
0.04

Iod-28100
Iod-15

Iod-30 / upper annular compartment

0.03
0.02

Iod-27A

0.01
Iod-30 / dome compartment

0.00

10

0

5

10

15

20

25

30

35

40

0

10

20

30

40

50

60

70

80

90

Relative humidity / %Time after iodine injection / hFig. 6: Distribution coefficients for gaseous I2
interaction with dry painted surfaces in THAI
tests.

Fig. 3. THAI test Iod-17: Specific iodine
loading on local painted surface.

Fig. 4. Two-step model of gaseous I2 reaction
with dry painted surfaces as realised in AIM-3.
THAI Iod-27A: Online scintillation detector / iodine deposition coupon(contribution of gas-borne iodine in the sonde focus between glass and coupon is subtracted)

1.E-1

stratified mixin

mixedSpecific iodine loading / g/m2

gwashing / vessel switched off

1.E-2
steel: detection limit

1.E-3
OS49F: Steel coupon
upper annular comp.
4.9 m / 225

OP49D: Painted coupon
upper annular comp.
4.9 m / 135

1.E-4
OP21E: Painted coupon
lower annular comp.
2.1 m / 180

1.E-5
0

5

10

15

20

25

30

35

40

45

Time after start of iodine injection / hFig. 5. THAI test Iod-27A: specific iodine
loading on painted coupons and steel coupon.

266

Fig. 7: Test Iod-24: Iodine concentrations in
the wall condensates of the painted bodies withhigh (upper) and low (lower) condensation rate.

100

NEA/CSNI/R(2016)5

Fig. 8: Test Iod-24: Iodine concentrations in
the gas phase (only gas scrubbers 2, 3, and 4),
only the first 13.5 hours of are considered.

Fig. 11. THAI test Iod-26: Decrease of gasborne iodine by chemisorption of I2 onto settling
Ag particles.

Fig 9. THAI test Iod-26: Ag aerosol massconcentrations.

Fig. 12. THAI test Iod-26: specific iodine
loading on Ag foil and steel coupon.

OECD-THAI-2 laboratory tests: I2 deposition on particles
I2 distribution coefficient KD = I-surface.final / I2-gas conc.
80 C
N2, rh 4 %120 C
N2, rh 1 %140 C
N2, rh 0.5 %Paint in THAI tests
60 - 100°C; 0-80 % rh

1.E+3

1.E+2
air/ steam
rh 90 - 70 %KD / dm

1.E+1

1.E+0
Ag, d.50 = 19 µm
Ag, d.50 = 2.6 µm
Fe, d.50 = 1.8 µm
Fe det.lim., d.50 = 1.8 µm
SnO2 det.lim., d.50 = 1.2 µm
CsI, d.50 = 18 µm
CsI det.lim., d.50 = 18 µm

1.E-1

1.E-2

70

80

90

100

110

120

130

140

Temperature / °C

Fig. 10. THAI test Iod-25: Decrease of gasborne iodine by physisorption of I2 onto settling
SnO2 particles.

Fig. 13. Distribution coefficients forinteraction of gaseous I2 with dry deposited
aerosol particles in laboratory tests.

267

150

NEA/CSNI/R(2016)5

TABLE I
Test matrix on THAI test to study I2 interactions with dry and wet paint, and with aerosol.
Test

Gas temp.

Rel. humid.

/ °C

/%

PaintedsurfaceCond. ratepaintstate / m2

/ g/(m2.s)

Aerosol

Iod-15100

40-45

dry, 20--

Iod-1775

70

dry, 10.3

-

-

120

20-15

100

70

dry, 11.3

-

-

130

30

100

not eval.

dry, 9--wet, 90.15

dry, 10.4-

Iod-20

Iod-21

Iod-27A

60<1-

Iod-2860

40

dry, 10.4--

Iod-3085

45dry, 10.4

-

Ag

80

45

dry, 10.4

Iod-24100

90

wet, 7.4

0.13 / 1.2-

Iod-2570

air, no steam

-

-

SnO2

Iod-2670

air, no steam

-

-

Ag

268

NEA/CSNI/R(2016)5
EFFECT OF WATER-DROPLET SIZES ON THE MIGRATIONOF VOLATILE I2 AND CH3I
Hee-Jung Im(1)*, Jei-Won Yeon(1), Jin-Ho Song(2)Nuclear Chemistry Research Division, Korea Atomic Energy Research Institute,
150 Deokjin-dong, Yuseong-gu, Daejeon 305-353, Republic of Korea(2)
Severe Accident & PHWR Safety Research Division, Korea Atomic Energy Research Institute,
150 Deokjin-dong, Yuseong-gu, Daejeon 305-353, Republic of Korea.
*Corresponding author, tel: (+82) 428684740, Fax: (+82) 428688148, Email: imhj@kaeri.re.kr(1)

Abstract – Volatile iodine and organic iodide are commonly known to move in gaseous or several types ofaerosol forms. Therefore, to study how iodine-related aerosols are formed and behave in the containmentof a nuclear power plant and further in the environment, a lab-scale set-up including an I2 (or CH3I) gas
generator, a water droplet generator, and an aerosol collector was installed as a single system with steadycontrol. Experimental findings on the migration of I2 and CH3I on water droplets of different sizes arereported using two different analysis methods: a quantitative analysis based on UV-vis and GC-MS. In
addition, Ag nanoparticle-embedded blank and ligand-anchored silica gels were synthesized for the futureadsorption-property studies of iodine species to investigate the chemical behaviors of iodine from water
droplets–air flow.

269

NEA/CSNI/R(2016)5
The methods used to generate volatile iodine
species (such as I2 and CH3I) that are typicallydetermined in contaminated exhaust air were
properly chosen, and water droplets withconstant particle sizes were generated andevaporated using the Model 3450 Vibrating
Orifice Aerosol Generator (TSI Incorporated,
USA) [4]. Each experiment was performed atroom temperature, and the approximate inlet
iodine mass and volume were controlled to be
0.0254 g of solid I2 or 0.7 mL of liquid CH3I.
The generated volatile gases went through acylindrical water jet with water together and thenspread out to certain sizes of water aerosol basedon a periodic disturbance of a controlledfrequency, to look into their migration fromwater droplets–air flow [5].

I. INTRODUCTION
Iodine is a relatively high fission-yield productand volatile nuclide of nuclear fuel [1]. It canreadily interact with steam and water droplets ina nuclear reactor, and the contaminated iodine
aerosols formed can be exhausted into theatmosphere during emergency venting or after
nuclear power plant damage [2]. The aerosols
released can further interact both homogeneously
andheterogeneously
withtroposphericparticulate matter, such as smoke, sea salts, or
cloud droplets.
The following experiments were originallyplanned to look at the effect of water-aerosol
sizes on the migration of volatile I2 and CH3I that
may occur during an accident in the nuclear
reactor containment. The results may also be
helpful for an understanding of not only iodine
wash-out behavior by water spray but also the I2
and CH3I transfer during an SGTR (SteamGenerator Tube Rupture). During such an SGTR
accident, the I2 and CH3I are transferred from theprimary coolant (liquid phase) into the second
system (which contains the two phases) [3].

To absorb transferred volatile wet I2, a purewater filled bottle and a 0.1 M sodium hydroxide
solution filled bottle were composed into a flow
type experimental apparatus, as shown in
Scheme 1 and Figure 1. However, volatile CH3I,
unlike I2, is not absorbed well by a liquid phasesuch as a NaOH solution, and thus a Tenax tube,
which is filled with a solid sorbent, was used toadsorb transferred volatile wet CH3I directly
using a mini pump.

Since the contaminated iodine aerosols canbe easily released into the environment andaccumulated in the human body owing to its highvolatility, an evaluation of the behavior of theiodine aerosols and a stabilization anddetermination technique of the aerosols are
essential. Therefore, we also synthesized a newsorbent for future studies. Ag metal is aseffective as Ag+ in combining with various
iodine species, and inorganic-organic hybrid
silica gels are readily mixable, not only inaqueous or organic solutions, but also withgaseous molecules. Thus, Ag nanoparticleembedded inorganic-organic hybrid silica gelscan be good sorbents to adsorb a variety ofiodine species in various media. These materials
can be used further for the study of chemical
behaviors of the iodine and iodide species andstabilizing them.

Scheme 1. Schematic diagram of lab scalefor a behavioral study of volatile I2 fromwater droplets-air flow.

II. DESCRIPTION OF EXPERIMENT270

NEA/CSNI/R(2016)5
III. RESULTS AND DISCUSSION“The vibrating orifice monodisperse aerosol
generator is based on the instability and break-up
of a cylindrical liquid jet. A cylindrical liquid jet
broke up into droplets by mechanicaldisturbances.
When
these
mechanicaldisturbances were generated at a constantfrequency and with sufficient amplitude on aliquid jet of constant velocity, the jet broke up
into equal sized droplets [9].” Thus, 0, 38, 42,
and 48 µm sizes of water droplets could be
generated when 0, 80, 60, and 40 kHz
frequencies were applied at a nominal operating
condition (20 µm orifice diameter, 20 cm3
syringe capacity, 8.2 x 10-4 cm/s syringe pumprun speed, and 0.139 cm3/min liquid feed rate).
The formed monodisperse water droplets were
well dispersed with 15 x 100 cm3/min air anddiluted 40 L/min air before a significant
coagulation occurs. Gases of I2 and CH3I weregenerated at about 60oC and 4oC respectively,
and then led to a cylindrical water jet with avolumetric flowrate of 5cc/min (Figure 2).

Fig. 1. Photo of lab scale set-up for abehavioral study of volatile I2 from water
droplets-air flow.
Ultraviolet-visible (UV-vis) spectroscopy andgas chromatography-mass spectroscopy (GCMS) methods were adopted to analyze thetransferred I2 and CH3I qualitatively andquantitatively.
As mentioned in the introduction, thesorbents [6] were designed and synthesized in
our laboratory for the localization of volatile
iodine aerosols [7]. For the preparation of Ag
nanoparticle-embedded inorganic-organic hybridgels,
ligandsofethylenediamine(NH2CH2CH2NH-, TMSen) and mercapto (HS-)
functionalized three-dimensional porous SiO2
sol-gels were first synthesized through hydrolysis
and condensation reactions [8], and Ag
nanoparticles were then embedded into the
TMSen- and mercapto-anchored silica gels each,through electron-beam (e-beam) irradiation. The
whole process mentioned above is briefly shownin scheme 2.

Fig. 2. Monodisperse (48 µm size) water droplets
produced by an aerosol generator (left photo) andvolatile I2 produced from solid I2 by temperature
controlling and air flowing (right photo).
The concentrations of I-, I2 I3-, and CH3I in an
aqueous phase can be calculated from theabsorbancies at 226, 460, 288 (or 350), and 250
nm of obtained UV-vis spectra respectively.
Based on the following equations,
I2 + 2OH- → I- + IO- + H2O (k=30)
3IO- → 2I- + IO3(k=1020)

Scheme 2

the concentrations of transferred volatile wet I2
can be calculated from the absorbance of I- at

271

NEA/CSNI/R(2016)5
226 nm when the 0.1 M NaOH solution is anabsorbing liquid.

characterization of Ag nanoparticle-embedded
blank and ligand-anchored silica gels will be
discussed in detail during the IODE workshop.
The color of the blank and ligand-anchored gels,
1-3, changed from colorless to a brownish color
in gels 4-6 after e-beam irradiation in a AgNO3
solution, as shown in Figure 4. The ICP-AES
results of the remaining Ag concentrations ineach AgNO3 aqueous phase after the e-beamirradiation of gels 1-3 were as follows: The
starting Ag concentration of 516.5 (± 0.5) µg/mL
in the solution was significantly decreased to18.35 (± 0.15), 0.91 (± 0.01), and 3.645 (± 0.025)µg/mL, respectively. Around 4.618, 4.780, and4.755 mmol of Ag were embedded into each 10 gof a blank gel, and TMSen- and mercaptoanchored gels. Upon immersing the blank and
TMSen- and mercapto-anchored gels into the
AgNO3 solutions, Ag-nanoparticle suspensions,which were reduced as Ag metals from Ag+ ofAgNO3bye-beamirradiation
were
spontaneously adsorbed into these gels owing toa strong covalent bonding between the silver andligands of ethylenediamine and thiol groups, orpores produced in three-dimensional structures.

(6NaOH + 3I2 → 5NaI + 5NaIO3 +3H2O)

Fig. 3. Relationship between transferred volatile
I2 concentrations and water droplet sizes wheneach of the approximate inlet iodine
concentration was 0.0254 g of solid I2.
As shown in Figure 3, the amount oftransferred volatile iodine was strongly
influenced by the existence of water. However,small changes in water-droplet sizes did not
influence the transfer of volatile iodine muchcompared to the case of I2, but a larger amount of
I2 was transferred with a bigger sized water
droplet than a smaller one when the same amountof water was applied. Similar results were also
obtained for volatile CH3I transfer in the case of
size changes in the water droplets. Hydrolysis
reaction rates of I2 and CH3I with water are very
slow in comparison with physical weathering andphysical dissolution, and they are only slightly
soluble in water. Thus, smaller sized (higher
surface areas) water droplets did not help thetransfer of the volatile wet I2 and CH3I.

Fig. 4. Photo of blank and TMSen- andmercapto-anchored
gels
1-3,
and
Ag
nanoparticle-embedded blank and TMSen- andmercapto-anchored gels 4-6 (180 − 600 µm
sizes).
IV. CONCLUSION

In the case of our synthesized sorbents, theaddition of ligands yielded a larger average poresize than the absence of any ligand. Moreover,
the TMSen ligand led to looser structures andbetter access of the Ag nanoparticles to the
TMSen-anchored gel. As a result, more Ag
nanoparticles were introduced into the TMSenanchoredgel.
The
preparation
andThe generated volatile iodine gases have been
introduced into the water droplets-air flow tolook into aerosol related behavior, and several
spectroscopic methods were applied for aquantitative look. In addition, Ag nanoparticles
were successfully embedded into blank and
TMSens- and mercapto-anchored gels through ebeam irradiation. These Ag nanoparticle272

NEA/CSNI/R(2016)5
embedded blank and ligand-anchored gels areready for the adsorption-property studies ofiodine species to investigate the chemical
behaviors of iodine from water droplets–air flow,
and to define the chemically stable condition of
the products in it.

4. Instruction Manual, Model 3450 Vibrating
Orifice Aerosol Generator, P/N 1933450,
Revision L (TSI, October 2002).
5. G. Brenn and U. Lackermeier, “Dropformation from a vibrating orifice generator
driven by modulated electrical signals”,
Phys. Fluids, 9, 3658 (1997).

ACKNOWLEDGMENTS

6. H.-J. Im, B. C. Lee, and J.-W. Yeon,“Preparation and characterization of Ag
nanoparticle-embedded blank and ligandanchored silica gels”, J. Nanosci.
Nanotechnol. 13, 7643 (2013).

This work was supported by the Nuclear
Research and Development program of the
National Research Foundation of Korea (NRF)
grant funded by the Korea government (MSIP).
REFERENCES

7. D. R. Haefner and T. J. Tranter, Methods ofgas phase capture of iodine from fuelreprocessing off-gas: A literature survey,
Report INL/EXT-07-12299 (2007).

1. B. Clément, L. Cantrel, G. Ducros, F. Funke,
L. Herranz, A. Rydl, G. Weber, and C. Wren,
State of the Art Report on Iodine Chemistry,
Report NEA/CSNI, no. R1 (2007).

8. H.-J. Im, Y. Yang, L. R. Allain, C. E.
Barnes, S. Dai, and Z. Xue, “Functionalized
sol-gels for selective copper(II) separation”,
Environ. Sci. Technol. 34, 2209 (2000).

2. H.-J. Allelein, A. Auvinen, J. Ball, S.
Güntay, L. E. Herranz, A. Hidaka, A. V.
Jones, M. Kissane, D. Powers, and G.
Weber, State of the Art Report on Nuclear
Aerosols, Report NEA/CSNI, no. R5 (2009).

9. R. N. Berglund and B. Y. H. Liu,
“Generation of Monodisperse AerosolStandards”, Environ. Sci. Technol. 7, 147(1973).

3. A. Cartonnet, Contribution à l’étude de rejetà l’environnement de l’iode radioactif lorsd’une séquence accidentelle de type RTGV,
Doctor Thesis, University of Lille & the
IRSN (2013).

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Session 4: filtered containment venting systems

275

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WESTINGHOUSE MOLECULAR SIEVE TECHNOLOGY
Dr. Anders Andrén(1)*, Dr. Ralf Obenland(2)Westinghouse Electric Sweden AB, Engineering Equipment & Major Projects, 721 63 Västerås, Sweden,(2)
Westinghouse Electric Germany GmbH, 68167 Mannheim, Germany*Corresponding author, tel: (+46) 732367765, Fax: (+46) 21414190, Email:andrenal@westinghouse.com(1)

Abstract – In case of severe accident conditions such as core melt-down, there may be a need to releasepressure from the containment to the atmosphere to ensure containment integrity. To minimize theradioactive release to the environment during the venting process, Filtered Containment Venting Systems(FCVS) are generally back-fitted to nuclear power plants. Beside of radioactive aerosols, the vented gas
flow typically contains gaseous iodine, elemental as well as organic, that need to be efficiently removed
before the gas is released to the atmosphere. Westinghouse offers dry and wet FCVS. Dry FCVS consist ofa metal fiber filter for the removal of radioactive aerosols and a molecular sieve (MS) for filtering gaseousiodine, whereas wet FCVS use scrubber systems to remove aerosols and elemental iodine. As organic
iodine removal capability of scrubber systems is generally limited, Westinghouse developed and filed apatent for a special MS, supplementing wet scrubbers, in order to meet modern organic iodine retention
requirements.
Westinghouse molecular sieve designs are flexible such that they can meet plant-specific conditions
including design and operating pressure and temperature, amount and composition of gas and radioactive
contents, available installation location and required decontamination efficiency (DF). The filters aredesigned for small pressure losses and may easily be back-fitted to existing containment filtered ventsystems
After an introduction into the various venting filters of Westinghouse and an outline of the need for iodine
filtering, the Westinghouse MS technology is described in some more detail but also the limitation of
traditional wet scrubber systems based on thiosulphate to filter organic iodine.

276

NEA/CSNI/R(2016)5
I. INTRODUCTION

Fig. 1. Principle of the Westinghouse SVEN
filtered containment venting system.

Following a severe accident in a nuclear power
plant the need to preserve containment integrity
may require venting to the atmosphere. The ventgases typically contain, besides steam, aerosols
and gases of various compositions. To prevent
radioactive releases to the environment theventing lines may be equipped with filters. These
Filtered Containment Venting Systems (FCVS)
are typically designed to retain a certain fractionof an aerosol size distribution, and to retain
elemental and organic iodine.

The venting flow is first routed through a set ofsintered metal fibre filter cartridges submerged ina scrubber liquid inside a pressure vessel. The
venting flow forces the liquid out from the
manifold and the cartridges. The majority ofaerosols (>99%) in the vent flow are removed by
the submerged filter media as the vent flow flows
from the inside of the cartridges to the outside
through the filter media. The venting flow thenpass a liquid volume where the large interfacial
area between gas and liquid lead to scrubbing of
the gas flow from aerosols. The scrubber liquidconsists of sodium thiosulphate (Na2S2O3) andsodium hydroxide (NaOH) dissolved in demineralized water. During this bubbling phaseelemental iodine is removed by a very fastchemical reaction with thiosulphate solution.
Sodium hydroxide (NaOH) is used as a buffer toprovide a sufficiently high pH level. Stand-by pH
level is defined by the severe accident scenarios
defined for the actual plant installation anddepends on amount and nature of acidic materials
such as e.g. chlorine in the vent stream. A
detailed chemical analysis is made that ensures a
pH level above 7 during venting in order toprevent re-volatilization of iodine. Above thescrubber liquid, a splash shield and a demister toremove moisture from the vent flow are installed.
The demister prevents release of aerosols due tore-suspension when aerosols encapsulated inliquid droplets follow the gas flow from thescrubbing volume. A DF for aerosols >1,000 is
obtained by these first three steps, but the ventflow exiting the scrubber may still contain small
amounts of fine aerosol particles. A second set of
HEPA rated metal fibre filters is therefore
provided in the upper part of the SVEN tank,designed to capture a smaller load of log-normal
size distribution aerosols with MMD 0.3 μm anda geometric standard deviation of 2.

Westinghouse currently proposes two different
FCVS products; a wet scrubber systemcommonly referred to as “SVEN” (Safety
VENting system) and a dry filter system,
commonly referred to as “DFM” (Dry Filter
Method). The offering of a select choice of
products allows for an adjustment to the specificconstraints and needs of each nuclear power
stationWet FCVS of Westinghouse - SVEN
The SVEN wet scrubber system is based on a
combination of wet scrubbing and sintered metal
fibre filter separation technique [1]. Aerosol
filtering and retention of gaseous elemental andorganic iodine is in SVEN performed in fiveconsecutive steps, see Figure 1.

Typical aerosol retention efficiencies obtained inextensive testing of the integrated system arehigher than 99.99 % (DF > 10,000) [1]. Typicalelemental iodine retention efficiencies are larger
than 99.9 % [1]. Organic iodine retention in the
bubbling phase is moderate, about 50% (DF≈2).
The gas exiting from the SVEN tank may
therefore contain small amounts of organic
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NEA/CSNI/R(2016)5
iodine. To achieve a specified guaranteed DF fororganic iodine, respectively total gaseous iodine,an optional molecular sieve, consisting of beds ofsilver doped zeolite beads through which the ventflow is routed, is provided. Zeolite is a micro
porous ceramic material with a high inner
porosity. The filtering principle inside the zeolite
filter is based on the chemical reaction between
iodine (both, elemental and organic) and thesilver doping, which is called chemical sorption.
The molecular sieve beds are placed in a separate
housing downstream of the SVEN tank andprovide the fifth and last filtration step of the
SVEN system. The SVEN system molecularsieve is tailored to the chosen DF, typically 50,
based on the conditions of the specificinstallation.

contained in the DFM system has participated intheinternationallysponsored
Advanced
Containment Experiments (ACE) [3]. Main
objective of the ACE tests was the investigationof the performance of different containment
venting filtration devices. For this, the efficiency
for removal of fission product aerosols whichmight be potentially released during a postulated
severe reactor accident was measured for eight
filtration systems utilizing the same aerosols,
carrier gas, and test protocol. Among others anaerosol mixture consisted of CsI, CsOH, and
MnO was utilized. The ACE tests demonstrate an
excellent performance of the DFM aerosol filter
with decontamination factors up to 3,000,000 for
Cs. In addition, when considering the overall test
results of all three elements (Cs, I, and Mn) the
DFM aerosol filter showed the best performanceof the investigated venting filtration systems.

Dry FCVS of Westinghouse - DFM
The DFM system consists of a series of modular
filter stages, see Figure 2 [2].

In the second step, located downstream of theaerosol filter, gaseous iodine (both elemental andorganic) is retained in a molecular sieve. The
molecular sieve contains a silver-doped zeolite.
In order to optimize the retention efficiency forgaseous iodine, the venting gas flow is expanded
in a fully passive manner to eliminate thehumidity of the steam and superheat the venting
gas before entering the iodine filter. Typicallydesigned decontamination factors for the iodine
filter of the DFM are 1,000 for elemental iodine
and 50 for organic iodine. The depth of thezeolite bed can be designed to achieve higher orlower DF for gaseous iodine as required by thecustomer.

Fig. 2. Principle of the Westinghouse DFM
filtered containment venting system.

Due to the modular design, different
configurations of a DFM system are possible.
Aerosol and iodine filter modules can beinstalled separately but also in combined filter
housings,
accordingtoplantspecificrequirements and available space inside the plant.
In addition, several relatively small filter
modules can be installed inside the containment(see e.g. Figure 2), providing especially the
incomparable advantage among others thatfiltered radioactive particles always remain
inside the containment. Inside and outsidecontainment configurations had been installed
already more than 20 years ago in Germany. To
meet increased decay heat removal capability,
which is typically the case for other nuclear

For aerosol filtering and gaseous iodine
retention, two different types of filters areapplied. In a first step, aerosols are filtered fromthe venting gas flow using a specially designed
deep bed metal fibre filter. The deep bed fibre
filter consists of a multistage design containing
metal fibres which decrease in diameter over thedepth of the filter. The metal fibres in the initial
filter stages have relatively large diameters
whereas the later filter stages are composed ofsmaller metal fibres. Most of the aerosol massload is retained within the first filter stages, while
the last aerosol filter stages achieve the overallhigh filtering efficiency. The metal fibre filter
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power plants worldwide, additional cooling tube
modules have been added in recent installations.
The application of the DFM to several plantdesigns is described in [4,5].

varying such that the partitioning between
elemental and organic suggested in [7] may beunderestimating the organic iodine fraction. The
large scale PHEBUS experiments support thisobservation and show the presence of volatileorganic iodides in the containment atmosphere aswell as the importance of interactions withdifferent types of surfaces for the distribution ofiodine in the containment [10]. Together withresults from the THAI test facility on iodineoxides aerosol formation it is clear that thecomposition of the containment atmospherefollowing a potential severe accident may bestrongly time dependent. Predictions of iodine
species concentration are therefore still linked tolarge uncertainties. From an industrial
perspective this means that the FCVS design
must be robust and able provide filtration
efficiency margin. In practice that higher
decontamination factors than previously used forgaseous and especially for organic iodine mayneed to be considered.

II. IODINE FILTRATION NEEDSIn terms of requirements relating to ultimate
releases to the environment, there are nointernationallystandardized
regulatory
requirements for severe accidents and venting
system performance.
This implies that appropriate requirements forthe DF must be derived from severe accident
analysis by codes like MAAP or MELCOR. In
addition, considering the differing filtration
efficiencies for the different forms of iodine,various combinations of DF for aerosols,
elemental and organic iodine may be possible inorder to meet the overall release limit. There is
no stringent reason to impose the same DF
requirement on different chemical forms of afission product when these are present indifferent fractions.

III. WET SCRUBBER IODINE FILTRATIONNUREG-1465 [6] is widely used as a genericapproximation for source term to containment,.
According to [6] about 95 % of the iodine will bepresent in form of aerosols (CsI). The remaining
5 % are present in gaseous form. NRC
Regulatory Guide RG 1.183 [7], which is basedon NUREG-1465, suggests that 97 % of these
5 % (4.85 % of total iodine) are present in formof elemental iodine and 3 % (0.15 % of total
iodine) are present in form of organic iodine.

Gases vented from containment may containboth elemental iodine and to a lesser extent
organic iodides. When the first FCVS were
designed and installed in the 1980’s, following
the Three Mile Island accident, there were no
requirements on retention of organic iodides [2].
Wet scrubber systems therefore utilized ascrubber liquid consisting of sodium thiosulphate(Na2S2O3) and sodium hydroxide (NaOH)
dissolved in de-mineralized water. The elemental
iodine is in this scrubber liquid removed by avery fast chemical reaction with the thiosulphate
solution. Iodide ions created in the reactions aresoluble in water. This technology were used bothby ABB Atom (nowadays Westinghouse) in theirFiltra-MVSS system and by KWU (nowadays
Areva) in their wet scrubber system.

Since NUREG-1465 had been written, thebehavior of iodine in the reactor system and in
the containment has been investigated in moredetail. Aspects such as the complexity of surface
reactions, the mass transfer of gaseous iodine
species between aqueous and gas phase andradiolytic oxidation leading to formation of solidoxide products have been investigated within the
SARNET cooperation. An overview of some ofthese results can be found in [8]. The complex
interaction between iodine and paint used inside
the containment, especially in connection with
irradiation, was studied in the EPICUR, NROI
and BIP [9] programs. The results, besides
providing input to modelling efforts, shows thatthe interactions are very complex and time

The purpose of the sodium hydroxide is to createa buffer that maintains the pH level above 7
during venting. This is necessary to prevent revolatilization of iodine as acidic materials in thevent stream, typically hydrochloric acid formed
from pyrolysis or irradiation of cable isolations,paint films, etc., are captured in the scrubber.

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NEA/CSNI/R(2016)5
Wet scrubber retention efficiency for elemental
gaseous iodine in a sodium thiosulphate based
scrubber liquid is excellent. Retention efficiency
primarily depends on the generated gas-to-liquidinterface area and the residence time for the gas
in the scrubber. Quoted retention efficiencies [2]are typically better than 99.9%, corresponding toa DF of 1,000, or better. Retention efficiency
measurements are typically based on highly
accurate gas chromatography-mass spectrometry
techniques. Variations in obtained values are due
more to repeatability of test conditions during
large scale testing (temperature, pressure, flow
rates, etc.), than to concentration analysis errors.
Quoted DF values are generally therefore lower
bounds with significantly better values obtained
in individual cases.

Fig. 3. General layout of wet scrubber iodine
retention test rig.

With an increased focus on creation of organic
iodides in the containment during a severe
accident, e.g [8], has followed new requirements
on FCVS systems ability to filter also organic
iodides. Traditional alkaline scrubbers are known
to retain very little of organic iodides [11, 12]. It
has however been unclear as to which extent thesodium thiosulphate based scrubber liquid willretain organic iodides. To determine retention oforganic iodide in the commonly used sodium
thiosulphate based wet scrubber technology,
Westinghouse has performed prototype scaletesting [13, 14]. Two test rigs, A and B, wereerected to determine organic iodide retention.
The general layout of the test rigs are depicted in
Figure 3.

A full height scrubber equipped with either aventuri nozzle or a fibre filter cartridge was kept
heated and pressurized at pre-set temperature andpressure. Nitrogen was used as carrier gas andheated to the required test point temperature.
Methyl iodide (MeI) was used as trace gas todetermine retention efficiency. Flow rates ofcarrier gas, MeI feed, and sample flows wereused to control the experiments. Samples
representative of upstream and downstream
conditions were taken through sampling lines
connecting close to the inlet and outlet of thescrubber.
Test rig A used an aqueous solution of Nal,containing the radiotracer isotope 1-131, anddimethyl sulphate that was mixed to produce
radioactive Mel. Dosage of Mel was made bycontinuously bubbling nitrogen gas through theliquid in a cylinder. The volatile Mel was carriedby the nitrogen flow and debouched into themain flow gas stream. Sampled flows were dried
and routed through carbon filters which werethen analyzed for MeI content.
Test rig B used a nebulizer to atomize liquid MeI
into a vaporization chamber in which the MeI
was mixed with the carrier gas.
Several tests were made covering typicalpressure, temperature and pH operating
conditions. The ranges covered where 1.1 bar(a)<´pressure <5.3 bar(a), 73°C < liquid
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NEA/CSNI/R(2016)5
temperature < 129°C, 10.5 < pH < 13.4. Figure 4
shows retention efficiency in percent as functionof scrubber liquid temperature. It is clear that
retention is low, of the order of 15%-70%, withno clear trend with temperature. As shown in
Figure 5 the data show no trend with pH either.
Data point symbols indicates corresponding
points in Figures 4 and 5. The average retention
is found to be 36% corresponding to a DF equal
to 1.6.
It is clear from these tests that the traditional
sodium thiosulphate based wet scrubber
technology provides limited retention of organic
iodine. In order to meet modern organic iodine
retention requirements Westinghouse havetherefore developed an iodine filter product line
based on molecular sieve technology forsupplementing the scrubber system.

IV. WESTINGHOUSE MOLECULAR SIEVETECHNOLOGY
Westinghouse provides two alternative designs
for the molecular sieve: “cylindrical type”, see
Figure 6, and “box type”, see Figure 7, designs.
The cylindrical design was developed tosupplement a wet scrubber system especiallyregarding modern organic iodine retention
requirements and can be designed for highpressure (up to 10 bar). The box type has been
already installed in the past as part of the DFM
system and is therefore applied for nearatmospheric pressure. Compared to thecylindrical type, where only small amounts oforganic iodine exiting the SVEN scrubber remain
to be filtered, the box type design can be adjusted
by the means of passive cooling tubes (seeFigure 7) to cope with high decay heat.

Fig. 4. Prototypic scale testing of organic
iodide retention in sodium thiosulphate based
wet scrubbers as function of scrubber liquid
temperature.

Both the cylindrical design MS and the box
design MS use the same working principle forfiltering of gaseous iodine, which is the use ofsilver doped zeolites to clean the gas fromelemental and organic iodine.

Fig. 5. Prototypic scale testing of organic
iodide retention in sodium thiosulphate based
wet scrubbers as function of scrubber liquid pH.

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NEA/CSNI/R(2016)5
Fig. 6. Cylindrical Molecular Sieve. The redarrows symbolize the gas containing organic
iodine and the green arrows symbolize thecleaned gas, ready to exit to the atmosphere.

leading to an increase in dew point distance andsuperheated steam. At higher containmentpressure, a larger dew point distance can beachieved. At lower vent pressure, the distance todew point will go down and hence the apparent
efficiency of the iodine filter will be reduced.
This effect is, however, compensated by thelower flow rates at lower containment pressure
and the resulting longer residence times of thegas molecules inside the filter bed. Hence, theefficiency of the iodine filter remains constantand can be guaranteed over the full range of flow
and pressure of the venting cycle.
In the MS the gas is evenly distributed andpasses through zeolite beds. In the cylindricaltype design (Figure 6), vent flow is maintained
downward through the zeolite bed whereas, inthe box type design (Figure 7), a horizontal flow
is maintained through the zeolite beds. In thecylindrical design, gas flow is evenly distributedby the inlet distribution pipe openings and theflow path shapes. Multiple techniques are used in
the box design MS to provide even distributionof gas flow to the parallel filter modules and to
the single filter beds.

Fig. 7. Box Type Molecular Sieve.

Significant performance improvements withrespect to structural stability, weight andhygrophobic behavior, of doped zeolites havebeen obtained in recent years. Westinghousecooperates with the leading zeolite vendors toassure optimal selection of zeolite type for each
installation. Several qualification tests are donein order to qualify a specific zeolite batch to be
used. The zeolite material is tested at various
air/steam mixtures, and at various superheating
values (distance from dew point) to determine
retention efficiencies for various residence times.
In addition, the zeolite material is exposed toharsh steam, radiation and high temperature
conditions and then the efficiency is measured
again
to assureminimalperformancedegradation. The dew point distance is selected
to be representative for the conditions at the
specific
nuclearpower
plant.
Further
improvement of iodine filtering could beachieved by the use of next generation zeolitematerials, providing very good filter performance
also under saturated conditions.

The zeolite is a micro porous ceramic material
with a high inner porosity doped with added
silver. In a chemical reaction between iodine andsilver, silver iodide (AgI) is formed.
The zeolites are commonly made of a synthetic
crystalline aluminosilicate zeolite X type. Thiszeolite type is sensitive to moisture. The filter
efficiency for gaseous elemental and organic
iodine therefore depends both, on the residence
time of the gas inside the zeolite bed and on the
distance of the gas temperature to the dew pointof the steam contained in the gas mixture. The
venting gas is expanded in a fully passive
manner by a pressure reducing expansion orifice,Typically designed decontamination factors forthe iodine filter are 1,000 for elemental iodine
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and 50 for organic iodine. The depth of thezeolite bed can be designed to achieve higher orlower DF if required.
Experience from MS installations has shown nodegradation of the zeolite material over time
when nitrogen cover gas is used to maintain thezeolite material in a dry atmosphere.

REFERENCES
1. P-O. Nilsson, “SVEN Filtered Containment
Venting System, Design and Testing”,
Westinghouse Electric Sweden AB, Report SEB
14-040, rev 1, Proprietary, 2014.
2.

V. CONCLUSIONS
Following the Fukushima Daiichi accident theawareness of the importance of filteredcontainment venting as a part of plant defense indepth increased. Therefore, the installation or theenhancement of a FCVS has become arequirement in more and more countries
worldwide. New research results are in additionindicating a larger than previously assumed
presence of organic iodine compounds in thevented gases: To maintain a high level of safety
and protection to the environment in case of asevere accident Westinghouse have therefore
refined it’s filtration technologies to meetmodern requirements.
With the molecular sieve designs, which can be
designed to meet any gaseous iodine retention
requirements, integrated in a passive selfregulating filter system design, Westinghouse iswell positioned with its portfolio of FCVS,
meeting also requirements for organic iodine
filtering, for which especially wet scrubber
systems based on thiosulphate solutions showed
up in latest tests to be limited to adecontamination factor in the order of 2.
NOMENCLATUREDF = Decontamination factor
DFM = Dry Filter Method
FCVS = Filtered Containment Venting System
HEPA = High Efficiency Particulate Arresting
MeI = Methyl Iodide
MS = Molecular Sieve
SVEN = Safety VENting system“Status Report on Filtered Containment
Venting,”, Nuclear Safety, NEA/CSNI/R(2014)7,
July 2014.

3. I. Wall, M. Merilo, “Containment Filtration
Systems Tests - Advanced Containment
Experiments (ACE) Project Summary Report,”
Electric Power Research Institute (EPRI),
February 1992.
4. C. Hartmann, M. Bauer, J.C. Adams,“Experience from the application of the Dry
Filtered Method for Filtered Containment
Venting to several plant designs” Proceedings ofthe 33rd NACC, June 22-24 2014, St. Lois, MO.
5. C. Hartmann, M. Bauer, H. Ulrich, “Experiencefrom the Application of the Dry Filtered Method
for Filtered Containment Venting to Several
Plant Designs” atw - International Journal for
Nuclear Power- Issue 8/9 2014.
6. L. Soffer, S. B. Burson, C. M. Ferrell, R. Y. Lee,J. N. Ridgely, “Accident Source Terms for LightWater Nuclear Power Plants” NUREG-1465,
1995.
7. U.S.
Nuclear
Regulatory
Commission,“Alternative radiological source terms forevaluating design basis accidents at nuclear
power reactors”, Regulatory Guide 1.183, July
2000.
8. S. Dickinson, A. Auvinen, L. Bosland, G. Glowa,
D. Powers, B. Teisseire, S. Tietze, “Iodine
Chemistry in Nuclear Reactor Accident
Conditions: Recent Studies and Hypotheses”,
NPC Sapporo, 2104.
9. Glenn A. Glowa, Chris J. Moore, Joanne M.
Ball, “The main outcomes of the OECD
Behaviour of Iodine (BIP) Project”, Annals of
Nuclear Energy 61, pp. 179–189, 2013.

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10. B. Simondi-Teisseire, N. Girault, F. Payot, B.
Clément, “Iodine behaviour in the containment
in Phébus FP tests”, Annals of Nuclear Energy
61, pp. 157–169, 2013.
11. D. R. Haefner, T. J. Tranter, “Methods of Gas
Phase Capture of Iodine from Fuel Reprocessing
Off.-Gas: A Literature Survey”, Idaho National
Laboratory, INL/EXT-07-12299, February 2007.
12. P. Paviet-Hartmann, W. Kerlin, S. Bakhtiar,“Treatment of Gaseous Effluents Issued from
Recycling – A Review of the Current Practices
and Prospective Improvements”, Idaho National
Laboratory, INL/CON-10-19961 PREPRINT,
November 2010.

13. S. Kristensson, “FILTRA-MVSS removal of
methyl iodine at different pH and thiosulfate
concentration” Westinghouse Electric Sweden
AB, Memorandum SEB 14-070, rev 0,
Proprietary, 2014.
14. S. Kristensson, “SVEN pilot plant Iodine –description”, Westinghouse Electric Sweden
AB, Report SEB 14-119, rev 0, Proprietary,
2014.

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IMPROVED IODINE RETENTION AT FILTERED CONTAINMENT VENTING
P. Zeh*, S. Buhlmann, A. Stahl, N. Losch
AREVA GmbH, Erlangen / Offenbach, Germany*Corresponding P. Zeh, Tel: (+49) 913190097690, Fax: (+49) 913190097312,
Email:peter.zeh@areva.com
Abstract - During a severe accident in a NPP the containment pressure can exceed the allowablemaximum pressure. To avoid uncontrollable cracking of the containment shell, pressure relief is necessary.
To perform this AREVA developed and qualified a Filtered Containment Venting System (FCVS) already25 years ago. It contains a venturi scrubber whose scrubbing liquid has high potential for chemisorption ofiodine compounds followed by a subsequent droplet separator and a metal fiber filter unit. The
performance of the system has been verified to remove aerosols and iodine with very high decontamination
factors. Since once the aerosol and iodine release has been significantly reduced by an above describedfiltered venting system, the release of organic iodine becomes a leading contributor to the radiologicalconsequences, that still remain, but on a much lower level as compared to without a filtered venting system
Therefore, long before the Fukushima accident AREVA in 2009 decided to improve its qualified FilteredContainment Venting System (FCVS), composed of a venturi scrubber followed by a subsequent droplet
separator and a metal fiber filter unit. Aim was to improve the comparatively limited retention capabilityof organic iodide species of the AREVA FCVS to values of above 98 %.by adding an additional filtering
unit, focused on organic iodide retention, to the existing design.
Development tests were performed at a lab-scale scrubber test facility, called “VESPER34” which allows
testing retention of elemental and organic iodine species under flow conditions at temperatures up to
200°C and up to 10 bar. This VESPER34 test facility simulates the FCVS conditions at a lab scale in a
vessel of 9 l containing 3 l scrubbing liquid followed by variable filtering units downstream. A variable
steam / air-flow is purged through a venturi nozzle into a scrubbing liquid containing a solution of sodiumhydroxide and sodium thiosulfate for iodine retention and stabilization. Methyl iodide (MeI) was used as
test substance for investigating retention of organic iodine compounds, since MeI shows the lowest
retention compared to all other species of organic iodine. This MeI was traced with I-131 and injected
continuously into the gas flow upstream the venturi scrubber. The retention of MeI was improved byadding an additional molecular sieve filtering unit downstream the venturi scrubber. In parametric studies,
the best adsorption material was selected and operational conditions were optimized.
After the design phase, qualification tests were performed at the large scale test facility at AREVA
Karlstein. Here a filtered venting system of original height with a reduced flow cross section of approx.
1:10 was used for qualification tests. In this filtered venting system an additional molecular sieve filter
section was implemented downstream the metal fiber filter unit. Parametric tests were performed between
1 to 8 bar at a temperature range from 110°C to 160°C. As test substance unlabeled MeI was injected
continuously into the gas flow upstream the venturi scrubber. The qualification tests verified good organic
iodine retention under all operating conditions. The MeI-retention was above, mostly far above, 98%.
The large scale test results are used to design the additional sorbents stage for the requested application.
The dimensions of the sorbents stage can be adjusted to meet the customer request regarding the DF fororganic iodine. This form of qualification ensures reliable and verified performance values by excluding
relevant scaling errors from lab scale to industrial application. The performed lab scale tests complete thequalificationofusedsorbents
materialforsevere-accident
conditions.
I. INTRODUCTION

During a severe accident scenario in a NPP thecontainment pressure can exceed the allowable285

NEA/CSNI/R(2016)5
maximum pressure. To avoid uncontrollable
cracking of the containment shell pressure relief
is necessary. In order to minimize radionuclide
release to the environment due to containmentpressure reduction via venting, filtered venting
systems have been designed, internationallyqualified and implemented in manny NPP’s.
Main focus was given to the reliable and efficientaerosol retention. In addition also efficient iodine
retention was requested, as this element has
significant activity content in nuclear fuel in
combination
with
high
volatility
andradiotoxicity. Therefore, efforts are made to
reduce the radionuclide emission of iodine in theoff-gas during venting.

organic iodide species of the AREVA FCVS tovalues of above 98% by adding an additional
filtering unit, focused on organic iodide
retention, to the existing design.

In 2014 the state-of-the art-report of NEA on
FCVS [1] was updated with the collaboration of
AREVA. This is one of the good opportunities
AREVA uses to challenge the products onmarket requests. Currently several new topics are
more or less intensively discussed as sub-micronaerosols IOx, RuO4 and organic iodine. The latter
one is discussed in the paper on hand.

Iodine in the containment atmosphere under
severe accident conditions is expected to bepresent in various species whose species
composition and distribution changes over time[1]. The iodine species entering a filtered venting
system reflect the species distribution of iodinein the containment at a given time.

For system development and design numerous
tests at a lab-scale test facility were performed,
followed by qualification tests at a large scale
test facility of original height. The AREVA test
facilities capable of testing behavior of different
iodine species under venting conditions aredescribed and qualification results are given.
II. IODINE CHEMISTRY IN WET VENTINGSYSTEMS

Especially at first venting, a substantial part ofiodine is expected to be aerosol bound. Possible
aerosol species of iodine are CsI, AgI or IOx.
Iodine oxides (IOx), which are expected to form
small size aerosols (micro aerosols), are issue offurther research [3]. Such species can be captured
very efficiently in any liquid of a venturi
scrubber. But alkaline conditions are necessary to
avoid reformation of elemental iodine from
dissolved CsI under radiolytic conditions [1, 3].
Equilibrium of reversible disproportionation /conproportionation reaction of elemental iodine(Eq. 1) is under alkaline conditions almost
quantitatively on the disproportionation side of
the ionic species (IO3- and I-). High pH ensuresthat the formation of elemental iodine, the firststep for revolatilization, is hindered.

State-of-the-art containment wet venting
scrubbing liquids use a solution of sodiumhydroxide and sodium thiosulfate in order towash out volatile iodine species. Under severe
accident conditions such volatile iodine species
are aerosol bound iodide as well as gaseouscompounds of elemental iodine, hydrogen iodide
and organic iodide species. With alkaline
thiosulfate solution high retention efficiencies foraerosol bound iodide as well as elemental iodine
and hydrogen iodide are achieved.
Since once the aerosol and iodine release has
been significantly reduced by an above describedfiltered venting system, the release of organic
iodine becomes a leading contributor to theradiological consequences, that still remain, but
on a much lower level as compared to without afiltered venting system

3 I 2  6 OH 

IO3  5 I   3 H 2O Eq. 1

Further on the presence of thiosulfate (S2O32-) as
reducing agent prohibits the radiolytic oxidationof I- therefore IO3- cannot exist.

Therefore, long before the Fukushima accident
AREVA in 2009 decided to improve its qualified
Filtered Containment Venting System (FCVS),
composed of a venturi scrubber followed by asubsequent droplet separator and a metal fiber
filter unit. Aim was to improve thecomparatively limited retention capability of

Besides aerosol bound iodine there are various
volatile iodine species present in the vented gas,
first of all elemental iodine (I2) followed by HI
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and unknown organic iodide species. Suchspecies are formed in the containment byinteraction of iodine with organic material [5].
Due to its high volatility, gaseous organic iodide
is often dominated by methyl iodide [6].

RI  1 AA

Ix ,out

Eq. 4

Ix ,inwith:

A

Ix ,inTo assure the retention of volatile iodine species
chemical additives are necessary to assure highdecontamination factors of vented steam and gas
from the containment. Therefore sodiumhydroxide and sodium thiosulfate are added tothe scrubbing solution allowing chemisorption of
HI and elemental iodine with very high retention
factors. Thiosulfate works in that process as
reducing agent, transforming elemental iodine toiodide (I-). In parallel NaOH itself supports inprinciple the destruction of volatile I2 by
producing non-volatile iodide I- and iodate IO3according the disproportionation reaction of Eq.
1. HI, an acidic gas is captured by NaOH due to aneutralization reaction. By this the emission ofthese species (I2, HI) can be lowered by several
orders of magnitude.

the sum of activity (in Bq) of all iodine species x
entering the filtered venting system andA

Ix ,out

the sum of activity (in Bq) of all iodine species x
leaving the filtered venting system.
The iodine fraction leaving the filtered venting
system strongly depends on the species of iodine
compound and the individual decontamination
factor achieved for this species (DFIx). There aredifferent retentions expected for iodine aerosols,
elemental iodine, hydrogen iodide and organic
iodide. In theory for each species (Ix) the activity
release can be calculated using equation 5.
Practically the activity fraction of different iodine
species is not known in detail and usually only
three fractions are distinguished (aerosol bound,inorganic volatile and organic volatile iodide).

Organic iodide compounds will react in the
scrubbing solution with OH- and S2O32- bynucleophilic
displacement
reactions[7].
Exemplarily given for methyl iodide, thedominant representative of organic iodide
compounds, the decomposition reactions are
given in Equation 2 and 3.

AIx ,out  AIx ,in 

1
DFIx

Eq. 5

In case the decontamination factor of one species
is much lower than all others there is a risk that
this species dominates the overall release. This is
illustrated from following thought-experiment
where three species at different arbitrary
concentrations and decontamination factors aredefined and the species of lowest concentration
has at the same time the lowest decontamination
factors. In such a case the release can be
dominated by the species of lowest concentration(see Table I). In example of Table 1 species A
gives 90% of the total release even when its
fraction was only 0.9% in the gas entering thefilter system.

CH 3 I  OH   CH 3OH  I  Eq. 2

CH 3 I  S 2 O3 2  CH 3 S 2 O3   I  Eq. 3

Kinetic of methyl iodide decomposition reactions
are not fast enough to achieve complete
conversion
under
representative
venting
conditions. Due to limited residence time ofventing gas the retention of organic iodide islimited.
The overall retention efficiency of iodine
compounds strongly depends on those iodine
compounds of low retention. This can beclarified by the following equations.
The overall retention RI is given by:

The fraction of organic iodide species is usuallyminor compared to the fractions of aerosol boundor volatile inorganic iodine but it shows thelowest retention in filtered venting systems. This

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Operable parameter range of VESPER34 test
facility

was the motivation to further improve the
AREVA Filtered Containment Venting System.
TABLE I

Thought-experiment, showing the dependence of
release from decontamination factor of species

SpeciesA
B
C

entering
activity(arbitrary
unit)
1
10
100

arbitrary
DF

10
1000
100000releasedactivity(arbitrary
unit)
0.1
0.01
0.001

III. EXPERIMENTAL FACILITIES FOR TESTINGIODINE BEHAVIOUR UNDER VENTING
CONDITIONSParameter

Operable range

Temperature

RT – 200°C

Pressure (abs.)

1 -10 bar

Mass flow steam

0.01 – 1.1g/s

Mass flow gas(air)concentration ofiodine compound
Gas / steamcomposition

0.01 – 1.3 g/svariable, usually in the rangeof 10-8 to 10-9 mol/l
any gas / steam composition
possible at any mixing ratio

Fig. 1. Schematic set-up of the VESPER

34

test

facility with submerged injection nozzle.

For testing the behavior of iodine species underventing conditions AREVA has two test facilities
of different size, a lab scale test facility called
VESPER34 and a large scale test facility calledJAVA.
The lab scale test facility VESPER34 is located in
the AREVA radiochemistry laboratories in
Erlangen, Germany. This test facility simulates
the FCVS conditions in a vessel of 9 l containing
3 l scrubbing liquid followed by variable filtering
units downstream. A variable steam / air-flow ispurged through a venturi nozzle into a scrubbing
liquid containing a solution of sodium hydroxideand sodium thiosulfate for iodine retention andstabilization. Different iodine species such as
elemental iodine or organic iodide compounds
are added to the flow, and their retention at
variable parameters is tested. Usually the iodine
species are labeled with radioactive tracers of I123 or I-131. With this the retention behavior of
iodine compounds can be easily followed atconcentration ranges of 10-9 to 10-12 mol/l.

The large scale test facility JAVA is located in
Areva´s Technical Center in Karlstein, Germany.
The test facility is especially erected for thetesting of FCVS and includes dedicatedinstrumentation and process control. It includes
three different filtration stages, a Venturi
scrubber, a metal fiber filter (dry stage) and anadsorption filtration stage including a passive
superheating device. The scrubbing liquid of thewet venting stage of the venturi scrubber has ahigh potential for chemisorption of iodine
compounds and retention of aerosols. The
subsequent droplet separator and metal fiber
filter unit prevents the carry-over of droplets andgives high retention for remaining fine aerosols.

The schematic set-up of the VESPER34 test
facility is given in figure 1. Retention tests canbe performed under flow conditions attemperatures up to 200°C and up to 10 bar. The
operable parameter range is given in Table II.
Typical containment venting conditions oftemperature, pressure, flow velocity and iodine
concentration can be tested over a wide range
with this facility.
TABLE II

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NEA/CSNI/R(2016)5
To this filtered venting system additionally anadsorption filtration stage downstream the metal
fiber filter unit was implemented, which iscapable of absorbing and immobilizing organic
iodide compounds.
This JAVA test facility is scaled up to the FCVS
original height of 8 m with a reduced flow crosssection of 1:10. Since the single process sections
are integrated in the original scale, very
representative results can be produced. This test
facility allows retention tests at flow velocities
and gas residence times similar to realistic(calculated) venting scenarios. The operable
parameter range is given in Table III. Due to its
modular design the test facility enables thetesting of single or combined process steps undervarious test conditions. Numerous tests were
performed at this JAVA test facility to qualify
the AREVA FCVS system under representative
conditions. Retention of aerosols, elemental andorganic iodide as well as thermo hydraulicperformance were tested. In figure 2 a picture of
the test facility is given.

Parameter

Operable range

Temperature

RT – 200°C

Pressure (abs.)

1 -11 bar

Mass flow

0.05 – 1.5 kg/s

concentration ofiodine compound
Gas / steamcomposition

variable, usually in the rangeof 10-7 to 10-9 mol/l
any air / steam composition
possible at any mixing ratio

IV. QUALIFICATION TESTS FOR IMPROVEDFILTERED CONTAINMENT VENTING SYSTEM

Developing tests were performed at the
VESPER34 test facility. Methyl iodide (MeI) wasused as test substance for investigating retentionof organic iodide compounds. Due to its highest
partial pressure (544 hPa at 20°C) of organic
iodide compounds it is regarded to cover thisgroup of compounds conservatively. For labscale tests MeI was traced with I-131 andinjected continuously into the gas flow upstream
the venturi scrubber. The retention of MeI was
improved by adding an additional molecularsieve filtering unit downstream the venturi
scrubber. In parametric studies different
adsorption materials were tested and operation
conditions were optimized. . These data wereused for a new design of filtered venting system.
Finally one material was selected for further
more extensive qualification which was foundthe optimum regarding retention performance,long-life and stability under the predictable
operating conditions.

Fig. 2. Image of JAVA test facility.

After the design phase, qualification tests were
performed at the JAVA test facility at AREVA
Karlstein, at a filtered venting test facility inapprox. 1:10 scale of flow cross section andoriginal in height. Parametric tests were
performed between 1 and 8 bar in a temperature
range from 110°C to 160°C. The mass flow ofsteam and gas was around 1 kg/s, which isroughly 10% of the expected mass flow of
calculated venting scenarios.

TABLE IIIAs test substance, unlabeled commerciallyavailable MeI (methyl iodide) was injected

Operable parameter range of JAVA test facility289

NEA/CSNI/R(2016)5
continuously into the air / steam flow upstream
the venturi scrubber at a constant mass flow. Thiswas performed by injecting with a high pressure
pump a defined mass flow of liquid MeI into an
evaporator unit where MeI is evaporated
instantaneously and flushed with a nitrogen
carrier gas into the main air / steam flow. The
decision for unlabeled CH3I was made bybalancing between advantages and drawbacks.
The decision required amongst others to develop
the measuring and injection technology and tocheck the influence of CH3I concentration.

iodide. Thereby the qualification tests ensure
most reliable verified performance values by
excluding relevant scaling errors from lab scale
to industrial application which could easily resultin an overestimation of retention efficiency byorder(s) of magnitude. The performed lab scale
tests complete the qualification of used sorbentsmaterial for severe accident conditions.

At the same time fractions of the gas flow ofsteam and air were sampled up- and downstream
the filtering unit of venting system. MeI content
in the gas sample is filtered by absorption on
charcoal. Later on methyl iodide is extracted
quantitatively from the collected charcoal
samples and converted to iodide (I-). The iodine
content in the extraction solution was analyzed
by ICP-MS (Inductive Coupled Plasma Mass
Spectrometry). From iodine concentration, massof extraction solution and the portion of sampledgas (referred to the main flow), the formed
methyl iodide concentration in the main gas flow
can be calculated. Similar to Eq. 4 the retention
can be estimated from methyl iodide
concentration up- and downstream of the filteredventing system.

Pressure*(barabs.)
7.2
5.0
1.5

TABLE IV

Methyl iodine retention at different boundary
conditions
Temperature** (°C)
139
118
105

Steam
(wt.%)
70
50
90

DF
256
195
53

* upstream FCVS** inside sorbent section

V. CONCLUSION
Retention of iodine in a filtered containment wet
venting system was improved by focusing on theretention of organic iodide. Such species havemuch lower retention in a venturi scrubber thanaerosol bound or inorganic volatile iodine. By
implementing an additional adsorption filtration
section with a passive superheating device it waspossible to improve the overall retention ofiodine species.

From 2012 to 2014 more than 50 tests of methyl
iodide retention were performed at variable
pressure, temperature and mass flow conditions.
These qualification tests verified that the
improved FCVS gives, besides the alreadyverified high retention of aerosols and inorganic
volatile iodine (I2), also very good retention oforganic iodide compounds. Under all operating
conditions the MeI-retention was above, mostly
far above, 98%. In addition the adsorption
filtration section also improves the retention of
inorganic volatile iodine as such species are also
bound with high efficiency on the usedadsorbent.

Development tests were performed at the
VESPER34 test facility where the best adsorption
material could be selected and operation
conditions optimized during parametric studies.
After the design phase, qualification tests were
performed at the large scale JAVA test facility at
AREVA Karlstein, a filtered venting system oforiginal height. The large scale test results areused to design the additional sorbents stage forthe requested application. Based on the foundrelationships, the dimensions of the sorbents
stage can be adjusted to meet the requirement ofdifferent authorities or customer request
regarding the DF for organic iodine. Thereby thequalification ensures reliable and verified
performance values by excluding relevant scaling
errors from lab scale to industrial application.
The performed lab scale tests complete the

The large scale test results are used to design the
additional sorbents stage for the requested
application in detail. The dimensions of thesorbents stage can be adjusted to meet thecustomer request regarding the DF for organic
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NEA/CSNI/R(2016)5
qualification of used sorbents material for severe
accident conditions.
Under all operating conditions the MeI-retention
was above, mostly far above, 98% for the usedsorbents section dimensions. In addition theadsorption filtration section also improves theretention of inorganic volatile iodine as suchspecies are also bound with high efficiency on
the used adsorbent.
ACKNOWLEDGMENTS

The authors would like to thank all the teammembers for their commitment.
NOMENCLATUREFCVS = Filtered Containment Venting System
MeI = methyl iodide
REFERENCES

1. Jacquemain, D., et al. “Status report on filteredcontainment venting”, OECD / NEA report,
NEA/CSNI/R (2014)7

Phebus FP tests." Nuclear Engineering and
Design 236(12): 1293-1308 (2006).
3. Dickinson, S., et al. "Experimental andmodelling studies of iodine oxide formation andaerosol behaviour relevant to nuclear reactor
accidents“ Annals of Nuclear Energy 74 (2014)
200–207
4. Girault, N., Fiche, C., et al. "Towards a better
understanding of iodine chemistry in RCS of
nuclear reactors." Nuclear Engineering and
Design 239 (2009) 1162-1170.
5. Wren, J.C. , Ball, J.M., Glowa, G.A. , “Theinteraction of iodine with organic material incontainment”, Nucl. Technol. 125 (1999) 337362
6. Marchand, C., “Production d'ioduires organiquesa partir des surfaces peintes de l'enceinte deconfinement d'un reacteur a eau pressurisee ensituation accidentelle”, IPSN Note technique
DPEA I SEAC/97-04, October 1997
7. E.A. Moelwyn-Hughes, “The kinetic of certain
reactions between methyl halides and anions inwater”, Proc. Roy. Soc. (London), Vol. 196,
540ff (1949)

2. Girault, N., Dickinson, S., et al. "Iodine
behaviour under LWR accident conditions:Lessons learnt from analyses of the first two

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DESIGN ON THE PERFORMANCE TEST OF FILTERED CONTAINMENT VENTINGSYSTEM
Kwang Soon Ha(1)*, Jaehoon Jung(1), Sang Mo An(1), Hwan Yeol Kim(1), Jinho Song(1)(1)
Korea Atomic Energy Research Institute, Daejeon, Korea*Corresponding author, tel: (+82) 428688653, Fax: (+82) 428688256, Email:tomo@kaeri.re.kr
Abstract –A scaling analysis method is proposed for the design of a test loop for the qualification of theperformance of a Filtered Containment Venting System (FCVS). FCVS is employed in the nuclear power
plant to maintain the integrity of the containment by releasing the mixture of steam and fission products
from the reactor containment to the outside. During the release, fission products in the form of aerosol andgas are filtered to avoid substantial release of radioactive material into the environment. An integral test
facility is designed to verify the performance of a proposed FCVS. The proposed FCVS system consists ofpool venturi scrubber, cyclone separator, particulate filter, and molecular sieve filter. A scaling method
was developed to design a reduced scale FCVS test facility. Tests to quantify the performance of the FCVS
include a thermal-hydraulic test, aerosol removal test, elemental iodine removal test, organic iodine
removal test, re-suspension of aerosol test, re-volatilization of iodine test, and dynamic test under a highpressure gas ejection from the containment. The scaling method was designed to reproduce the same
fundamental phenomena in the scaled down test facility as the prototype.

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NEA/CSNI/R(2016)5
I. INTRODUCTION

The FCVS should decrease the pressure inside
the containment by releasing steam and noncondensable gases to maintain the integrity of thecontainment during severe accident. In addition,
the performance of the filtration should beconsidered in the FCVS design so thatradioactive materials such as cesium andradioactive aerosol are not released outside theenvironment. In addition, the FCVS was
designed to operate passively. Accordingly, the
FCVS immediately copes with increasing thepressure inside the containment and prevents thefailure of the containment. At this time, the flowrate of the discharge gas is changed according to
the sliding pressure inside the containment tomake a continuous operation in the wide pressure
range. In addition, the FCVS does not need offsite power because of the passive operation. For
this reason, the FCVS can operate in perfectworking order to maintain the integrity of thecontainment during a long SBO such as theaccident in the Fukushima nuclear power plant.
Furthermore, the FCVS was designed to prepare
for an earthquake.

In the case of a severe accident like the
Fukushima accident [1], the melted core material(corium) can be relocated to the lower plenum of
the reactor pressure vessel. If the reactor vessel isbreached the molten corium will be discharged
into the containment environment.
The defense in depth approach is used to prevent
significant release of radioactive material outside
the containment. In the case of a LWR (Light
Water Reactor), there are 4 step physical barriers.
The first barrier is a fuel pellet that can containfission gas and all radioactive materials. The
second barrier is fuel cladding enclosing the fuelpellets. During a severe accident, first and second
barriers are breached, and then the reactor vessel
and containment can prevent the release ofradioactive material as third and fourth barriers.
The discharge of corium would lead to
substantial releases of fission products in theform of aerosols and gases, which are
simultaneously released with steam and noncondensabe gases such as hydrogen. The
hydrogen is generated during the oxidation of
zircaloy in the core material while noncondensable gases are generated due to corium
concrete interaction.

However, still there is a risk of early containment
failure due to a steam explosion, directcontainment heating, or hydrogen explosion.
Also, the bypass sequence such as steam
generator tube rupture or interfacing system
LOCA cannot be mitigated by an installation of
FCVS. Additional provisions have to be made to
mitigate the consequences of bypass sequences.

During the severe accident pressure inside thecontainment can increase substantially, and theintegrity of the containment may be threatened.
In the Fukushima accident the last barrier isbreached, which led to a significant release ofradioactive material to the environment andcontamination of huge area including sea, soil,
and rivers. A prolonged station black out (SBO)
due to earthquake and tsunami caused loss offour barriers discussed above.

Korea has 23 reactors (19 PWR, 4 PHWR) on 4
sites (Hanbit (6), Hanul (6), Kori (6), Wolsong (1PWR, 4 PHWR)) providing about 30% of its
electricity. Fukushima accidents triggered a
discussion on the need to protect from acontainment failure by an over pressurization andmitigate an uncontrolled release of activity into
the environment.

Therefore, a use of additional system like FCVS
is considered in many countries to provide
additional barrier [2] to maintain the containment
integrity even during the prolonged SBO.

In the Wolsong-1 PHWR unit, the installation of
the first FCVS (High Speed Sliding Pressure
Venturi (HSSPV) type) was completed at the endof 2012 (during the preparation to acquireapproval for a continued operation after a 30 year
operation). The remaining 22 Korean NPP units(19 PWR, 3 PHWR) are planned to be equippedwith FCVS, and an open tender is being
prepared. Korea is now developing a new FCVS
for the light water reactors as a depressurization

FCVS (Filtered Containment Venting System) is
employed to maintain the integrity of thecontainment by releasing the high-temperatureand pressure gas from the containment to outside
through a pipe line while capturing andscrubbing radioactive gases and aerosols.

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system of the containments under a severe
accident.

II. DESIGN FEATURES OF A FCVS
General design features of current FCVSs are
summarized in Table I. Current major supplierson FCVSs are AREVA, Westinghouse, and IMI
in Switzerland [2].

To verify the performance of FCVS, tests [2]were performed in the scaled test facilities suchas ACE and JAVA. A new FCVS test facility
was designed and is under construction. The
FCVS test facility consists of a test vessel,thermal-hydraulic, and aerosol/iodine generationand measurement parts. .

Korea is now developing a domestic FCVS for
light water reactors as a depressurization systemof the containments. As shown in Fig. 1, theKorean CFVS consists of the four main
components. The venturi scrubber including thepool, droplet or particle separator, metallic fiber
filter (MFF), and molecular sieve are main
components of the CFVS.

In the same manner, an integral test facility isdesigned to verify the performance of a proposed
FCVS. The proposed FCVS system consists ofpool venturi scrubber, cyclone separator,
particulate filter, and molecular sieve filter.

During the severe accident, if the pressure inside
the containment reaches desired set point, thevent valve pipe will be opened by the operator or
the operation of the rupture disk. Then, theradioactive gas mixture which consists of steam,gases, and aerosols are discharged to thefiltration tank filled with water through the ventpipe.

In this paper, a scaling method is discussed for adesign of a reduced scale FCVS test facility.
Tests to quantify the performance of the FCVS
include a thermal-hydraulic test, aerosol removal
test, elemental iodine removal test, organic
iodine removal test, re-suspension of aerosol test,
re-volatilization of iodine test, and dynamic test
under a high-pressure gas ejection from thecontainment. The scaling method was developed
to reproduce the same fundamental phenomenain the scaled down test facility as the prototype.

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Table I Comparison of current FCVS systems

Supplier

Filtration
process

AREVA

Westinghouse

Westinghouse

DFM

FILTRA-MVSS

Venturi scrubber,
metal filber filter,chemical retention Metal fiber filter,of iodine,
Molecular sieve
Molecular sieve
testedIMI

Sparger assemblies,
Venturi scrubber, co-current scrubber,
metal fibre filter,
recirculation zone,
chemical retention
droplet separator,of iodinechemical retention ofiodinesMax. designpressure

10bar(g)6bar(g)6bar(g)6bar(g)Max. design
Temperature

200℃

550℃

70~150℃

128℃

Max. design
flow rate

4~6kg/s

3~4.5kg/s

0.1~13kg/s

13.8kg/s

Reference
plant

1000MWe PWR

German PWRMARK II

GE BWR

Qualification
test

ACE/JAVA

ACE, without
molecular sieve

ACE, venturi only

PSI

The species for the fission product, sizes, andevent sequences leading to the release ofradioactive gases are discussed in reference 3.
While the mixture gas passes the venturi nozzle,
a swarm of droplets coming through the hole in
the throat of the nozzle is supplied to the mixture
gas. During this process, impaction occurs
between the gas and the droplet due to a velocity
difference, therefore the radioactive materials are
removed from the gas.

hydraulic condition of pool are dominant factors
affecting the decontamination [4, 5].

The mixture gas including the aerosols will passthrough a pool of water. While the gases passthrough the pool in the form of bubbles,
scrubbing process occurs. It will result in a
scrubbing of gas such as elemental iodine andaerosols. Scrubbing of gas is by a mass diffusion
process, while scrubbing of aerosols is driven by
the inertial impaction and settling in the gas
bubbles. The gas bubble dynamics and thermal

Fig.1. Conceptual Design of proposed CFVS
system295

NEA/CSNI/R(2016)5
Most of the radioactive aerosol and iodine areremoved in the venturi scrubber, however the gas
passing the pool still includes tiny aerosol andentrained water droplets. In the cyclone, theheavy particles and hygroscopic moisture of gas
are removed by centrifugal force. As shown in
the Figure 1, a number of cyclones are containedin a closed space like a box while the inlet of
cyclone is exposed to the gas space such that the
mixture gas containing the water droplet andaerosols could pass through the cyclones. The
box has several holes for the cyclone inlet. The
exit of cyclone is contained in the closed space,at the bottom of which a drain line is connected
deep into the pool such that the droplets would
drain into the lower part of the water pool.
Addition of chemicals is known to augment the
scrubbing process for the elemental and organic
iodine gases.

aerosol filter. The aerosol filter installed after the
pre-filter consists of the metallic fiber with athickness of 2 – 8μm in order to remove the tiny
particles. The thickness of the metallic fiber is aparameter affecting the efficiency of thecapturing performance; the performance is betteras the thickness is thinner.
The molecular sieve box is located outside the
CFVS vessel to remove the organic iodine. The
gas passing the metallic fiber filter goes to themolecular sieve in order to remove the iodine.
The molecular sieve plays a role in filteringorganic iodine. The molecular sieve consists ofthe synthetic zeolite, silver zeolite is used so thatthe radioactive iodine is removed in the hightemperature process. There are a lot of uniform
tiny holes, therefore the gas coming from themolecular sieve will pass the holes to remove theiodine.

In next stage, the metallic fiber filter into
removes very tiny aerosols and droplets whose
size is less than 2 μm. The metallic fiber filter
can be the pre-filter and the aerosol filter. The
pre-filter consisted of the metallic fiber with athickness of 6 – 10μm is installed in front of the

Finally an orifice and valve is installed after theexit of the molecular sieve. The location oforifice has to be optimized as it will change thevolumetric flow into the molecular sieve, which
would affect the amount of molecular sieve,
which is quite expensive as it contains Ag.

Table II Local phenomena and scaling parameters
Components

Local Phenomena

Scaling parameters

Pool scrubber

Submerged Jet mixing
Pool circulation
Separation of aerosols-iodine
Iodine absorption
Entrainment

Jet Reynolds number, Nozzle diameter,
Pool height and gas/liquid density ratio
Iodine/aerosol density ratio, concentration flux
Contact Surface area
Pool height, gas velocity

Droplet separator

Droplet impact –breakup
Re-entrainment

Entrained droplet and gas velocity
Gas velocity at separator, flow path

Dry aerosol filter

Particle Capture

Filter pore size and flow path

III. DESIGN OF A REDUCED SCALE TESTFACILITYAs described in the previous chapter, the key
components of the FCVSs are a pool scrubbing
chamber, droplet separator such as a cyclone, anddry aerosol filtration system. Each of these hasvarious phenomena associated with them. These
local phenomena associated with each of thesecomponents are summarized in Table II. Also inthis table the pertaining scaling parameters witheach of the local phenomenon are listed. The
major scaling parameters for designing a test
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facility are a gas residence time and pressure
drop.

the suppression tank, which has deep water pool.
The venturi scrubber is operated in self primingmode such that water is injected from the poolthrough the holes located in the throat section ofventuri. The pressure difference between the pooland inside the throat section of the nozzle drives
the liquid inflow. The size of dispersed liquid
droplet in the throat section depends on the size
and number of holes, pressure difference, gasvelocity, and thermos-physical properties ofliquid.

III.1. Scaling principle and design requirement
Based on the discussions in previous sections,scaling principle and design requirement for the
integral test facility to simulate the performanceof prototypic FCVS is provided. It is divided into
source term, system performance, and local
phenomena at the components including pool,venturi, cyclone, and Metal Fiber Filter.
The scaling principle for the source term and
FCVS system performance is briefly summarizedin Table III. The scaling principle is developed topreserve the important integral phenomena anddecontamination principle in the prototype be
preserved in the scale-down test facility.

Below is a discussion on the requirement onventrui dimensions and gas flow rate to enableself-priming mode of operation.

The next step is to preserve the local
phenomena in each components of FCVS
system, which are discussed blow.

∆Pc = ∫ (D) 2 ρv 2 dl (1)III.2. Venturi with Self priming mode

The

The pressure drop due to skin friction in aconverging nozzle is calculated as below;f1

𝑚̇ = 𝜌𝑣 (𝜋𝐷 2 )/4 = 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡.(2)pressuredrop

iscalculatedas;

A schematic of venturi nozzle is shown in Fig 2.
The ventrui nozzle is located at the lower part ofTable III Scaling Principle for the source term and FCVS system performance

Sourceterm
EarlyventingLateventing

Prototype FCVS

Scaling Principle

Scaled Model

Discharge of steam and noncondensable gases withaerosols
Radioactive gas (noble,
elemental and organic
iodine),aerosols (lognormal, 3 μm
AMMD)
Penetrating small aerosols(0.5 μm AMMD)

Use the same fluid and/or
similar non-toxic fluid

Use Steam, nitrogen, air, andaerosols

Preserve type and size
Use of non-radioactive
material

Gas (elemental and organic
iodine),Particles 3 μm AMMD
(in-soluble particles such as
SiO2)
Particles at smaller size
(in-soluble particles such as
SiO2, 0.5 μm AMMD)
Low density could be used, if
measurement accuracy for thedecontamination is warranted

Aerosol Typical Concentration in thedensity containment (10 g /m3N, m3
N means the volume at 0°C,
1bar)
System Discharge of a mixture of
steam and non-condensablegas from the containment at
FCVS opening pressure,
where aerosols are contained

Preserve type and size
Use of non-radioactive
material
Preserve the typical density

Preserve thermal-hydraulicconditions
Maintain the range of mass
flux

297

Full pressure
Reduce scale in flow area (1/N)
Preserve the mass flux

NEA/CSNI/R(2016)5
At the exit of the diffuser, the gas flow will bedischarged to the pool with the total pressure of
𝑃𝑑 + 1/2𝜌𝑣𝑑2 , which will be static equilibriumwith the pool of water. Pressure drop balancenear venturi nozzle, for the liquid injection
through the holes in the nozzle section, thepressure drop in the diffuser should be smaller
than the hydrostatic head. To maintain the higherhydrostatic head the pool of liquid near theventuri should be maintained as liquid. The
pressure at the nozzle inside the venturi is
calculated as ;

Dd

Ld

Dn

1

1

𝑃𝑛 = 𝑃𝑑 + 2 𝜌𝑣𝑑2 + Δ𝑃𝑑 − 2 𝜌𝑣𝑛2 .(8)The pressure in the pool outside the nozzle iscalculated as ;Fig.2. Schematic of venturi nozzle∆𝑃𝑐 = 8𝑓𝑚̇ 2 /(𝜌𝜋 2 )𝐿𝑖 (𝐷𝑛2 + 𝐷𝑖2 )(𝐷𝑛 + 𝐷𝑖 )/(𝐷𝑛4 𝐷𝑖4 ).

1

𝑃𝑛𝑜 = 𝑃𝑑 + 2 𝜌𝑣𝑑2 + 𝜌𝑝 𝑔𝐿𝑑 .(9)(3)

There is an assumption that the gas steam is inmechanical equilibrium with pool of water just at
the exit of the diffuser. It is reasonable because
the density of gas is much smaller than theliquid. Since the pressure inside the nozzle
should be less than that the pool at the sameelevation, we obtain the equation below;

The pressure drop at the diffuser section can becalculated in a similar manner;∆Pd = 8fṁ 2 /(ρπ2 )Ld (D2n + D2d )(Dn + Dd )/(D4n D4d ).

(4)However, this is a crude approximation. In
reality, due to the injection of liquid into throughthe holes in the nozzle section, there will be aflow of two phase mixture of gas and liquid
droplet. It will increase the pressure drop in the
diffuser. So, the above equation should be
modified as;

1
2

Δ𝑃𝑑 − 𝜌𝑣𝑛2 < 𝜌𝑝 𝑔𝐿𝑑 .(10)
From the expression, 𝑚̇ = 𝜌𝑣𝑛 (𝜋𝐷𝑛2 )/4 ,Φ𝑓/𝐿𝑑 (𝐷𝑛2 + 𝐷𝑑2 )(𝐷𝑛 + 𝐷𝑑 )/𝐷𝑑4 − 1∆𝑃𝑑 = Φ8𝑓𝑚̇ 2 /𝜌𝜋 2 𝐿𝑑 (𝐷𝑛2 + 𝐷𝑑2 )(𝐷𝑛 +𝐷𝑑 )/(𝐷𝑛4 𝐷𝑑4 )< 𝐷𝑛4 (𝜌𝜋 2 𝜌𝑝 𝑔𝐿𝑑 )/(8𝑚̇2 ) (11)(5)Φ𝑓𝐷𝑑 /𝐿𝑑 (𝜁 2 + 1)(𝜁 + 1) − 1 < (𝜌𝑝 𝑔𝐿𝑑 )/(1/2𝜌𝑣𝑛2 ) (12)where Ф is a two-phase multiplier to incorporate
the effect of droplet gas mixture flow.

, where ζ = Dn/Dd.

The pressure in the nozzle is calculated as;

It tells that the kinetic energy at the nozzle
should be much smaller than the hydrostatic headto have a wide range of operation. As the fractionof droplet increases, we have bigger pressure
drop in the diffuser section due to two phasemixture.

1
1
𝑃𝑖 + 2 𝜌𝑣𝑖2 − Δ𝑃𝑐 = 𝑃𝑛 + 2 𝜌𝑣𝑛2 .(6)The pressure in the diffuser is calculated as
1

1

𝑃𝑛 + 2 𝜌𝑣𝑛2 − Δ𝑃𝑑 = 𝑃𝑑 + 2 𝜌𝑣𝑑2 .(7)Also, if we have bigger nozzle velocity it isbetter in terms of droplet break up, but it reduces
298

NEA/CSNI/R(2016)5
where λ is the removal rate coefficient for a fixedand defined volume V of carrier gas.

the range of operation. So, there is a range of
optimal operation, which has to be determined
experimentally. By installing cooling fins at theouter surface of the FCVS tank is recommended
to have subcooled water pool at least at thebottom of the tank to enable water injection into
the nozzle section to enable self-priming mode ofoperation.

DF = 𝑁𝑜 /𝑁1 = exp(𝜆𝑡) (15)Results of previous experimental program [6]related to the pool scrubbing suggested that thepool depth, pool thermal–hydraulic condition andinjector configuration is the major parameters
affecting the decontamination factor. Therefore,
maintaining the same pool height, the same unit
cell injector with multi nozzles, and unit-cellthermal inertia will preserve the pool scrubbing
phenomena for the particles and iodine vapor.

Lehner [6] indicated that the separation
efficiency depends on the inlet gas velocity, andparticle diameter and number of injection
nozzles. Therefore, range of operation should bedetermined in a separate component test.
III.3. Pool Scrubbing Phenomena

Pool scrubbing performance can be preserved by
the unit cell of pool scrubber section to consider
venturi to venturi interaction can be simulated.
Also it is required to preserve pressure drop
between each component while maintaining thesame inlet velocity and K-factors.

A brief discussion on the pool scrubbing
phenomena is provide below to understandfundamental physics which influence the designof pool. Most information is taken from
Reference 7.

III.4. Droplet separator - Cyclone
Vapor scrubbing model on the removal ofvolatile gas like iodine is described in equation(13).

Cyclone separators are gas cleaning devices thatutilize the centrifugal force created by a spinninggas stream to separate particles from a gas. A
standard tangential inlet vertical reverse flow
cyclone separator is shown in Fig. 3.

SF = 1 − exp[−3𝑘2 𝑡𝐵 /𝑅𝐵 ] (13)In equation (13), k2 is overall mass transfer
coefficient for vapor, tB rise time of bubble, RB
radius of bubble.

The gas flow is forced to follow the curved
geometry of the cyclone while the inertia ofparticles in the flow causes them to move toward
the outer wall, where they collide and arecollected. The gas stream may execute several
complete turns as it flows from one end of thedevice to the other.

Particle scrubbing mechanisms within a gas
bubble are categorized in an inertial impaction
for particles bigger than 0.5 μm due tocirculation of gas in a bubble, diffusion due toBrownian motion for small particles 0.1 μm, andsedimentation for big particles larger than 1.0
μm. If Brownian motion can be neglected, thescrubbing factor is expressed as ;Correlations for typical pressure drop across thecyclone and cut off diameter can be found from
Altmeyer et; el.[8] and Dirgo and Leith [9]. The
pressure drop can be expressed as below;

SF = 1 − exp[−653.33𝑡𝐵 𝜌𝑝𝑟 𝑟𝑝2 /(𝜇𝐺 𝑅𝐵 )] (14)ΔP = 0.003ρ(16𝑄𝑜2 )/(𝑎𝑏𝐷𝑒2 ) (16), where ρp particle density, μG viscosity of gas, rp
particle radius in cgs unit.

, where Qo is inlet volumetric flow rate in m3/s.
And cut size diameter is given by;

Decontamination factor is the ratio of initial (No)to final amount (N1) of fission product after the
scrubbing. It can be represented by equation (15),299

NEA/CSNI/R(2016)52𝑑𝑝𝑐= (9𝜇𝐷𝑎𝑏)/[4𝜋𝑁𝑡 𝑄𝑜 (𝜌𝑝 − 𝜌)]less than the gravity. So, the test with single
nozzle has to go with reduced scale cyclone.
However, as the performance of cyclone is nonlinear a separate effect test is required to compare
the performance of full scale cyclone and scaled
cyclone.

(17)

𝑁𝑡 = 𝑉𝑎𝑏 (0.1079 − 0.00077𝑉𝑎𝑏 + 1.924
2
× 10−6 𝑉𝑎𝑏)(18)

If we go with multi nozzle such as three nozzles,
the centrifugal force is greater than the gravity.
Use of one full scaled cyclone is justified as it
will give us a conservative result in terms ofcollection efficiency and cut off diameter.

where Vab = Qo/(ab).

Fig.3. Typical cyclone dimension

If we use a prototype cyclone in a reduced scale,the inlet velocity of the cyclone will decrease if
the scaling ratio M is bigger than the number ofcyclone Nc.

De
b

a

Please note that if we want to the inlet velocity
for the scaled down cyclone, the ratios of (ab)between the prototype and scaled model should
be ;

S
h

D

(𝑎𝑏)𝑒𝑥𝑝𝑒𝑟𝑖𝑚𝑒𝑛𝑡 /(𝑎𝑏)𝑝𝑟𝑜𝑡𝑜𝑡𝑦𝑝𝑒 = 𝑁𝑐 /𝑀 .(19)
If Nt and Vab is maintained, then cutoff diametercan be maintained with the same D between the
prototype and scaled facility. If the cyclone
diameter decreases, it will result in a smaller cut
off diameter, which is not conservative. And
equation (16) indicates that the pressure drop willchange with ratio of (Qo/De)2. So, the pressure
drop will decrease in a scaled facility.

H

B

It is shown that the pressure drop and cut off
diameter is a strong function of volumetric flow
rate. If we reduce the volumetric flow rate thepressure drop will decrease in a quadratic manner
while the cut off diameter is inversely
proportional. If we reduce the volumetric flowrate 1/4, the pressure drop will be 1/16. And thecutoff diameter will increase about 2 times.

III.5. Summary of Scaling Principle
A brief summary of scaling principle is describedin Table IV. Prototypic venturi nozzles, cyclone,metal fiber filter and molecular sieves are used tomaintain the local phenomena. The number andsize of component is adjusted to preserve system
performance at a reduced scale.

As the number of cyclone is expected to besmaller than N, the mass flux to the cyclone at
the reduced scale facility will be lower thanprototype. Then the collection efficiency will belower and the pressure drop will also be lower
than the prototype. In terms of decontamination,
this is conservative.

Separate Test for aerosols and iodine areconsidered. Mixture gas without steam could beconservative in terms of decontamination. Effectof steam could be simulated by a nearly saturated
pool.

However, if we go with single nozzle the inlet
velocity is small such that centrifugal force is300

NEA/CSNI/R(2016)5
It is noted that the test facility is designed to be
operated at full pressure and full temperatureusing the same fluid. So, there will be no scaling
distortion in terms of material property. Also it isnoted that the test facility is designed to have thesame height as the prototype FCVS such that
residence time of gases in the pool is maintained.

By scaling the mass flow rate to have the same
mass flux, flow velocity is preserved. Also, thesame fluid level in the pool would maintain thesame thermal inertia of the fluid. How the local
phenomena is preserved was already discussed.

Table IV. Summary of Scaling Principle for the Test Facility
Parameter
CFVS
Test facility
Decay
heat

SystemScrubberDropletseparator
MFF
Molecular
Sieve

Remarks

Decay heat fromthe FPs

Accumulate in thepool and MFF

Heating of MFF may berequired

Decay heat from the FP
particles has to beevaluated

Diameter

Dp

Dp/M1/2

M is scaling ratio

HeightHp

Hp

Full height

Design Pressure

Pd

Pd

Full pressure

DesignTemperature

Td

Td

Water subcooled andnearly saturated

Flow rate

Qo or 𝑚̇𝑝Qo/M, 𝑚̇𝑝 /M

Inlet dia./ Outlet
dia.

Dip , Dop

Dip/M1/2, Dop/M1/2

No. of Nozzle

Ns

Ns/M

Water Pool
Height

Hpool

Hpool

Volumetric flow

Qo

Qo/M

Inlet velocity

Qo /(AdsxNds)(Qo/M)/Ads

Hmf×Wmf×Dmf ×Nmf

Hmf×Dmf×(WmfxNmf/M)

Hms×Wms×Dms×Nms

Hms× Dms
×(Wms×Nms)/M

Height×Width×
Depth ×EA
Height×Width×
Depth×EA

301

System K factor has to be
maintained
Use of multiples ofprototypic venturi
Use a prototypic cyclone
could be an option, M >Nds is conservative
Adjust number of MFF
and width
Adjust number of MS andwidthNEA/CSNI/R(2016)5
IV. TEST CATEGORY FOR PERFORMANCE
TEST

operation condition from a start-up. The noncondensable gas effect will be investigated.

The following test category need to be performed
to verify the performance of the FCVS. A briefsummary of test and measurement requirement is
described in Table V.

Aerosol removal test: In a high inlet pressure
condition, the collection efficiency of thescrubber and cyclone increases while theefficiency of the metal fiber filter and molecularsieve decreases. In contrast, the collection
efficiency of the scrubber and cyclone decreases
while the efficiency of the metal fiber filter andmolecular sieve increases in the low inlet
pressure condition. Thus, an aerosol removal test
will be performed under the same conditions as athermal hydraulic test. The decontamination
factor (DF) of the FCVS is measured. To
simulate the most severe case, a test will be
conducted at a maximum aerosol concentration
condition (less than 10g/m3). Non-soluble
particles such as SiO2 will be used. Furthermore,because it is hard to remove a 0.5-μm particle,
one aerosol removal test using small particles(near 0. 5μm) will be conducted.

Thermal hydraulic test: An investigation into
the thermal hydraulic phenomena in the FCVS
will be performed. To measure the pressure dropat each component within the operating pressure
range of the FCVS, at least two cases, which are
high/low inlet pressure conditions, will be
performed using 100% saturated steam. In
addition, a test to observe the non-condensablegas effect in the scrubber pool will be performed,
using the steam/nitrogen or steam/air gas.
Dynamic test under high pressure gas
discharge from containment: During a severe
accident, if the pressure inside the containment isreached at the specific opening value, the ventpipe will be opened and high-pressure gas will
release into the FCVS. Through a dynamic test, it
will be confirmed that the components in the
FCVS are functioning normally during thetransient process. It also needs some time toreach normal operation from a start-up. Thus, themeasuring parameters are the change of thescrubber pool height, the pressure at eachcomponent, and the time to reach the normal

Re-suspension of aerosol test: As a change ofthe vessel temperature and pressure, the particles
are re-suspended from the scrubber pool andsurface of the component. In this test, it will be
confirmed that the re-suspension of aerosol isremoved in the FCVS. Most of this test will be
conducted after an aerosol removal test.

302

NEA/CSNI/R(2016)5

Table V. Summary Test and Measurement requirement
Design Requirement and Measurement requirement
Extended operation

Re-suspension/
re-entrainment ofaerosols from the pool
Retention of Iodine Since the pool has to be heated up to the saturated condition, the duration of
the test has to be estimated.
 During the heat up process, the steam has to be discharged either to the ambi
ent or to the suppression pool
 The aerosols at the exit of the FCVS should be collected and should not be discharged to the ambient.
 The duration of the test at saturated pool condition should be estimated to determine the size of the suppression tank. It could be in the range of hours.
 Extended operation for the re-suspension test would require more hours for t
he operation
 The test with iodine would require closed loop operation for the safety reason. However, closed loop system would result in a very big facility like cooli
ng tower Extended operation of aerosol test approaching the maximum particle densit
y in the pool
 Measure DF just before MFF
 Monitor Liquid droplet entrainment and removal by cyclone
 Iodine gas concentration at inlet and exit of CFVS at given pool height, tem
perature, pH, and mixture velocity
 Contaminated pool water should be treated carefully for the safety Iodine concentration at the exit of the CFVS should be monitored, if it is ab
ove the safety level the test has to be terminated The pipe should be coated to minimize iodine deposit on the piping.

iodine is re-volatilized from the scrubber pooland the surface of the component. In this test, it
will be confirmed that the re-volatilization ofelemental/organic iodine is removed in the
FCVS. Most of this test will be performed after
an elemental/organic iodine removal test.

Elemental iodine removal test: As either thewater content of radioactive gas increases or thevelocity of gas increases, the collection
efficiency of the molecular sieve decreases. For
this reason, an iodine removal test will be
conducted in a high inlet pressure with 100%
steam. To simulate the most severe case, a test
will be conducted at the maximum elemental
concentration condition. Although the remove
efficiency of iodine in a scrubber pool isdependent on the PH, this effect will not be
observed in this performance test.

V. PROPOSED DESIGN IMPROVEMENTS
In addition to the need for the increase indecontamination of organic iodine, there areissues related to the performance of the system as
the prolonged operation of FCVS, like more than
72 hours, are required after the advent of the
Fukushima accident, where power recovery to
the site took several days [1]. Some of the issues
are already pointed out in Reference 2. In thissection design improvements are suggested tosolve outstanding issues.

Organic iodine removal test: This test will be
conducted according to the similar procedures tothe elemental iodine removal test.
Re-volatilization of iodine test: As the changeof the vessel temperature and the pressure, the303

NEA/CSNI/R(2016)5
V.1. Collection efficiency of venture-scrubberin the FCVS system. The retention of Iodine in
the liquid pool is highly affected by the pH of theliquid pool. Maintaining high pH helps to retain
the iodine in the pool. However, the gamma
radiation coming from the fission product tends
to reduce the pH of the pool such that the iodine
retention in the liquid pool is deteriorated while
the fission product aerosols are accumulated in
the liquid pool. To solve this problem it issuggested that separate tank be used for theremoval of fission product aerosol and iodine
retention [6, 10]. In the first tank the fission
product aerosols be removed. Then in the secondtank gaseous iodine is removed by poolscrubbing. As most of the fission productaerosols are removed in the first tank, the pH of
the liquid in the second tank would not change.
There will be no deterioration of gaseous iodine
removal. Conceptual picture of proposed design
is shown in Fig. 5.

The collection efficiency of venture scrubber is astrong function of liquid inlet flow into thenozzle. During the prolonged operation of the
FCVS, the fission product aerosols areaccumulated in the lower part of the scrubbing
pool. The decay heat from the fission productaerosols will heat up the liquid such that thelower part of the pool could be heated up until
reaching the saturated condition. If the liquid in
the lower part of the scrubbing pool becomes twophase liquid, the liquid inflow into the venture
nozzle will decrease significantly due to increasein pressure drop through the nozzle holes andincrease in void fraction. This would certainly
result in a significant decrease in collection
efficiency.
By providing a cooling fin on the surface of thetank and locating the lower part of the pool in acavity where liquid can be flooded, the above
issue can be minimized. The cooling provided by
the boiling of the flooded water would maintainthe temperature of the lower part of the liquid
pool below saturation temperature. Then theperformance of venture scrubber will not be
deteriorated during long term operation.
Conceptual picture of the proposed solution isshown in Fig. 4.

V.3. Provision for the bypass scenarios
Though the containment integrity is maintained,
there are event sequences which would result in a
bypass scenario such as steam generator tube
rupture [11]. To mitigate these sequences it issuggested that the steam generator be flooded,
which would result in a significant
decontamination of fission product due to water
scrubbing. However, the release to theenvironment in a long term cannot be avoided,
because the atmospheric dump valve (ADV) of
steam generator has to be opened to prevent
steam generator overfill due to continuous
leakage from the primary side of the reactor. This
direct release to the environment has to be
mitigated. Another usage of FCVS can beconsidered to handle this issue. It is suggested
that the exit of ADV be connected to the FCVS
to prevent direct release of radioactive material.
Then there could be technical challenges. The
design pressure and capacity of FCVS has to be
increased to accommodate release from the steam
generator.

Fig.4. Cooling of scrubbing vessel
V.2. Retention of gaseous iodine in a liquid pool
Retention of Iodine gas in the liquid pool is the
major contributor for the gaseous iodine removal304

NEA/CSNI/R(2016)5
4. P. C. Pwczarski and K. W. Burk, “SPARC90: A Code for Calculating Fission ProductCapturein
Suppression
Pools”,NUREG/CR-5765, Oct. 1991.
5. M. E. Berzal, et al., “State-of-the-art Review
on Fission Products Aerosol Pool Scrubbing
under Severe Accident Conditions”, EUR
Report 16241, 1995.
6. M. Lehner, “Aerosol Separation Efficiency ofa Venturi Scrubber Working in Self-Priming
Mode”, Aerosol Science and Technology,
Vol. 28, No.5, pp. 389-402, 1998.

Fig.5. Proposed two tanks solution
VI. CONCLUSION

7. M. Escudero Berzal, M.J. Marcos Crespo, M.
Swiderska-Kowalczyk,
M.
MartinEspigares, J. Lopez Jimenez ISSN 10185593, State of the art review on fission
products aerosol pool scrubbing under
severe accident conditions, Report EUR
16241, 1995

An integral test facility at reduced scale isdesigned to verify the performance of a newly
developed FCVS. A scaling principle is proposed
where integral system behavior and local
phenomena is preserved. A full pressure,temperature and full height test facility isdeveloped. A test and measurement requirement
is also developed for thermal-hydraulic test,
aerosol removal and iodine gas removal test.
Finally design improvements are proposed tosolve the deficiencies of the current generation of
FCVS.

8. S. Altmeyer, V. Mathieu, S. Jullemier, P.
Contal, N. Midoux, S. Rode, J.-P. Leclerc,“Comparison of different models of cyclone
prediction
performance forvarious
operating conditions using a generalsoftware”, Chemical Engineering andProcessing 43, pp. 511–522, 2004.

ACKNOWLEDGMENTS

This work was supported by the Korea Instituteof Energy Technology Evaluation and Planning(KETEP) grant funded by the Korean
government (Ministry of Trade, Industry, andEnergy) (No. 20131510101700).

9. John Dirgo, David Leith, “Cyclone
Collection Efficiency: Comparison ofExperimental Results with Theoretical
Predictions”,
Aerosol
ScienceandTechnology, 4:4, pp. 401-415, 1985.

REFERENCES

10. Sang-Hyuk Jung, Jei-Won Yeon, Sue
Young Hong, Yong Kang, Kyuseok Song,“The Oxidation Behavior of Iodide Ion
under Gamma Irradiation Conditions”,
Nuclear Science and Engineering, 2015, inpress

1. Report of Japanese Government to the IAEA
Ministerial Conference on Nuclear Safety The Accident at TEPCO’s Fukushima
Nuclear Power Stations -, June 2011,
Nuclear Emergency Response Headquarters,
Government of Japan.

11. Y. Liao, S. Guentay, “Potential steam
generator tube rupture in the presence ofsevere accident thermal challenge and tube
flaws due to foreign object wear” , Nuclear
Engineering and Design, Vol.239, pp.1128–
1135, 2009.

2. Committee on the Safety of Nuclear
Installations, “Status Report on FilteredContainment
Venting”,
NEA/CSNI/R(2014)7, July 2014.
3. Committee on the Safety of Nuclear
Installations, “Status-of-the-art Report on
Nuclear Aerosols”, NEA/CSNI/R(2009)5,
Dec. 2009.
305

NEA/CSNI/R(2016)5
THERMAL HYDRAULIC EFFECT ON IODINE IN THE FILTERED CONTAINMENTVENTING SYSTEM
Young Su Na(1)*, Kwang Soon Ha(1), Sungil Kim(1), Jin Ho Song(1), and Song-Won Cho(2)(1)
Korea Atomic Energy Research Institute, Korea, (2) NSE, Korea*Corresponding author, tel: (+82) 42-868-8522, Email:ysna@kaeri.re.kr

Abstract – The effect of thermal hydraulic conditions on iodine scrubbing in a FCVS (FilteredContainment Venting System) was estimated in this study. The decontamination factor of metal iodide
aerosols especially cesium iodide (CsI), on a scrubbing solution in the FCVS was calculated by the
MELCOR computer code, where SBO in the OPR 1000 was simulated and the FCVS operates when thepressure in the containment building approaches 5 bars. The dimensions of a cylindrical vessel for the
FCVS are 3 m in diameter and 6.5 m height and a vessel includes a scrubbing solution of 21 tons. The
decontamination factor increased rapidly as soon as the FCVS operated at 33 hours after the start of SBO.
When the temperature of a scrubbing solution nearly reached its saturation temperature, thedecontamination factor started to decrease. Steam condensation in a scrubbing solution can occur in theearly FCVS operation, and a scrubbing solution is then constantly evaporates owing to the consecutive
steam supply with high temperature from the containment building. The variation of the decontamination
factor during the accident progress process can be determined by both the steam condensation and poolevaporation in the FCVS.

306

NEA/CSNI/R(2016)5
I. INTRODUCTION

II. METHODSThe FCVS (Filtered Containment Venting
System) has the main objectives of both thedepressurization in the containment building andthe decontamination of fission products
generated under a severe accident. One of thecommercial FCVS consists of a cylindricalpressure vessel including a scrubbing solution
and filters (Ref. 1). A FCVS vessel can beinstalled on the outside of the containmentbuilding, and is connected with the containment
through a venting pipe closed under normal
conditions. When the pressure in the containmentbuilding approaches a setting value, whichshould be below the failure pressure of thecontainment, a venting pipe opens to operate the
FCVS. The amount of steam and gas mixturesgenerated in the containment building under asevere accident can be released into the FCVS
through a venting pipe, where the exit nozzles ofa pipe are submerged into a scrubbing solution in
the FCVS vessel. Non-condensable gases andfine aerosols can pass a scrubbing solution. They
are released into the atmosphere, and they thenenter the filters in the FCVS vessel. The
decontaminated gases are finally discharged fromthe FCVS to the outside environment, and thepressure in the containment building decreases.

The MELCOR computer code simulated that oneof the severe accident scenarios, an SBO (Station
Blackout), occurred in a target nuclear power
plant, an OPR 1000. The reactor type is a PWR(Pressurized Water Reactor) with a thermal
power of 2,815 MWt. It is assumed that the
FCVS (Filtered Containment Venting System)
operates when the pressure in the containmentbuilding reaches 5 bars. This study applied thesame MELCOR input file and the modeling of
the FCVS used in the previous studies (Refs. 2
and 3). In the summary of the FCVS modeling, acylindrical vessel for the FCVS has a 3 m
diameter and 6.5 m height, and it includes a
scrubbing solution of 21 tons, i.e., the pool depth
is 3 m. The inlet of the FCVS vessel is connectedwith the containment dome through a venting
pipe with a 25 cm diameter, where the control
volume hydrodynamics (CVH) and flow path(FL) packages in the MELCOR computer code
were used (Ref. 4). The exit of a venting pipe issubmerged into a scrubbing solution in the FCVS
vessel, and is located 2 m below the surface of apool. A venting pipe includes a multi-holesparger, which is defined in the radionuclide(RN) package (Ref. 4). The outlet of the FCVS
vessel is linked to the outside environment
through an exhaust pipe with a 25 cm diameter,where aerosol filters on this flow path (Refs. 2
and 3) were removed to estimate thedecontamination factor on a scrubbing solution.

Previous studies (Refs. 2 and 3) calculated thethermal hydraulic variations in the FCVS vessel
during a long operating time of over 24 hours
using the MELCOR computer code. It wasobserved that a scrubbing solution in the FCVS
vessel was constantly evaporating owing to hightemperature steam released continuously fromthe containment building. A scrubbing solution
in the FCVS vessel was completely evaporated atabout 31 hours after the FCVS operation. Poolevaporation in the FCVS vessel can negatively
affect the decontamination feature of the FCVS
because it reduces the scrubbing depth for fission
products in an aerosol form. This study carefully
evaluated the decontamination factor of metal
iodide aerosols especially cesium iodide (CsI),
on a scrubbing solution in the FCVS underthermal hydraulic variations during FCVS
operation.

The decontamination factor of a cesium iodide(CsI) aerosol on a scrubbing solution in the
FCVS is defined by the ratio of the input mass of
CsI aerosol to its output mass. The MELCOR
computer code can calculate the mass of the CsI
aerosol in the atmosphere and a pool in the
FCVS vessel and the outside environment. The
input mass of CsI aerosol into a scrubbing
solution in the FCVS is the sum of CsI aerosol
mass in a pool in the FCVS vessel and the output
mass of CsI aerosol from a scrubbing solution,
where the output mass can be estimated by CsIaerosol mass in the FCVS vessel atmosphere plusthat in the outside environment.

307

NEA/CSNI/R(2016)5
III. RESULTS AND DISCUSSION

Figure 2 shows the accumulated mass of CsIaerosol in the FCVS vessel atmosphere. It wasobserved that there are no CsI aerosols in theatmosphere in the FCVS vessel from the start of
the FCVS operation to about 59 hours. It can be
explained by a thorough scrubbing of CsI aerosol
in a pool below the atmosphere in the FCVS
vessel until 59 hours in Fig. 1. The accumulated
mass of CsI aerosol in the FCVS vessel
atmosphere increases from 59 hours to 72 hours
in Fig. 2, where the elevation of a scrubbing
solution approached the venting pipe exit at 59
hours, and a pool was completely evaporated at
64 hours (Refs. 2 and 3). The first peak of theaccumulated mass at 64 hours in Fig. 2 can occur
because the remaining CsI aerosol in a pool canbe released suddenly into the atmosphere in the
FCVS vessel. The accumulated mass of CsIaerosol reaches the maximum value at 72 hours,
and then decreases in Fig. 2. This can be causedby the reduction of CsI aerosol generation in thecontainment building under a severe accident at
72 hours in Fig. 3, which shows the accumulated
mass of CsI aerosol in the atmosphere in thecontainment building dome.

To assess the thermal hydraulic effect on cesium
iodide (CsI) aerosol scrubbing in the FCVS(Filtered Containment Venting System), thevariation of aerosol mass during the accident
progress process was calculated by the
MELCOR computer code.
Figure 1 shows the accumulated mass of CsIaerosol in a scrubbing solution in the FCVS
vessel, where the aerosol mass deposited on theheat structure, i.e. a FCVS vessel wall, was notincluded. As soon as the FCVS operates at 33
hours after the start of SBO, the CsI aerosol mass
increases continuously until 59 hours. The
increment rate of the aerosol mass in Fig. 1
eventually slows. The accumulated mass of the
CsI aerosol in a scrubbing solution is unvaried
from 59 hours to 64 hours in Fig. 1. This
indicates that the mass released from thecontainment building into a scrubbing solution in
the FCVS is identical to the mass discharged
from a pool into the atmosphere in the FCVS
vessel, i.e., there is no scrubbing of CsI aerosol
in a pool in the FCVS vessel. In addition, theelevation of the pool level in the FCVS vessel at
59 hours decreased by that of the venting pipe
exit, which was initially submerged into ascrubbing solution in the FCVS vessel, i.e., thedistance to the scrub aerosol became zero (Refs.
2 and 3). The accumulated mass of CsI aerosol ina scrubbing solution falls sharply at 64 hours in
Fig. 1 because a pool in the FCVS vessel wascompletely evaporated at that time (Refs. 2 and3), i.e., no aerosol particles remain in the
scrubbing solution.

Figure 4 shows the accumulated mass of CsIaerosol in the outside environment, where theaerosol mass deposited on the heat structure, i.e.,
the environment wall, was not included. CsIaerosol mass discharged from the FCVS to theoutside environment through the exhaust pipe
starts to increase at 59 hours. When the pooldepth to scrub aerosol in the FCVS becomes zeroat 59 hours (Refs. 2 and 3), CsI aerosol appears
in the atmosphere above a scrubbing solution in
Fig. 2, as well as in the outside environment in
Fig. 4. The accumulated mass of CsI aerosol
increases from 59 hours to 90 hours in Fig. 4,
where the amount of CsI aerosol generated in thecontainment building under a severe accident
reduces near 90 hours in Fig. 3. The accumulated
mass of CsI aerosol in the outside environment is
kept from 90 hours because the amount of CsIaerosol discharged from the containmentbuilding is constantly decreasing, and it finallybecomes zero, which is shown in Fig. 3.

Fig. 18. Accumulated mass of CsI aerosol in apool in the FCVS vessel.
308

NEA/CSNI/R(2016)5
decontamination factor of CsI aerosol on a
scrubbing solution in the FCVS, the input mass
was divided by the output mass. The input mass
increases continuously from 33 hours to 64
hours, where the FCVS operation started at 33
hours, and a scrubbing solution was completely
evaporated at 64 hours (Refs. 2 and 3). As soon
as the pool level approaches the bottom of the
FCVS vessel at 64 hours, the inlet mass falls
sharply, and its variation then follows theaccumulated mass of CsI aerosol in the outsideenvironment in Fig. 4. The output mass in Fig. 5
also depends on the accumulated mass of CsIaerosol in the outside environment in Fig. 4,
where the amount of the accumulated mass of
CsI aerosol in the atmosphere in the FCVS vessel
in Fig. 2 is relatively too small. The output massin Fig. 5 starts to increase when the pool level isreduced by the level of a venting pipe exit
initially submerged in a scrubbing solution in the
FCVS vessel. The accumulated output mass isunvaried from about 90 hours because theamount of CsI aerosol mass generation in thecontainment building under a severe accident isconstantly decreasing from 72 hours in Fig. 3.

Fig. 19. Accumulated mass of CsI aerosol in the
FCVS vessel atmosphere.

Fig. 20. Accumulated mass of CsI aerosol in thecontainment atmosphere.
Figure 5 shows the input and output of theaccumulated CsI aerosol mass on a scrubbing
solution in the FCVS in the left axis, where theinput indicates the accumulated mass of CsIaerosol released from the containment building
to a scrubbing solution, and the output presents
the accumulated mass of CsI aerosol discharged
from a scrubbing solution in the CFVS. In
addition, the right axis of Fig. 5 shows thedecontamination factor of CsI aerosol on a
scrubbing solution in the FCVS. The output massis the sum of accumulated mass of CsI aerosol in
the FCVS vessel atmosphere in Fig 2, and that in
the outside environment in Fig 4. The input masscan be calculated by the accumulated mass of CsIaerosol in a scrubbing solution in Fig. 1 plus theoutput mass in Fig. 5. To estimate theFig. 21. Accumulated mass of CsI aerosol in theoutside environment.
The decontamination factor of CsI aerosol on a
scrubbing solution in the FCVS presented in theright axis of Fig. 5 starts to dramatically increaseat 33 hours as soon as the FCVS operates. It
peaks at 50 hours, where the pool temperature in
the FCVS vessel reached its saturation
temperature at that time (Refs. 2 and 3), and it
309

NEA/CSNI/R(2016)5
then decreases sharply by unity, i.e., noscrubbing of CsI aerosol in a pool in the FCVS,
at 64 hours, where a scrubbing solution wascompletely evaporated at 64 hours (Refs. 2 and
3). In the early FCVS operation, thecondensation of high-temperature steam released
from the containment building to a roomtemperature scrubbing solution in the FCVS
vessel occurs due to the temperature difference.
Steam condensation can enhance the ability tocapture particles in a scrubbing solution. In
addition, aerosol particles can stay longer in a
scrubbing solution because steam condensation
increased the pool level (Refs. 2 and 3). Opposite
the steam condensation in the early FCVS
operation, the evaporation of a scrubbing
solution took place owing to a consecutive
supply of high-temperature steam from thecontainment building (Refs. 2 and 3). The
evaporation negatively affects the particle
scrubbing in a FCVS pool, where the pool level
was continuously decreased from 36 hours to 64
hours (Refs. 2 and 3). In addition, the scrubbing
depth also decreases during the pool evaporation,i.e. the time for particles staying in a scrubbing
solution becomes short. The decontamination
factor of CsI aerosol on a scrubbing solution canbe finally dependent of the thermal hydraulicconditions in the FCVS vessel.

IV. CONCLUSION
This study estimated the decontamination factorof metal iodide aerosols, especially cesium
iodide (CsI), on a scrubbing solution in the
FCVS (Filtered Containment Venting System).
The MELCOR computer code simulated that an
SBO (Station Blackout) occurred in the OPR
1000, where the FCVS operates when thepressure in the containment building approaches
5 bars. The FCVS consists of a cylindrical vessel
with a 3 m diameter and 6.5 m height, and it
includes a scrubbing solution of 21 tons.
Accumulated mass of CsI aerosol was calculated
in a scrubbing solution and the atmosphere in the
FCVS vessel and the outside environment. The
variations of CsI aerosol mass during a severe
accident progress can be affected by the thermal
hydraulic conditions in the FCVS. The CsI
aerosol was released into a scrubbing solution in
the FCVS as soon as the FCVS operated at 33
hours after the start of an SBO, and it increasedby 59 hours. The scrubbing solution wasconstantly evaporating because of the amount ofhigh-temperature steam released from thecontainment building to a pool in the FCVS
vessel. The pool level in the FCVS vessel
decreased by the elevation of the exit of aventing pipe, which was initially submerged in apool, and a scrubbing solution was thencompletely evaporated at 64 hours.
The decontamination factor of CsI aerosol on a
scrubbing solution in the FCVS vessel can be
determined by the thermal hydraulic conditions.
In
the
early
FCVS
operation,
thedecontamination factor of CsI aerosol rapidly
increased owing to steam condensation in a
scrubbing solution. When the temperature of thescrubbing solution approached its saturation
temperature, the decontamination factor of CsIaerosol started to decrease because of poolevaporation in the FCVS. Particle capture in a
scrubbing solution can be affected by both thesteam condensation in a scrubbing solution in theearly FCVS operation and the pool evaporation
owing to a consecutive supply of hightemperature steam from the containmentbuilding. In this study, it is clearly observed thatthe decontamination factor of CsI aerosol on a
scrubbing solution can be dependent on thethermal-hydraulic conditions in the FCVS.

Fig. 22. Decontamination factor calculated by
input and output of CsI aerosol mass on
scrubbing solution.

310

NEA/CSNI/R(2016)5
It is necessary to add a pool chemistry model toestimate the gas phase iodine which was notconsidered in this study. We are developing ananalysis tool to assess the performance of the
FCVS,
i.e.,
depressurization
anddecontamination, where the MELCOR computercode can calculate the behaviour of the fission
product under a severe accident, as well as
thermal hydraulic conditions. The sensitivity
analysis is required to enhance the reliability ofcalculation results. Further studies on an
operating procedure of the CFVS and theinteraction among safety systems can contributeto improve a strategy for severe accident
management.

Overview and Applicability to CANDU
Plnats,” ML12206A266 (2012).
2. YOUNG SU NA, KWANG SOON HA,
REA-JOON PARK, JONG-HWA PARK,
and SONG-WON CHO, “Thermal hydraulicissues of containment filtered venting
system for a long operating time,” Nuclear
Engineering and Technology, 46, 6, 1-6(2014).
3. YOUNG SU NA, KWANG SOON HA,
RAE-JOON PARK, JONG-HWA PARK,
and SONG-WON CHO, “Design ofcontainment filtered venting system for
depressurization in containment under
severe accidents,” Proc. of ICAPP 2014,
Charlotte, USA (2014).

ACKNOWLEDGMENTS
This work was supported by the National
Research Foundation of Korea (NRF) grant
funded by the Korea government (Ministry of
Science, ICT, and Future Planning) (No. NRF2012M2A8A4025893).

4. R. O. Gauntt et al., “MELCOR Computer
Code Manuals Vol. 1: Primer and Users
Guide Version 1.8.6 September 2005”,
SAND 2005-5713, Sandia National
Laboratories, USA (2005).

REFERENCES
1. AREVA's presentation to US Nuclear
Regulatory
Commission,“FilteredContainment Venting Systems: General311

NEA/CSNI/R(2016)5
INVESTIGATION OF IODINE RETENTION IN A FILTERED CONTAINMENT VENTINGSYSTEM IN THE VEFITA TEST PROGRAM
D. Suckow(1)*, M. Furrer(1), J. Yang(1), B. Jäckel(1), T.Lind(1)(1)
Paul Scherrer Institute, Villigen-PSI, Switzerland.
*Corresponding author, tel: (41)563104145, Fax: (+41)3104481, detlef.suckow@psi.ch
Abstract – An extensive experimental and analytical program has been conducted to study the iodine
chemistry for both molecular iodine and organic iodide (i.e. methyl iodide) in aqueous solutions. Based onthese investigations a novel chemical process was established which uses a phase transfer catalyst(Aliquat336®) as a co-additive to sodium thiosulfate widely used in water pools of filtered containment
venting systems (FCVS). Experiments under a variety of conditions have shown the process effectively
decomposes volatile gaseous organic iodide into non-volatile iodide ions and effectively hinders theirrevolatilization by radiolytic oxidation. First experiments in a dynamic system of a small lab-scale liquid
scrubber column demonstrate promising decontamination factors for methyl iodide. It is foreseen to apply
the process in a commercial FCVS. To assess the retention for gaseous iodine species under anticipatedsevere accident flow conditions a full height, reduced diameter model of a FCVS was erected andmeasurement techniques developed to measure molecular and methyl iodide concentrations at the inlet andoutlet side of the test facility. Planned experiments are presented to determine the decontamination factors
for both iodine species. An outlook to accompanying analytical and experimental work is given to model
the two-phase flow hydrodynamics in water pools of FCVS.

312

NEA/CSNI/R(2016)5
I. INTRODUCTION

the VEFITA (Venting Filter Assessment) test
program which aims to experimentally determine
the enhanced retention of organic iodide species
using this process in a representative, scaled
model of a technical scrubber type FCVS under
prototypical thermal-hydraulic flow conditions.
Tests are planned with elemental iodine or withorganic iodide loaded carrier gas consisting ofpure non-condensable gas, pure steam and of
typical mixtures of non-condensable gas/steam.

Modern filtered containment venting systems(FCVS) can considerably reduce the leakage ofradioactive materials to the environment. In
many countries the implementation of FCVS’s isunder discussion to mitigate fission productrelease not only in the short-term but also in thelong-term view. In severe accidents elemental
and organic iodides are the main gaseous iodine
species in the containment atmosphere. Releaseof the gaseous species in sufficient quantities
from containment filter systems generates a riskfor public health.

II. THE NOVEL PROCESSThe objective of the PSI research anddevelopment program was to establish a process
for a fast and effective decomposition of gaseous
methyl iodide and to reduce iodine volatility to aminimum or, ideally, to hinder it completely.
Research since the 1970s ([3]) has suggested that
sodium thiosulfate Na2S2O3 (THS) dissolved inspray water would increase the decomposition ofmolecular iodine and organic iodide. It was also
concluded that the CH3I decomposition rate is
not high enough to reduce gaseous iodine activity
in the containment using THS in the aqueous
solution in spray droplets. Additionally, the useof a high pH in water pools was suggested toretain the iodide formed as a non-volatile productof gaseous CH3I decomposition or I2 hydrolysis.
To enable a fast and complete CH3I
decomposition in an aqueous solution a chemical
reagent (additive) should be sufficiently
thermally and radiolyticaly stable, must enhanceand not suppress the thermal and radiolytic
decomposition of CH3I, be usable in a wide pH
range, in the presence of impurities in the
aqueous solution and at anticipated boundary
conditions of a scrubber type FCVS. Ideally theadditive should help to bind iodide, thus
suppressing its subsequent thermal and radiolyticoxidation to volatile elemental Iodine.

Experimental programmes have confirmed theexistence of gaseous organic iodide in the longterm in even higher concentrations than forgaseous molecular iodine (I2), [1]. Methyl iodide(CH3I) is probably the most dominant volatileorganic iodide compound in relevant accident
conditions. Its reaction with surfaces and theremoval by i.e. containment filters and scrubbers
is less efficient in comparison to moleculariodine.
In the recent years an experimental andanalytical research program has been conductedat the Paul Scherrer Institute (PSI) to develop a
process leading to a fast, comprehensive andreliable retention of volatile iodine species inaqueous solutions, i.e. in the water pool of ascrubber type FCVS.
A summary of the PSI work on the iodine
chemistry since 2002 in the presence of
impurities and additives in aqueous solutions
under the influence of different conditions
including radiation is presented in a separate
paper in this publication [2].
The result of this extensive work is a novel
established process to fast decompose volatile
iodine species into non-volatile ions in aqueous
solution. The process has been tested in small
lab-scale under different boundary conditions
including the influence of radiation, pH,temperature and impurities.

In the development program PSI found asecond additive to act as a co-additive to thewidely used THS to increase sufficiently high the
CH3I decomposition rate, a quaternary long chain
ammonium salt, Aliquat336®. It contains a
mixture of C8 (octyl) and C10 (decyl) chainswith C8 predominating. Aliquat336® (ALI) is acommercial reagent [4], which is successfully
applied in nuclear technological processes, such

In this paper the novel process and the main
experimental findings are presented together with313

NEA/CSNI/R(2016)5
as spent fuel reprocessing and other metallurgicalprocesses for metal extraction. The high stability
to ionizing radiation was demonstrated by the
spent fuel reprocessing industry.

experiments the radiotracer technique wasapplied using the iodine isotope 131I to label
methyl iodide (CH3131I) and to track CH3I
decomposition and its products. An overall massbalance was performed for each test bydetermining the deposited activity in the different
components of the test setup which could usually
be closed by an uncertainty of typically ±15%.
This technique provided sufficient sensitivity todetect near complete decomposition. Tests were
performed in small lab scale reaction vessel
typically of a few cm3 in volume. The pH in the
different test solutions were usually bufferedusing boric acid and sodium borate mixtures, orby adding sodium hydroxide to boric acidsolutions. The main findings are presented in thefollowing sections.

The removal process of volatile gaseousiodine species from gas bubbles in an aqueous
solution takes place by the nucleophilic reductionof elemental iodine and organic iodides with
THS, Eq.(1) and Eq.(2).
I2 + 2S2O32-→ 2I- + S4O62-(1)CH3I + S2O32- → I- + CH3S2O3-(2)While the first reaction is fast and effective,
the second reaction needs to be accelerated.
Aliquat336® has a dual function.

Maximum decomposition rates for CH3I
were obtained for specific THS and Aliquat336®
concentrations in aqueous solutions, subject tothe patent. Figure 1 shows the CH3I
decomposition rate at 22°C and pH 9 versus
Aliquat336® aqueous concentrations for different
THS concentrations. Aliquat336® becomes
effective once the concentration is higher thanabout 1.10-5 mol/dm3. The data confirm theeffectiveness of Aliquat336® even when the CH3I
initial concentration is increased 15 times and the
optimized THS concentration remain unchanged.
The rates for Aliquat336®-free reaction solutions
are shown in the log-scale diagram at a fictive
concentrationof
10-7 mol/dm3.
Low
concentrations of Aliquat336® (< 2.10-5 mol/dm3)
do have only a small beneficial effect. The effectis pronounced for ratios of THS:Aliquat336® ofabout 10:1.

Aliquat336® functions as a phase transfer
catalyst by clustering both reactants, i.e. THS
ions and the CH3I molecules get together toaccelerate
theirinteraction.
Thus
thedecomposition of CH3I is enhanced.
In addition Aliquat336® contains an
inorganic anion (chloride) which can act as anion exchange for iodide ions and thus preventing
their radiolytic oxidation into I2. In this way the
CH3I decomposition product iodide can bebound. The ion exchange takes place as aconcurrent reaction to the nucleophilic reduction
by a covalent reaction with an exchange of iodide
ion for the chloride ion to form a tertiary amine
Eq.(3) and quaternary long chain ammonium saltEq.(4).
R2CH3N + I- → R2CH3NI-(3)andR3CH3N+Cl- + I- → (R3CH3NI) I-(4)This novel process developed at PSI was
successfully patented ([5]) and is foreseen to be
applied in a commercial FCVS ([6], [7]).
Many experiments were conducted toachieve an optimum mixture of THS andAliquat336® under a wide range of conditions,such as temperature, pH, interference from coexisting ions, thermal and radiation ageing. In the314

NEA/CSNI/R(2016)5

-2

.

-3

.

-3

.

-3

0

10

[THS] 4.0 10 mol dm
.
-3
.
-3
[THS] 1.0 10 mol dm
.
-4
.
-3
[THS] 2.5 10 mol dm
.
-3
.
-3
pH9, [CH3I] 4.0 10 mol dm

-2

10

-1

10

-2

10
-1

Decomposition rate (s )-1

Decomposition rate (s ).

[THS] 6.2 10 mol dm
.
-2
.
-3
[THS] 6.2 10 mol dm 15x[CH3I]

-1

10

-3

10

-4

10

-5

10

-3

10

-4

10

-5

10

.

-7

10
-6

10

-7

-6

10

-5

10

-4

10

-3

10

-2

10
.

10

-2

.

-3

[THS]: 6.2 10 mol dm
.
-4
.
-3
[ALI]: 4.0 10 mol dm
.
-2
.
-3
[THS]: 6.2 10 mol dm only
no additive
-5
-4
.
-3
pH 9, [CH3I] 1.0 10 - 5 10 mol dm

-6

10

-8

10

20

-3

Aliquat336 Concentration (mol dm )

30

40

50

60

70

80

90

100

o

Temperature ( C)

Fig. 1. CH3I decomposition rates vs. Aliquat336 and THS
concentrations.
Fig. 3. CH3I decomposition rates vs. temperature w/wo THS®
and w/wo Aliquat336 at pH 9.

Figure 2 presents the influence of pH of thesolution on the decomposition rate of CH3I in thepresence or absence of Aliquat336® at the
optimized THS concentration of
6.2 10-2
3mol/dm at a temperature of 22°C. Different
CH3I concentrations were tested. The
significantly enhanced CH3I decomposition ratein the presence of Aliquat336® is apparent and isnearly 2 orders of magnitudes higher than with
THS only. The rate is independent of the pH in
the range of 4 to 14 and is even effective atstrongacidicconditions.
The
slowerdecomposition rate for pH < 4 may be due to®
Aliquat336
decomposition or net yet known
effects in more acidic solutions.

Decomposition rates for CH3I are
temperature dependent. The decomposition rates
at temperatures between 22°C and 90°C obtainedfor pH 9 buffered solutions are shown in Figure 3
for mixtures of Aliquat336® and THS, for THS
only and are compared to additive-free CH3I
solutions. The additives clearly bring a rateincrease of several orders of magnitude, although
the decomposition rates had increased more
slowly with temperature than without additives.
The addition of Aliquat336® to the THS solution
increased the rate for low temperatures by afactor of about 10 but the rate increase at higher
temperatures is smaller, apart from the higherhydrolysis rates.
.

-1

-2

.

-3

.

-4

.

-3

[THS]: 6.2 10 mol dm [ALI]: 4 10 mol dm
.
-3
.
-3
.
-5
.
-3
[THS]: 1.7 10 mol dm [ALI]: 5 10 mol dm
.
-3
.
-3
typical NPP Solution: [THS]: 1.7 10 mol dm

-1

10

10

-2

10

-2

10
-1

Decomposition rate (s )-1

Decomposition rate (s )

-3

10

-4

10

-5

.

10

-2

.

-3

.

-4

.

[THS]: 6.2 10 mol dm , [ALI]: 4.0 10 mol dm
.
-2
.
-3
.
-3
[THS]: 6.2 10 mol dm , [ALI]: 0.0 mol dmoT: 22 C
-7
-4.
-3
[CH3I] 1.10 - 2.10 mol dm

-6

10

-7

-3

-3

10

-4

10

10

-8

10

-5

0

2

4

6

8

10

12

10

14

-8

10

pH

-7

10

-6

10

-5

10

-4

-3

10

.

10

-2

10

-3

CH3I Concentration (mol dm )Fig. 2. CH3I decomposition rates vs. pH, with THS, w/wo®
Aliquat336 .

Fig. 4. CH3I decomposition rates vs. CH3I concentration at®
pH 9 and room temperature, w/wo Aliquat336 .

315

NEA/CSNI/R(2016)5
The measured decompositions rates at 22°C
and pH 9 as a function of the initial CH3I
concentration derived for the optimum
concentrations of THS and Aliquat336® in thesolution is presented in Figure 4. For comparison
the decomposition rate obtained for a typicalscrubber solution in a FCVS of a nuclear power
plant using THS only with a concentration of
1.7 10-3 mol/dm3 is shown. By adding
Aliquat336® at an aqueous concentration of 5.105mol/dm3 an increase of the rate by one order ofmagnitude can already be achieved. Using afurther increase of the concentrations of THS andAliquat336® to the optimal values of 6.2 102
mol/dm3 and 4.10-4 mol/dm3, respectively, an
overall enhancement in the decomposition ratesof 2 to 3 orders of magnitudes compared to atypical scrubber solution can be achieved.

50

no Aliquat336
with Aliquat336opH5, 22 C-5-3[I2]: 4 10 mol.dm-4-3[ALI]: 4 10 mol.dm

I2 Release [%]

40

30

20

10

0
0

200

400

600

800

1000 1200 1400 1600 1800 2000

Time [s]®

Fig. 5. Influence of Aliquat336 on I2 fraction released vs.
sparging time at 22°C and pH 5.

III. BASIC EXPERIMENT
To obtain an estimate of the potential
decontamination factor (DF) in a scrubber typefilter system containing the optimized THS andAliquat336® aqueous solution mixture several
small-scale laboratory tests in a column typescrubber were performed. The DF is defined as
the ratio of the mass of a species at the inlet of asystem to the one at the outlet. The test setup
consists of two different vessels. Table I provides
the main dimensions and experimental
conditions.

Separate experiments were performed toconfirm the effectiveness of Aliquat336® to bind
iodide ions as a result of the exchange of I- forCl- in the quaternary ammonium compound
including irradiation conditions. The fixation
ability of Aliquat336® for I2 is demonstrated in
Figure 5, which shows the I2 release fraction
versus time from a solution where only I2 or I2
and I- ions are present. The tests were performed
at 22°C and pH 5 under un-irradiated conditions.
The solution was sparged with Argon gas for acertain time. In the presence of Aliquat336® therelease of I2 was greatly reduced. This gives aninsight into the effect of Aliquat336® inhindering the radiolytic oxidation of iodide ionsto form volatile I2.

Total Height
Sparger Submergence
Liquid volume
Pressure, inlet
Pressure, test section
Gas temperature, inlet
Water temperature
Argon gas flow ratemmlbar(abs)bar(abs)°C°C
ln/min2.0
0.035 - 1.40
0.2 - 0.8
1.0 - 1.2
ambient
22
22 - 80
0.052 - 0.103

Table I. Column scrubber dimensions and boundary
conditions.

The first vessel (donor vessel) contained anaqueous solution of liquid CH3I labelled with131
I, without any additives and with a gas phase
above the liquid. The second vessel (scrubbercolumn) was filled with an aqueous solution of
the decomposing substances for CH3I retention.
Both vessels were connected by a flexible small
PTFE (Polytetrafluoroethylene) tube as shown in
Figure 6a. As scrubber a 2 m long glass tube witha heating jacket was used to perform tests at
different liquid temperatures. At the bottom of316

NEA/CSNI/R(2016)5
each vessel a gas discharge unit (frit) was
installed producing small gas bubbles in the size
range of 1-2 mm. A gas flow of Argon at room
temperature, controlled by an electronic mass
flow controller, was used to sparge the CH3I
from the donor vessel into the scrubber column.
The remaining CH3I released into the gas stream
leaving the scrubber column was trapped in
activated charcoal (ACC) filter impregnated bytri-ethylene-diamine (TEDA) placed at the outlet.
Dedicated
experiments
werepreviously
conducted to determine the liquid CH3I transfer
rate from the aqueous solution in the donor
vessel into gas bubbles. Before and after the tests
samples were taken from the solutions in thedonor and scrubber vessel using a specifichandling procedure. The samples and the ACCfilters were measured with a NaI γ-ray detector
and analyzed to achieve a complete massbalance.

Fig. 6. View of bubbles in column scrubber at different
conditions.

Figure 7 shows the measured decontamination
factors DF* normalized by the height of thescrubber solution as a function of the scrubber
liquid temperature. A strong influence of thetemperature on the decomposition of methyl
iodide was found. The decontamination factor atroom temperature varied from 3 to 19 anddepended on the water height in the scrubber andwhether THS was used alone or a mixture of
THS and Aliquat336®. The solution with
Aliquat336®
atthederivedoptimum
concentrations showed an increase in the DF* bya factor of about two at the different
temperatures. Without the additives (pure water)
no decontamination took place. The DF* wasabout 2600 when using the additive mixture at
75 °C. This value is about 150 times higher thanat room temperature.

Tests were performed at temperatures of theaqueous solution in the scrubber column from
22°C to 80°C. The required pH in the scrubber
solution was buffered and obtained using boric
acid (H3BO3) and sodium hydroxide (NaOH)
mixtures. Different solution heights wereinvolved in the scrubber, which determine
together with the bubble diameters the bubble
residence time. The residence time was estimatedto be between 5 s up to 35 s depending on theconditions used. A bubble diameter reduction
from 1-2 mm to less than 1 mm was observed
when either pure water (Figure 6a) or a mixtureof THS and Aliquat336® (Figure 6b) was used,
respectively. The reduction in bubble size
diameter is due to the lower surface tension in
the presence of Aliquat336®.

DF* (per meter column height) (1/m)

10000

1000

100

10

no [THS],no [ALI].
-2.
-3[THS]: 6.2 10 mol dm no [ALI]
.
-2
.
-3
.
-4
.
-3
[THS]: 6.2 10 mol dm [ALI]: 4.0 10 mol dm

1

.

-5

-5

.

[CH3I] 2.2 10 - 3.510 mol dm

0.1
20

30

40

50

60

70

-3

80

90

100

o

Temperature [ C]

Fig. 7. Normalized decontamination factor DF* vs.
®
temperature in a column scrubber, w/wo Aliquat336 , w/wo
THS.

317

NEA/CSNI/R(2016)5
These experiments provided first results on the
sub-mergence and temperature dependent
decontamination factor of a dynamic scrubber
system, where a mass transfer of methyl iodide
from the gas phase through rising bubbles in aliquid column into the liquid phase took place.
The bubble size directly influences the totalinterfacial area of mass transfer of the gas species
for scrubbing. Another important aspect was thequite low terminal rise velocity of the very small
bubbles causing an enhanced transfer time for thegas species in the liquid and thus promoting highdecontamination factor.
IV. APPLICATION IN FCVS
In the dynamic system of a technical scrubber
type FCVS the flow conditions in the aqueous
solution are much more complex than in theexperiments performed at lab-scale. The species
transfer rate from the gaseous phase (bubbles)
into the liquid phase depends on the heat andmass transfer properties, which are dominated by
the bubble hydrodynamics. The main controllingparameters are the gas bubble size (interfacialarea), the bubble residence time and thethermodynamic properties of the gas mixture andsteam content, which is especially important if
steam condensation takes place. In addition theliquid temperature and surfactants in the liquid,
affecting the gas and liquid side of the bubble
surface for heat and mass transfer rates.

Fig. 8. Schematic view of the industrial FCVS system of
IMI- Nuclear (CCI AG), Switzerland.

The filter ([8], [9]) consists of a stainlesssteel vessel comprised of a wet scrubber section
with a gas injection assembly, an inner riser
column with specific internal structures(structured mixing elements) to disintegrate gas
bubbles into smaller ones to enhance theinterphase mass transfer. The lower wet scrubber
section is followed by a gas space and an end
separator section with several stages to remove
droplets and fine moisture from the cleaned gas
stream leaving the filter to the stack. The filter
has a height of about 9 m, a diameter of about
3.5 m and a liquid volume typically of about
30 m3.

At PSI the analytical and experimental program
VEFITA is conducted to study the iodine
retention behavior for both elemental and organic
iodide species in a large scale technical wet filter
system. Tests will investigate the anticipatedenhancement in iodine retention using the novel
process under prototypical thermal-hydraulicflow conditions of a FCVS, but in the absence ofradiation.

In the wet scrubber section the gas sparger
assembly is composed of a large number of
special designed injection nozzles, where the size
and number is depending on the required flowrate to depressurize the containment. The
scrubbing water is doped normally with THS andeither with sodium hydroxide (NaOH) or withsodium carbonate (Na2CO3) at certain aqueousconcentrations for pH control.

The test facilities used in this investigation are
based on a commercial FCVS developed by the
IMI-Nuclear
CCI
AG,
Switzerland,
schematically shown in Figure 8.

1:1 full height FCVS test facility VEFITA
The retention of iodine and organic iodide
will be performed in the new established318

NEA/CSNI/R(2016)5
VEFITA test facility, which is a scaled-down,
full height, reduced diameter mock-up of the CCIFCVS, Figure 9.

To preserve the thermal-hydraulic behaviour
and phenomena of the industrial FCVS, VEFITA
has all the internal structures, including one
prototypical injection nozzle, the water pool with
the inner riser column and mixing elements, thegas space and the three stages of the end
separator. These three stages are arranged inseries after each other to allow access formeasurement systems and probe sampling. The
full height of the test facility enables similar gas
residence times and velocities as in the real scale
FCVS. Table II provides the main geometricaldimensions together with the achievable
experimental boundary conditions. The water
submergence of the nozzle can be varied up to
3.5 m. The maximum operation pressure in frontof the nozzle is 9.5 bar and the temperature is
200°C. The test section can be operated fromambient pressure up to 4.5 bar by a pressure
control system in order to simulate pressure
losses due to long pipe works downstream of thegas outlet of a real FCVS. By pressurizing thegas space tests with elevated water saturation
temperatures up to 140°C can be performed,
depending on the gas mixture used.

Fig. 9. The 1:1 height, reduced diameterscale FCVS facility VEFITA to determine
aerosol, I2 and CH3I retention; a) lower part wet
scrubber, b) upper part droplet and end separator
sections.

The test facility schematically shown in
Figure 10 is intensively instrumented to measure
the fluid, gas and wall temperatures, the absolute,gauge and differential pressures along the test
section and the flow rates at the inlet and outletof the test section. The collapsed water level andthe water level swell are monitored online bydifferential pressure probes and by a special
guided radar probe, respectively. The pH of thescrubber solution is determined on-line during
the test by a pH electrode device. The inlet andoutlet piping as well as the whole test section canbe trace heated to compensate for heat losses, toprevent steam condensation and to reduce iodine
losses at cold walls.

Table II VEFITA facility dimensions and operating
conditions.
Total Heightm

12

Inner Diameterm

0.58 / 0.40

Cross sectional flow area, wet
scrubber section

m2

0.266

Number of injection nozzles-

1

Sparger (nozzle) submergencem

2.0 - 3.5

End separator section-

Yes

Pressure, inlet

bar(abs)

1.3 - 9.5

Pressure, test section

bar(abs)

1.0 - 4.5

Gas temperature, inlet°C

35 - 200

Water temperature°C

35 - 140

Nitrogen flow rate, max.

kg/h

1200

Steam flow rate, max.

kg/h

250

Steam mass fraction%

0 - 100

Gaseous molecular iodine is generated bydissolving solid iodine crystal in a certain
amount of ethanol. The liquid solution is fed byan electronic controlled dosing pump to a twofluid spray nozzle attached to a heatedevaporation chamber where the solution is
atomized by a hot nitrogen flow into extremely
fine droplets. The liquid ethanol evaporates andgaseous molecular iodine is formed and injected319

NEA/CSNI/R(2016)5
Off-Gas VA2810
VA2807

TW

HE6511, H55 6522
(220 W)

HE2803TW TW
2806 2805

HE2803

DN65,PN16

HE6511

into the main flow in the inlet piping of the test
facility. The spray system can be operated atpressures up to 10 bar and temperatures of
200°C. To generate gaseous organic iodide thesame two-fluid spraying system is used but pureliquid CH3I, sucked from a small container, isdirectly injected by the dosing pump into theevaporation chamber and transported by a further
added flow of hot nitrogen to the inlet piping.

VA2806 VR2803

PA
6502

VA2804(HE6510.2), H51 TW(529 W)
6521

L
PD
6508
H

org. Iodide (CH3I):
PID-2
MS-2
Sample-3
Sample-4

TW
6520
TF
6507

SS-6M0-1-4W

L
PD
6506
H

HE6510.1, H51 TW(529 W)
6519

SS-6M0-1-4W

TW
6518
PG
6503

HE6509, H48(1165 W)

VM6503
TW
6517

TW
6516
TW
6515

FV
6501
VM6502V

Gaseous iodine and organic iodide
concentrationsofabout
5·10-5 mol/dm3,
depending on the experimental conditions will be
used. Such high concentrations are required at
the test section inlet in order to detect expectedhigh DF’s using the measurement techniquespresented in the next section.

TF
6508

SS-6M0-1-4W

HE6510.1

L
PD
6505
H

L
PD
6504
H

SS-6M0-1-4W

HE6509

L
PD
6502
H

HE6510.2

VR2801.B

Iodine (I2):
ISE-2
Sample-3
Sample-4

PG
6504

TF
6506

Droplet:PDPA

VM6505
PG
6505

TF
6509

L
PD
6507
H

DN400
PN6
DN400
PN6,BF

TF
6505

L
PD
6509
LF6503 H

Droplet:
PDPA

L
PD
6503
H
TW
6514

HE6508, H49(3450 W, 400 V)

VM6504

TW
6513

HE6508TW6512

Due to the experimental environment thepreviously applied radiotracer technique using
131
I labeled iodine or methyl iodide could not be
used in the VEFITA facility. Different
measurement techniques based on non-active
samples will be used for I2 and CH3I
concentration
measurement
andDF
determination. Gas samples are taken
simultaneously from the inlet and outlet piping of
the test section.

Not-AusPG PG PA
6501 6502 6501

VS6501Kabel

Ans chlussStec ker

Ans chlussStec ker

filled with H2O

VM6511

max swell level
TF
6504 LF

TW
6509

HE6505

nominal level

HE7001, H21(200 W)
HE6504, H45(2099 W)

TF
7001

TW
6506
HE6504

TW
7002

TW
6506.2HE7001

TF
6510

TF
6503

VM6506

HE6503, H44(2099 W)

TW
6505

HE6503filled with H2O

VM6510TW
6504

HE6502

Gas Mixture
Inlet

HE6502, H43(2099 W)

TW
6503

HE2706

VM6602

TF
6502

TW
6503.2

TF
6601

TW
6502

CO PG
6601 6601

TW
2714

H PD L
6501 LF6501
H PD L
6511

TW
7003

HE7002, H22(375 W)TW
7004

pH-Measurement

TF
6501

TF TF
7707 7708

PG
7001

TF
7002

PA
7001

HE7002H0 = 0 (aN)

VM7711

VM6507org. Iodide (CH3I):
PID-1
MS-1
Sample-1
Sample-2

VM6601
Air

VP6601

TW
6501

TW
2713

PG
6602

HE6501

HE6501, H42(2099 W)

Iodine (I2):
ISE-1
Sample-1
Sample-2

HE6507, H47(1890 W)

TW
6507

TW
7001

VC2701

TW
6508

TW
6523

Radar
Probe
TW
6511

HE6505, H50(1200 W)

TF
6512

6502

HE6507

TW
6510

HE6506

HE6506, H46(1890 W)

HE7004

TF
7003

TW
7007
TW
7008

HE7004, H31(110 W)

VA7001Purge GasV

N2/Air

FV
1803

VC1803

VM1806

VA1810

PG
1816

TF PA
1811 1803

Resuspension Gas
VA2206
TW
2210

TW
2211

TF
2210

TW
7006
TW
7005

HE7003, H24(748 W)

VA2205 VC7001 HE7003
HE2204, H11(150 W)

Fig. 10. Process and instrumentation scheme of
VEFITA.

320

NEA/CSNI/R(2016)5
Molecular iodine will be sampled to liquid
gas scrubber columns with a coolant jacket andanalyzed by iodide ion selective electrodes (ISE)in continues and batch sampling mode. Several
gas scrubber columns will be operated in parallel
and in series during the test providing multiple
data points.

saturation (boiling) in case of pure steam tests.
Different sizes of injection nozzles were tested.
In a series of tests special attention was focusedon the two-phase mixture level-swell of thescrubber solution, which is of particular interest
to the design and operation of the filter system.
The gas composition of the venting gas (massfraction of NC gas and steam), the water pooltemperature and the pressure in the gas space ofthe filter (ambient up to 4 bar) were varied. The
level-swell gave insights into the gas hold-up andthe residence time of the gas bubbles, [10].

Methyl iodide is characterized on-line withhighly sensitive UV photo ionization detectors(PID) installed in special gas sampling trains.
The sample gas is mixed with a flow of hot
nitrogen directed to a condenser where the gas iscooled down to the maximum operating
temperature of 50°C of the PID sensor. Thereby
any steam present in the sample is removed andthe added nitrogen is used to keep the partialpressure of CH3I in the sample gas flow low
enough so that no condensation of CH3I can takeplace. Parallel to the PID measurements gas
samples are taken to a second type of samplingwhere the gas sample is mixed with nitrogen anddirected to a condenser where any steam present
is condensed and collected in a liquid trap. The
residual gas with the contained CH3I is bubbled
through two liquid gas scrubbers arranged inseries and operated with toluene. The condensed
steam sample and the two samples from thetoluene scrubbers are analyzed off-line using agaschromatography/mass
spectrometer(GC/MS). This method can detect CH3I
concentrations down to ppb level. In addition to
the PID and GC/MS method a continuous gas
sample flow is directed from the inlet and outletof the test section to a mass spectrometer (MS).

In the experimental iodine test program tests
are foreseen either with elemental iodine or withorganic iodide loaded carrier gas consisting ofpure NC gas (Nitrogen), pure steam and of
mixtures of NC gas/steam at different steam
mass fractions. To investigate the effect of thenovel process tests with and without Aliquat336®
will be done with aqueous concentrations of THS
and Aliquat336® defined by the novel process. In
all tests an initial pH of 11 (given at 25°C) will
be used by adding NaOH to the scrubber solutionof de-ionized water.
Based on analyzed containment venting
scenarios for the CCI FCVS some representativeand some for the filter performance challenging
test conditions have been selected for theexperiments in VEFITA. For a given injection
nozzle size and nominal mass flow rate the gas
mixture inlet pressure and temperature will be
about 3.5 - 4 bar and 165°C, respectively. The
gas space above the wet scrubber section is atambient pressure and the water temperature will
range from ambient to 90°C and to saturation
temperature (boiling) is case of pure steam
injection. A nominal water level of about 3.1 m
above the injection nozzle will be used. All tests
are planned at stationary conditions and withiodine injection phases lasting 4-5 hours tocollect an appropriate number of I2 and CH3I
samples at the inlet and especially at the outlet,
respectively. Thermal-hydraulic (T - H) pretests
without iodine feeding at the selected test
conditions are used for detailed test planning.
The following iodine retention tests are planned,
Table III.

Tests in VEFITAIn VEFITA a series of thermal-hydraulicexperiments have been performed to characterize
the pressure and temperature behavior and thewater inventory behavior due to heat-up, steam
condensation and evaporation as a function of thegas composition of the venting gas with respect
to the mass fractions of non-condensable (NC)gas and steam. The pressures in the inlet piping
upstream of the injection nozzle were between
1.3 and 9.5 bar at mass flow rates up to
1200 kg/h and temperatures of the gas between
35°C and 165°C, respectively. The water pooltemperatures were between ambient up to321

NEA/CSNI/R(2016)5

test type Twater Hwater gas comp. Tgas flow rate
low
NC
T-H
nominalhigh
NC+steam

CH3I

close
to Tsat

CH3I
mult. highcyclesclose
I2
to TsatNC
NC+steamnominalnominallownominalCH3I

nominalIn the two venting cycles high and low mass flow
rates corresponding to high and low inlet
pressures will be used.

ALI
w/o
w
w/o
w
w/o
w

NC+steam

Cy1: highCy2: loww

NC+steamnominalw- I2 retention at a water temperature close tosaturation of approximately 90°C using an inlet
gas mixture of NC gas and steam at the nominal
flow rate and temperature. The scrubber water
will be doped with both additives.
In addition tests are under discussion to study theeffect of a reduced water level above theinjection nozzle on the DF and the retention
behavior if pure steam is injected where thewater pool is at boiling conditions.

Table III. Main tests planned in VEFITA.
- CH3I retention at nominal flow rate andlowest feasible water temperature of about 35°C
using only NC gas at nominal gas temperature of
165°C. One test run will be performed without
and a second test with the co-additive
Aliquat336® (ALI). These conditions will
provide a relative lower limit of the DF fororganic iodide at nominal flow rate and at themost challenging conditions when the water poolis cold, a condition present during start of thefirst venting cycle in a FCVS, and no credit canbe given for the enhancing effect of steam
condensation to the mass transfer of the gaseous
phase to the liquid phase. The level swell will
have a relative maximum.

Pre-test facility mini-VEFITA
In a small pre-test facility, mini-VEFITA(Figure 11), with a further reduced height andvolume and without the upper end separator
sections of VEFITA, dedicated experiments havebeen started to evaluate the measurement
techniques foreseen for the measurement of I2
and CH3I, which will be implemented in
VEFITA.
The geometrical dimensions and boundary
conditions are given in Table IV. The test section
is not a scaled-down representation of the real
FCVS but mainly used for demonstration
purpose and instrument testing.

- CH3I retention at a water temperature close
to saturation of approximately 90°C using a gas
mixture of NC gas and steam. Tests with andwithout Aliquat336® will be carried out. These
tests will give the DF when steam condensation
takes place in the presence of NC gases withcomplex two-phase hydrodynamic behavior. The
high pool temperature will cause steaming andinfluence the water inventory by water
evaporation into the NC gas leaving the test
section.

Total Height
Inner Diameter
Cross sectional flow
area
Number of injection
nozzles
Sparger(nozzle)submergence
Pressure, inlet
End separator section
Pressure, test section
Gas temperature, inlet
Water temperature
Nitrogen flow rate
Steam flow rate
Steam mass fraction- In severe accident management procedures
multiple venting cycles are also foreseen, with anintermediate stand-by without flow for several
hours. A CH3I retention test with two venting
cycles with a stand-by time of 10 h will study the
behaviour of the chemical additives (THS andAliquat336®) in the scrubber water during restart.
The inlet gas mixture will be composed of NC
gas and steam with a high steam mass fraction.

m
m

1.5
0.2

m2

0.03141m

0.8 - 1.2

bar(abs)

1.0 - 2.0
No
atmosphere35 - 100
20 - 80°C
100 - 300
0-5
0 - 100bar(abs)°C°C
ln/minkg/h%Table IV. Mini-VEFITA pre-test facility dimensions
and operating conditions.

322

NEA/CSNI/R(2016)5
The test section consists of a borosilicate
glass tube of 0.2 m in diameter, a height of 1.5 m
and an inner acrylic glass riser holding only one
structured mixing element layer in contrast to thetaller three layer arrangement used in the fullheight VEFITA test facility. At the bottom aninjection nozzle assembly similar to the ones
used in the VEFITA (FCVS) is installed. The
water level above the injection nozzle can bevaried from 0.8 m to about 1.2 m. The gas outlet
is at the top. Nitrogen as well as steam andmixtures of both can be used at limited flow
rates.

zone formed by the annulus between the inner
riser and the outer glass tube.
The test section will be used for short lasting
tests of about 30 minutes to perform parametricstudies on a relative basis not providing absolutevalues of the DF with respect to the full heightmodel. Different effects will be studied such as
the effect of the water, the relative change in
retention of I2 and CH3I with and without
Aliquat336® and the influence of different steam
mass fractions in the gas mixture.
V. FURTHER WORKa) Stagnant

Although the processes of iodine transfer fromthe gas phase to the bulk liquid are relatively
well described in existing iodine codes mainly
representing the sump of a PWR, the complexhydrodynamic characteristics in water pools areeither not included in the codes or only in a very
rudimentary approach. In a further step anexperimental investigation is considered to bettercharacterize the two-phase-flow hydrodynamics
in pools under turbulent to churn-turbulent flow
conditions typically found in FCVS or wetwells
and coupling the hydrodynamics with existinggaseous iodine mass transfer models. The
experiments are foreseen to be conducted in the
TRISTAN test facility of PSI, [11]. TRISTAN
consists of a large square water pool (0.5 m x
0.5 m) with an overall height of 6.2 m, withtransparent side walls allowing access for opticalmeasurement devices. The facility is operated
under ambient conditions. NC gas can beinjected at ambient temperatures throughexchangeable injection devices up to high flow
rates (max. 450 kg/h). The water level inside thepool can be controlled and adjusted. The
characterization of the bubble-hydrodynamics(i.e. bubble size distribution, interfacial area,
bubble velocity, etc.) is measured by a wiremesh-sensor (WMS). The WMS technique isessentially based on the differences of theelectrical conductivity of a dispersed andcontinuous phase, detected by two planes of wire
grids placed in the flow in a short distance fromeach other. The grids act as electrodes, while one
plane acts as a transmitting and the other as thereceiving plane. It is planned to use the WMS oranother suitable technique to describe the massb) With gas injection

Fig. 11. Small scale pre-test facility Mini-VEFITA, a)stagnant and b) with gas flow and level swelling.

Figure 11b demonstrates the flow behaviourin the scrubber at a gas flow rate of 200 ln/min
causing a water level swell of about 0.1 m.
Several bubble sizes can be observed ranging
from a few mm to about 20-30 mm, which arequite different to the observed nearly constantbubble sizes of 1-2 mm in the lab-scale scrubbercolumn tests (Figure 6). Directly at the injection
nozzle big gas globules are formed. At the
impaction plates of the nozzle the bubbles breakdown to smaller ones which are further
disintegrated when rising in the riser and passing
through the mixing element. Some small gas
bubbles are even trapped in the recirculation

323

NEA/CSNI/R(2016)5
transfer and retention of gaseous iodine using agas phase simulant.

FCVS type water pools to better model the masstransfer process of gaseous iodine from the gas
phase into the liquid phase.

The experimental work is accompanied withcode simulations. The very complex two-phaseflow behavior in the wet scrubber section of the
FCVS is challenging to code models. The
thermal-hydraulic data of the VEFITA facility
are used for the assessment of system codes suchas RELAP5/Mod3.3-p3 and TRACE/V5.0-p2.
Models are developed for the facility andexperiment transients are simulated, [10].

ACKNOWLEDGMENTS

A large fraction of the work on the novel iodine
process was carried by R. Cripps, H.
Bruchertseifer and S. Güntay who have already
retired. Their work is gratefully acknowledged.
Without the support of the PSI hot laboratory theexperimental programme using radiotracer would
not have been possible. The VEFITA test facility
was erected in collaboration with IMI-Nuclear(CCI AG) which provided specific internal
structures of the test section. SwissNuclear, theNuclear Energy Section of swisselectric(Switzerland) supports financially the ongoingexperimental programme in the large-scale test
facility.

VI. CONCLUSIONS
The retention of gaseous organic iodides in wet
scrubber type filtered containment venting filters
using only chemical additives typically present in
the water pools, i.e. sodium thiosulfate andsodium hydroxide or sodium carbonate is limited
even at high water temperatures. In an extensive
research and development program a novel
process was established to suppress the release of
gaseous iodine species from aqueous solutions.
The process enables not only the fast andefficient decomposition of organic iodides into
non-volatile iodide ions but also a fixation ofiodide ions so that their radiolytic and thermaloxidationissuppressed.
Experimental
investigations
havedemonstratedtheeffectiveness of the process under a broad rangeof pH, temperature, organic iodide (CH3I)
concentrations, dose and in the presence of
impurities. Optimized aqueous concentrations of
the additives used in the process have been
identified. First tests in the dynamic system of alab-scale scrubber column in which volatile CH3I
is transferred from the gas phase into the aqueous
phase during sparging have shown the potentialof the process for FCVS application.

NOMENCLATURE

ACC = Activated Charcoal
ALI = Aliquat336®
DF = Decontamination Factor
DF* = Decontamination Factor per meter length
FCVS = Filtered Containment Venting System
GC/MS = Gas Chromatography / Mass
Spectrometer
ISE = Ion Selective Electrodes
NC = non-condensable
PID = Photo Ionization Detector
PWR = Pressurized Water Reactor
PTFE= Polytetrafluoroethylene
TEDA= Tri-ethylene-diamine
THS = Sodium Thiosulfate (Na2S2O3)
TRISTAN = Tube Rupture In Steam generator
multi-phase flow investigations
VEFITA = Venting Filter Assessment
WMS = Wire Mesh Sensor

To study the application of the novel process in atechnical FCVS a full height, reduced diameter
test facility with all internal components was
erected, in which retention tests for both
molecular iodine and methyl iodide will be
performed under prototypical conditions present
during containment venting scenarios. A set ofexperiments is planned to assess theeffectiveness of the chemical additives on theretention under complex two-phase flow
conditions. A new experimental and analyticalproject will study the bubble hydrodynamics in

REFERENCES

1. B. Clément et al., “LWR severe accident
simulation: synthesis of the results andinterpretation of the first Phébus FP
experiment FPT0”, Nuclear Engineering and
Design, Vol. 226, p. 5-82, (2003).

324

NEA/CSNI/R(2016)5
2. T. Lind, B. Jaeckel, S. Güntay, “A summaryof the PSI Investigations on Iodine Chemistry
in the Presence of Impurities and Additives,”
OECD-NUGENIA/ SAR-NET workshop on“Progress in Iodine Behaviour for NPP
Accident Analysis and Management”,
Marseille, March 30 - April 1, (2015).

7. H. Bruchertseifer, R. Cripps, S. Güntay, B.
Jäckel, “Experiments on the retention of thefission product iodine in nuclear reactor
accidents”, Scientific Report Volume IV:
Nuclear Energy and Safety, Paul Scherrer
Institut, ISSN 1423-7334, Switzerland (2004).
8. D. Grob, “Filtered Containment Venting
System”, NRC Public Meeting, Rockville
MD, September 4, USA (2012).

3. L.F. Parsly, “Chemical and physicalproperties of methyl iodide and its occurrence
under reactor accident conditions (A summary
and annotated bibliography)”, ORNL-NSIC82, (1971).

9. D. Jacquemain et al. “OECD/NEA/CSNI
Status Report on Filtered Containment
Venting”, NEA/CSNI/ R(2014)7, Appendix 3,(2014).

4. BASF, “Aliquat336, Technical information”,
TI/EVH 0125/4e, (2013).

10. J. Yang, D. Suckow, H.-M. Prasser, M.
Furrer,T.
Lind,“Assessmentof
RELAP5/TRACE against VEFITA ThermalHydraulic Level Swell Tests”, Proceedings ofthe International Congress on Advances in
Nuclear Power Plants (ICAPP), paper No.
14154, Charlotte, April 6-9, USA (2014)

5. H. Bruchertseifer, S. Güntay, “Fast Reductionof Iodine Species to Iodide”, United States
Patent, Patent No. US 8,142,665 B2, March
27, (2012).
6. S. Güntay, H. Bruchertseifer, H. Venz, F.
Wallimann, B. Jaeckel, „A Novel Process for
Efficient Retention of Volatile Iodine Species
in Aqueous Solutions during Reactor
Accidents”, NEA Workshop on the
ImplementationofSevereAccident
Management Measures (ISAMM), Oct.26-28,
Paul Scherrer Institute PSI, Villigen,
Switzerland, (2009).

11. T. Betschart, D. Suckow, V. Brankhov, H.-M.
Prasser, „High resolution two-phase flow
structure investigations under pool scrubbing
conditions“,ProceedingsInternational
Topical Meeting on Nuclear ThermalHydraulics (NURETH), paper NURETH15191, Pisa, Italy, May 12-17, (2013)

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NEA/CSNI/R(2016)5
THE EUROPEAN PASSAM PROJECT ON ATMOSPHERIC SOURCE TERMMITIGATION: HALF-WAY PROGRESS AND MAIN RESULTS
T. Albiol(1), L. Herranz(2), E. Riera(3), S. Guieu(4), T. Lind(5), S. Morandi(6),T. Kärkelä(7), N. Losch(8), B. Azambre(9)(1)
IRSN, Centre de Cadarache, BP3, 13115 Saint Paul les Durance Cedex, France (thierry.albiol@irsn.fr)(2)
CIEMAT, Avda. de la Complutense, 40, Madrid 28040, Spain(3)
CSIC, Serrano 144, Madrid 28006, Spain(4)
EDF, 12 - 14 avenue Dutriévoz, 69628 Villeurbanne Cedex, France(5)
PSI, 5232 Villigen, Switzerland(6)
RSE, via Rubattino 54, 20134 Milano, Italy(7)
VTT, BI7, Biologinkuja 7, P.O. Box 1000, FI-02044 VTT, Finland(8)
AREVA GmbH, Kaiserleistrasse 29, 63067 Offenbach, Germany(9)
Université de Lorraine - LCPA2MC – Institut Jean Barriol – Rue Demange, 57500 Saint Avold, France
Abstract - The PASSAM (Passive and Active Systems on Severe Accident source term Mitigation) project
has been launched in the frame of the 7th framework programme of the European Commission.
Coordinated by IRSN, this four year project (2013 – 2016) involves nine partners from six countries
having a strong experience on severe accidents: IRSN, EDF and university of Lorraine (France); CIEMAT
and CSIC (Spain); PSI (Switzerland); RSE (Italy); VTT (Finland) and AREVA GmbH (Germany).
It is mainly of an R&D experimental nature, aiming at studying phenomena that, under severe accident
conditions, might have the potential for reducing radioactive atmospheric releases to the environment.
Improvements of existing source term mitigation devices (pool scrubbing systems; sand bed filters plus
metallic pre-filters), as well as innovative systems which may achieve larger source term attenuation(Acoustic agglomeration systems; High pressure spray agglomeration systems; Electric filtration systems;
Improved zeolite filtration systems; Combined Filtration Systems) are studied.
After two years, the current status of the project is as follows. A state-of-the-art report has been completed,allowing a precise definition of the experimental test matrices in direct link with the identified gaps ofknowledge. The experimental work is in progress and in some domains (e.g. pool hydrodynamics, use ofzeolites for iodine trapping, combined wet and dry filtrations systems, etc…) major results have alreadybeen obtained. We can also quote the organization of an open workshop mainly targeted to R&D
organizations, to National Safety Authorities and their Technical Support Organizations, to utilities and tovendors, held on February 26th, 2014 in Madrid. A second workshop will be organized at the end of the
project, in December 2016.
Simple models and/or correlations will result from in-depth analysis of PASSAM experimental results.
Once implemented in severe accident analysis codes, these models should allow enhancing the capabilityof modelling Severe Accident Management measures and developing improved guidelines.
The project’s final outcomes will constitute a valuable database which may be strategic for helping theutilities and regulators in assessing the performance of the existing source term mitigation systems,
evaluating potential improvements of the systems and developing severe accident management measures.
Keywords: nuclear, safety, accident, mitigation, FCVS, PASSAM

326

NEA/CSNI/R(2016)5
I. INTRODUCTION

aerosol particles and make them more easily
trapped by solid filters or liquid pools).

After the Fukushima-1 accident one of the mainconcerns of the nuclear industry has been the
search for improved atmospheric source term
mitigation systems. The motivation underneathstems from two major evidences: venting of thecontainment building might be an essential
accident management measure in order toprevent the loss of its mechanical integrity, but
this containment venting might result in
substantial radioactive releases if no efficient
source term mitigation system is implemented.
Many countries have recently considered theimplementation of Filtered Containment Venting
Systems (FCVS) as a mean to further enhancetheir Nuclear Power Plants (NPPs) safety; whilesome others, like Sweden, Switzerland, Finland,
Germany and France, had already adopted thatmeasure in their NPPs.

These systems, some of which are installed on
NPPs, have been well characterized as regards
aerosol retention efficiency, but to a far lesser
extent as regards volatile iodine retention. In thiscontext the Advanced Containments Experiment(ACE) can be considered as the largest
international test programme [3]. Since then, thepotential high contribution of gaseous organic
iodine to the radioactive release in case of a lossof the containment building leak-tightness has
been evidenced. Additional tests have been
performed on FCVS, and improvements of thesystems have been proposed for organic iodide
retention in water pools. Nevertheless theresearch on this specific aspect needs to becomplemented. A lack of knowledge also
appears clearly for organic iodine retention in dry
systems. Finally, gaps may also remain in theinvestigation of systems performance that havenot been properly addressed, either because
evolution of anticipated working conditions
and/or because of new insights coming fromrecent research on fission product behaviour.
Hence, some specific experiments could provide
meaningful
insights
intothesystems
performance that could result in an optimizationof their working conditions.

The renewed interest for FCVS has pushed newinternational activities, like writing a status
report by the working group on the Analysis andManagement of Accidents of the CSNI(Committee on the Safety of Nuclear
Installations) of the OECD-NEA [1], or
launching the European Commission (EC)project on “Passive and Active Systems on
Severe Accident source term Mitigation”(PASSAM) [2].

In parallel, several alternative and innovative
approaches are appearing in the literature or areeven already proposed by some vendors. Electric
filtration systems are already widely used inindustry, out of the nuclear field, for separationof particles, for instance, from flue gases of coalfired power plants or from product gases in ironmanufacturing or in cleaning indoor air
applications. There are also a number of
proposed solutions based on molecular sieves(improved zeolites, metallic organic filters,
etc…) that are widely used as filter in manyindustrial processes. Many kinds of zeolite exist;
the key point would be to find the more efficientzeolite for iodine species in severe accident
relevant
conditions.
Anotherpromisinginnovative solution consists of the combinationof wet and dry filter systems. Besides, as it iswell known that, generally speaking, thefiltration systems are less efficient for aerosolThis paper gives an overview of the objectives,
work structure, current status and main results of
the PASSAM project two years after its startingdate.
II. PASSAM OBJECTIVES
The industrial FCVS are essentially based on two
approaches: “dry” and “wet” systems. In dry
systems, large trapping surfaces are provided
either by gravel or sand beds, or by metal fibers
or by molecular sieves. In wet systems, trapping
occurs in a liquid (water plus additives) pool as aconsequence of several removal mechanisms theefficiency of which depends mainly on thermalhydraulic conditions. These systems can beenhanced by including venturi scrubbers: water
droplets injected in the gas stream capture327

NEA/CSNI/R(2016)5• a deeper understanding of the phenomena
underlying
theirperformanceandmodels/correlations that allow modelling6 of thesystems in accident-analysis codes, e.g., ASTEC[4],

particles of some tenths of microns, somesystems are being developed, aiming at
agglomerating aerosol particles in order to getbigger particles which will be better filtered.
Implementing such a device upstream of thefiltration system would overcome the decrease offiltration efficiency of sub-micron particles.

• an estimation of the order of magnitude forsource term reduction for each filtration systemand suggestions for improved filtration systems7.

Another subject which has poorly been studied isthe long term behaviour of the trapped fission
products (i.e., potential for re-vaporisation and/or
re-entrainment of radioactive material) due tosurrounding conditions relevant to an accident
and more especially, due to continuous
irradiation, coupled to continuous flow-rate (ifthe venting system remains open), temperature,humidity, etc… Indeed the re-entrainment oftrapped aerosols was tested in different small
scale facilities and, at larger scale, in theinternational ACE programme where the aerosol
loaded filters (dry and wet filters) were operated
with clean gas and the aerosol concentrations of
the gas downstream of the filters were measured.
Nevertheless, no data on long term revaporisation under irradiation, both for aerosols
and for gaseous iodine forms, could be found in
the open literature.

III. PASSAM WORK STRUCTURE
The PASSAM project launched under 7thframework programme of EURATOM has
started in January 2013. Participated by 9organizations (IRSN, EdF and University of
Lorraine, France; CIEMAT and CSIC, Spain;
PSI, Switzerland; RSE, Italy; VTT, Finland; andAREVA GmbH, Germany) and coordinated by
IRSN, the project involves nearly 400 personmonth and the associated cost is estimated to be
more than 5 M€. Fig.1 shows a map of EUPASSAM partners across Europe.

In this context, the PASSAM project has been
built to explore potential enhancements ofexisting source term mitigation devices andcheck the ability of innovative systems toachieve even larger source term attenuations.
Heavily relying on experiments, the PASSAM
project aims at providing new data on thecapability and reliability of a number of systems
related to FCVS: pool scrubbing systems, sandbed filters plus metallic prefilters, acoustic
agglomerators, high pressure sprays, electrostatic
precipitators, improved zeolites and combinationof wet and dry systems. Nonetheless, the scopeof some of the ongoing research - as fission
products and aerosol retention in water ponds goes beyond FCVS and might be applied for
accident situation other than containment
venting.

Fig. 1. The PASSAM partnership
The PASSAM project is organized into 5 Work
Packages (WP) that are described next:

6

The implementation of models in Severe Accident
codes is out of the scope of PASSAM.
7

Note that the hydrogen risk within the FCVS,
which has to be taken into account, is not studied in
PASSAM which focuses more on the retention ofaerosols and gaseous iodine and on their potential
release at mid or long term.

The main outputs of the project will be:• a relevant extension of the current database on
these existing or innovative mitigation systems,

328

NEA/CSNI/R(2016)5• WP1:(MANAG) Project Management (leader:
IRSN). Project scientific, administrative andfinancial coordination, quality management
included.

experimental work-package [6, 7]. It ended up
with an open workshop held in Madrid (Spain) in
February 2014 [8].

• WP2:(THEOR) State of the art and modelling(leader:
CIEMAT).
Analyticalwork
supporting both test matrices definition anddevelopment of models and/or correlations
from the experimental results.

The experimental phase, which constitutes thelargest part of the PASSAM activities, ispresently ongoing and will last up to mid-2016. It
will result with the development of robust andsound databases for each system tested. The
main results already obtained are presented in thenext section of this paper.

• WP3:(EXIST) Existing filtration systems(leader: PSI). Two types of devices will be
experimentally studied: pool scrubbers(WP3.1) and sand bed filters - plus metallic
pre-filters(WP3.2).

The last phase will focus on managing the
project output in the best way so that nuclear
community can easily benefit from the research
conducted. In particular a final PASSAM
workshop will be organized by end 2016 and thefinal synthesis report of the project will beavailable in open literature.

• WP4:(INNOV) Innovative filtration systems(leader: VTT). Experimental campaigns will
be conducted on devices that might enhance
mitigation of source term, either through preagglomeration of the radioactive load thatwould reach the filter stage (acoustic
agglomerators (WP4.1) and high pressure
sprays(WP4.2)) or alternative filter systems(electric precipitators (WP4.3), improved
zeolites (WP4.4) and combination of wet anddry filters(WP4.5)).

Fig. 2 illustrates how the project development isrelated to specific activities considered within thevarious work-packages and their interaction.

• WP5:(DKS) Dissemination of knowledge andsynthesis (leader: IRSN). Matters concerning
PASSAM visibility, the communication of its
results and the sharing of the experience
gained.
IV. PASSAM CURRENT STATUS

Fig. 2. PASSAM Timing and major activities

The experimental nature of the project and thefinal outcomes foreseen, make its planning to berather linear as a whole, with three conceptual
project phases:• set-up of technical and organizational bases;

V. PASSAM MAIN EXPERIMENTAL
RESULTS
WP3.1: Experimental studies of pool scrubbing
systems (leader PSI)

• execution of experimental campaigns;

Removal of contaminating particles and/or
vapours from a carrier gas when passing throughan aqueous pond is known as pool scrubbing.
Most pool scrubbing investigation dates back to
80’s and 90’s of last century [9].
In the last 20 years, there have been few specificstudies focused on hot pools [10] and jet regime[11]. These experimental campaigns, however,were limited in their scope. More recently, some• project wrap-up.
The first phase included a literature survey on theexisting and innovative systems to beexperimentally studied in the project [5]. Thissurvey confirmed, with more details, theanticipated gaps of knowledge and so it allowed
to optimize the test matrices for each329

NEA/CSNI/R(2016)5
additional information on pool scrubbing ofaerosols has been gained through the ARTIST
projects [12, 13], in which the most distinctive
element was the presence of submerged surfaces
representing the secondary side of a steam
generator.
The gathered database allowed drawing someinsights into pool scrubbing but there are still
significant weaknesses: lack of systematic
analysis of the parameters influencing poolscrubbing (i.e., submergence, particle size, steam
content, etc.); no experimental tracking ofvariables like bubble size and shape; conditions
hardly addressed in the past, as jet injection or
in-pool gas rise under churn-turbulent regime;
few experiments on scrubbing of fission products
vapors. No less important, since the experimental
programs were performed, some information has
been lost throughout the years.
There are a good number of studies concerning
bubble hydrodynamics out of the nuclear safety
community. However, they mostly addressconditions far from those anticipated in severe
accidents. Anyway, the existing significant
information will be brought on-board, so that it istaken into account during results discussion andmodelling.
As a consequence, the following issues areinvestigated in PASSAM:• Pool scrubbing of aerosols in the jet injection
regime (CIEMAT). The experimental phase,
involving SiO2 particles, is planned from
April to October 2016.

wire
mesh
sensortechnology
formeasurement of 3D flow patterns and void
fractions was successfully developed,
complemented by application of imaging
techniques for very small bubbles. Clear
differences were observed between different
flow regimes and channel geometries. The
highly three dimensional nature of the flow
was clearly visible. Large gas agglomerates
were evidenced next to small bubbles. Large
spreading of bubble sizes throughout thechannel was observed and bubble shapes
could be reconstructed. A so-called “Gradient
Methods” was implemented for determining
interfacial area and for estimating the velocityof the gas phase. New experimental data werecollected for bundle and large square channel
geometries. These data are used for thevalidation of 3D hydrodynamic models andimprovement / development of the poolscrubbingmodelsregardingpoolhydrodynamics. New experience was gainedin understanding the phenomenology of twophase flows. More details can be found in[14].
• Mid/Long term behaviour of iodine in thewater pools under irradiation (IRSN). The
first two tests have been performed in
December 2014 and March 2015 using the
IRSN EPICUR facility. The results are still
being analysed, but under the experimental(severe) conditions, a significant release ofiodine from the “scrubber solution” wasobserved for the first test. An additional test isscheduled in 2015.

• Pool scrubbing of aerosols in the presence ofadditives in the water, simultaneous
determination of aerosol scrubbing efficiency
and pool two-phase flow hydrodynamics(RSE). Preliminary experiments on bubble
studies are ongoing up to May 2015. In
parallel the first experiments involving SiO2
aerosols have started and the wholeexperimental set should be completed by mid2016.

• Retention of organic iodine in the pool,including in the presence of scrubbing
additivesandsubmerged
structures(AREVA). Experiments at laboratory scale(VESPER34 facility) were performed on theinfluence of scrubbing liquid composition, of
mixing elements and of temperature. The
additives often used in commercial scrubbers
are NaOH for pH control and Na3S2O3 as
primary reactant. The CH3I retention
performance could not be further enhanced byincreasing
thesodiumthiosulfate
concentration. Yet the CH3I retention
performance in the scrubber could be
moderately increased by: Aliquat 336• Pool hydrodynamics under churn-turbulent
two-phase flow representative of the steam
generator tube rupture (SGTR) conditions
when the tube rupture is submerged (PSI).
Many experiments were performed in the
Tristan facility in 2013 and 2014. A specific330

NEA/CSNI/R(2016)5(quaternary ammonium salt serving as phasetransfer catalyst); mixing elements (increaseof residence time and of metallic surface);increased temperature. However, if Aliquat
scrubbing liquid is used, the retention
performance is not further increased bymixing elements nor by increased temperature(a reduction of CH3I retention was evenobserved above 110°C). It was not possible toreach high CH3I retention performance with
thestudiedparameters(additive
concentrations, geometries …), under theconditions used in the tests. The applicationof sliding pressure to AREVAs FCVS
(scrubber is operated close to containmentpressure) results already in increased pooltemperatures and thus the potential of CH3I
retention is seen as maxed out. So AREVA’s
conclusion is that alternative filtration
methods (e.g. dry filtration) are necessary toreach a qualitative step towards high CH3I
retention (see PASSAM WP4.5).

According to the experiments performed, the DF
attributed to the sand bed plus metallic pre-filter
system is:•1000 for aerosol particles (metallic pre-filter
DF assumed to be 10),•

10 for inorganic gaseous iodine, 1 fororganic gaseous iodine (no test done).

Fig. 3. Sketch of the French FCVS (from [15])WP3.2: Experimental studies of sand bed filters
plus metallic pre-filters (leader IRSN)

Despite the qualification of the system, twodomains appeared not (or not enough) to becovered and are presently investigated in the
PASSAM project:

Sand bed filters were decided to be implemented
on the French PWRs (Fig. 3) in the early 90’s oflast century. Their initial design was based on aset of boundary conditions: containment pressure
and temperature of 5 bar and 140ºC, respectively;
gas flow rate through the FCVS line of 3.5 kg/s;
gas mixture composed of air (33%), steam(29%), CO2 (33%) and CO (5%) (H2 risk wasaddressed later on); aerosol concentration of 0.1
g/m3 with an Aerodynamic Mass median
Diameter (AMMD) of 5 µm. The initial objective
consisted of obtaining a Decontamination Factor(DF) of 10 for aerosol particles. The system wastested at two different scales: PITEAS andFUCHIA [15]. The PITEAS-filtration program
enabled to determine the characteristics of thefilter material as well as its domain of use, andcheck its ability to meet the filtering efficiency
criterion under accident conditions. The fullscale FUCHIA program validated the thermohydraulic and filtering behaviour of the filteredventing line (without pre-filtration)
Then, it was decided to add a metallic pre-filter
upstream of the sand filter, in the containmentbuilding.

•

•

331

Filtration efficiency of gaseous molecularand organic iodides (IRSN). First laboratory
scale experiments recently performed
showed no trapping of gaseous moleculariodine (I2) in the tested conditions on puresand. A dedicated iodomethane Gas
Chromatography analysis method waslargely improved and allows now to reachextremely low detection limit for CH3I (10ppt) without pre-concentration. Next
experiments planned in 2015 and 2016 will
focus on I2 retention on sand filter system inmore representative conditions and on I2 andCH3I retention by metallic prefilters.
Mid/Long term stability of filtered fission
products, in particular iodine, under severe
accident conditions (temperature, flow-rate,pressure, humidity, irradiation) (IRSN). The
corresponding experiments are foreseen in2015 and 2016 using the IRSN EPICUR
facility.

NEA/CSNI/R(2016)5
WP4.1: Experimental studies of acoustic
agglomeration systems (leader CIEMAT – main
contributor CSIC)

WP4.2: Experimental studies of high pressure
spray agglomeration systems (leader RSE)
High pressure sprays have been used in multiple
industrial applications [20], including NPPs
(High Pressure Core Spray System of BWR/5
and BWR/6). This type of sprays works between
50 and 200 bar and can generate droplets smaller
than 100 µm at velocities near 100 m/s. Under
these conditions, in addition to clean-up and cool
atmospheres, they can also foster growth ofsubmicron particles [21]. However, few studies
have addressed this potential capability and mostinvestigation on sprays performance has been
conducted for low pressure sprays [22].
Therefore, high pressure sprays application as an
agglomerator in the pre-filter stage of a FCVS
would require addressing generic studies:• Determination of the agglomeration efficiency as
a function of injection pressure and droplet sizeand velocity.
• Enhancement of agglomeration efficiency byadding water additives and/or by droplet
charging.

Enhancement of particle agglomeration would
have the potential of enhancing mitigation. On
one side, agglomeration of sub-micron particles
in the containment building would result in larger
particles that would be much faster depleted by
sedimentation (settling velocity is proportional toaerodynamic diameter squared). On the otherside, if the agglomeration is done in the venting
line to a wet/dry containment filter, theperformance of the filtration devices would be
improved by enlarging particles, since theefficiency of those systems show a minimum inthe range of 0.1 to 0.3 microns.
Acoustic agglomeration (AA) was firstexperimentally investigated in the 30´s of lastcentury and has been extensively studied overmany years to understand its basic mechanisms.
Its practical applications came a little later in thelate 40’s with the development of powerful airjet sound generators. Since then a broad researchof potential applications have been investigated[16, 17, 18, 19]. The development of acoustic
high-intense chambers as pre-conditioners offilters was specifically tested in some of theseworks. These studies found out optimum
working conditions as regards exposure times;
particle concentrations and distributions in the
agglomeration chamber. As for the ultrasonicfield optimization in nuclear severe accident
conditions, it is known to be largely dependent
on particle size (micron/sub-micron) andhumidity while no degradation should be
expected under high pressure and temperatureenvironments.
Therefore, the main challenge for a potential
application of AA in nuclear severe accident
conditions will consist of checking the system
performance under the anticipated working
conditions during a severe accident. In the
PASSAM frame, preliminary tests have been
carried out in 2014 with an Acoustic
Agglomerator (21 KHz, 300W/unit) developedby CSIC and implemented in the CIEMATPECA facility. They allowed setting up the test
protocol. The experimental phase in the PECA
facility is ongoing since November 2014 andplanned up to May 2015.

These studies are planned in the frame of
PASSAM. The RSE SCRUPOS facility has been
set-up and preliminary tests have been carriedout for checking the good performance of the
instrumentation. The experimental phase in the
SCRUPOS facility is foreseen up to mid-2016.
WP4.3: Experimental studies of electrostatic
precipitators (leader VTT)
Potential of electric filtration systems werediscovered early 19th century, when corona
discharge was found to remove particles fromgas streams. Today many types of these filters
exist including electrostatic precipitators (ESP),
air ionizers and ion wind devices. Typically they
are very efficient filtration systems with minimal
resistance to the gas flow. They are applied for
reduction of many industrial emissions, including
coal and oil fired power plants, salt cakecollection from black liquor boilers in pulp mills,
and catalyst collection from fluidized bed
catalytic cracker units in oil refineries
An ESP removes aerosols from gas flow due toforces induced by strong electric fields [23]. In
case particles do not get high enough charge for
their collection, a pre-charging water mist can be332

NEA/CSNI/R(2016)5
sprayed to enhance particle charging: Thissystem is called Wet ESP (WESP). The
operation temperature of a typical WESP at
atmospheric pressure is limited to 90 ºC and thepressure drop through the system is small. The
reported energy requirement for the filtration of 1
m3 of gas by ESP varies from 10-5 to 10-3 kW.h.
The atomization of water is probably the main
power drain of WESP. In case of the atomizationof small droplets, it is approximately 400-800 W
for a gas flow-rate of 1 m3/s.

WP4.4: Experimental studies of improved
zeolites (leader IRSN – main contributor UniLor)
Zeolites are crystalline materials containing
pores of molecular size (5-12 Å). They arecomposed of [SiO4]4- and [AlO4]5- tetrahedra
linked by oxygen atoms. The substitution of theSi4+ by Al3+ leads to a negative charge, which iscompensated by the presence of a metallic (M)cation of n valence (Mx/n [AlO2]x [SiO2]y. zH2O,
where y/x should be higher than 1; i.e., more Si
than Al tetrahedral). Several zeolites families arepresently being used for different industrial
processes: Mordenite, Ferrierite, Faujasite andothers. A number of studies have been conducted
to assess the zeolites capability for iodine
retention [25, 26, 27, 28] and among all themetallic cations tested (Cd, Pb, Na, Cu …), foriodine trapping, silver seems to give the best
results for potential nuclear applications. Twotypes of silver-containing solids have beensuccessfully tested for iodine trapping: silver
mordenite (AgZ) and silver faujasite (AgX). The
trapping mechanism seems to involve chemical
redox reactions with some embedded Ag species,
which results in the formation of a stable
insoluble iodine AgI precipitate.

Typical collection efficiency of a WESP for
particles is between 99 to 99.9 %. Nonetheless,collection efficiency can be increased by
maximizing the strength of the electric field [24]and the residence time within the precipitator.
Other factors affecting efficiency are dust
resistivity,
gas
temperature,chemicalcomposition (of the dust and of the gas flow),
and particle size distribution. WESP performancehas been hardly tested for gaseous iodine,
although preliminary tests were conducted on
gaseous iodine filtration by VTT in 2011. In thiscase higher efficiency might be reached byadding an oxidation stage (ozone generator) thatturns gaseous iodine species into iodine oxide
particles.
Application of WESP under the foreseen
conditions of a nuclear severe accident poses two
major challenges:•
Conditions still not explored, like radiationand high temperatures (>90ºC), andpressures (>1 bar).
•
Filtration of iodine volatile species
including organic iodides.
The status of the PASSAM experiments on thisdomain is as follows. The facility has been set-up
and preliminary tests have started in April 2014
using WESP at room conditions. Experimental
phase in more representative conditions began
recently using TiO2 aerososls and showing astrong decrease of the trapping efficiency forvoltages below 15 kV (negative). Preliminary
retention tests for gaseous and particulate iodine
have also been conducted.

Investigation has given insights into the iodine
capture by zeolites: structural and chemical
features of zeolites heavily affect trapping; theFaujasite Ag-X adsorbents showed highdecontamination factors (DF) for both moleculariodine and organic iodides, but their efficiency
depends on steam and nitrous oxides presence; inMordenite I2 chemisorption is enhanced if
metallic silver is present [29]; high humidity
affects negatively organic iodide trapping; silver
zeolites are robust and withstand harshtemperature, humidity and radiation conditions.
In spite of all the knowledge gathered concerning
iodine trapping in zeolites, little information isavailable concerning impacts of zeolite structure
and cation loading as well as the persistence ofhigh iodine DF under oxidizing conditions
resulting from radiation.
More precisely, the experiments in the frame of
the PASSAM project are focused on:• the optimization of the nature of the zeolite inorder to avoid poisoning effects and to obtainhigh iodine DFs;333

NEA/CSNI/R(2016)5• the definition of optimal active sites for thetrapping of iodine and organic iodides(deeper understanding of the trapping
mechanisms);• providing extensive and sound data on theeffect of expected severe accident working
conditions including influence of chemical
effects (steam and gas contaminants) andpotential irradiation effects on iodine
trapping capacity.
The experimental programme started in 2013
and is foreseen up to 2016. The main results
acquired up to now by the French University of
Lorraine are as follows:• Up to 20 metal-exchanged zeolites wereprepared and characterized.
• Adsorption tests of CH3I in gaseous phase
under different conditions were performed.
About the trapping efficiency, silver zeolites
lead to the best adsorption performances in
the absence of contaminants and theexperiments are now oriented to a better
assessment of the effects of structure andsilver incorporation method. About thetrapping stability, the formation of a highly
stable AgI phase was evidenced as well as
less stable products.

(even those smaller than 0.5 µm) and higher than
99.5% for I2. Despite this performance, as it isreminded in the PASSAM objectives section(Section II), there are studies [30] which indicatethat a significant generation of organic iodine in
the containment atmosphere might occur during
a severe accident. Consequently the AREVAFCVS Plus (Fig. 4) was developed bysupplementing the existing AREVA FCVSStandard with a sorbent (zeolite) section as athird retention stage with combined passive
superheating [31].

Fig. 4. AREVA High Speed Sliding Pressure
Venting Plus

WP4.5: Experimental studies of “combined”
filtration systems (leader AREVA)

In link with the PASSAM project, the AREVA
experimental programme on this “FCVS PLUS”
combined system was completed in 2013. The
main results on organic iodine retention as statedby AREVA are:

The AREVA FCVS is based on an extensive
series of laboratory tests and further large-scale
investigations. As a result, a combined retention
processwasselected,
consistingofcomplementary wet scrubbing and particulate
filtering. The AREVA FCVS standard comprises
a venturi scrubber unit consisting of a venturi
section (wet section), a combined droplet
separator and metal fibre filter section (drysection) combined with a throttling orifice for asliding pressure process. By the entrainment anddispersion of the scrubbing liquid, large reaction
surfaces are created in the venturi nozzles which
result in an additional effective sorption of
gaseous iodine. For optimum iodine retention,
the scrubbing liquid is conditioned with caustic
soda and other additives.

• Retention efficiency of zeolite depends upon
many parameters: sorbent material, geometry/ flow dynamics, superheating / relative
humidity, heat repartition, residence time, gas
composition, flow velocities, etc.
• Extensive large-scale test campaigns wereconducted at the JAVA PLUS test facility.
The performed JAVA PLUS tests verified
that this combined filter is able to reach thetarget DF, e.g. superior to 50 for organic
iodine (more than 98% retention).
• The studied parameters allowed to develop amodel in order to customize the DF fororganic iodine by appropriate design of the

The AREVA FCVS provides already retention
efficiencies higher than 99.99% for aerosols
334

NEA/CSNI/R(2016)5
sorbent stage for the requested operating
range.
The performed test campaign results in a fully
qualified combined filter design of this “FCVS
PLUS”.
VI. CONCLUSION

interest in the PASSAM Project, and for funding
it in the frame of the 7th framework programme
FP7/2007-2013 under grant agreement n°
323217.

The four year (2013-2016) European PASSAM
project devoted to mitigation of atmosphericsource term is being conducted as initiallyplanned. After two years of operation, theexperimental work is in progress and in somedomains (e.g. pool hydrodynamics, use ofzeolites for iodine trapping, combined wet anddry filtrations systems) major results havealready been obtained
Thanks to the PASSAM project, databases on
existing or innovative mitigation systems will be
extended. Among the various topics studied,three of them may be quoted for their potential
direct impact on severe accident management:
gaseous iodine retention (molecular and organic
iodine), hydrodynamics for scrubbers, and longterm stability of trapped compounds in thevarious systems.
It is expected to get a deeper understanding of
the phenomena underlying the performance of
these mitigation systems, and to be able topropose simple models or correlations whichshould be easy to implement in accident analysis
codes, like ASTEC.
Orders of magnitude for source term reduction
for each mitigation system studied, including on
the long term, in accident conditions, as well as
general advantages and drawbacks of all thesesystems (efficiencies, passive behaviour,robustness, long term retention, etc…) is being
assessed. It should allow to give hints for
improved filtration systems.
The main technical outcomes of the project will
be documented in a final synthesis report andpresented in a final workshop to be held in Paris
area by end 2016. Finally, all the public
documents of the project are uploaded on line onadedicated
web
site
at
https://gforge.irsn.fr/gf/project/passam/ .

1. D. JACQUEMAIN, S. GUENTAY, S. BASU,
M. SONNENKALB, L. LEBEL, H. J.
ALLELEIN, B. LIEBANA MARTINEZ, B.
ECKARDT
andL.
AMMIRABILE,
OECD/NEA/CSNI Status Report on FilteredContainment Venting, NEA/CSNI/R(2014)7.

ACKNOWLEDGMENTS
The authors thank the European Atomic Energy
Community (Euratom) for showing a strong

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RICHTER, S. FOURCAUDOT, P. BARRAUX,P. LAJARGE, J. A. GALLEGO-JUÁREZ, E.
RIERA, G. RODRIGUEZ and N. SEYFERT,“Characteristics
ofanelectro-acoustic
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F. COLINET, W. R. FOX, R. J. FRANTA, J.
JOY, W. R. REED, P. W. REESER, J. R.
ROUNDS and M. J. SCHULTZ, Dust Control
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21. T. CHUEN-JINN, L. CHIA-HUNG, W. YUMIN, H. CHENG-HSIUNG, L. SHOU-NAN, W.
ZONG-XUE and W. FENG-CAI,. “An Efficient
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Particles in Exhaust Gas”, Journal of Air &Waste Management Associacion, Vol. 55, p.
319–325 (2005).
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27. B.S. CHOI, G.I. PARK, J. H. KIM, J. W. LEE
and S. K. RYU, “Adsorption equilibrium anddynamics of methyl iodide in a silver ionexchanged zeolite column at high temperatures”,
Adsorption, Vol. 7, p. 91-103 (2001).

23. C. RIEHLE, Applied electrostatic precipitation,
Chapter 3, p 25-88, “Basic and theoreticaloperation of ESP’s”, Edited by K.R. PARKER,
Chapman & Hall, London, UK (1997).

28. B.S. CHOI, G.I. PARK, J. W. LEE, H. Y.
YANG and S. K. RYU, “Performance test ofsilver ion-exchanged zeolite for the removal of
gaseous radioactive methyl iodide at hightemperature
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256, p. 19-26 (2003).

24. J. CHANG, Y. DONG, Z. WANG, P. WANG, P.
CHEN and C. MAN, “Removal of sulphuric acidaerosol in a wet electrostatic precipitator withsingle terylene or polypropylene collection
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29. K. W. CHAPMAN, P. J. CHUPAS and T. M.
NENOFF, “Radioactive iodine capture in silvercontaining mordernites through nanoscale silver
iodide formation”, Journal of American
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25. D. T. PENCE, F.A. DUCE and W. J. MAECK.,“A Study of the Adsorption properties of metal
zeolites for airborne iodine species”, Proceedingof the 11th AEC Air Cleaning Conference,
Richland (WA, USA), August 31 – September 3,1970.
http://rsearch.hitechsvc.com/nuclearsafety/qa/hep
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26. H. G. DILLMANN, H. PASLER AND J. G.
WILHELM, “Filtered venting for German powerreactors”, Nuclear technology, Vol. 92, p. 40-49(1990).

30. B. CLÉMENT, L. CANTREL, G. DUCROS, F.
FUNKE, L. E. HERRANZ, A. RYDL, G.
WEBER and C. WREN, State of the art reporton iodine chemistry, NEA/CSNI/R(2007)1.
31. N. LOSCH, S. BUHLMANN and C.
HUTTERER, “High speed sliding pressure
venting PLUS (AREVA’s FCVS with enhanced
organic iodine retention) design principles andqualification of severe accident equipment”,
Nuclear Plant Chemistry Conference, SFEN,
Paris (France), September 24 – 28, 2012.

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Session 5: recent developments in iodine chemistry modelling in severe accidents and applications tosource term and radiological consequences evaluations

339

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IODINE CHEMISTRY IN DESIGN BASIS FAULTS(1)

A Zodiates(1)*, S Dickinson(2)EDF Energy, Barnwood, Gloucester, UK, (2) National Nuclear Laboratory, Harwell, UK*Corresponding author, tel: (+44) 1452653915, tasos.zodiates@edf-energy.com

Abstract.
Iodine is the dominant contributor to off-site doses in Design Basis Faults (DBFs) in Pressurised Water
Reactors. The volatile nature of iodine makes the assessment of iodine release very complicated.
This paper considers the range of conditions and the potential iodine release pathways applicable totypical design basis faults, and identifies the iodine chemistry parameters that need to be known in order tocarry out an assessment of the iodine release. The paper also identifies potential gaps in our
understanding of iodine chemistry parameters which may be addressed by a future experimental
programme.

340

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I. INTRODUCTIONi)

Primary coolant discharges through thebreach at a pressure and temperaturedependent on the system affected and thestatus of the reactor. Depending on thesystem, the fault can occur during reactor
operation, during normal shutdown,
following a primary circuit fault with fuelfailures or during long-term post faultcooling operations.
ii) The water jet discharging from the leakpoint fragments to form a droplet
distribution suspended in the room
atmosphere, an aerosol, and may also
partially flash to form steam.
iii) On detection of a release of activity into theauxiliary building, the emergency exhaustsystems (containing iodine filters) areswitched into operation.
iv) Operator action isolates the leaking coolant
system, although a discharge continues until
the isolated pipe is empty.

The Sizewell B safety case probabilistic safetyassessment (PSA) considers in detail the Design
Basis (Accidents) Faults (DBFs). The reactor
event tree analysis and the consideration of
hazards and radwaste faults resulted in the orderof ~5000 DBFs. The DBFs were grouped into
~1000 Limiting DBFs (LDBFs) based on theevent tree endpoints and the scope anddescription of the initiating faults. A further
grouping process based on the fault
characteristics and the radiological aspects of thefaults (source term, release path, retention
mechanisms), resulted in ~200 Bounding LDBFs(BLDBFs).
Detail radiological consequences assessments
including all nuclides were carried out for
representative BLDBFs. These assessments
showed that iodine is the dominant contributor tothe off-site doses. Therefore, the radiologicalconsequences assessments of the BLDBFs, witha few exceptions, were carried out based on
iodine consideration only.

In the auxiliary building LOCA the relevant
parameters span the following typical ranges:

This paper considers the range of conditions andthe potential iodine release pathways applicableto typical design basis faults, and identifies theiodine chemistry parameters that need to be
known in order to carry out an assessment of theiodine release. These are identified byconsidering a typical auxiliary building Loss Of
Coolant Accident (LOCA); for example, leakage
through a Residual Heat Removal System(RHRS) valve or pump in an RHRS pump room.

-

Water leak rate: 0.5 – 60 kg/s
T of discharging water: 70 – 200 oC
P of discharging water: 6 – 155 bar a
T of droplets/room atmosphere/sump water:
70–100oC
pH = 4.6 or 8
D = few mGy/hr to few tens Gy/hr
Iodine concentration = 10-6 – 10-11 moles/kg
III. IODINE RELEASE PATHWAYS

The paper also identifies potential gaps in our
understanding of iodine chemistry parameterswhich may be addressed by a future experimental
programme.

Iodine in a water solution exists in two forms interms of volatility: volatile (e.g. I2) which canescape from solution and involatile (e.g. iodide
or iodate) that remains in solution. If theconditions in the solution are constant or evolve
only slowly, the volatile and involatile forms willreach a steady state depending on dose rate andthe pH and temperature of the water solution.

II. OVERVIEW OF AUXILIARY BUILDING
LOCA
Some parts of the primary circuit systems arelocated in the auxiliary building adjacent to thereactor building. Loss of coolant from leaks inthese systems can result in airborne activity inthe auxiliary building followed by a release to
the external atmosphere via the ventilation
systems or direct leakage. A general descriptionof the fault is as follows:

The volatile iodine that escapes from the water
may accumulate in the room atmosphere.
Depending on the physical processes taking
place, i.e. room ventilation rate, the iodine
concentration in the gas side and the volatile
iodine concentration in the water may reach anequilibrium.
341

NEA/CSNI/R(2016)5
Figure 1 illustrates the various iodine processes
and release pathways considered in an auxiliarybuilding LOCA.

discharged from the room via the ventilation
system where iodine retention may be claimed on
charcoal filters should they be available in theventilation system. The water loading on thefilters and the resultant reduction in filtration
efficiency is taken into account.

There are no significant retention mechanisms
for volatile iodine released into the room
atmosphere and all volatile iodine is taken to be

Leakage through pipe
breach.

Primary circuit
water flowing
though pipe.

Extracted asgaseous.
Extracted
as aerosol.

Some of iodine inleaking water
becomes
immediately gaseous
when it enters room.

Some of leaking
water becomesairborne asaerosol, andinvolatile iodine
remains within it.
Some involatile
iodine oxidizes andmay escape fromthe aerosol.

Auxiliary building room.
Aqueous iodine in contact
with surfaces reacts to form
organic iodine.
Volatile, organic
iodine.

Sump
Water droplets settle tosump with any remaining
iodine.

Volatile iodine insump water may bereleased to theroom.

Oxidised iodine inaerosol becomesgaseous.

Fig 1: Iodine Processes in an Auxiliary Building LOC

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NEA/CSNI/R(2016)5
The iodine release pathways are:i)‘gaseous from primary’ii)‘extracted as dissolved in droplets’iii)‘gaseous from droplets’iv)‘partitioned from sump’v)‘organic from surfaces’

The room ventilation flow will carry out with it
airborne droplets and any iodine that is present in
the droplets. The iodine concentration in thedroplets depends on:a) the iodine that remained in the droplets after
the volatile iodine has been released at the
point of discharge, see IV above, andb) on the rate of formation of volatile iodine in
the droplets and its rate of release from thedroplets, see VI below.

In order to make the assessment manageable it isnecessary to use simplified iodine models andthis, by nature, tends to lead to a conservative
assessment which can be easier to support andjustify in the absence of consistent experimental
data.
These pathways are discussed in the sections
below.

Therefore, the iodine release from the room as‘extracted as droplets’ depends on the initial
droplet size distribution, the droplet settlingprocesses, the ventilation rate and the iodine
droplet concentration (which may have been
modified by release of volatiles as in (a) above)..

IV. GASEOUS FROM PRIMARY

VI. GASEOUS FROM DROPLETS

Oxidation of iodine to form volatile I2 or I occurs
in the radiation fields of the primary circuit.
Other potentially volatile iodine species, such as
HOI, are also formed but this has a much higher
partition coefficient than the other forms,
particularly atomic I, so is considered to be lesssignificant. The equilibrium volatile iodine
fraction, Fvol, depends on the water pH andtemperature and is derived in a conservative way
from experimental results at high doserates [1].

The droplets produced at the moment the water isdischarged into the room contain involatile
iodine only as it is assumed that all volatile
iodine partitions from the water at that moment.
However, in the presence of the radiation field in
the room, volatile iodine is produced in thedroplets.
Given the small size of the droplets and the large
area to volume ratio of the fine dropletsremaining airborne, it is conservatively assumed
that the volatile iodine partitions out of thedroplets as it is produced. This is likely tooverestimate the release from the droplets,
particularly if the discharged water contains pH
control chemicals.

The volatile fraction is considered for two pH
ranges: ‘buffered’ pH > 7, and ‘unbuffered’ pH<7.
The temperature range covered
experimental results is 25 to 140 oC.

by

the

All the volatile iodine in the water discharging
through the leak is conservatively assumed to bereleased into the room atmosphere and extracted
by the room ventilation.

The volatile iodine fractional production rate isdependent upon the iodine ‘G value’ and it isassumed to be proportional to the doserate andindependent of the iodine concentration and thewater pH. The ‘G value’ was determined fromexperimental results at relatively high dose
rates, > 100 Gy/hr, and concentrations, >10-5
mol/kg.

V. EXTRACTED AS DISSOLVED IN
DROPLETS
The water jet discharging into the room from theleak, fragments into droplets of various sizes
depending on the water temperature and systempressure. The larger size droplets fall out of theroom atmosphere faster, and are collected in theroom sump or floor pool. The fine dropletsremain airborne for longer.

The airborne doserate in the room depends on the
total activity in the discharging water which is
fault dependent and can be as low as a few
mGy/hr. The iodine concentration also can varyby orders of magnitude. Extrapolation of theexperimentally-observed behaviour over such a343

NEA/CSNI/R(2016)5
wide range of conditions is not straightforward,
as discussed below in Section IX.

A = water/gaseous interface area (m2)

It would be beneficial in the modelling of thesefaults to extend the experimental data to lower
doserates and a wider range of iodine
concentrations.

IPC = overall Iodine Partition Coefficient = R/ FvolV = water volume (m3)

Fvol = iodine volatile fraction
R = iodine partition coefficientVII. PARTITIONED FROM SUMP

Consideration is given to the release of both
inorganic iodine and organic iodine from thesump.

As the water discharges through the leak the vast
majority settles to the room floor and sump very
quickly. Volatile iodine will build up in the waterand be available to escape from the water into theroom atmosphere. Retention of iodine on thesurfaces of the sump walls and room floor orsubmerged equipment is conservatively ignored.
Although deposition on sump surfaces is apotential source of volatile organic iodine, theassumptions made in calculating the organic
iodine production are sufficiently conservativethat this additional source does not need to beconsidered.

The complexity in the assessment arises in theidentification of appropriate values for IPC and
Kt. IPC depends on the volatile iodine species, I2,
I or organic iodine, and can also depend on watertemperature, pH and doserates. Kt depends(among other things) on the volatile iodine
species, the geometry of the water volume andthe room air flow convection patterns.
For buffered solutions, pH>7, IPC is calculated
based on the doserate. For un-buffered solutions,pH<7, IPC is calculated conservatively from theexpression R/Fvol on the assumption that I2 is thevolatile species, or for low iodine concentrations
by a value that bounds the available experimental
results.

In the auxiliary building LOCA calculations thefollowing simplifying assumptions are made:- the volatile iodine concentration in the wateris always at its equilibrium value, Fvol, forthe conditions;

Additional experimental data for low iodine
concentrations, <10-8 mol/dm3, and lower
doserates, 0.01 – 10 Gy/hr, may be useful inremoving conservatisms in the interpretation of
the experimental results.

- the gaseous side iodine concentration in theroom atmosphere, which can slow down therelease from the water, is neglected.
The first assumption can be very conservative if
the volatile iodine production rate is less than thevolatile iodine release rate, which may be thecase at low doserates. Thus, there is a need toextend the range of experimental results to low
doserates as identified in the previous section.

VIII. ORGANIC FROM SURFACES
An additional pathway for releasing iodine to theroom atmosphere is the formation of organic
iodine through surface reactions of iodine insolution deposited on the room surfaces which
are not under water.

The iodine release rate from the sump is
determined by mass transfer and is evaluatedfrom the following equation, using values of A
and V appropriate to the sump as a whole:d [ I total ] aqdtThe organic iodine formation rate, as a fractionof the iodine activity on the surfaces, is given by: Kt A[ I total ] aq .
Vw IPCk orgI  ( K  K r D)

where:

A
Vl

where:[Itotal]aq = total iodine concentration in the water(Bq)

K / Kr = thermal and radiolytic production
rates,

Kt = overall iodine mass transfer coefficient(m/s)

D = dose rate344

NEA/CSNI/R(2016)5
A/Vl = water area to volume ratio of the surface
water film taken to be 0.1mm.

there will be other radical reactions whichremove volatile iodine e.g.
I + e- = I-

The thermal and radiolytic production rates are
based on experimental measurements of methyl
iodine production from painted surfaces loadedwith iodine [2]. The values used in theassessments have been periodically checkedagainst more recent data from the EPICUR [3]and BIP [4] programmes to ensure that theassessment methods remain conservative.
Although some differences have arisen between
the new data and those used in the assessments,
these are not very large and, particularly given
the as-yet unexplained variations between
different data sets, a change to the values used is
not merited. Also, the more recent studies havebeen directed towards severe accident conditions
where the surface concentrations may be ordersof magnitude higher than those expected in somedesign basis faults.

or which compete for OH radicals and reduce theformation rate of I atoms. The net result is thatthe production rate of volatile iodine becomesmuch less than the OH radical production rate athigh dose rates. At very low dose rates, however,the concentration of other reactive species is very
low so the volatile iodine production rate may
approach its limiting value.
The production rate of volatile iodine has notbeen measured at dose rates below about 100 Gy
hr-1 so the range of validity of the formula usedin the DBF assessments is unknown. The iodine
radiation chemistry model INSPECT [5] predicts
that it is valid at dose rates above 1 Gy hr-1 but atlower dose rates the limiting value applies (Fig.
2).

IX. VOLATILE IODINE PRODUCTION AT
LOW CONCENTRATION AND DOSE RATE
Parts of the assessment described above require a
production rate for volatile iodine in irradiatedsolution. The formula currently used is based on
measurements in irradiated solutions using iodine
concentrations up to 10-4 mol dm-3 and dose rates
of 0.1 to 1 kGy hr-1. Whilst these are appropriate
to severe accidents, the values for design basis
faults can be very much lower. Although it isusually conservative to assume that behaviour
observed in experiments will apply under muchless severe conditions, this may not be the casehere.
Fig 2. Production rate of volatile iodine in 10-8
mol dm-3 I- solution: comparison of INSPECT
predictions with the DBF assessment method.

The initial reaction in the radiolytic production ofvolatile iodine species (I2 or atomic iodine) fromI- ions in irradiated water involves the hydroxyl
radical:Although INSPECT is a mechanistic model
which uses a large number of chemical rate
constants to simulate the iodine behaviour, it is
not validated at such low dose rates and its
predictions should therefore be treated with somecaution. Experimental data at low dose rates
would be very helpful in validating the model
prediction and improving the assessment
methods.

I- + OH = I + OHIf it is assumed that every OH radical producedin the irradiation of water reacts to convert aniodide ion into volatile form then the production
rate of volatile iodine will be equal to the Gvalue of OH radicals, which is approximately
3×10-7 mol dm-3 Gy-1 at 70°C. This is the upper
limit for the volatile iodine production rate.
However, if the dose rate is sufficiently high345

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REFERENCES

X. SUMMARY
Iodine is the dominant contributor to off-site
doses in Design Basis Faults (DBF) in
Pressurised Water Reactors. The volatile natureof iodine makes the assessment of iodine release
very complicated.
This paper uses the example of auxiliary building
LOCA to identify the range of conditions and thepotential iodine release pathways applicable totypical design basis faults. It also identifies theiodine chemistry parameters that need to be
known in order to carry out an assessment of theiodine release. Suitable values for the iodine
chemistry parameters need to be specified tocover the whole range of conditions that exist in
typical faults. These values are derived based on
experimental results. In general, the experimental
data tend to cover the range of temperatures up to
below 100oC and some extends to about 140oC.
Temperatures in many design basis accidents
exceed this range and it is worth considering
extending the experimental temperature range tobeyond 140oC.
The paper also identifies that there are potential
gaps in our understanding of iodine chemistryparameters that are relevant to the conditions of
low doserates and very low iodine
concentrations. It is worth considering whether
these conditions can be the subject of future
research.

1.

W. G. BURNS et al, “Iodine chemistry innuclearreactorenvironments”,
Water
Chemistry of Nuclear Reactor Systems 5, p.
281, BNES, (1989)

2. A. M. DEANE, “Volatile organic iodine
formation by surface processes”, Proc. 3rd CSNI
Workshop on Iodine Chemistry in Reactor
Safety. p181, NEA/CSNI/R(91)15 (1992)..
3. J. COLOMBANI et al., “Experimental study oforganic iodide formation from painted surfaces
in the containment atmosphere during a severe
accident”, ICAPP-2011, Nice. France, 2-5 May
2011..
4. G. A. GLOWA, C. J. MOORE and D.
BOULIANNE, “The main outcomes of theOECD Behaviour of Iodine project”, Thisconference, (2015).
5. S. DICKINSON and H. E. SIMS, “Developmentof the INSPECT Model for the Prediction of
Iodine Volatility from Irradiated Solutions”,
Nuclear Technology 129, p374, 2000.

NOMENCLATURE
PSA = Probabilistic Safety Assessment
DBF = Design Basis Fault
LDBF = Limiting Design Basis Fault
BLDBF = Bounding Limiting Design Basis Fault
LOCA = Loss of Coolant Accident
RHRS = Residual Heat Removal System
IPC = Iodine Partition Coefficient346

NEA/CSNI/R(2016)5

IODINE BEHAVIOUR IN THE CIRCUIT AND CONTAINMENT: MODELLING
IMPROVEMENTS IN THE LAST DECADE AND REMAINING UNCERTAINTIES
L. Bosland(1)*, L. Cantrel(1)(1)
Institut de Radioprotection et de Sûreté Nucléaire, Saint-Paul Lez Durance, France*Corresponding author, Tel: (+33)4.42.19.94.46, loic.bosland@irsn.fr
Abstract
Within the past decade, international experimental programmes were performed in the area of iodine
behaviour in the Reactor Coolant system (RCS) and containment during a severe accident (SA) in a
Nuclear Power Plant (NPP). The objectives were to better understand involved physico-chemical
processes and develop relevant models in SA codes like ASTEC which are used to estimate Source Term
(i.e., radioactive releases to the environment). Significant progresses have been made in the modelling of
the iodine behaviour in the RCS and the containment. This article provides a synthesis of the progresses
made in the last decade, what has been understood from the experimental data and which models havebeen developed, validated and implemented in the ASTEC code. Significant progresses have been made onunderstanding and modelling of key processes for iodine behaviour which gives more confidence on iodine
Source Term (ST) assessments and which highlights remaining major source of uncertainties for suchassessments. These uncertainties mostly concern processes that have been recently identified as potential
important contributors to the releases at mid-term, notably for some NPPs in conjunction with the use of
Emergency Containment Filtered Venting Systems (EFCVS). These processes are discussed and are thesubject of on-going R&D programmes (e.g., OECD-STEM, ANR-MIRE, EC-PASSAM) and future R&D
initiatives (e.g., OECD-STEM2, OECD-BIP3, OECD-THAI3) in the ST field.

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I. INTRODUCTION

few percents of the i.b.i (initial bundle inventory)
whereas in FPT3, it is up to 30% of the i.b.i (andrepresenting 90% of the iodine entering in thecontainment). The understanding of this result
has been pursued through the ISTP-CHIP
program, whose objective was to investigate theiodine transport through the RCS. Semi-global
experiments [13] as well as more analytical ones
[14, 32] were carried out to study the influenceof the most relevant elements on iodine
speciation (gaseous or particulate forms) at the
break. These experimental efforts have been
coupled with the development of theoreticalchemistry methodologies which allow us getting
more fundamental data as thermodynamic
properties [33, 34] and rate constants forelementary reactions [35].
For the “iodine system”, a kinetic reaction
scheme was developed [36] and validated on awide range of experimental conditions either inpure steam atmosphere or in hydrogen
atmosphere. It appears that for temperature
below  1000 K, in oxidizing conditions, thesystem cannot be considered to be at chemical
equilibrium whereas in reducing conditions, thekinetics are faster and very close to theequilibrium even at low temperatures.
In presence of cesium (and without any otherelement), the formation of CsI is very fast
because the reaction CsOH + HI  CsI + H2O isbarrier-less. As there is an excess of Cs withrespect to I, CsI is formed without any possible
existence of gaseous iodine.
If Mo is added to the Cs-I system, the reactivity
completely changes because Mo has a highchemical affinity with Cs and forms some cesium
molybdates [15] (CsxMoOy compounds) but inpractice Cs2MoO4 is the most stable gaseous
molybdates. For a steam atmosphere, up to 80%
of gaseous iodine is transported in a thermal
gradient tube with an outlet temperature of
150 °C with a low fraction under aerosol form.
The gaseous iodine flow is mainly composed ofmolecular iodine (~ 2/3); the remaining iodine
fraction can be reasonably attributed to HI or
HOI. The Mo aerosol forms are CsxMoOy andMoO3. In hydrogen atmosphere, CsxMoOy cannot
be formed and the results obtained regarding theiodine speciation are similar to those without
Mo, i.e. the formation of CsI aerosols dominates.

Iodine behaviour during a nuclear reactor severe
accident has been investigated within the past
decadethroughseveralinternational
experimental programs that have significantly
contributed to develop the knowledge in thisfield. These include the PHEBUS-FP integral
tests [1-4] and more analytical programs such as
ISTP [5], PARIS [6, 7], OECD-STEM [8],OECD-THAI [9, 10] and OECD-BIP [11, 12]
programs which were focused on iodine
behaviour either in the Reactor Coolant System(RCS) [13-15] or in the gaseous phase of thecontainment [6, 7, 9-12, 16, 17] (the liquid phase
iodine chemistry was mostly investigatedbetween 1994 and 2005 [18]).
For the RCS, the kinetics of the I-O-H system,
the influence of fission products (Cs, Mo) and
Boron (B) on iodine transport has been
investigated. Concerning the potential impact of
the control rod material (Ag, In, Cd), the studies
are ongoing at IRSN. In the containment, most of
the efforts have been put on iodine interaction
with Epoxy paint under irradiation [16, 17, 1921], iodine oxides (IOx) formation [6, 7, 9, 2224] and radiolytic decomposition [7, 12, 25],
iodine interaction with steel [26, 27] and iodine
adsorption on aerosols [10]. For each phenomena
investigated at the experimental level, asignificant effort has also been put on theinterpretation of the data in order to model eachphenomenon with the objective to implement
these new models in the ASTEC severe accident
code (jointly developed by GRS and IRSN) [2830]. This paper recaps in Section II the progress
made for iodine modeling in the RCS whereas
section III synthesizes the progress made at
IRSN for gaseous iodine chemistry modeling in
the containment for about 10 years. Section IV
identifies new containment phenomena whose
influence on iodine volatility might be significant
and needs to be studied.
II. IODINE CHEMISTRY IN THE RCS
One unexpected result coming from PHEBUSFP tests is that a significant gaseous iodine
amount enters in the containment from the circuit
during the fuel bundle degradation [31]. In FPT0,
FPT1 and FPT2, this gaseous fraction reaches
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If Boron is added to the Cs-I system, with steam,gaseous iodine is promoted but in a lower extent
than for the Mo case, due to the formation of
CsBO2.
For control rod material, Ag and Cd are expected
to be reactive with respect to iodine and toprevent the formation of gaseous iodine along the
RCS. An IRSN experimental program is ongoing
to check this. A preliminary result obtained with
the Cd-Cs-I-Mo-H2O gaseous system seems to bea bit surprising as the amount of CdI2 formed islower than expected.
From a qualitative point of view, the trends are
summarized in Table 4.

Another point is the possible release of gaseousiodine resulting from remobilization of somemetallic iodides deposits onto RCS walls that canlead to gaseous delayed releases inside thecontainment. Analytical tests performed in theframe of the French MIRE project [38] will be
performed in a near future to check and quantify
this phenomenon.
At the end of 2017, the improvements expected
are to:- get a good understanding and modelling of theinfluence of the control rod materials onto theiodine transport;- better model possible revaporization from metal
iodides deposited along the RCS.

Table 4: influence of elements on iodine
speciation at the RCS cold-leg break
SystemOxidant
gaseousconditions

Reducing
gaseousconditions

IodineNot inequilibrium

Tend toequilibrium

Iodine +cesiumTend to equilibrium, CsI
formation

Iodine +cesium +boron[13, 37]

Not in equilibrium for BCsO2
formation

Iodine +cesium +Mo [15]
Iodine +cesium +Mo + CdCsI + I2
formation

III. IODINE CHEMISTRY IN THE
CONTAINMENT
III.1. Iodine interaction with paintThe equilibrium between iodine adsorption andradiolytic release by Epoxy type paint(commonly used in most of the reactors) has
been identified as a main phenomenon
contributing to iodine volatility. Some studies
have been performed with immerged and dry
paints in the 1990’. For the gaseous phase,different experimental conditions have beenstudied by separate teams and have led to asignificant scattering of the data. The
uncertainties on the optimized kinetics constants
are thus important [39, 40] and this has been an
obstacle to the development of a relevant modelthat would fit more accurately the whole set ofdata. EPICUR irradiation facility has been set up
in 2004 to provide the possibility to measure online gaseous organic and inorganic iodine species
at the same time after being desorbed from paints
under irradiation. Some tests were conductedwithin the ISTP [5] and STEM [8] projects withthe objective to evaluate the release rate oforganic and inorganic iodine from Epoxy paintunder irradiation and for different conditions(temperature, dose rate, dose, humidity andiodine loading on the paint). The whole set ofdata has led to the development of a new iodinepaint model [41] in ASTEC. It was applied todifferent test cases and especially to PHEBUS
FPT-3 test for which more than 90% of theCsI formation

Tend to equilibrium for Cs2MoO4
formation
I2 + HIformation

CsI formation

CdI2 not in equilibrium, to be
confirmed by additional tests

From a modelling point of view, these data haveled us to implement a kinetic I-O-H scheme in
ASTEC and also complete/review thethermodynamic database (for example, CdMoO4
was missing whereas it has been experimentallyobserved).

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NEA/CSNI/R(2016)5
iodine entered in the containment under agaseous inorganic form (and representing 386mg) [4] whereas iodine multi-component
aerosols mass settling on dry containment
surfaces is not significant (4 mg). Under
irradiation, it has been shown in the OECDSTEM program [8, 25] that CsI aerosols are notstable under irradiation and lead to the
production of volatile iodine. The fate ofdeposited multi-components iodine aerosols in
PHEBUS is not well known and a significant
uncertainty exists on their influence on iodine
volatility for FPT-0/1/2 tests as their deposited
mass is more important than in FPT-3. FPT-3 testis thus an interesting test to identify to what
extent the new iodine-paint interaction model
modifies our understanding of iodine volatility as
the decomposition of deposited multi-component
iodine aerosols coming from the RCS is notexpected to have a significant influence.

the conclusions as both calculated iodine
concentrations remain significantly outside theexperimental uncertainty margins that are
estimated to be close to ± 30-50 % for inorganic
iodine and ± 40-100 % for organic iodide (thecalculation of these uncertainties is not detailed
in this paper). It means that other phenomena
govern iodine volatility in the long term.
Two phenomena have been identified as
eventually contributing to iodine volatility in
PHEBUS tests:--

To perform this calculation, the dose rate (DR)has been estimated all along the test with theactivities detected by the gamma-counters in thegaseous phase and in the sump.

Iodine aerosols radiolytic decomposition:
iodine aerosol (IOx and multi-components
aerosols) might be partly unstable underirradiation and might contribute to revolatilize gaseous I2.
The radiolytic conversion of I2 into CH3I:
this reaction was reported to happen as
volatile organics (CH3R compounds type)create °CH3 radicals under irradiation thatquickly reacts with gaseous O2 and I2.

Both phenomena are explained in more details in
the next two sections. Their influence on iodine
volatility has also been checked.

A stand-alone calculation was then performed
with the ASTEC/IODE module. Fig. 23 and Fig.
24 show the comparison between the gaseous
inorganic and organic iodine data with the
ASTEC modeling.

III.2. Iodine oxides formation and radiolytic
decomposition
Considering the formation of iodine oxides
compounds (IOx) gained in importance within
the past decade. Air radiolysis leads to theformation of air radiolytic products (ARP) likeNO2, O3 or HNO3 [6] that can oxidize iodine andlead to IOx particles that sediment onto surfaces[7, 22]. From PARIS tests [6, 7], a radiolytic
model of IOx formation-destruction was set up in
ASTEC (I).

The initial inorganic iodine concentration peak iswell modelled with the injection of 386 mg of I2
from the circuit. Its evolution in the long term isunderestimated up to one order of magnitude
even if the tendency is well caught. The initialorganic iodine peak is quite well reproduced with
the new iodine-paint interaction model until theend of the aerosol phase ( 35.000 sec) whereas
the long term concentration is also
underestimated by one to two orders ofmagnitudes.

I2(g)  ARP(g)  I2O5(aer) (I)As significant discrepancies appear after theaerosol phase for both volatile species, it waschecked if the uncertainties on the DR estimationand the experimental data could explain thosediscrepancies. The uncertainty on the DR isestimated to be close to a factor 2. Setting up ahigher (x 2) or lower DR (x 0.5) does not change
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anhydrous forms where detected like I2O5, I4O9
or hydrated forms like I2O5.HIO3 (or HI3O8) thatmight lead to the formation of HIO3 at highhumidity [44].
More studies are still being conducted to get abetter estimation of their size, composition andbehaviour under different conditions within theobjective to better consider their behaviour in thesevere accident codes. The main recent progressthat has been implemented in ASTEC V2.1 is toconsider IOx as aerosols (possibly agglomeratingwith other kind of aerosols) as they used to beconsidered as a volatile species before. This leads
to the possibility of interaction and condensation
with other aerosols, especially in the short termof an accident when they settle down.
A more complete understanding of theirradiolytic and thermal stability once deposited on
the containment dry surfaces is also needed as i/OECD-STEM project results [8] have shown that
IOx partly decompose under irradiation and ii/
VTT has shown that volatile iodine is released
from IOx uptake of water from humid air andheat [43]. This is planned to be further addressedin the OECD STEM2 project [45].

Fig.
23.
Gaseousinorganic
iodineconcentration in FPT3 containment with the
new iodine-paint model

Another uncertainty that remains about IOx istheir filtration efficiency by filtered containment
venting system (FCVS) as their size was found to
be under 1 µm [42]. Some studies are being
performed within PASSAM project [46] toevaluate the sand filters IOx trapping efficiency
installed on French PWRs as well as theirradiolytic stability in the sand filter (radioactive
aerosols trapped in the sand filter would deliver asignificant local irradiation field that might leadto IOx and iodine aerosols decomposition into
volatile iodine, which would increase the iodine
Source Term).
Finally, the Fukushima accident has highlighted
some modifications of iodine speciation (gas particle) in the environment during its transport
in the atmosphere. The influence of photolysis,
atmospheric conditions and the presence ofpollutants are suspected to play a significant
influence on iodine speciation. Some work is
being performed at IRSN to quantify thoseFig. 24. Gaseous organic iodide concentration
in FPT3 containment with the new iodinepaint modelMore studies have been launched since and weresummarized recently [12, 42, 43] in order tobetter understand IOx composition and behaviour
under irradiation and thermal conditions. IOx aresmall aerosol particles (< 1 µm), whose
composition is not well known as several
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NEA/CSNI/R(2016)5
atmospheric modifications that are crucial for theiodine ST estimation [47].

Nevertheless, this model needs to be completed
as the temperature, humidity, dose, dose rate andreactants concentration might influence thereaction kinetics. Their effect have thus to bechecked and quantified. This is planned to beaddressed in the OECD BIP3 project [51].

III.3. Radiolytic gaseous conversion of I2 into
CH3I
The reaction between methane (CH4) (and moregenerally volatile organics CH3R) and I2 (II) has
been discussed several times and the knowledge
on this reaction has been summarized several
times [40, 48, 49]:

I2

 CH 4

 CH3 I

III.4. Influence of IOx decomposition and CH3R
reaction with I2 under irradiation on the modeling
A simple model of radiolytic decomposition of
IOx deposited on dry surfaces has been set up to
check how IOx decomposition into I2 could
influence iodine volatility in PHEBUS FPT-3
test. Fig. 25 and Fig. 26 show how thecontainment iodine volatility modeling is
modified for PHEBUS FPT-3 when we consider
these 2 reactions (developed in section III.2. andIII.3.). It leads to a better estimation of inorganicand organic iodine for FPT-3. The agreement isbetter for both species, with some remaining
discrepancies that cannot be explained by theuncertainty on the multi-components iodine
aerosols radiolytic stability as their mass is not
significant. A possible explanation resides in the
degradation of IOx by carbon monoxide (CO)that is produced in significant amount by FPT-3
fuel bundle degradation (containing B4C controlrod), further detailed in section IV.2.

 HI (II)CH3R concentration is at least that of methane in
the atmosphere (3.10-8 mol.L-1) and could reach
1.10-6 mol.L-1 [48]. However, no relevant model
has ever been published. It is known that theforward thermal reaction is not likely atrepresentative containment temperature (T <200 °C) as it would start to be significant for T > 400 °C. At representative containment
temperatures, the radiolytic reaction would takeplace instead with the following mechanism (III)[40, 48, 49]:

CH 3 RI2

 CH 3g CH 3

 R

 CH 3 I

 I

(III)

I° radical formed would then contribute to theformation of gaseous iodine oxides. Most of theexperiments performed in this area have led to a
result expressed in terms of conversion
percentage so that no kinetics data were everpublished before Bartonicek [50] obtained somedata in representative conditions. From these
data, a model was set up (Eq. 1) that was
assumed to follow an order one on each reactant
and recently implemented in the ASTEC V2.1
code.
d CH 3 I dt k.DR .CH 3 R . I 2 

However, for FPT-0, FPT-1 and FPT-2, theinorganic iodine is still underestimated after theaerosol phase by one order of magnitude when
we consider these 2 phenomena. In these tests,
IOx aerosols are one type of iodine aerosols
being formed in the containment. They co-exist
with multi-component iodine aerosols that comefrom the circuit, enter in the containment,
agglomerate and settle down in the dry surface
and the sump. Their composition is more
complex as they involve different fission
products and structure materials [1-3]. As CsIaerosols were found to be unstable underirradiation in OECD-STEM project [25], anestimation of the radiolytic decomposition
kinetics of these multi-component aerosols was
made. It could range between one to two ordersof magnitude lower than IOx decomposition
kinetics to model accurately the inorganic iodine
concentrations. Consequently, these deposited
multi-component aerosols could be the main

(Eq. 1)

DR: dose rate (Gy.s-1)
k: radiolytic reaction kinetics (L.Mol-1.Gy-1)

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NEA/CSNI/R(2016)5
source of inorganic iodine in the long term. For
organic iodides, CH3I concentrations could
mostly depend on I2 concentration via reactionsof mechanism (III) that might be the main sourceof organic iodides whereas, in the short term,
paint would contribute more.
As the main gaseous iodine source in the longterm could take its origin in the multi-component
aerosols decomposition, the determination of
their radiolytic decomposition kinetics is of
crucial importance in order to complete themodeling in ASTEC also for iodine Source Term
estimation.
Other reactions have been updated or developed
in ASTEC V2.1 recently. They are detailed in thenext two sections.
Fig.
25.
Gaseousinorganic
iodineconcentration
in
FPT3
containment
considering IOx radiolytic decomposition andI2 radiolytic conversion into CH3I

III.5. I2 interaction with steel
Until recently, I2-steel interaction model has beentreated in a simple way with a physicaladsorption and desorption reaction [52].
However, some updates on the mechanisms havebeen published, highlighting the fact that someiodine is definitively trapped on the steel underan oxidized form (IV) [53].

I 2( gas )  I 2(sdry)  FeI 2( sdry )  FeI 2O( sdry ) (IV)It considers two reversible steps leading to theadsorption of iodine under I2 and FeI2 forms.
Then, an irreversible step leads to the chemical
trapping of an oxidized species (FeI2O).
The kinetics model has thus been recently
updated in ASTEC, based on GRS work [54] andis now available in the ASTEC V2.1 version
which will better model iodine interaction withsteel surfaces. It does not change ourinterpretation and understanding of iodine
volatility in PHEBUS test as paint surfaces playa more important role on iodine volatility thansteel.

Fig. 26. Gaseous organic iodide concentration
in FPT3 containment considering IOx
radiolytic decomposition and I2 radiolytic
conversion into CH3I

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NEA/CSNI/R(2016)5
III.6. I2 adsorption on aerosols

questions, in particular for their influence on
iodine volatility. For paints, natural ageing innormal operating conditions as well as theirageing during the accident (under irradiation,high temperature and humidity) might influenceiodine adsorption and/or release kinetics so thatiodine volatility could be modified. Aleksandrov[55] has shown that pre-irradiating an Epoxy
paint between 0.1 kGy and 1 MGy before
adsorbing iodine leads to an I2 adsorption
kinetics that is decreased by one order ofmagnitude. Glowa has found the same tendency
for iodine adsorption on pre-irradiated couponsat 1 Gy.s-1 for 160h [11] and that this preirradiation phase favors CH3I release kinetics
under irradiation [12]. In case of a severe
accident, the cumulated dose absorbed by the
paint is expected to reach 1 to 10 MGy (theupper value being the dose recommended bythree standards [56-59] in such a case). This
raises the question about how paint ageing (innormal operating conditions followed byirradiating conditions) might influence iodine
volatility and Source Term. In the forthcoming
OECD STEM2-BIP3 program [45, 51], sometests are planned in this field to complete ourunderstanding and eventually adapt our models
in the ASTEC code.

Recently, THAI 2 tests were performed (as partof the OECD-THAI 2 program) with silver
aerosols (considered to be one of the mostreactive aerosols towards I2) and tin oxide (SnO2)(expected to be chemically inert). They haveshown that I2 adsorbs faster on silver (by chemisorption) than on tin oxide aerosols (by physicsorption) [10] in suspension. During the shortterm of a severe accident, aerosols sediment andmight contribute to lower I2 volatility if I2
adsorption on aerosols competes significantlywith others gaseous phenomena like iodine
adsorption on paint or IOx formation. In order toevaluate how important this could be for iodine
volatility (especially in the short term) a simple
model (V) of I2 adsorption-desorption on
aerosols has been developed in the ASTEC V2.1
version.

I 2( gas ) I 2(aer) (V)It will help evaluating if this phenomenon might
affect iodine volatility in the short term whenaerosols settle down and if we need to get moreexperimental data in this field. Preliminary
evaluations performed for the most probable
accident sequences have shown that iodine
adsorption on aerosols do not affect significantly
iodine ST. Nevertheless, uncertainties remain
about the iodine adsorption kinetics on
representative multi-component aerosols that has
not yet been studied due to experimental
limitations in producing such kind of multicomponent aerosols.

IV.2. Iodine oxides decomposition by carbon
monoxide (CO)Various authors have reported iodine pentoxide(I2O5) decomposition by carbon monoxide [60,61] as being complete at ambient temperature(VI).

I 2O5  5CO  I 2  5CO2 (VI)

The next section deals with containment
phenomena that have been identified as
eventually having a significant influence on
iodine volatility but whose quantification need tobe completed.

Other organic compounds (like benzene,
propane, butane…) also react with iodine
pentoxide. Nevertheless, the reaction with COmight play a significant influence on iodine
volatility as carbon monoxide might be formed insignificant amount in the containment by:

IV. Other containment phenomena whose
influence on iodine volatility needs to bequantified

IV.1. Paint ageing
Most of power plants are now between 20 and 40
years old and materials ageing raises some354

the B4C control rods or plates (present innumerous NPPs) degradation in the bundle(like in FPT3 for which the COconcentration in the containment could havereached 6.10-5 mol.L-1 i.e. at least 3 orders of

NEA/CSNI/R(2016)5-

magnitude over the iodine volatileconcentration)
the molten corium-concrete interactions in
the containment.

- A model of iodine oxides aerosols formation
and radiolytic decomposition has beendeveloped- A preliminary model of gaseous conversionof I2 into CH3I through volatile organic
compounds has been created- The model of I2 interaction with steel has
been refined- The adsorption of I2 on aerosols has beenmodelled

It is not clear whether the iodine oxides formed
inside the containment are pure I2O5 or a mix
between hydrated forms and non-hydrated forms(as mentioned in section III.2) and if those
hydrated forms would also react with CO. CO
might also be converted to CO2 under irradiation
so that its concentration would decrease along
the transient. The influence of this reaction on
iodine volatility has thus to be estimated.

They have helped in the research of theremaining uncertainties in iodine volatility
estimation and iodine ST estimations. Those
uncertainties are listed below:

V. CONCLUSION- Paint ageing: their ageing in normal operation
and/or during the course of an accident (withbeta and gamma rays) could change the paintcapacity to trap iodine which could lead to ahigher iodine volatility- Iodine oxides radiolytic decomposition: moreparameters need to be investigated to consider
their influence in the modeling (temperature,humidity, presence of CO etc)
- More generally, the radiolytic oxidation ofmulti-component iodine aerosols, or at least
others metallic iodide aerosols than CsI, need
to be checked to evaluate how much volatile
iodine could be released under irradiationfrom deposited iodine aerosols on dry
surfaces. Iodine adsorption on those aerosols
is also of interest- The gaseous radiolytic conversion of I2 into
CH3I through volatile organic components has
been highlighted and need to be completedwith the influence of more parameters like thehumidity, dose rate, dose and temperature- Filtered containment venting system (FCVS)
efficiency: it is currently being studied in theEuropean PASSAM project [46] and French
MIRE program [37] with the objectives toestimate their efficiency under irradiation andbetter evaluate the iodine and ruthenium
gaseous Source Term
- In-containment delayed releases: iodine
remobilization can come from boiling sumpor thermal and mechanical loading effects
resulting from transient phenomena on FP
deposited onto inner walls (such as H2
combustion, thermal radiation from passive
autolytic recombiners and large steam

Within the past 10 years, significant progresses
have been made in the modeling of iodine
volatility in the RCS and the gaseous phase in thecontainment.
In the RCS, chemical understanding of iodinetransport has been improved with PHEBUS-FP
tests as well as ISTP CHIP tests. The iodine
phenomenology in the RCS appears to be much
more complex than expected two decades ago
with the simple “CsI” assumption i.e iodine
being transported as CsI aerosols in the RCS. A
mechanistic modeling is needed to take into
account the large range of conditions that can be
encountered in various scenario of severe
accident.
The first improvements were made considering
kinetics of the “iodine system” and inextending/refining the thermodynamic databasein ASTEC with new element. Some
complementary models have to be developed tocover the influence of structure materials andotherfissionproducts.
The
possibleremobilizations of FP deposits have also to bemodeled, considering chemical transformation
onto RCS surfaces.
In the containment, significant progresses havebeen made as the modeling of five phenomena
has been recently updated or added in ASTEC
code. They are listed below:- A new model of iodine (I2 and CH3I)
interaction with paint under irradiation has
been set up
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production during corium flooding (in and exvessel)). Their influence has to be checked- The modification of the iodine speciation in
the environment that has been highlighted by
the Fukushima accident. Iodine transport andits reactivity in the atmosphere under thephotolysis and atmospheric conditions withpresence of pollutants [47] is being currently
studied in order to better evaluate its
speciation in the environment.

PASSAM: Passive and Active Systems on
Severe Accident source term Mitigation
PWR: Pressurized Water Reactor
RCS: Reactor Coolant System
ST: Source Term
STEM: Source Term Evaluation and Mitigation
THAI: Thermal-hydraulics, hydrogen, Aerosols
and Iodine

Those phenomena are listed in Table 5 with theobjectives to orientate the research programs inthis field, fill in the remaining gaps by setting
up/refining relevant models and gain confidence
in ST computations.

Table 5: Phenomena whose influence on
iodine ST needs to be completed

ACKNOWLEDGMENTS
Authors acknowledge the partners of the
PHEBUS-FP,
ISTP, NUGENIA-SARNET,
OECD-THAI and THAI2, OECD-BIP and BIP2
and OECD-STEM programs whose results havelargely contributed to increase our understanding
and develop relevant models of iodine behaviourin the RCS and containment.
NOMENCLATUREASTEC: Accident Source Term Evaluation Code
BIP: Behaviour of Iodine Project
CHIP: Iodine Chemistry in the Primary coolingsystem
FCVS: Filtered Containment Venting System
FP: fission Product
ISTP: International Source Terme Project
MIRE : MItigation des Rejets à l’Environnement
OECD : Organization for
operation and Development

Economic

Co-

PARIS: Program on Air Radiolytic, Iodine
adsorption on Surfaces

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NEA/CSNI/R(2016)5

Phenomena

Justification

Project name

In RCS
Progress with CHIP –Iodine transport in the more data expected
IRSN project
RCS
on the influence ofAg, In and Cd
Delayed FP releases
from RCS deposits

Scarce data

MIRE [38]In containment
Paint ageing
Iodine oxides
decompositiong
IOx 
 I2

Radiolytic
decomposition ofiodine multicomponent aerosols

Literature review +BIP-BIP2 data[1]. N. Hanniet and G. Repetto, "FPT-0 final
report",CEA/IPSN/DRS/SEA/PEPF
Report PHEBUS PHPF IP/99/423, (1999).
[2]. D. Jacquemain, S. Bourdon, and al., "FPT-1
final report", CEA/IPSN/DRS/SEA/PEPF rapport SEA 1/100 - Report PHEBUS FP
IP/00/479, (2000).
[3]. A. C. Gregoire, P. March, and al., "FPT-2
final report", IRSN/DPAM/DIR 2008 - 272 Report PHEBUS-FP IP/08/579, (2008).

STEM & BIP2 data

STEM data[4]. F. Payot, T. Haste, and al., "FPT-3 final
report", DPAM/DIR-2010-148 - PF
IP/10/587, (2010).

OECD
STEM2-BIP3[45, 51]g
I aer 
 I2

Gaseous homogeneous
CH3I radiolytic
Literature review &production
STEM datag
0.5I 2  CH3 R 
 CH3 I

Effect of CO on IOx
decomposition
IOx  CO  0.5I 2

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361

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HETEROGENEOUS CHEMISTRY IN REACTOR ACCIDENTS
Michael Salay*, Richard Lee, and Dana Powers
U. S. Nuclear Regulatory Commission,
Rockville, Maryland, USA*Corresponding author, tel: (+1) 301-415-2408, Fax: (+1) 301-415-6671, Email:Michael.Salay@nrc.gov

Abstract – The effects of heterogeneous chemistry on reactor accident behavior are discussed.
Heterogeneous chemistry is one of the three elements of the Nuclear Regulatory Commission’s approach
to modeling containment chemistry. The other elements are gas-phase chemistry and liquid phasechemistry. Heterogeneous chemistry is typically not considered when predicting fission product behavior
although it can influence fission product behavior and the release of fission products to the environment.
Gas-solid, gas-liquid, and liquid-solid heterogeneous chemistry are reviewed with examples provided foreach. These examples focus primarily on iodine interactions or phenomena that affect the prediction ofiodine behavior. Particular attention is given to the influence of heterogeneous chemistry on containment
modeling.

362

NEA/CSNI/R(2016)5
I. INTRODUCTION

fractions of released iodide in the containment
atmosphere as particulate material or as vapor.
Clearly, experimental results show that there canbe interchanges between particulate and vapor
iodine. Furthermore, iodine trapping by basic
solutions in containment sumps may beineffective if iodine interacts preferentially withsurfaces in the containment.

Experiments in the Phébus-FP program have put
a premium on garnering a better understanding of
the chemistry of fission products in reactor
containments following severe reactor accidents[Jacquemain, et al., 1999]. Prior to the conductof these experiments, the behaviors of various
fission products released to the containments
were treated simply. Noble gas fission products(xenon and krypton) were thought to have nosignificant chemistry aside from very modest
solubility in water. Most other radionuclides
released to containment were thought to be in theform of aerosol particles that could settle or beremoved using engineered safety systems such as
sprays and suppression pools. Iodine was viewed
as the exceptional fission product that could be
released to containment either as vapor or as
aerosol particles. Mitigation of the inventory of
iodine in aerosol form available for release to theenvironment could be done using the sameengineered safety systems and natural processes
as were applied to other radionuclides in aerosol
form. Mitigation of iodine in vapor form focusedon solvating the iodine in strongly basic solutions[Soffer, et al., 1995].

The impetus to better understand the chemistryof fission products in reactor containments
following reactor accidents has grown as a resultof the accidents at the Fukushima Dai-ichi sites
in Japan. The recovery and decommissioning of
the three damaged plants at this site arecomplicated by the presence of highconcentrations of contaminated water in thecontainment. Radiation fields produced by theradionuclides in the reactor containments and thereactor buildings must be reduced substantiallybefore major manual recovery operations canbegin. Water cycling through purification
systems as well as natural radioactive decay have
reduced the fields, but the reduction has not beenenough for recovery to begin. Indeed, it appears
that there are continuing sources ofcontamination of the water inventories in thedamaged reactors. More effective cleanup will beneeded that will necessarily involve more thanjust the purification of water. Surface cleansing
may also be required.

The Phébus-FP experiments showed, however,that a residual, essentially steady-state, amountof iodine vapor could persist in the containment
atmosphere for days despite temperatures or pHof sump waters. Much of the attentions toaccount for the experimental observations havefocused on the heterogeneous interactions ofiodine with epoxy paints on surfaces within thereactor containments. It has been known for
decades that paint can absorb molecular iodine[Rosenburg et al., 1969]. The salient finding ofseparate effects tests is that iodine absorbed bypaint can desorb as either molecular iodine (I2) ororganic iodide (nominally CH3I) vapor underirradiation [Guilbert, et al., 2007]. Furthermore,paint can absorb molecular iodine from irradiatedsolutions of iodide ion. Finally, it has been
observed that both molecular iodine and organic
iodide vapors in the containment atmosphere canbe converted to exceptionally fine particles ofiodine oxide (IOx) during irradiation [Dickinson,
et al., 2014]. These results cast some doubt about
regulatory schemes that envisage nominal

In support of its systems-level accident analysis
model, MELCOR, the U.S. Nuclear Regulatory
Commission has undertaken an effort to better
understand radionuclide chemistry in reactor
containments following severe reactor accidents.
This effort is not restricted to understanding thechemistry of iodine, but iodine is an importantfocus. The effort also seeks to understand theradiological implications of the use of emergency
coolants such as seawater used during the
Fukushima accidents.
Chemistry of interest includes aqueous-phasechemistry,gas-phasechemistry,andheterogeneous chemistry. The aqueous-phasechemistry of iodine has been investigated for
decades within the reactor safety community[Clément, et al., 2007]. There now exists anunderstanding of aqueous-phase chemistry363

NEA/CSNI/R(2016)5
adequate for the safe regulation of nuclear power
plants. The gas-phase chemistry of iodine has
been studied extensively over the last decade
within the atmospheric sciences community[Vogt, et al., 1999]. It appears that theunderstanding is sufficient that it can be adapted
to the needs for safe regulation of commercial
nuclear power plants. Heterogeneous chemistry
has not received as much attention, though evermore is being learned about the heterogeneousinteractions of iodine with paint [Bosland, et al.,2014]. The rest of this paper identifies somepertinent aspects of heterogeneous chemistry andprovides some examples that merit considerationin the design of computer models for predictionof severe reactor accident processes.

strategy has been under some debate because of
precipitates formed by phosphate ions during
design basis accidents that can clog screens used
to protect intakes for water circulated through thereactor coolant system. Vapor-liquid interactions
also arise in decontamination of bubbles sparged
through steam suppression pools and indecontamination of containment atmospheres bysprays.
The interaction of iodine vapors with water
surfaces is primarily an issue of mass transfer
both in the aqueous phase and in the gas phase.
Mass transfer considerations also affect theprediction of liquid acidification by nitric acidgenerated by radiolysis of the reactor
containment atmosphere or by hydrochloric acidgenerated by radiolysis or pyrolysis of cableinsulationssuchaschlorosulfonatedpolyethylene. Water acidification can affect thepropensity for iodine to be retained in liquidwater. In the third section of this paper, someconsiderations of the mass transport of solutes toand within liquid water are presented.

II. HETEROGENEOUS CHEMISTRY
Heterogeneous chemistry of interest for the
analysis of reactor accidents includes gas-solidinteractions, gas-liquid interactions, and liquidsolid interactions. Within the general category ofgas-solid interactions are processes such as vapor
condensation or reactions with structural surfaces
and with the surfaces of aerosol particles [Beardet al., 1999]. For the most part, these processes
are expected to be complete in the reactor coolant
system prior to the escape of fission products to
the reactor containment. Most vapor-solidinteractions of fission products are, then, outside
the scope of this paper. The exception is theinteraction of iodine vapor with painted surfaces
within containment that has been the topic of somuch recent study [Bosland et al., 2014]. The
understanding of this interaction of iodine withpaint is not complete even for engineering
purposes. The topic is excluded here from further
consideration only because it merits a more
complete discussion other authors can give it.

The interaction of contaminated liquids withsolid surfaces in the reactor containment is amulti-facetted topic. Of current interest inconnection with the recovery from the
Fukushima accident is the leaching of fission
products from reactor fuel exposed to water. Fuelmay be exposed to water in the reactor coolant
system or in the containment if core debris wasable to penetrate the reactor vessel as is thought
to be the case in Unit 1. Fuel leaching by water
has been a topic of considerable interest within
the community addressing the issues ofradioactive waste disposal. A considerable andrather sophisticated understanding of theleaching processes has been developed within
this context. It will be of considerable interest to
see if this understanding of leaching for waste
disposal can be adapted adequately for the
purposes of reactor accident analysis and forrecovery of the damaged Fukushima reactors.
Leaching is not discussed further here.

The interaction of fission product vapors withliquid water is fundamental to many of theimportant engineered safety systems for
mitigation of iodine release from reactor
containments to the environment. Vapor
interactions can occur with water pools, water
films, and water droplets. Iodine absorption inwater pools kept basic with sodium phosphatebuffers has been an important, passive, safety
strategy for many nuclear power plants. This

Recovery from severe reactor accidents can takea rather long time. Decontamination of thedamaged reactor at Three Mile Island took nearlya decade. It is anticipated that recovery of the364

NEA/CSNI/R(2016)5
damaged reactors at the Fukushima site may takeat least as long. The long-term exposure of thereactor coolant system and the containment
boundary to contaminated water raises the issueof corrosion. Corrosion over long periods cancompromise the containment boundary and thecontinued cooling of the damaged reactor fuel.
Corrosion products can affect the operability ofthe system for the post-accident cooling of coredebris by both obstructing the flow of coolant orby coating heat transfer surfaces. The corrosion
products may augment the load of solid
contaminates in coolant waters brought on byother heterogeneous chemical processes in thecontainment including the corrosion and leachingof concrete.
Corrosion per se is not discussed here. What is ofinterest is the adsorption of dissolved fission
products on corrosion products. To be sure,contaminated corrosion products and otherprecipitates in water may have to be removedprior to major decontamination of the Fukushima
reactors. Removal of these corrosion products
and precipitates may be challenging relative tothe removal or purification of containment
waters. Indeed, contamination of the precipitates
may well frustrate and delay efforts to purify
containment water. It is, then, useful to havesome appreciation of the radionuclide
contamination of suspended and deposited solids
in the containment. Some aspects of this issue are
discussed further in later sections of this paper.

𝑁𝑔𝑎𝑠 =

𝑘𝑔[𝑃− 𝑃𝑖𝑛𝑡 ]𝑅𝑇 𝑏𝑢𝑙𝑘

where:
𝑘𝑔= 𝑔𝑎𝑠 𝑝ℎ𝑎𝑠𝑒 𝑚𝑎𝑠𝑠 𝑡𝑟𝑎𝑛𝑠𝑓𝑒𝑟 𝑐𝑜𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑡 (𝑚⁄𝑠)= 𝐷𝑔 ⁄𝛿𝑔
𝐷𝑔= 𝑔𝑎𝑠 𝑝ℎ𝑎𝑠𝑒 𝑑𝑖𝑓𝑓𝑢𝑠𝑖𝑜𝑛 𝑐𝑜𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑡 (𝑚2 ⁄𝑠)𝛿𝑔 = 𝑔𝑎𝑠 𝑝ℎ𝑎𝑠𝑒 𝑓𝑖𝑙𝑚 𝑡ℎ𝑖𝑐𝑘𝑛𝑒𝑠𝑠 (𝑚)𝑅= 𝑔𝑎𝑠 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡 (8.3144621𝑥10−5 𝑏𝑎𝑟 − 𝑚3⁄𝐾)
𝑇 = 𝑎𝑏𝑠𝑜𝑙𝑢𝑡𝑒 𝑡𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒 (𝐾)𝑃𝑏𝑢𝑙𝑘 =
𝑝𝑎𝑟𝑡𝑖𝑎𝑙 𝑝𝑟𝑒𝑠𝑠𝑢𝑟𝑒 𝑜𝑓 𝑡ℎ𝑒 𝑠𝑜𝑙𝑢𝑏𝑙𝑒 𝑣𝑎𝑝𝑜𝑟
𝑖𝑛 𝑡ℎ𝑒 𝑏𝑢𝑙𝑘 𝑔𝑎𝑠 𝑝ℎ𝑎𝑠𝑒 (𝑏𝑎𝑟)𝑁𝑔 = 𝑣𝑎𝑝𝑜𝑟 𝑓𝑙𝑢𝑥 (𝑚𝑜𝑙𝑒𝑠⁄𝑚2 − 𝑠)
The mass flux of solute away from the interface
into the bulk aqueous phase, 𝑁𝑙𝑖𝑞 , is given by:
𝑁𝑙𝑖𝑞 = 𝑘𝑙𝑖𝑞 [𝑐𝑖𝑛𝑡 − 𝑐𝑜 ]where:
𝑘𝑙𝑖𝑞 =
𝑙𝑖𝑞𝑢𝑖𝑑 − 𝑝ℎ𝑎𝑠𝑒 𝑚𝑎𝑠𝑠 𝑡𝑟𝑎𝑛𝑠𝑝𝑜𝑟𝑡𝑐𝑜𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑡 (𝑚⁄𝑠) = 𝐷𝑙𝑖𝑞 ⁄𝛿𝑙𝑖𝑞
𝐷𝑙𝑖𝑞 =
𝑙𝑖𝑞𝑢𝑖𝑑 𝑝ℎ𝑎𝑠𝑒 𝑑𝑖𝑓𝑓𝑢𝑠𝑖𝑜𝑛 𝑐𝑜𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑡 (𝑚2 ⁄𝑠)𝛿𝑙𝑖𝑞 =
𝑙𝑖𝑞𝑢𝑖𝑑 𝑝ℎ𝑎𝑠𝑒 𝑓𝑖𝑙𝑚 𝑡ℎ𝑖𝑐𝑘𝑛𝑒𝑠𝑠 (𝑚)𝑐𝑜 = 𝑐𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛 𝑜𝑓 𝑠𝑜𝑙𝑢𝑡𝑒 𝑖𝑛 𝑡ℎ𝑒 𝑏𝑢𝑙𝑘
𝑙𝑖𝑞𝑢𝑖𝑑 𝑝ℎ𝑎𝑠𝑒 (𝑚𝑜𝑙𝑒𝑠⁄𝑚3 )𝑁𝑙𝑖𝑞 =
𝑠𝑜𝑙𝑢𝑡𝑒 𝑓𝑙𝑢𝑥 𝑖𝑛 𝑙𝑖𝑞𝑢𝑖𝑑 𝑝ℎ𝑎𝑠𝑒 (𝑚𝑜𝑙𝑒𝑠⁄𝑚2 − 𝑠)III. GAS-LIQUID MASS TRANSFER
Consider a fission product vapor with watersolubility characterized by Henry’s Law:
𝑃𝑖𝑛𝑡 = Ƕ𝑐𝑖𝑛𝑡
where:
𝑃𝑖𝑛𝑡 =
𝑖𝑛𝑡𝑒𝑟𝑓𝑎𝑐𝑖𝑎𝑙 𝑝𝑎𝑟𝑡𝑖𝑎𝑙 𝑝𝑟𝑒𝑠𝑠𝑢𝑟𝑒𝑜𝑓 𝑡ℎ𝑒 𝑠𝑜𝑙𝑢𝑏𝑙𝑒 𝑣𝑎𝑝𝑜𝑟 (𝑏𝑎𝑟)𝑐𝑖𝑛𝑡 = 𝑖𝑛𝑡𝑒𝑟𝑓𝑎𝑐𝑖𝑎𝑙 𝑐𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛𝑜𝑓 𝑠𝑜𝑙𝑢𝑡𝑒 (𝑚𝑜𝑙𝑒𝑠⁄𝑚3 )Ƕ= 𝐻𝑒𝑛𝑟𝑦 ′ 𝑠 𝑙𝑎𝑤 ′constant' ( 𝑏𝑎𝑟 − 𝑚3 ⁄𝑚𝑜𝑙𝑒)

Within the reactor safety community, it has beencommon to solve this mass transport problem
using the so-called “two film” model [Weber, etal., 1992]. A quasi-steady state is assumed.
Fluxes to the gas-liquid interface and away fromthe interface are equated and:𝑁𝑔𝑎𝑠 = 𝑁𝑙𝑖𝑞 = 𝑁 =The mass flux of vapor to the interface, 𝑁𝑔𝑎𝑠 , isgiven by:

𝑃𝑏𝑢𝑙𝑘 − 𝑃𝑜𝑅𝑇Ƕ[𝑘 + 𝑘 ]𝑚𝑙𝑖𝑞

where 𝑃𝑜 = Ƕ𝑐𝑜 . Introduced in 1924 by Lewis
and Whitman [1924] and re-derived many times,
the two-film model has typically adequate
365

NEA/CSNI/R(2016)5
𝜇𝑙𝑖𝑞
𝜌𝑙𝑖𝑞 𝐷𝑙𝑖𝑞
𝑈𝑆 = 0.03𝑈 ∞ = 𝑠ℎ𝑒𝑎𝑟 𝑣𝑒𝑙𝑜𝑐𝑖𝑡𝑦 (𝑚⁄𝑠)flexibility to account for observations inparticular experiments. It has been abandoned,
however, by the chemical engineeringcommunity because it has been found that results
derived from experiments of one type do nottransfer accurately to other circumstances.
Instead, the surface-renewal model of Higbie[1935] where:

𝑆𝑐 =

𝑈∞ =
𝑔𝑎𝑠 𝑣𝑒𝑙𝑜𝑐𝑖𝑡𝑦 𝑓𝑎𝑟 𝑓𝑟𝑜𝑚 𝑙𝑖𝑞𝑢𝑖𝑑 𝑠𝑢𝑟𝑓𝑎𝑐𝑒 (𝑚⁄𝑠)
Water pool agitated by falling raindrops:

𝐷𝑙𝑖𝑞
𝑁𝑙𝑖𝑞 = 2 [𝑐𝑖𝑛𝑡 − 𝑐𝑜 ]√= 𝑘𝑙𝑖𝑞 [𝑐𝑖𝑛𝑡 − 𝑐𝑜 ]
𝜋𝑡𝑒

𝑘𝑙𝑖𝑞

0.00135 𝑀𝑓 0 < 𝑀𝑓 < 0.0111 𝑘𝑔⁄𝑚 − 𝑠 2= {0.00035(𝑀𝑓 )0.7
0.0111 < 𝑀𝑓
where:
𝑀𝑓 = 𝜌𝑙𝑖𝑞 𝑄̇ 𝑣𝑑𝑟𝑜𝑝𝑄̇= 𝑣𝑜𝑙𝑢𝑚𝑒𝑡𝑟𝑖𝑐 𝑑𝑟𝑜𝑝𝑙𝑒𝑡 𝑓𝑙𝑢𝑥 (𝑚3 𝑑𝑟𝑜𝑝𝑙𝑒𝑡𝑠 𝑝𝑒𝑟 𝑚2
𝑒𝑥𝑝𝑜𝑠𝑒𝑑 𝑤𝑎𝑡𝑒𝑟 𝑠𝑢𝑟𝑓𝑎𝑐𝑒 𝑝𝑒𝑟 𝑠𝑒𝑐𝑜𝑛𝑑)
𝑣𝑑𝑟𝑜𝑝 =
𝑖𝑚𝑝𝑎𝑐𝑡 𝑣𝑒𝑙𝑜𝑐𝑖𝑡𝑦 𝑜𝑓 𝑑𝑟𝑜𝑝𝑙𝑒𝑡 𝑜𝑛 𝑝𝑜𝑜𝑙𝑠𝑢𝑟𝑓𝑎𝑐𝑒 (𝑚⁄𝑠)𝑡𝑒 = 𝑡𝑖𝑚𝑒 𝑜𝑓 𝑠𝑢𝑟𝑓𝑎𝑐𝑒 𝑟𝑒𝑛𝑒𝑤𝑎𝑙 (𝑠)or Dankwerts’ [1951] penetration model where:
𝑁𝑙𝑖𝑞 = [𝑐𝑖𝑛𝑡 − 𝑐𝑜 ]√𝐷𝑙𝑖𝑞 𝑠 = 𝑘𝑙𝑖𝑞 [𝑐𝑖𝑛𝑡 − 𝑐𝑜 ]𝑠 = 𝑝𝑎𝑟𝑎𝑚𝑒𝑡𝑒𝑟 𝑓𝑜𝑟 𝑝𝑟𝑜𝑏𝑎𝑏𝑖𝑙𝑖𝑡𝑦 𝑑𝑒𝑛𝑠𝑖𝑡𝑦𝑜𝑓 𝑠𝑢𝑟𝑓𝑎𝑐𝑒 𝑟𝑒𝑛𝑒𝑤𝑎𝑙 (𝑠 −1 )have been adopted. More complicated models
with additional parameterization exist [See forexample Seo and Lee, 1988]. These modelingdifferences are significant especially when asimulant is used in place of iodine vapor forparameterization of the model. This is especiallytrue when correlations are used to derive masstransport coefficients based on a gas other than
the fission product vapor of interest.

An extended discussion of terminal velocities offalling water droplets is provided by Clift, Grace,and Weber [1978].
Falling wavy liquid films in turbulent flow
[Banerjee, Scott and Rhodes, 1968]:

The chemical engineering and oceanographic
communities have been developing correlations
for mass transport coefficients pertinent to theanalysis of vapor absorption in water during
reactor accidents. Some that may be pertinent
include:𝑘𝑙𝑖𝑞 = 0.00293 𝑅𝑒 0.933 √𝐷𝑙𝑖𝑞
An alternative, more complicated, formulation
proposed by Won and Mills [1982] is:1⁄32
𝑘𝑙𝑖𝑞 𝜇𝑙𝑖𝑞
𝑅𝑒 𝑚 𝑆𝑐𝛽= 6.97𝑥10−9( 2 )𝐷𝑙𝑖𝑞 𝑔𝜌𝑙𝑖𝑞√𝐾𝑎
𝑔 = 𝑎𝑐𝑐𝑒𝑙𝑒𝑟𝑎𝑡𝑖𝑜𝑛 𝑑𝑢𝑒 𝑡𝑜 𝑔𝑟𝑎𝑣𝑖𝑡𝑦= 9.80665 𝑚⁄𝑠 2
𝑚 = 3.49𝐾𝑎0.0675
0.137
𝛽 = 1 − 0.055
𝐾𝑎
4
𝜇𝑙𝑖𝑞
𝑔
𝐾𝑎 =
3
𝜌𝑙𝑖𝑞 𝜎𝑙𝑖𝑞

Laminar flow over a shallow pool of water[Cuest, et al., 1999]:
𝐿𝑘𝑙𝑖𝑞
2
𝜑 1⁄3
=
) ) √𝜑𝑆𝑐𝑅𝑒𝐻(1 − 0.35 (
𝐷𝑙𝑖𝑞
𝑅𝑒𝐻
√𝜋
where:
𝐿 = 𝑙𝑒𝑛𝑔𝑡ℎ 𝑜𝑓 𝑝𝑜𝑜𝑙 (𝑚)𝐻 = 𝑑𝑒𝑝𝑡ℎ 𝑜𝑓 𝑝𝑜𝑜𝑙 (𝑚)𝜑 = 𝐿⁄𝐻
𝜌𝑙𝑖𝑞 𝐻𝑈𝑆
𝑅𝑒𝐻 = 𝜇The problem of using simulant solutes to deducethe behavior of iodine mentioned briefly above
extends to the treatment of diffusion in the liquid
phase. Molecular iodine is not inert in water. A
rapid reaction in solution is:𝑙𝑖𝑞

𝜌𝑙𝑖𝑞 = 𝑙𝑖𝑞𝑢𝑖𝑑 𝑑𝑒𝑛𝑠𝑖𝑡𝑦 (𝑘𝑔⁄𝑚3 )
𝜇𝑙𝑖𝑞 = 𝑙𝑖𝑞𝑢𝑖𝑑 𝑣𝑖𝑠𝑐𝑜𝑠𝑖𝑡𝑦 (𝑃𝑎 − 𝑠)366

NEA/CSNI/R(2016)5
precipitated by reaction with carbon dioxide
dissolved from the containment atmosphere:I2(gas) ↔ I2(aq)I2(aq) + H2O ↔ HOI + I- + H+

CO2(gas) ↔ CO2(aq)CO2(aq) + H2O ↔ H2CO3
H2CO3 + OH- ↔ HCO3- + H2O
HCO3- + OH- ↔ CO32- + H2O
CaOH+ + HCO3- ↔ CaCO3(solid) + H2OConsequently, the diffusion of the uncharged
species I2 and HOI is affected by the ionic
diffusion produced by reaction with water. The
complication of ion diffusion does not arise instudies with oxygen as a simulant and can besuppressed when carbon dioxide is used as asimulant. Methods such as those discussed byWiens and Leaist [1986] must be adapted toaccount for the electrochemical potential of theions and to deduce effective diffusion
coefficients for the uncharged species
particularly for water films on surfaces incontainment that are neither borated nor treatedwith buffer or other indifferent electrolyte.

It is not uncommon for calcium carbonate toprecipitate initially as aragonite and transform
into calcite [Morse, et al., 1997]. This
precipitation is of great interest in bothoceanography and geology, so it has been muchstudied. Among the findings of these studies isthat there can be the co-precipitation of strontium
carbonate:y SrOH+ + (1-y) CaOH+ ↔ (Ca(1y)Sry)CO3(solid) + H2OThis implies that calcium carbonate crusts thattypically form near the interface between the gas
phase and the saturated water phase in reactor
containments can be contaminated withradioactive isotopes of strontium and barium.
Experimental investigations show that thepartitioning of strontium and barium ions
between the precipitating aragonite and the fluidin a closed system follows the so-called DoernerHoskins relationship [Gaetani and Cohen, 2006]:

Complications also arise in gas phase diffusion
when water vapor is either evaporating from orcondensing on water surfaces which will nearly
always be the case during reactor accidents. The
flux of evaporating or condensing water canretard or enhance solute transport to water pools
or films. The coupling between diffusion of thesolute and the water vapor in an immobile
background gas (air or nitrogen) requires the useof Stefan-Maxwell formulations of the diffusion
equation rather than the Fickian formulation,
which requires determination of many morediffusion coefficients, or, more crudely, simple
binary diffusion.

𝑙𝑛 (1 +𝑚𝑆𝑟 (𝑠𝑜𝑙𝑖𝑑))𝑚𝑆𝑟 (𝑓𝑙𝑢𝑖𝑑)𝑆𝑟= 𝐾𝐷𝐻
𝑙𝑛 (1 +

IV. CO-PRECIPITATION OF CALCIUMCARBONATE AND FISSION PRODUCTS

where:𝑚𝐶𝑎 (𝑠𝑜𝑙𝑖𝑑))𝑚𝐶𝑎 (𝑓𝑙𝑢𝑖𝑑)𝑚𝐶 (𝑝ℎ𝑎𝑠𝑒) = 𝑚𝑜𝑙𝑒𝑠 𝑐𝑎𝑡𝑖𝑜𝑛 𝐶 =𝐶𝑎2+ , 𝑆𝑟 2+ 𝑜𝑟 𝐵𝑎2+ 𝑖𝑛 𝑝ℎ𝑎𝑠𝑒 =𝑠𝑜𝑙𝑖𝑑 𝑜𝑟 𝑓𝑙𝑢𝑖𝑑
𝐶
𝐾𝐷𝐻=
𝐷𝑜𝑒𝑟𝑛𝑒𝑟 − 𝐻𝑜𝑠𝑘𝑖𝑛𝑠 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡 𝑓𝑜𝑟 𝑐𝑎𝑡𝑖𝑜𝑛 𝐶 =𝑆𝑟 2+ 𝑜𝑟 𝐵𝑎2+ 𝑖𝑛 𝑎𝑟𝑎𝑔𝑜𝑛𝑖𝑡𝑒
Gaetani and Cohen found:605 ± 47
𝑆𝑟
ln 𝐾𝐷𝐻=− 1.89 ± 0.15
𝑇(𝐾)2913 ± 154
𝐵𝑎
ln 𝐾𝐷𝐻=− 9.0 ± 0.5
𝑇(𝐾)Hexavalent uranium and, presumably, rare earth
ions can partition into the precipitating calcium
carbonate crystals [Gabitov, et al., 2008].

If water pools come into contact with concrete
within the reactor containment, the water willleach the concrete and become saturated incalcium ion. Emergency coolant water fromnatural sources such as seawater may already besaturated in calcium ion. Additives to the coolant
such as borate or phosphate buffer may
precipitate calcium from the solution. The
potential effects of these flocculent precipitates
will be discussed in the next section of thispaper. Once the additives have been exhausted,
there can be continued dissolution of calcium ionfrom the concrete. This calcium ion can be367

NEA/CSNI/R(2016)5
and, consequently, negatively charged anions
such as molybdate [Goldberg, et al., 1996] andiodate [Machesky, et al., 1989] can absorb underacidic conditions when the surface hydroxides
are protonated:

Depending on the level of contamination, it may
be necessary to remove calcium carbonate
encrustations near water-atmosphere interfaces
within the Fukushima reactor containments andbuildings prior to the start of intensive
remediation.

S-OH2+ + HMoO4- ↔ S-OH2-O-MO3H
In the case of flocculent, calcium phosphate
precipitates suspended in solution, absorbed
molybdate anion may be incorporated into theprecipitate as a phospho-molybdate species.
Iodate adsorption can distort via the iodine massbalance equation the equilibria in solution thatlead to the formation of aqueous moleculariodine.
Suspended solids are not the only possible sitesof fission product ion adsorption from solution.
The so-called “green rusts” formed by corrosionof mild steel can absorb ions. These surface
layers are nominally ferrous hydroxide, but can
incorporate other ionic species such as chloride,
carbonate and sulfate to form different structures.
Depending on the ambient conditions, outerlayers can be converted to ferric and ferricferrous species such as goethite, (α-FeO(OH)),and magnetite, (Fe3O4). Surface hydroxides onthese species can be protonated or deprotonatedto form sites for ion absorption.

V. ADSORPTION OF FISSION PRODUCTIONS ON SURFACES
The current understanding of the aqueous
chemistries of fission products is based on
experimental and analytic results for relatively
dilute and simple aqueous mixtures. Typically,
only buffers such as borate have been included in
the solutions. Waters in reactor containments will
not be so simple. Many species will exist in
solution. But, as noted above, there will also be
suspended solids coming from precipitation
reactions and corrosion products. The sumps atThree Mile Island were a milky white fromsuspended corrosion products of aluminum –mostly Al(OH)3 and γ-AlO(OH)– as well as zinccorrosion products. Over a longer term, corrosion
products of iron will contribute to the complexityof aqueous solutions in containment.
Typically, oxides suspended in solution are
surface active. The activity can be understood interms of surface hydroxides. Let S-OH denote a
surface species such as iron ion or aluminum ion
on a suspended solid with an associatedhydroxide group pointed into the aqueous phase.
The hydroxide species can protonate or
depronate depending on both the nature of thesolid and the ambient pH of the solution:

There are well developed models for theprediction of ion adsorption on surfaces exposed
to aqueous solutions [See for examples: Bousse,
et al., 1983; Hiemstra, et al., 1989]. Thesemodels are all highly empirical and require data
for the particular solids and conditions. Amongthe most studied solids are goethite, which can beproduced by corrosion of mild steel in the post
accident environment, and aluminum hydroxide,
which can be produced by aluminum corrosion[See for example: Hingston, et al., 1972].
Experimental data do tend to be scatteredperhaps because of unmodeled peculiarities ofthe solids.

S-OH + OH- ↔ S-O- + H2O
S-OH + H+ ↔ S-OH2+
Dissolved fission product ions can absorb on
these surface sites and contaminate thesuspended solids with radionuclides. The more
highly charged the solute the more likely it is toabsorb. Cations such as Cs+ and Sr2+ absorb
under basic conditions when the surfacehydroxides are deprotonated:

Aqueous solutions expected to be present inreactor containments following reactor accidents
will be chemically complicated and include both
radioactive and nonradioactive solutes – all ofwhich can compete for adsorption sites onsuspended solids or corrosion products exposed

S-O- + Sr2+ ↔ S-OSr+368

NEA/CSNI/R(2016)5
to the solution. Rather detailed chemical
speciation models will be needed to predict theextent of radionuclide contamination of thesuspended solids and the surface materials by ion
adsorption. Some iteration between models anddata may be required to obtain reliablepredictions.

and C. Wren, State of the Art Report on
Iodine Chemistry, NEA-CSNI-R-2007-01,
Nuclear Energy Agency, Paris, France, 2007
6.

R. Clift, J.R. Grace, and M.E. Weber,
Bubbles, Drops, and Particles, Academic
Press, New York, 1978.

7.

Cuesta, F.X. Grau, F. Giralt, and Y. Cohen,“Air-water mass transfer of organics fromshallow ponds under laminar recirculation”,
International Journal of Heat and Masstransfer, 42, 165-179, (1999).

8.

P.V. Danckwerts, “Significance of liquidfilm coefficients in gas absorption”,
Industrial and Engineering Chemistry, 43,
1460-1467, (1951).

9.

S. Dickinson, A. Auvinen, Y. Ammar, L.
Bosland, B. Clément, F. Funke, G. Glowa,
T. Kärkelä, S. Tietze, G. Weber, andS. Zhang, “Experimental and modelingstudies of iodine oxide formation andaerosol behavior relevant to nuclear reactor
accidents”, Annals of Nuclear Energy, 74,
200-207, (2014).

VI. CONCLUSIONSHeterogeneous
chemistry
involvingtheinteractions of species across phase boundaries
must be considered along with aqueous-phaseand gas-phase chemistry to model fission productbehavior in reactor accidents. Heterogeneous
chemistry must also be considered in the cleanup
and remediation of reactor facilities following
accidents. Example considerations are the coprecipitation of radionuclides with nonradioactive materials such as calcium carbonate
and the adsorption of ions on solids suspended insolutions or deposited on surfaces within thereactor.
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S. Banerjee, D.S. Scott, and E. Rhodes,“Mass Transfer to Falling Wavy Liquid
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A.B.
Beard,
C.G.
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B.R. Bowsher, “Fission product vapouraerosol interactions in containment:
simulant fuel studies”, AEEW-R-2449,
UKAEA Atomic Energy Establishment,
Winfrith, UK, 1999.

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10. R.I. Gabitov, G.A. Gaetani, E.B. Watson,
A.L.
Cohen,
andH.L.
Ehrlich,“Experimental determination of growth rate
effect on U6+ and Mg2+ partitioning between
aragonite and fluid at elevated U6+
concentration”,
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11. G.A. Gaetani and A.L. Cohen, “Element
partitioning during precipitation of aragonite
from seawater: A framework forunderstanding paleoproxies”, Geochimica et
Cosmochemica Acta, 70, 4617-4634, (2006).

L. Bousse, N.F. De Rooij, and P. Bergveld,“The Influence of Counter-ion Adsorption
on the ψo/pH Characteristics of Insulator
Surfaces”, Surface Science, 135, 479-496,(1983).

12. S.
Goldberg,
H.S.
Forster,and
C.L. Godfrey, “Molybdenum Adsorption on
Oxides, Clay Minerals, and Soils”, Soil
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L. Bosland, S. Dickinson, G.A. Glowa,
L.E. Herranz, H.C. Kim, D.A. Powers, M.
Salay, and S. Tietze, “Iodine-paintinteractions during nuclear reactor severe
accidents”, Annals of Nuclear Energy, 74,
184-199, (2014).

13. S. Guilbert, C. Gomez, L. Martinet,
K. Boucault, and A.-L. Imbert, “S2-6-3
Epicur Test Report”, DPAM-SEREA-2007148, ISTP No 46, Institut de Radioprotection
et de Sûreté Nucléaire, Cadarache, France,
2007.

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22. Y.G. Seo and W.K. Lee, “Single-eddy
model for random surface renewal”,
Chemical Engineering Science, 43, 13951402, (1988).

14. T. Hiemstra, W.H. Van Riemsdijk, and G.H.
Bolt,
“Multisite
ProtonAdsorption
Modeling at the Solid/Solution Interface of(Hydr)oxides: A New Approach”, J. Colloid
and Interface Science, 133, 91-104, (1989).

23. L. Soffer, S.B. Burson, C.M. Ferrell,
R.Y. Lee, and J.N. Ridgely, Accident Source
Terms for Light-water Nuclear Power
Plants, NUREG-1465, U.S. Nuclear
Regulatory Commission, Washington, DC,
USA, 1995.

15. R. Higbie, “The rate of absorption of a puregas into a still liquid during short periods of
exposure”, Transactions American Instituteof Chemical Engineers, 35, 365-388, (1935).
16. F.J. Hingston, A.M. Posner and J.P. Quirk,“Anion adsorption by Goethite and Gibbsite
1. The Role of the Proton in Determining
Adsorption Envelopes”, Journal of Soil
Science, 23, 177-192, (1972).

24. R. Vogt, R. Sander, R. von Glasow, and
P.J. Crutzen, “Iodine Chemistry and its Rolein Halogen Activation and Ozone Loss in
the Marine Boundary Layer: A Model
Study”, J. Atmospheric Chemistry, 32, 375395, (1999).

17. D. Jacquemain, N. Hanniet, C. Poletiko,
R. Cripps, C. Wren, D. Powers, Y.
Drossinos, and F. Funke, “An Overview of
the Iodine Behaviour in the Two First
Phebus Tests FPT-0 and FPT-1”,
Proceedings of the OECD Workshop on
Iodine Aspects of Severe AccidentManagement, Vantaa, Finland, 1999.

25. C.F. Weber, E.C. Beahm, and T.S. Kress,
Models of Iodine Behavior in Reactor
Containments, ORNL/TM-12202, Oak
Ridge National Laboratory, Oak Ridge, TN,
USA, 1992.
26. B. Wiens and D. Leaist, “Multicomponent
diffusion of double salts. Sodium hydrogensulphate in aqueous solution”, J. Chemical
Society, Faraday Transactions 1, 82, 247253, (1986).

18. W.K. Lewis and W.G. Whitman, “Principles
of Gas Absorption”, Industrial andEngineering Chemistry, 16, 1215 -1220,(1924).

27. Y.S. Won and A.F. Mills, “Correlation of
the Effects of Viscosity and Surface Tension
on Gas Absorption Rates into Freely Falling
Turbulent Liquid Films”, International
Journal of Heat and Mass Transfer, 25, 223,
(1982).

19. M.L. Machesky, B.L. Bischoff, and
M.A. Anderson, “Calorimetric Investigationof Anion Adsorption onto Goethite”,
Environmental Science and Technology, 23,
580-587, (1989).
20. J.W. Morse, Q. Wang, and M.Y. Tsio,“Influences of temperature and Mg:Ca ratio
on CaCO3 precipitates from seawater”,
Geology, 25, 85-87, (1997).
21. H.S. Rosenburg, J.M. Genco, and
D.L. Morrison, Fission-Product Deposition
and its Enhancement Under Reactor
Accident Conditions: Deposition on
Containment-System Surfaces, BMI-1865,
Battelle Memorial Institute, Columbus,
Ohio, 1969.

370

NEA/CSNI/R(2016)5
APPLICATION OF THE COCOSYS IODINE CHEMISTRY MODEL AIM-3 TO PWRCONTAINMENT ANALYSES USING NEW VALIDATION RESULTS FROM RTF AND
CAIMAN EXPERIMENTS
A. Danilevich(1) , P. Levi(1), F. Funke(1), M. Bendiab(1), H. Dimmelmeier(1)*(1)
AREVA GmbH, Erlangen, Germany, Germany*Corresponding author, Tel: (+49)913190095035, Fax: (+49)913190097507, Email:harald.dimmelmeier@areva.com
Abstract – In the severe accident containment code COCOSYS, the module AIM is used for modelingiodine chemistry to predict a best-estimate radiological source term. In the current version AIM-3, 70
chemical and physical processes are calculated for 26 iodine species and 8 other chemical species. The
iodine transport between compartments by gas and water flows is considered as well according to theresults provided by the thermo-hydraulics module in COCOSYS.
In contrast to the previous version AIM-2, AIM-3 utilizes new validation data from large-scale experiments
performed for instance in the THAI and PHEBUS facilities. A particular focus is the iodine deposition on
surfaces (including the reaction with paint that can form organic iodine compounds) and possible resuspension processes as well as iodine-silver reactions in sumps. This has also led to the modification ofcertain reaction parameters and even the structure of underlying kinetic equations.
In order to quantify and analyze the difference between AIM-2 and AIM-3, and to extend the validationbasis for AIM-3, the experiments ACE/RTF 3b (performed in Whiteshell, Canada) and CAIMAN 2001/03(performed in Cadarache, France) that were calculated with AIM-2 were recently repeated by AREVA
with AIM-3. By modifying some default parameters in AIM-3 for the iodine reaction with paint to accountfor the specific properties of the paints used in the experiments, we were able to acquire very good
agreement of the COCOSYS calculations with the experimental results.
Finally, calculations of the iodine behavior in the containment during a typical severe accident scenario ina generic pressurized water reactor were performed with AIM-3 using both the original and the newparameter selection to assess the impact of the different AIM parameters on the relative distribution of thedifferent iodine species and the radiological source term.

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NEA/CSNI/R(2016)5
I. INTRODUCTION

II. ACE/RTF 3b EXPERIMENTIn the severe-accident containment code
COCOSYS, the module AIM is used formodelling iodine chemistry for predicting a bestestimate radiological source term. In the current
version AIM-3, 70 chemical and physicalprocesses are calculated for 26 iodine species and8 other chemical species. Chemical and physicalreactions between iodine species as well as
between iodine and other chemical species aremodelled by AIM. Furthermore, interfacial masstransfer between the gas phase and the sump is
considered. Intercompartment iodine transport bygas and water flows is considered as wellaccording to the results provided by the thermohydraulics module in COCOSYS.

The experiment ACE/RTF 3b was conducted in
the Canadian Radioiodine Test Facility (RTF) in
Whiteshell. The experimental steel container has
a total volume of 386 l, of which the sump takes
up 35 l. The walls of the container arecompletely painted with epoxy decontamination
paint. The surface area between the gas phaseand the painted walls amounts to 2.4 m2 and thesurface area of the immersed painted surfaces is
0.6 m2. The interfacial area between the sumpand the atmosphere is 0.41 m2. The temperatureof the sump is kept constant throughout theexperiment at 60°C and the temperature of thegas phase at 53°C. The experiment was
conducted at a constant pressure of 1 bar. At thestart of the experiment, a CsI solution is injected
into the sump to reach an initial iodine
concentration of 1.3  10−5 mol/l. In order toachieveandmaintainhomogeneousconcentrations, both the gas phase and the sump
are continuously mixed through recirculation
loops at rates of 3 l/min for the sump and 23
l/min for the gas phase. 73 h after initiation of theexperiment the pH value of the sump is changed
from 5.7 to 9.0 by injecting LiOH. Throughout
the course of the experiment both the sump andthe gas phase are subject to a constant radiationdose rate of 2 kGy/h.

In contrast to the previous version AIM-2, AIM3 utilizes new validation data from large-scale
experiments performed for instance in the THAIand PHEBUS facilities. A particular focus is theiodine deposition on surfaces (including thereaction with paint that can form organic iodine
compounds)
andpossiblere-suspension
processes as well as iodine-silver reactions insumps. This has also led to the modification ofcertain reaction parameters and even the structureof underlying kinetic equations.
In order to quantify and analyse the difference
between AIM-2 and AIM-3, and to extend thevalidation basis for AIM-3, the experiments
ACE/RTF 3b (performed in Whiteshell, Canada)
and CAIMAN 2001/03 (performed in Cadarache,France) that were calculated with AIM-2, wererecently repeated by AREVA with AIM-3.

II.1 COMPARISON BETWEEN EXPERIMENT
AND AIM-2/AIM-3 CALCULATIONS
Concentrations of iodine compounds in the sumpand in the gas phase were measured in theexperiment and can be compared with calculation
results. In Fig. 1 through Fig. 3 one can see thetotal concentration of iodine compounds in thesump, the total concentration of iodine
compounds in the gas phase and theconcentration of organic iodine in the gas phase,respectively. In addition to the iodine sources inFig. 1 through Fig. 3, iodine is deposited on thepainted surfaces in the sump and in theatmosphere. Furthermore, iodine oxides arepresent in the atmosphere. No separate
experimental values are available for iodineoxide. However, in the total concentration of all
iodine compounds in the gas phase, Fig. 2, bothIn this paper, a comparison of AIM-2 and AIM-3
results with the experiment is outlined.
Moreover, the difference between the AIM-3
results and the experimental values is addressed:
The results of a parameter variation study of theiodine reaction with painted surfaces arepresented. Key iodine-paint reactions which
affect the iodine concentration in the sump andthe gas phase are identified both for the
CAIMAN and the ACE/RTF experiment.

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NEA/CSNI/R(2016)5
the experimental values and the numerical results
include iodine oxides.
As shown in Fig. 1, the COCOSYS calculations
with both AIM-3 and AIM-2 over-estimate thetotal amount of iodine in the sump, AIM-3 being
closer to the experimental values. In Fig. 2 AIM2 provides a slightly better overall prediction of
the total amount of iodine in the gas phase. On
the other hand, AIM-2 under-estimates theexperimental values for organic iodine
compounds in the atmosphere (see Fig. 3). Here
AIM-3 is more precise than AIM-2. As a reason
for the over-prediction of the sump iodine
concentration by both versions of AIM, it wasconsidered that different paints were used for thevalidation of the AIM paint reaction constants
and in the ACE/RTF 3b experiment.

-

Reaction 37, which describes the depositionof I− on the immersed painted surfaces.

-

Reaction 19, which is the reverse to thereaction 37 and corresponds to the resuspension of I− in the sump.

The reactions 19 and 37 which govern thedeposition of I- on paint and the re-suspension
from paint were adapted to laboratory tests using
painted epoxy surfaces, see 2 for details. Apartfrom the adsorption / desorption steps 37 / 19
considered to describe fast physisorption, themodel also includes a chemisorption step
occurring at the paint, where physisorbed iodine
is slowly converted into a chemisorbed iodine
species (reactions 79 / 80. and 76 / 77). The rate
constants of the corresponding overall I- / paintmodel include a complex dependence of thereaction constants on the parameters temperature,
pH, age of the paint, etc. It is important to pointout that the model was adapted to experiments
where GEHOPON epoxy paint was used.
Therefore iodine reactions with different types ofpaints (as e.g. employed in the RTF and
CAIMAN experiments) could lead to otherreaction constants of paint-iodine reactions.
Therefore the finding that reactions 19 and 37
turn out to have significant impact on the
numerical values is in agreement with the
expectations, since most of the iodine iscontained as I− in the sump or as I− adsorbed on
paint in the sump, and an inappropriate modelingof the corresponding reaction constants would
cause to a noticeable inaccuracy in the iodine
concentrations. The age of the paint in the AIM
simulation was assumed to be 5 years. The age ofthe paint also affects the reaction constants forreaction 19 and 37 and can be another source forsystematic errors. The age of the paint cannot be
set below 5 years in the implemented iodine
model.

II.2 AIM-3 PARAMETER STUDY
It was assumed that the main difference between
experiment and simulations stems from a wrong
set of constants for iodine reactions with paint in
the sump and in the gas phase. These reactions
include deposition and re-suspension of I2 and I−
on the painted surfaces in the sump and in thegas phase, transformation between chemisorbed
and physisorbed I− in the sump and between
chemisorbed and physisorbed I2 in the gas phase,
as well as thermal and radiolytic formation oforganic iodine compounds from iodine species
on painted surfaces. In order to estimate theinfluence of these reactions on the total amountof iodine in the sump and in the gas phase, a
step-by-step variation of the corresponding
reaction constants over several orders ofmagnitude in both directions from the respective
default values was performed. The reaction
constants for different types of reactions werevaried independently such that crossdependencies could be excluded.

When considering the total amount of iodine in
the sump and in the atmosphere, other iodinepaint reactions turned out to play a minor or norole at all in the framework of the ACE/RTF 3b
experimental conditions. By adjusting reactions
19 and 37 it is possible to change the amount of
iodine deposited on immersed painted walls andconsequently the total amount of iodine in thesump. The reaction constants for reaction 19 andAs a result the amount of iodine-paint reactions
which have a significant impact (within theboundary conditions of the experimental setup)on the total amount of iodine in the sump and in
the atmosphere could be reduced to a single
reaction pair:373

NEA/CSNI/R(2016)5
37 were fine-tuned such that numerical results
for the total amount of iodine compounds in thesump are in good agreement with the experiment,
see Fig. 4. As a result of increasing the amountof I− adsorbed on the paint in the sump, adjusting
reactions 19 and 37 leads to a reduced amount of
iodine available in the sump for interfacialtransfer to the gas phase. The change of theaqueous phase parameters does not largely affect
the good agreements of gaseous iodine
concentrations, as seen in Fig. 5 and Fig. 6.
Overall, adjusting reactions 19 and 37 leads to agood agreement of experiment and simulation for
the iodine concentrations in the sump and in theatmosphere, which are both crucial for theradiological source term in containment
applications.

created in the sump and transported to the gas
phase, is being adsorbed on the painted surface inthe gas phase via the fast physisorption reaction.
Therefore no changes in the iodine
concentrations in the gas phase are observed after
increasing the radiolytic I2 formation. In contrast
to RTF, in CAIMAN there is much less painted
surface available in the gas phase – most of thesurface area is steel. The corresponding
chemisorption of I2 on steel is slower than thephysisorption reaction on paint. Consequently, asignificant change in the iodine concentration in
the gas phase can be observed when I2 is beingtransported from the sump. Also, suppressing thethermal formation of organic iodine from iodine
adsorbed on painted surfaces in the gas phase(reaction 14 in AIM-3), which is done as avariation in the calculation of the CAIMAN
2001/03 experiment, has only little effect on the
ACE/RTF 3b calculation and is thus notdiscussed here.

After the variation of reactions 19 and 37 an
additional calculation was performed, in which
the reaction constant for the radiolytic formationof I2 from I− in the sump (reaction 20 in AIM-3)
was enhanced by a factor of 20. This calculation
was carried out with the newly modified
constants for reactions 19 and 37. This additionalvariation was performed in order to bring thechanges to the reaction constants in consistency
between the ACE/RTF 3b and CAIMAN
2001/03 calculations. In the CAIMAN 2001/03
simulations, adjusting reaction 20 is crucial for
properly balancing the total amount of iodine in
the gas phase (see Section III for a detailed
discussion). Enhancing reaction 20 leads to anincreased rate of formation of molecular iodine I2from I− in the sump. As a result, the volatile I2 isbeing transferred from the sump to the gas phase,increasing the total iodine concentration in theatmosphere and in turn reducing the iodine
concentrations in the sump. Thus, reaction 20 canbe used as an adjusting mechanism for balancing
iodine between the sump and the atmosphere.

Fig. 1. ACE/RTF 3b. Total amount of iodine compounds inthe sump. Comparison between AIM-2/AIM-3 calculations
and the experimental values. Both AIM-2 and AIM-3
severely over-estimate the total amount of iodine in thesump.

The results of the additional calculation arepresented in Fig. 4 through Fig. 6. In contrast to
the CAIMAN 2001/03 experiment (see Section
III), enhancing reaction 20 by a factor of 20 for
ACE/RTF 3b calculations does not lead to asignificant change in the total iodine
concentrations in the sump or the atmosphere.
One possible reason for this insensitivity is thatin the ACE/RTF 3b calculation I2, which is374

NEA/CSNI/R(2016)5
reaction 37 (adsorption of I− on immersed painted surfaces)
enhanced by a factor of 1.368 and the reaction constant for
the reverse reaction 37 (re-suspension of I− from immersed
painted surfaces to the sump) reduced by a factor of 1000.
Apparently the modified AIM-3 calculation was brought
into very good agreement with the experimental values. The
four experimental values in the time range around 50 h areclose to zero due to a measurement error, they are plotted
but not considered for the comparison between experimentand simulation. In addition to the variation of reactions 19
and 37, an additional calculation was performed where thereaction constant for the radiolytic formation of I2 from I− inthe sump (reaction 20) was enhanced by a factor of 20 (ontop of the modified constants for reactions 19 and 37). Thisvariation was performed in order to bring the changes for
the ACE/RTF simulation in consistency with the variationsof the CAIMAN 2001/03 simulations. In the CAIMAN
2001/03 simulations reaction 20 is crucial for balancing thetotal amount of iodine in the gas phase (see Section III for a
detailed discussion). The results of this variation are
represented by the dashed green curve.

Fig. 2. ACE/RTF 3b. Total amount of iodine-containing
compounds in the gas phase. Comparison between AIM2/AIM-3 calculations and the experimental values. Bothnumerical calculations lie within an acceptable range aroundthe experimental values. The AIM-3 model seems to overestimate the total amount of iodine in the gas phase.

Fig.3. ACE/RTF 3b. Concentration of organic iodine in thegas phase. Comparison between AIM-2/AIM-3 calculations
and the experimental values. In contrast to the AIM-3
results, the AIM-2 calculations seems to under-predict theconcentration of organic iodine compounds in the gas phase.

Fig. 5. ACE/RTF 3b. Total amount of iodine-containing
compounds in the gas phase. The red line represents theoriginal AIM-3 results. The solid orange line corresponds tothe modified AIM-3 calculation as described in Fig. 5.
Apparently the modified AIM-3 calculation is in good
agreement with the experimental values. The dashed green
curve presents results for the additional variation of reaction
20 as also described in Fig. 5.

Fig. 4. ACE/RTF 3b. Total amount of iodine compounds insump. The red line represents the original AIM-3 results,
which significantly over-predict the experimental
concentration. The solid orange line corresponds to amodified AIM-3 calculation with the reaction constant for375

NEA/CSNI/R(2016)5
T = 110°C, p = 3.52 bar (absolute) for 47 h
< t < 68 h.

-

T = 90°C, p = 2.68 bar (absolute) for 72 h <t < 161 h.

The radiation source is located in the sump
such that the radiation dose rate is 5 kGy/h in thesump and 5 Gy/h in the atmosphere. Due totechnical difficulties, the experiment was not
subjected to radiation for 4.5 h < t < 19.5 h. At
the initiation of the experiment, CsI is dissolvedin the sump, resulting in the initial iodine
concentration of 2.92  10−5 mol/l. Throughout
the experiment homogeneous concentrations aremaintained by mixing gas phase and sumpthrough separate recirculation loops at rates of
150 l/h and 30 l/h, respectively.

Fig. 6. ACE/RTF 3b. Concentration of organic iodine
compounds in the gas phase. The red line represents theoriginal AIM-3 results. The solid orange line corresponds tothe modified AIM-3 calculation as described in Fig. 5.
Apparently both AIM-3 calculations are in very good
agreement with the experimental values. The dashed green
curve presents results for the additional variation of reaction
20 as also described in Fig. 5.

The concentrations of I2 and organic iodine
compounds were measured in the gas phase.
Furthermore, the amount of iodine absorbed on
paint coupons in the gas phase was measured aswell. In the sump the total concentration of all
iodine compounds was measured. In addition tothese measured concentrations, iodine isadsorbed on steel walls in the sump and in thegas phase and can be present as iodine oxides in
the gas phase, although the concentration ofiodine oxides is expected to be very low due tothe low radiation dose rate in the gas phase.
These additional concentrations were notmeasured during the experiment and therefore
cannot be compared with numerical values. It
should be pointed out that the experimental
measurements of the total iodine concentration in
the sump, see Fig. 7 and Fig. 11, are subject to asystematic error: for t > 42 h the total
concentration of iodine compounds in the sumpexceeds the total initial amount of iodine which
was dissolved in the sump at the beginning of theexperiment. Therefore only experimental datafrom the initial phase t < 42 h are used for thecomparison of iodine in the sump between
experiment and simulation.

III. CAIMAN 2001/03 EXPERIMENTThe experimental setup of the CAIMAN
2001/03 experiment features several qualitative
similarities to the ACE/RTF 3b experiment. The
experimental steel container has the total volumeof 300 l, of which the sump takes up 3.3 l. The
sump is connected to the atmosphere through anarrow bottleneck. The interfacial surface area isonly 0.0013 m2 and therefore significantly
smaller than in ACE/RTF 3b. In contrast to
ACE/RTF 3b, painted coupons are placed on thesteel surfaces in the atmosphere and in the sump
(instead of having a completely painted surface
as it is the case in ACE/RTF 3b). The total
painted surface area in the gas phase is 0.018 m2,
while with 1.9 m2 the total surface area between
the steel walls and the gas phase is much larger.
The total painted immersed surface area is0.0082 m2 and the total immersed steel surface
area is 0.014 m2. The pressure is set to 1.6 barabsolute pressure by injecting nitrogen at 20°C
before initiating the experiment. After adjusting
the pressure, the container is isolated against
particle exchange with the environment
throughout the course of the experiment. The
temperature is adjusted by heating the walls inthree phases:--

III.1 COMPARISON BETWEENEXPERIMENT AND AIM-2/AIM-3
CALCULATIONS

T = 130°C, p = 4.91 bar (absolute) for t <42.5 h.

Experimental results are compared with
AIM-2 and unmodified AIM-3 calculations in

376

NEA/CSNI/R(2016)5
Fig. 7 through Fig. 10. In all four cases AIM-2
provides better results than AIM-3. AIM-3
systematically under-predicts the experimental
values. In the case of the I2 concentration in thegas phase and the amount of iodine adsorbed on
paint in the gas volume the difference between
AIM-3 and the experiment is several orders ofmagnitude. In contrast to AIM-3, AIM-2 is ingood agreement with the experimental values.
Similar to the ACE/RTF 3b experiment, the paintused in the CAIMAN experiments was different
from the paint used for the validation of the AIM
paint reaction constants. Therefore it can again
be assumed that the difference between
experiment and AIM-3 stems from inappropriate
reaction constants for iodine-paint reactions
implemented in AIM-3. The postulatedconsequence would be that an unrealistic amountof iodine is adsorbed on the immersed painted
surfaces, therefore leading to an under-predictionof all other iodine concentrations.

37, which govern the deposition and resuspension of I− on immersed painted surfaces,
were found to have an influence on the balance
between the total amount of iodine in the sumpand the amount of iodine adsorbed on the painted
walls. The reaction constants for reactions 37 and
19 were fine-tuned such that the total amount of
iodine in the sump is in good agreement with theexperimental values. As a result, the deposition
rate of I− on the immersed surfaces was reduced
by 50% and the re-suspension was enhanced by afactor 10 (see Fig. 11). It is interesting to notethat the reaction constants for reactions 19 and
37 for the CAIMAN 2001/03 experiment have tobe adjusted in the opposite direction than theadjustments for the ACE/RTF 3b simulations as
described in Section II. The AIM-3 paint
reactions constants were modelled on theexperimental
basisofGermanTHAIexperiments. The corresponding model forreaction 19 and 37 incorporates dependencies ontemperature, pH and age of the paint.
Consequently, the supposed reason for thedifferent adjustments of reactions 19 and 37
between RTF and CAIMAN simulations is thefact that three different types of paint anddifferent pre-conditioning of the paint were used.

In the time interval between 5 h and 20 hafter the initiation of the experiment, where theradiation source in the experiment was notoperational, it can be expected that the strong
discrepancy between the experimental data andthe AIM-3 results (of around 2 to 8 orders ofmagnitude for amount of iodine adsorbed on thepaint and I2 in the gas phase) is not only caused
by variations in the paint type, but points to
deficiencies of the AIM-3 reaction set.

In addition, removing reaction 14, whichcorresponds to thermal formation of organic
iodine from physisorbed I2 in the gas phase, wasfound to improve the numerical results towards
the experimental values at the time before 20 h,where the gap between original AIM-3 results
and the experiment is exceptionally large(compare the solid and dashed red curves inFig. 12 and Fig. 14). Most likely, AIM-3 withdefault parameters strongly underestimates
chemisorption of iodine on the paint. This
process is currently modeled to be preceded byprevious physi-sorption, but chemisorption needs
to be accelerated independently from thephysisorption process. The deactivation ofreaction 14 closes the normally rather effective
reaction channel for desorption of physisorbed
iodine, thereby artificially retaining iodine on thepaint which in reality should remain bound as
chemisorbed iodine on paint.

III.2 AIM-3 PARAMETER STUDY
In order to test this hypothesis the influenceof iodine-paint reactions on the total amount of
iodine in the sump and in the gas phase wastested. A step-by-step variation of thecorresponding reaction constants over several
orders of magnitude in both directions from therespective default values was performed. The
reaction constants for different types of reactions
were varied independently such that crossdependencies could be excluded.
As a result, again most of the iodine-paint
reactions were found to have no significant
impact on the iodine concentrations in the sumpand in the atmosphere. In a similar manner to the
ACE/RTF 3b simulations, the reactions 19 andEven after adjusting the thermal formation oforganic iodine from adsorbed I2 in the gas phase377

NEA/CSNI/R(2016)5(reaction 14) and balancing the iodine in thesump via reactions 19 and 37, the simulation still
under-predicts
theexperimental
iodineconcentrations in the gas phase by about 1 to 2
orders of magnitude, as seen in the green curvein Fig. 12 through Fig. 14. Since all other paint
reactions were ruled out to have any significant
impact, the only remaining way to adjust theconcentrations in the gas phase was by balancing
the iodine concentration between sump and gas
phase. Two possible ways to achieve this goal
were identified: Increasing radiolytic formationof I2 in the sump or increasing the interfacial
mass transfer between the sump and the gas
phase. By increasing the radiolytic formation ofI2 from I− in the sump, which is governed byreaction 20 in AIM-3, the much more volatile I2
is being transferred to the gas phase, thus
effectively balancing the total amount of iodine
between sump and atmosphere. Since reaction 20
in AIM-3 is still subject to an inherent
uncertainty, it was decided to adjust theradiolytic formation of I2 in the sump via reaction
20 in order to balance iodine between the sumpand the gas phase. Reaction 20 was enhanced bya factor of 20 such that the AIM-3 iodine
concentrations in the gas phase were brought inagreement with the experimental results (seeFig. 12 through Fig. 14).

Fig. 8. CAIMAN 2001/03. Total amount of I2 in the gas
phase. In contrast to AIM-2 (blue line), AIM-3 (red line)
severely under-predicts the experimental values. In
particular, around t = 20 h the difference is around 4 to 5
orders of magnitude.

Fig. 9. CAIMAN 2001/03. Total amount of organic iodine
compounds in the gas phase. On the logarithmic scale, bothAIM-2 and AIM-3 are in agreement with the experiment.

Fig. 7. CAIMAN 2001/03. Total amount of iodinecontaining compounds in the sump. The experimental
values in the time period t > 42 h are not used for thecomparison between experiment and simulation (for details
see the main text). Both AIM-2 and AIM-3 results underpredict the experimental values.

Fig. 10. CAIMAN 2001/03. Total amount of iodine
adsorbed on paint in the gas phase. On the logarithmic scale,AIM-2 is in good agreement with the experiment. In
contrast concentration values calculated by AIM-3 underpredict the experiment by several orders of magnitude. In
particular around t = 20 h the difference is around 8 ordersof magnitude.

378

NEA/CSNI/R(2016)5
under-prediction of the experiment by the original AIM-3
calculation is corrected by this adjustment.

Fig. 11. CAIMAN 2001/03. Total amount of iodinecontaining compounds in the sump. The experimental
values for t > 42 h exceed the maximum possible iodine
concentration of 2.92  10−5 mol/l and therefore are not used
for comparison between experiment and model. The original
AIM-3 calculation (dashed red line, almost identical withthe solid red line) is almost identical to the AIM-3
calculation with disabled reaction 14 (solid red line,
corresponds to disabled reaction 14, i.e. disabled formationof organic iodine from I2 adsorbed on paint in the gas
phase). Both red lines under-predict the experimental
values. After additionally adjusting the iodine adsorption
behavior in the sump via reaction 19 and 37, theexperimental data are reproduced sufficiently well (solid
green line). Additionally enhancing the radiolytic I2
formation in the sump via reaction 20 (orange line) does notlead to a significant change in the total sump iodine
concentration in the time interval 0 h < t < 42 h.

Fig. 13. CAIMAN 2001/03. Total amount of organic iodine
compounds in the gas phase. The dashed red line represents
the original AIM-3 calculation, while the solid red line
corresponds to AIM-3 with disabled reaction 14, see Section
III. The solid green line corresponds to the AIM-3
calculation with disabled reaction 14 and adjusted reactions
19 and 37. Increasing the radiolytic formation of I2 in thesump via reaction 20 (orange line, corresponds to disabled
reaction 14, adjusted reactions 19, 37 and enhanced reaction20) leads to a better agreement between experiment andAIM-3.

Fig. 14. CAIMAN 2001/03. Total amount of iodine
adsorbed on paint in the gas phase. The AIM-3 calculations
with disabled thermal formation of organic iodine on
painted surfaces, i.e. disabled reaction 14 (solid red line),yield a significant improvement of the original AIM-3
results (dashed red line) around t = 20 h. The falling slopeof the solid red line at 15 h < t < 20 h (no radiation, nothermal formation of organic iodine from physisorbed
iodine) can be attributed to re-suspension of physisorbed
iodine (I2) from the painted surfaces to the gas phase. As
expected, additionally adjusting adsorption and resuspension of iodine in the sump via reactions 19 and 37(green line) does not influence the amount of iodine
adsorbed on the painted surfaces in the gas phase. However,increasing the radiolytic formation of I2 in the sump leads toincreased I2 deposition on the painted surfaces in theatmosphere (orange line, corresponds to modified reactions
14, 19, 37 and 20, as described in Section III).

Fig. 12. CAIMAN 2001/03. Total amount of I2 in the gas
phase. The original AIM-3 calculations (dashed red line)
significantly under-predict the experiment. Disabling thethermal formation of organic iodine from adsorbed iodine in
the gas phase (reaction 14, solid red line) leads tosignificantly increased concentration around t = 20 h ascompared to the original AIM-3 results. However, adjustingiodine adsorption and re-suspension in the sump does not
significantly affect the iodine concentration in the gas phase(solid green line, corresponds to adjustment of reactions 14,
19 and 37, as described in Section III). Finally, enhancing
radiolytic I2 formation from I− in the sump (reaction 20) by
a factor of 20 leads to an increased transport of the volatile
I2 to the atmosphere (solid orange line). Therefore the379

NEA/CSNI/R(2016)5
IV. COCOSYS SIMULATION OF A SEVERE
ACCIDENT WITH ADJUSTEDPARAMETERS FOR AIM-3

the accident no significant differences in theiodine chemistry are observed.
During the first part of the ex-vessel phase,
however, the differences in the total mass of
iodine compounds in the sump between the three
AIM-3 parameter sets can be as high as 20% (seeFig. 15). In the simulations of the ACE/RTF 3b
experiment, modifying the paint reaction
parameters resulted in sump iodine concentration
changes by a factor of 4 (see Fig. 4), whereas in
the simulation of the CAIMAN 2001/03
experiment the maximal changes amounted toapproximately 40% (see Fig. 11). The variationsin the containment simulation are thus not as
pronounced as in the experiment, which is fortwo possible reasons: a) the experimental setups
feature temperature and pH conditions which do
not necessarily closely match those during theaccident and b) the impact of the iodine-paint
reactions is more limited in the containment
simulation due to geometrical reasons, since therelative influence of surface reactions onvolumetric concentrations is reduced as the total
considered volume grows and therefore thesurface-to-volume ratio shrinks.

The AIM-3 paint reaction parameters have tobe adjusted in different ways to reproduce theexperimental results for the simulation of the
ACE/RTF 3b and CAIMAN 2001/03
experiments. In order to estimate the impact ofthese parameter changes on a containment
simulation and the corresponding radiologicalsource term, a COCOSYS simulation of a typical
severe accident sequence was carried out. As thebasic scenario a loss-of-offsite-power accident ina generic pressurized water reactor was selected.
In the course of the accident a core meltdown
and the corresponding release of mass andenergyintothecontainment
occurs.
Consequently the containment is contaminatedwith fission products, among others iodine. As aresult of the leakage from the primary circuit andthe accident mitigation measures, water is
injected into the containment which leads todeposition of fission products in the sump in thelowerlevelsofthecontainment.
COCOSYS/AIM-3
simulatestheiodinechemistry in the containment sump and theatmosphere and couples the chemical reactions to
the thermo-hydraulic mass and energy transfer
inside the containment.

It is interesting to note that when the set ofparameters used to match the CAIMAN 2001/03
experiment is applied in the containment
simulation, the total amount of iodine
compounds in the sump decreases compared tothe original AIM-3 set of parameters (see Fig.
15). This is in contrast to the behavior in thecalculations of the CAIMAN 2001/03
experiment itself, where the modifications in
AIM-3 lead to an effect in the opposite direction(see Fig. 11). This is probably caused by thedifferent initial and boundary conditions(geometrical scales, ratio of sump and gas phasevolumes, surface-to-volume ratio, iodine
concentrations, temperature, pH value, level ofradiation, etc.) and the fact that not only thereaction constants of the paint reactions but alsoof other reactions were adjusted.

In order to estimate the difference between
an AIM-3 calculation with original paint reaction
parameters and two calculations with paint
reaction parameters adjusted to match the
ACE/RTF 3b and CAIMAN 2001/03
experiments, the total mass of iodine in thecontainment sump and the total mass of iodine in
the containment atmosphere are compared
between the three cases. The results can be seenin Fig. 15 through Fig. 17. Fig. 17 is amagnification of Fig. 16 with respect to thevertical axis to allow a better assessment of thedifferences in the time period after 10 h, i.e. after
the initial fission product deposition phase when
the relative differences are most pronounced.
Note that in this severe accident scenario the startof core oxidation is at about 3 h 30 min, and thereactor pressure vessel fails at about 8 h 40 min,which means that during the in-vessel phase of

The maximum change in the total mass of
iodine in the containment atmosphere amounts toapproximately 30% (see Fig. 17). In the
ACE/RTF simulations the change wasapproximately between 10% at t = 60 h and 50%
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NEA/CSNI/R(2016)5
at t = 120 h, see Fig. 5, while modifying paint
reaction constants and radiolytic I2 formation in
the CAIMAN 2001/03 simulation resulted inchanges of the atmospheric iodine concentrations
over several orders of magnitude (see Fig. 12 andFig. 13).

Fig. 17. Accident simulation with COCOSYS/AIM-3. Totalamount of iodine compounds in the containment
atmosphere. Comparison between the original AIM-3
calculations (red line), AIM-3 with reaction parameters as
adjusted for the RTF experiment (green line, see Section II)
and the CAIMAN experiment (blue line, see Section III).

V. CONCLUSION
The differences between AIM-2 and AIM-3
were studied on the basis of experimental
measurements of iodine behavior in the
ACE/RTF 3b and CAIMAN 2001/03
experiments. For ACE/RTF 3b, AIM-3 withdefault parameters provides better results for the
total amount of iodine in the sump and theconcentrations of organic iodines in the gas
phase, while AIM-2 predicts the overall iodine
concentration in the gas phase more precisely.
For the CAIMAN 2001/03 experiment AIM-2
yields better prediction of all experimentallymeasured concentrations than AIM-3 withdefault parameters. As a hypothesis for thisbehavior it was assumed that the reaction
constants for iodine-paint reactions in AIM-3
have to be re-evaluated due to different types ofpaints used in the experiments. In order to test
this hypothesis, for both experiments parameterstudies were carried out with AIM-3 in order toinvestigate the influence of iodine-paint reactions
on the total amount of iodine in the sump and theatmosphere and the corresponding radiologicalsource term in containment applications.

Fig. 15. Accident simulation with COCOSYS/AIM-3. Totalamount of iodine compounds in the containment sump.
Comparison between the original AIM-3 calculation (redline), AIM-3 with reaction parameters as adjusted for the
RTF experiment (green line, see Section II) and the
CAIMAN experiment (blue line, see Section III).

Fig. 16. Accident simulation with COCOSYS/AIM-3. Totalamount of iodine compounds in the containment
atmosphere. Comparison between the original AIM-3
calculations (red line), AIM-3 with reaction parameters as
adjusted for the RTF experiment (green line, see Section II)
and the CAIMAN experiment (blue line, see Section III).

For both experiments deposition and resuspension of I− on immersed painted surfaces(reactions 19 and 37 in AIM-3) were identified
as the main mechanisms for balancing the total
amount of iodine in the sump. Fine-tuning of thecorresponding reaction constants was performed
to bring the simulation values in agreement with381

NEA/CSNI/R(2016)5
the experiment. The reaction constants had to be
adjusted in opposite directions from the AIM-3
default values for ACE/RTF 3b and CAIMAN
2001/03. This result is interpreted to stem fromthe fact that three different types of paint wereused in RTF, CAIMAN and previous AIM
validation experiments.

atmosphere and in the sump. This is not
surprising as the suppression of this reaction only
had an effect during the non-irradiated period of
the CAIMAN 2001/03 experiment. However, no
additional sensitivity analysis for reaction 14 was
performed for the ACE/RTF 3b calculation after
applying the modified set of reaction parameters
for reactions 19, 37 and 20.

In addition to iodine adsorption and resuspension, radiolytic I2 formation from I− in thesump (reaction 20 in AIM-3) was identified to be
an important mechanism for balancing iodine
between the sump and the atmosphere. By
adjusting the corresponding reaction constant theamount of iodine species in the gas phase could
be brought in agreement between the CAIMAN
2001/03experimental
valuesandthecorresponding AIM-3 simulation. Applying thesame modifications to the ACE/RTF 3b
simulation did not lead to significant changes in
the already good results for the iodine
concentrations in the sump or the atmosphere. A
possible explanation for this is that in the
ACE/RTF 3b calculation I2 that is created in thesump and transported to the gas phase is rapidly
being adsorbed on the painted surface in the gas
phase via the fast physisorption reaction, whichis also not significantly affected by increasing theradiolytic I2 formation. In contrast to RTF, in
CAIMAN there is much less painted surface inthe gas phase available for such fastphysisorption, as most of the surface area is steel.
The corresponding chemisorption of I2 on steel isa much slower process, and thus stronger
radiolytic I2 formation can considerably increase
the iodine concentrations in the gas phase.

All other iodine-paint reactions implemented
in AIM-3 were found to have no significant
impact on the total amount of iodine species inthe sump or the atmosphere.
Summarizing, paint reaction constants
apparently have to be precisely adjusted in AIM
depending on which type of paint is being
simulated with COCOSYS. With manually
adjusted paint reaction constants, AIM-3
outperforms AIM-2 when comparing simulations
and experiments.
Additionally a COCOSYS analysis of atypical severe accident scenario in a genericcontainment was carried out with three different
sets of reaction parameters, as used in theoriginal AIM-3 model, in the ACE/RTF 3b
simulation and in the CAIMAN 2001/03
simulation. In order to estimate the impact of theadjustments on the radiological source term,
iodine concentrations in the containment sumpand in the containment atmosphere werecompared between the three cases. As expected,
the impact of the variations is less pronounced in
the containment simulation than in theexperiments. However, still differences in thesource-term-relevant concentrations of about
20% to 30% in the ex-vessel phase wereobserved between the three sets of reaction
parameters. These effects must be set in relation
to other possible uncertainties that affect theradiological source term during a severe
accident. Additional studies could be used toestimate an exact (or at least a conservative) setof reaction parameters for the most common
types of decontamination paint.

For the CAIMAN 2001/03 experiment,
adjusting the thermal formation of organic iodine
from iodine adsorbed on painted surfaces in thegas phase (reaction 14 in AIM-3) was proven tohave a significant influence on the overall
amount of iodine in the gas phase at hightemperatures. The corresponding reaction
constant was set to zero in order to achieve thebest agreement between experiment andsimulation. For the ACE/RTF 3b experiment,
reaction 14 was tested within the parameter study
and was found to have no significant impact(under the boundary conditions given by theexperiment) on the iodine concentrations in the

ACKNOWLEDGMENTS

The simulations reported in this document were
funded by the German Federal Ministry of382

NEA/CSNI/R(2016)5
Economics (BMWi) in the frame of the project“Analyses of the impact of containment typicalphenomena on fission product distribution”,
project number 1501457.

-

REFERENCESNOMENCLATURE-

AIM-3 = Advanced[Weber 2009]-

CsI = Caesium iodide-

COCOSYS = Containment Code System[Klein-Heßling 2009]-

Iodine = term used without specificallyconside-ring the iodine species; dependingon the type of measurement in specificboundary conditions, it may represent thesum of several or of all iodine species-

IodineTHAI = Thermal Hydraulics, Hydrogen,Aerosols, Iodine experimental facility

1. [Bauer2007]: M. BAUER et al., "Validierungdes Iod-Modells von COCOSYS", AREVA
Report NGPS4/2005/de/0004 Rev. A, 2007

Model

2. [Weber 2009]:
G. WEBER
etal.,"Description of the iodine model AIM-3 in
COCOSYS",
GRS
reportA-3508,Gesellschaft
fürAnlagenund
Reaktorsicherheit
(GRS)
mbH,
Schwertnergasse 1, Cologne, 2009
3. [Klein-Heßling 2013]:
W. KLEINHESSLING et al., "COCOSYS V 2.4 user's
manual", Gesellschaft für Anlagen- undReaktorsicherheit
(GRS)
mbH,
Schwertnergasse 1, Cologne, 2013

I2 = Molecular iodine; the term is used if
this species is known, or if it is to bediscriminated from other iodine species (i.e.
iodine in aerosol form, organic iodide)383

NEA/CSNI/R(2016)5

IODINE SOURCE TERM COMPUTATIONS WITH ASTEC, LINK WITH PSA2-TOOLSAND FAST RUNNING SOURCE TERM TOOLS FOR EMERGENCY ORGANISATION
K. Chevalier-Jabet * (1), F. Cousin(1), L. Bosland (1), L. Cantrel (1), V. Créach (1), S. Fougerolle(1), J.
Denis(1), E. Veilly(1)(1)

Institut de Radioprotection et de Sûreté Nucléaire (IRSN), Saint Paul Lez Durance, 13115, France,*Corresponding author, tel: (+33) 42199364, Email:karine.chevalier-jabet@irsn.fr

Abstract – Severe accident (SA) phenomenology related to iodine release to the environment has beenclosely investigated at IRSN over the last decades, by conducting semi-integral or analytical experimental
programs like PHEBUS FP, ISTP (CHIP, EPICUR) and OECD/STEM and contributing to others such as
OECD/BIP and THAI. These programs have led to the improvement of knowledge and modelling of iodine
behavior in the reactor coolant system and the containment during a SA at a nuclear power plant (NPP).
Computational tools developed at IRSN for source term (ST) assessment have benefited from these
improvements.
Three kinds of codes are developed and used at IRSN for different purposes:- The Accident Source Term Evaluation Code, ASTEC, is a dedicated tool used to simulate and predict
accident evolution from the initiating event to the releases to the environment with a detailed modelling of
the accident phenomenology tackling the coupling between various processes. The accuracy of theprediction depends on the status of knowledge and uncertainties of the involved processes;- The fast running code MER is developed in order to get a rapid assessment of releases for Level 2
Probabilistic Safety Assessment. MER provides the associated iodine release for a large number andvariety of accident situations, with a reasonable accuracy;- In the field of emergency response, the fast running tool PERSAN provides a relative conservative
assessment of the releases to the atmosphere, also for a wide range of conceivable situations.
The short calculation times needed for MER and PERSAN to achieve an evaluation of a release are
obtained by imposing some features of the accident progression scenario, evaluated aside, and by using asimplified modeling. After a presentation of these three source term assessment tools, the article explains
the implemented methodology at IRSN to build suitable models for each of the three applications.
Simplifications are driven by the final need of realism or conservatism, using uncertainty modeling whennecessary. Calculation examples are given for MER and PERSAN.

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I. INTRODUCTION: PRESENTATION OF THE
TECHNICAL CONTEXT

In some situations time is a limiting factor forsource term assessment, and choices have to bemade regarding the way phenomenology is taken
into account: in L2PSA, a large number ofsituations are to be quantitatively assessed.
IRSN, acting as a Technical Safety Organization
for the French Nuclear Safety Authority, isdeveloping L2 PSAs, using its own selfdeveloped probabilistic tools. MER tool isdeveloped in support to PSA and is dedicated to
fast ST computation and it is extended(MERCOR) to also calculate radiologicalconsequences. These tools can be run from
KANT, the IRSN probabilistic Accident
Progression Event Tree (APET) development
and quantification tool. In that case, theassessment of the associated consequences ofeach given accidental sequence of the several
thousands of sequences of the whole L2PSA leadto a hierarchized vision of the overall plantsevere accident related risk : it leads to rank thecontribution to the global risk of all the initiating
events or containment failure modes, using
various risk metrics; it allows hierarchicalrepresentation of the plant safety as well as acomprehensive analysis of certain scenarios,
such as the impact of iodine chemistry andaerosols release uncertainties on emergency
planning.
Avalidation/comparisonof
MER/ASTEC results on a limited number ofaccident scenarios is currently ongoing, and ispart of the MER qualification process.

In case of severe nuclear accident, the estimation,
at a given time of the amount and nature of theradioactivity released to the environment, named“source term” (ST) hereafter in this paper, is a
key issue in several ways : emergency center
staff elaborate recommendations related tosanitary measures (evacuation of the public forexample) for decision makers ; consideringsafety assessment, for a given plant, anestimation of the source term associated to alarge variety of situations, given their frequency,
gives an overview of the risk associated to theplant. These estimations are based on the use of aset of calculation tools, each of them providing amore or less approximated description of theaccident processes and integrating to some extent
four decades of research and development but
without necessarily be at the state of the art.
The Accident Source Term Evaluation Code[1,2] (ASTEC), jointly developed by Institut deRadioprotection et de Sûreté Nucléaire (IRSN)
and
GesellschaftfürAnlagenund
Reaktorsicherheit mbH (GRS), aims at
simulating an entire Severe Accident sequence
on a nuclear power plant from the initiating eventto the release of radioactive substances to theenvironment. Historically, ASTEC has been
designed for several purposes: the first one is thevalidation and the capitalization of thecomprehension of the severe accidental
phenomenology, leading to predictability at thereactor scale. It is an integral code, as it takes
into account in a unified framework a large partof the severe nuclear accident phenomenology,
ensuring that the interactions between the various
phenomenological aspects are taken into account.
It is a multi-purpose code, as it is used at IRSN
directly for safety assessment, support to Level 2
Probabilistic Safety Assessment (L2PSA)
development, contribution to emergency
preparedness, or source term assessment.
Considering source term assessment, as ASTEC
capitalizes the last Research and Development
results on SA topics, the ASTEC source term
calculations can be seen as representative of thestate of the art.

In
the
case
of
managementofnuclear/radiological emergency situations, one of
the fundamental roles of IRSN is technicalsupport for French public authorities; for thatmatter, the assessment of the situation of the
impacted nuclear facility and its possible
evolutions in term of accident progression andradiological consequences to the public and theenvironment is performed. The main IRSN
expertise outputs are a diagnostic andprognostics of the plant status, including source
terms
and
the
associatedradiologicalconsequences, i.e. doses to the public. In that
context, source term simulations are repeatedly
performed over time as and when comprehensionof the accident progresses.

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NEA/CSNI/R(2016)5
For these two lastassumptions are made:···tools,

simplification

through comparison with experimental results;
they are presented afterwards, although thevalidation experimental base is not presented indetail for severe accident progression modules(details in this field are available in [1]).

physical models are simpler or less complete,a decoupling is performed between thephysical models: from the source term
assessment
point
of
view,
some
phenomenologicalaspects(degradation
thermo-hydraulics, ...) can indeed be seen as
boundary conditions for the fission products
behavior and transport part, no feedback
being taken into account from the latteraspects to the first ones on a first orderapproximation basis. This decoupling process
is associated to a prior assessment of theboundary conditions, which are given as input
data to these source term tools;
the plant geometry is simplified (compared to
an ASTEC representative plant input deck),
which is legitimate if the decouplingassumption validity is correct.

As previously mentioned, some modules can beconsidered as providing boundary conditions
evaluation modules: they refer to thermal
hydraulics, fuel and vessel degradation, basemat
erosion, residual power calculation, and are
related to the accident progression. Others are
directly involved in the modeling of radioactive
elements release, transport, and chemical
behavior. The description of the version 2.1 of
ASTEC, to be released in April 2015, is detailed
hereafter.

This paper gives a more detailed insight of eachof the three previously mentioned source term
tools, starting with ASTEC. An example ofsource term preliminary assessment using the
newly developed version 2.1 of the code (to bereleased in April 2015) for a 900MW PWR ispresented.
The L2PSA MER code system is described
afterwards, with its validation process; an
example of MER results based on a PWR900
example is given afterwards.
The emergency source term assessment tool andits use in Fukushima crisis context is described ina third part.
II. ASTEC
Fig. 1. ASTEC code organization scheme
ASTEC has been designed to perform SA
calculations for various light water plant types;
plant applications were for instance performed
for PWR-900, PWR-1300, KONVOI-1300,
VVER-440, VVER-1000, and EPR. New
developments are currently under progress within
the CESAM FP7 project to also deal with BWRs.
Regarding ST assessment with ASTEC, thewhole accidental phenomenology is involved;
each part of the addressed severe accident
phenomenology is handled via a dedicatedmodule, which can work either in a stand-alone
mode as well as coupled to the other modules(see figure 1). Each module has been validated

II.1 Severe accident progression modules
CESAR [3] module simulates the thermalhydraulics in the reactor vessel, the primary andthe secondary circuit. The CESAR diphasic
thermal-hydraulics modeling is based on 1Dmodeling (allowing cross flows) using control
volumes, and takes into account water, steam,and non-condensable gases (hydrogen, helium,
nitrogen, argon and oxygen). Thermal nonequilibrium is considered between phases, withthe possibility of sub-cooled liquid andsuperheated steam; a mechanical non-equilibrium
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NEA/CSNI/R(2016)5
between phases is considered too, with thepossibility of counter-current as well as stratified
flows. Specific swollen water level volumes areused to model large two-phase domains such aspressurizer. Hydro-accumulators, pumps, valves,
breaks, pressurizer spray and heater systems aremodeled specifically.

pool formation, corium slump into the lower
plenum are also modeled, including corium jet
fragmentation on contact with water.
In the lower plenum, a 2D meshing of the vessel
lower head wall, combined with a 0D approachwithin the plenum volume accounting for 3
stratified liquid corium layers (light metallic
layer, oxide pool, heavy metallic layer) and 2
possible debris layers.

CPA [4] module simulates thermal-hydraulics(including related hydrogen combustion pressurebuild up) in the containment. The discretization
through a lumped-parameter approach (0D zones
connected by junctions and surrounded by walls)
simulates multiple compartments with possible
leakages, filtered or not, to the environment or toother buildings. CPA calculates gases and water
distribution, temperature field, pressure build-up,
local heat and mass transfer to walls (free andforced convection, radiation, condensation), 1D
heat conduction in structures, as well as gas(hydrogen and carbon monoxide) combustion.
Equilibrium model or non-equilibrium model canbe used in the various compartments. Masstransfer between zones is described separately forgas and liquid flows by momentum equations
accounting for the height differences between
zone centres. Engineered safety systems such as
passive auto-catalytic recombiners (PAR) ofdifferent types, containment sprays, have beenmodeled.

At vessel lower head rupture, melt-through or
mechanical failure (plastic or creep rupture) ismodeled, accounting for the corium and waterloading on the lower head wall and also for the
possible vessel wall partial melting.
MEDICIS [6] module simulates Molten-CoreConcrete Interaction (MCCI) after vessel rupture
and corium slump in the cavity, using a lumpedparameter 0D approach with averaged corium
layers. It models concrete ablation, coriumoxidation and release of steam and noncondensable gases (H2, CO, CO2) into thecontainment. Corium remaining in the cavity
interacts with concrete interfaces. Either a wellmixed oxide/metal pool configuration or poolstratification into separate oxide and metal layers
can be assumed; the possible switch between
homogeneous and stratified pools is evaluatedthrough criteria using the superficial gas velocity,and density differences between oxide and metal.
In both cases the possibility of an upper crust
formation is taken into account. Element
speciation, thermodynamic data as liquidus
temperatures or enthalpies, thermo-physicalproperties as density, viscosity are taken into
account and assessed via the Material Data Bankof ASTEC.

ICARE[5] module describes in-vessel early andlate degradation phases phenomena. It is basedon a control volume modeling. For early phasedegradation, ICARE enables the simulations ofmechanical as well as chemical aspects ofdegradation: ballooning, creep and burst of
zircaloy fuel rod claddings, creep of control rod
stainless steel claddings for the mechanical part.
As far as chemistry is concerned, oxidation of
zircaloy or stainless steel by steam, dissolution of
UO2 by solid and liquid Zr, dissolution of Zr byliquidSilver–Indium–Cadmium
alloy,
dissolution of Zr by solid steel, oxidation,
degradation and release of B4C control rods,
oxidation/dissolution of relocating and relocated
U–O–Zr magmas, oxidation of solid debris
particles.

Between the pool bulk and the concrete interface,a convective zone and a possible conductive zone
described as a crust and a slag layer are modeled
as the corium/concrete interface; the energy
exchange between the corium and thecontainment involve radiative heat transfer, as
well as gas production.
For EPR plants, special modeling has been
performed to account for corium pouring kinetics
from the cavity towards the core-catcher; theevaluation of the melt spreading capability

Material melting, 2D movement of magmas,
debris formation and vertical collapse, molten
387

NEA/CSNI/R(2016)5
allows to evaluate the spreading radius and, thus
the spreading duration of corium, the thicknessof the spread corium and of the spread solidified
part, and finally the duration for corium to reachthe trigger and the time for the beginning of
corium quenching by water injection above
corium.

Low-volatile species include actinides and othernotable species such as technetium. Their releaseis based on the UO2 volatilization as UO3
species.
This classification and associated modeling isused either for early degradation fuel rod
configuration, but also for debris bed
configuration, the difference lying mostly insome macroscopic properties such as surface tovolume ratios.

ISODOP module, if provided an initial fuelinventory, simulates radioactive isotopes decay
and its associated power production, in different
zones of the reactor or containment. The
assessment is based on the JEFF (Joint EvaluatedFission and Fusion) database [7], including about
3800 isotopes.

Consideringthemoltencoriumpool
configuration, given the high-temperature
conditions, the chemical equilibrium is assumed
in the magma so that the release is governed bymass transfer and evaporation processes from thefree surface. Central to the modeling is thecalculation of the vapor pressures of the elementsin the molten pool, a non-ideal solution
chemistry assumption being made for phase
distribution.

II.2 Fission products related modules
ELSA [8,9] simulates the release of fission
products (FP) and structural materials from thedegraded core. It is tightly coupled with the
ICARE module.
Considering actinides and fission products, aclassification based on volatility has been
elaborated and associated to a limiting processmodeling: the volatile, the semi-volatile and the
low-volatile elements.
The release of volatile elements (noble gases,
Iodine, Cesium, Tellurium for example) is
assumed to be related to solid-state diffusion
through grains of UO2 fuel matrix, taking into
account
fueloxidation
andgrain-size
distribution. Tellurium trapping in the cladding,depending on the temperature and on the degreeof cladding oxidation is modeled. At fuelmelting, remaining volatile species located in theliquid part of the fuel are supposed to beinstantaneously released.
The release of semi-volatile fission products,
among which notable elements such areruthenium, molybdenum, strontium, barium areincluded, is assumed to be related to evaporationand mass transfer processes at the interface ofcarrier gas/solid matrix. Species like ruthenium
are, according to this modeling, slightly released
in reducing conditions and largely released when
the fuel oxygen potential is high.

388

NEA/CSNI/R(2016)5

TEST

during their candling along the rod externalsurface. Iron, nickel, chromium are released
during the candling of steel materials, using thesame type of modeling.

Description

CANDU UO2 fuel fragment,
MCE-1,
low burn up (except for
HCE2-CF2,
HCE2-LM2)
HCE2-CM2
max. temperature 1623-2273
HCE3 [30,29]
K
Air, Ar/(H2 or O2), steam
Short re-irradiated cladded
PWR fuel rod 36.7 GWj/tU
max. temperature 2070-2370
HEVA [11-14]
K
steam/H2, except HEVA6 in
H2
Cladded re-irradiated PWR
fuel rod,
VERCORS,
37 to 55 GWj/tU
VERCORS HT
max. temperature 2130-3170
and RT [16]K
Steam, steam + H2
Cladded PWR fuel rod
ORNL HI or VI 10 to 47 GWj/tU[17,26]
max. temperature 1675-2720
steam, H2 or H2/ steam
Cladded PWR fuel rod,ceramic melt
WP2-2/1 [27]57 GWj/tU
2928 K
Inert atmospherecladded SIC control rod +EMAIC [28]tube guide
Steam or steam + H2
Cladded re -irradiated PWR
fuel rod
FPT1 to 3 [9]max. temperature 2450-2650
K
Integral test
Debris bed of PWR fuel rods
+ cladFPT4 [9]max. temperature 3273 K
Integral test

ELSA models have been tested on an
experimental database involving about forty tests(Table 1), analytical or integral, including
different types of configurations, from fuelfragments up do debris beds or molten pools. The
database will be extended in a near future to the
CEA VERDON tests including MOX fuels.
Considering fuel degradation and fission
products release, the influence of fuel type, burnup, thermal conditions and carrier gas
composition was assessed. In a large number ofcases, the fuel was re-irradiated in order tofollow short lived fission products.
SOPHAEROS [10] module simulates thetransport of vapours and aerosols in the reactor
coolant system and in the containment,
approximated to control volumes, accounting forthe chemical reactions in vapour or liquid phase,and aerosol physics. Using five physical states(suspended vapours, suspended aerosols, vapour
condensed on walls, deposited aerosols, sorbed
vapours), SOPHAEROS is based either on amechanistic or a semi-empirical approach tomodel the main aerosol and liquid or vapourphase phenomena.
Considering aerosol phenomena, agglomeration,
gravitational, brownian diffusion, turbulent
diffusion are modeled. For deposition
mechanisms, eddy impaction, sedimentation,
thermophoresis, diffusiophoresis, impaction inbends, or flow contraction can be simulated.
Revaporisation, mechanical resuspension aretaken into account, based either on the “Force
Balance” model or on the “rock and roll” one. At
last, a dedicated pool scrubbing model accounts
for the retention of aerosols in the secondary sideof flooded steam generators for Steam Generator
Tube Rupture scenarios.

Table 1 Experimental validation base of ELSA
moduleFinally, for Silver Indium Cadmium (SIC)
elements and steel materials, release is describedby evaporation and mass transfer processes:
silver, indium, cadmium are released fromdegraded SIC control rods at the control rod
failure. It is followed by the release from freesurface of the control rod and its molten alloys

Liquid or vapour-phase phenomena take into
account chemisorption of vapours on walls,

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NEA/CSNI/R(2016)5
homogeneous or heterogeneous nucleation,condensation/revaporisation on/from aerosols
and walls. For chemical reactions, two different
assumptions may be taken into account: gasequilibrium chemistry, assessed through theMaterial Data Bank, or kinetics chemistry, whichmight not be neglected compared to transportphenomena when concentrations or temperatures
are too low ; this might indeed be the case in the
primary circuit, and influence the gaseous iodine
mass fraction at the break, which is an
unexpected result of PHEBUS-FP programme,
especially the FPT3 experiment involving boron.

Aerosolphysics

VapourIodineChemistryincontainment

carbide control rods : a gaseous iodine fraction of
30%/c.i. was observed at the Reactor Coolant
System (RCS) break. It was afterwards
considered that iodine chemistry in the RCS may
potentially be affected by other elements like Cs,
Ag, In, or Cd, Mo, B and this led to separate
effects tests : CHIP programme focused on
iodine interactions with caesium, molybdenum
and boron, confirming the possibility ofsignificant gaseous iodine mass fraction at the
RCS break. The effect of SIC is being
investigated in the ongoing CHIP+ program
conducted
at
IRSN.

Experiments

phenomena

TUBA

Thermophoresis, diffusiophoresis

DEPAT

Eddy impaction, resuspension in turbulent flow

LACE

Eddy impaction, bend impaction

TRANSAT

Settling, eddy impaction, bend impaction

STORM

Thermophoresis, eddy impaction, mechanical resuspension

ADPFF

Settling, eddy impaction, bend impaction

DEVAP
AERODEVAP

Condensation and chemisorption
Heterogeneous nucleation, condensation and vapour-aerosolinteraction

FALCON

Chemistry, condensation and vapour-aerosol interaction

REVAP-ASSESS

Revaporisation

CHIP

Kinetics of Cs-I-O-H system in RCSCAIMAN

Mass transfer phenomena

ACE PHEBUS RTF

Liquid and gaseous chemistries and mass transfer phenomena

Radiolysis tests

Liquid chemistry (oxidation of I−)

EPICUR

Gaseous chemistry (Org-I formation)

PARIS

Gaseous chemistry (I2 oxidation)

SISYPHE

Mass transfer phenomena (liquid-sump)

BIP

Gaseous chemistry (Org-I formation)

PhD academic studies

Liquid and gaseous chemistries, mass transfer

CARAIDAS
Iodine removal by spray
European projects of the 4th and 5th
FwP
Liquid and gaseous chemistries
Literature data

Liquid and gaseous chemistries

VERCORS [16]IntegralHCE [30]
Phébus.FP

Overall phenomenology
Table 2 Experimental Validation base of SOPAHEROS390

NEA/CSNI/R(2016)5
capture of I2 for each compartment (for CH3I theremoval is supposed to be negligible).

Considering ruthenium behaviour, OECD
STEM/START (Study of the TrAnsport of
RuThenium in the primary circuit) experimental
programme shall supplement the validation
database.

At last, adsorption/desorption of moleculariodine on painted, steel and concrete walls, ismodeled.

In ASTEC v2.1 version, the former IODE
[31,32] module reactions have been integrated asa part of SOPHAEROS module, and therefore
one single solver is used for iodine chemistry in
RCS and in containment; SOPHAEROS enables
the simulation of all iodine species incontainment, aerosol or gaseous ones, as well as
ruthenium behaviour.

Chemical reactivity in the liquid phase (i.e.
sumps) is taken into account through a number ofreactions listed hereafter:····

Iodine modelling in containment involves about
40 phenomenological models focused on the
predominant chemical reactions in the sump, in
gaseous phase, and mass transfer reactions at theliquid-sump interface or at the surfaces. The
kinetics of the chemical transformations ofiodine in the reactor containment building take
into account three kinds of processes: thermal
reactions, radiolytic reactions and mass transfer
processes. The iodine species involved in thereactions in the containment are I2, CH3I, I-, IO3-,
HOI, AgI and I2O5. Other relevant species, likeAg (from in-core control rods), Ag2O (Agoxidised in the containment), CH3R (organics), R(organic radicals), O2 and O3 are also taken into
account. For the ruthenium compounds in thecontainment chemistry, species are gaseous RuO4
and aerosol RuO2.

·
·

·

·

Considering mass transfer, exchanges between
sump and gaseous phase for diffusion/convection
processes are modeled, as well as the effect ofsteam condensation on the walls and on the sumpsurface. Transfer of non-volatile iodine oxides
towards the sump are also accounted for.

hydrolysis of molecular iodine I2,
HOI dissociation/disproportionation,
Oxidation of I- by the oxygen dissolved in thesump water,
radiolytic oxidation of I- into I2 in the sump,where oxidation depends on the production
rate of OH radicals, and where I2 reduction istemperature, pH and [I-] dependent,
reduction of iodates by radiolysis intomolecular iodine,silveriodide(AgI)
formationbyheterogeneous reactions (both Ag2O/I- andAg/I2reactions are considered, the reactionofAg2O with I- depending on the solubility of
Ag2O and AgI),
formation of organic iodide RI by
homogeneous reaction in the liquid phasewith the Taylor’s homogeneous model:
solvents are released from paint in liquid
phase, then oxidised under radiation to form
organic acids, and finally RI are formed byinteraction between I2 and solvents or subproducts,
decomposition of organic iodides in the liquid
phase, according to two possible destruction
processes (either radiolysis or hydrolysis).

The gaseous phase chemical reactivity has been
improved in the last version of ASTEC, bytaking into account latest results of the EPICUR
programme as follows :·The effect of spray on molecular iodine ismodeled through mass transfer between gaseous
phase and droplet, interfacial equilibrium at the
droplet surface and liquid mass transfers insidethe droplet being assumed; chemical reactions in
the bulk liquid are also taken into account. The
module computes kinetics of the overall gaseousiodine removal process during the droplet fall
down and the output information is the rate of

·

·

391

Ozone (O3) formation, considered as asurrogate of the air radiolysis products,
organic iodide formation, either by ahomogeneous model or by a heterogeneousmodel linked to iodine adsorption onto paint.
It consists of a 3 sites adsorption surface fromwhich organic and inorganic iodine arereleased under irradiation (Figure 2),oxidation of molecular iodine into iodineoxides (simplest form iodate) by air radiolysis

NEA/CSNI/R(2016)5··

products.- Decomposition of iodine oxides
into organic iodides,
radiolytic oxidation of organic iodine into
iodine oxides (simplest form iodate),
Decomposition of iodine oxides into organic
iodides.
(1)x

RI (short term)

kfast(RI)y

I2 desorption(short term)

kfast(I2)

I2 adsorption

Kads,I2

CH3I adsorption

(2)

(3)

z

RI (long term)

kslow(RI)

I2 (long term)

kslow(I2)

time
0s
0,2 s
3,5 s

event
12" break opening on cold leg
Reactor shutdown
HP Injection on containment highpressure criterion
3,8 sPrimary pumps shutdown on highcontainment pressure and low
pressurizer pressure criterion
1 min 07 s Injection by water tanks
1 h 21 min Loss of injection when switching torecirculation
2 h 09 min Clad burst
2 h 13 min Material structure release
2 h 16 min First corium slump
5 h 04 min Vessel rupture ; MCCI onset
3 d 07 h Containment venting system
activation
11 d
Basemat rupture

Kads,RI

Fig. 2. ASTEC 2.1 modeling of organic iodine
production in gaseous phase
Regarding ruthenium,
reactions are:
·

·
·

the

main

modeledTable 3 : PWR 900 LOCA sequence description
Reactor automatic shutdown occurs at 0.2 s on
low primary flow criterion, which triggers
turbine insulation. Steam generators normal
feedwater circuit is then isolated at 2.2 s on highwater level criterion. In the containment, thepressure build up triggers safety injection at 3.5s, itself triggering in turn the onset of safety
steam generators feedwater circuit. Primary
pumps stop at 3.8 s on high pressure level incontainment and low pressure level in primarycircuit. The accumulators discharge starts at 1
min 07 s when primary circuit pressure reaches
42 bars, and are isolated when primary pressure
passes below 15 bars. Safety injection is lost at 1
h 21 s when asking recirculation on low waterlevel criterion reached in water tank.

Decomposition in bulk phase (dry and moistair) of the ruthenium tetroxide RuO4(g)coming from the RCS into ruthenium
deposits,
ruthenium ozonation (reaction between
ruthenium deposits and ozone).
oxidation of ruthenium deposit due to theactionof
airradiolytic
products(revolatilisation from RuO2 surface deposits
producing RuO4(g) at low temperature).

DOSE module was designed to supply dose rates
for gaseous radiolytic reactions. This module
allows evaluating the dose rate in bulk gaseous
phase for each zone of the containment, as wellas the inner wall dose rate.

Sand bed filter
efficiency with prefiltering
Aerosols 99,9%
I2
90,0%
CH3I
0.
I2O5
90,0%

II.3 An example source term calculation using
ASTEC
The hereafter described sequence is one of thefirst ST sequences calculated using ASTEC
version 2.1. It is a PWR 900 MW sequence (seeTable 3 for scenario description), the initiator is a12"LOCA on the cold leg of the pressurizer loop.
Containment spray system is unavailable, as well
as safety injections systems when switching torecirculation mode.

Sand bed
filterefficiency
99,0%
90,0%
0.
90,0%

Table 4: assumed filtration efficiencies for iodine
species
Fission products and structural material are
released in a short period of time near 2 h 13 min

392

NEA/CSNI/R(2016)5(Table 3); the available silver is entirely releasedin the primary circuit, 988.28 kg being released
to the containment; considering iodine, 16.70 kg
are released from the core, among which about
43% deposits on the cold legs, under CsI, Cs2I2,
I3La, INi forms. The iodine release to thecontainment amounts to 9,5 kg, 2% of the total
being under gaseous form.
The first corium slump in the reactor vessel
occurs at 2 h 16min, and the vessel breaks down
at 5 h 04min. 65 tons of corium slump down in
the reactor pit. Secondary slumps occurring over
two hours duration period lead to a final total
amount of 140 t of corium in the reactor cavity.
The reactor pit being made of limestone iron
reinforced concrete, the gas emission tend to
swirl and homogenize the corium, and therefore
radial and axial ablation have the same kinetics.

Fig. 3. Iodine release from core

The MCCI induced gas production tend toaccelerate pressure build up in containment,
leading to containment venting system activation(CVS) at 3d 7h on the reach of high pressure
criterion in the dome (Figure 4). At this time, theiodine species in the containment atmosphere aregaseous species, the aerosols being entirely
deposited (Figure 5 and 6). In sumps the pH is
5.0 because no soda is injected, and thetemperature is high (~400 K) because of theabsence of recirculation cooling in the consideredscenario. Therefore, after CVS activation, whenpressure starts to decrease, evaporation
conditions in sumps are met, and iodine istransferred to the containment atmosphere; thesand bed filter is supposedly inefficient fororganic iodine, and an efficiency of 90% formolecular iodine and oxides is assumed (Table4). In these conditions, the overall iodine releaseto the environment through the FCVS is ~0.15g,
leading to a global iodine mass release of 0.19 g
considering direct leaks (Figure 7).

Fig. 4. Pressure in containment

Fig. 5. Iodine distribution in containment393

NEA/CSNI/R(2016)5
III. LEVEL 2 PROBABILITIC SAFETYASSESSMENT: MER
III.1. Presentation
The risks reduction obtained by improved or new
accident prevention and mitigation measures isassessed at IRSN using Level 2 PSA bycomparing the situation before and after taking
into account safety improvements, notably interms of reduction of radiological consequences.
The MER code [33,34] for environmental
releases estimation is fast-running with a
precision fitted to the purpose.
MER takes into account the boundary conditions
elaborated by the APET and its associatedsupport calculations, summarized in Table 5.
Fig. 6. Iodine speciation in containment
gaseous phase

For a given sequence, MER can produce somedetailed post-processing. Last years have been
devoted to the achievement of a realistic
modeling of nuclear accident consequences andto enhancement of legibility, for acceptance indecision making process. That includes theextension of the traditional radioactive releaseassessment to the evaluation of radiologicalconsequences, based on MERCOR module.
There are two MER models at IRSN, one for the
900 MWe French PWR and the other for the1300 MWe French PWR, a model for EPR being
planned for the future. The 1300 MWe PWR
model includes recent results from the IRSN
R&D on ST modeling, notably those of the ISTP
and STEM programs. The 1300 MWe version
includes a modeling of double containment andmore detailed modeling for the chemical species.

Fig. 7. Iodine release to environment (U5 =through FCVS, direct = direct leaks)

MER is based on the resolution of massconservation equations, allowing the descriptionof different kind of phenomena such asmechanical phenomena like the deposition, or
chemical phenomena like radiolysis reactions.

This simulation is a preliminary one with therecent release of ASTEC v2.1. It has been
performed with best estimate modeling using
most recent research and development data. The
order of magnitude of remaining iodine related
uncertainties is ongoing and their influence will
be evaluated in the future for an entire accidental
sequence, using the IRSN uncertainties
propagation tool, SUNSET.

According to the importance of the different
phenomena on ST, several approaches can beused to construct these models for MER:·

394

a first approach consists in integrating thecomplex models and simplifying the
calculation of some non-preponderant
physical parameters of the models; theNEA/CSNI/R(2016)5··

identification of non-preponderant physicalparameters can be performed by different
methods like experimental designs or expert
judgments;a second approach consists in integrating
directly R&D program results; to illustrate
this approach, in case of SGTR accident,
MER evaluates the retention in the primaryand secondary circuits thanks to ARTIST
results;
an approach consists in designing amathematic correlation based on a particularassessment; for example, MER uses a
correlation for modeling the aerosol behavior
based on ASTEC support studies for reactor
cases. The aerosol behavior within thecontainment building is subjected to different
phenomena
like
agglomeration
anddeposition; it is not possible to evaluate
simply the particle size evolution to model theagglomeration. This is the reason why we use
a correlation for the evaluation of deposition
coefficient which englobes all the deposition
processes; the last approach consists in
integrating a model based on engineer
judgment because of a lack of knowledge.
This approach allows the evaluation of theimportance of the phenomenon through a risk
or consequence assessment.
The simplified models related to FP behavior are:
·
·
·
·

·

·

395

the in-vessel release of fission products;
the fission products retention in the primaryand secondary circuits;
aerosols resuspension mechanisms inside theprimary system;
ex-vessel release of fission products: releaseof aerosols from a boiling sump, releaseduring molten corium-concrete interaction,
aerosols resuspension mechanisms inside thecontainment building (in case of hydrogen
combustion, in-vessel steam explosion,
reflooding);
the behaviour of the aerosols within thecontainment building: agglomeration anddeposition, aerosol depletion by spray;ruthenium and iodine chemistry.

NEA/CSNI/R(2016)5

In-Vessel core degradation

APET Phase

RC variable
CHRS availability

Range
Yes/ partially/ No

Containment break size

0.065 cm2 to total failure

Delay before containment failure
Delay before vessel breakNo failure
At the beginning of the phase
At the end of the phase
From 1 hour to 16 hours

RCS fission products retention

Yes/noCore reflooding

Yes/noEnergetic phenomena in RCS

Yes/noAverage primary pressure

From 3 to 80 bar

SGTR

No SGTR,
SGTR with empty SG
1 or 2 tube(s)10 tubes
Induced SGTR
0.065 cm2 to 50 cm2

Venting system functioning

yes/partially/noFiltration type

Iodine filters
High efficiency filters
No filtering
Yes/no

V-LOCANo energetic phenomenon
ex-vessel steam explosionhydrogen combustion
0.065 cm2 to total failure

Energetic phenomena in containment

Ex-vessel phase

VesselruptureContainment break size

CHRS availability

No rupture
Non energetic rupture
Steam explosion
Yes/ partially/ No

Containment break size

0.065 cm2 to total failure

Leakage by containment isolation failure ( mode)

0.065 cm2 to 50 cm2First containment failure mode

Delay before containment failure

No failure,
H2/CO combustion,
U5 filter (FCVS),
Containment slow pressurization
Failure of containment isolation valveshydrogen combustion,
ex-vessel steam explosion,
0 hours to up to 30 days

Venting system functioning

yes/partially/noFiltration type

Iodine filters
High efficiency filters
No filtering

vessel rupture mode

Energetic phenomena in containment

Table 5 accident progression variables

The modelling of iodine behaviour in MER takes
into account the following issues:·

the fraction of gaseous iodine released fromthe RCS;
the iodine oxides (IOx) formation anddestruction in the gaseous phase by thereactions between I2 or organic iodine with
air radiolysis products;

·

·
·

·

396

the iodine adsorption/desorption onto paints
in gaseous phase;the production of organic iodine in theatmospheric part of the containment fromiodine adsorbed onto paints;
destruction of organic iodides in the gaseous
phase;

NEA/CSNI/R(2016)5····

the radiolytic oxidation of iodide ions intomolecular iodine (I2) in the liquid phase;the iodine adsorption onto paints in liquid
phase;
I2 formation by metallic iodine reaction into
the recombiner.
the formation of AgI by reaction with iodide
ions and molecular iodine (I2)···

The modeling of ruthenium is based on:
·
·
·
·

·

·

the release in case of degraded fuel contact
with air,
the release into the containment building ingaseous form (ruthenium tetroxide),
the adsorption and desorption of gaseous
ruthenium on metal and painted surfaces,
the re-volatilisation of ruthenium depositedon the surfaces, under irradiation and ozone
action,the re-volatilisation of ruthenium trapped in
the liquid phase under irradiation and ozone
action.

·

L2 PSA allows the risk assessment by integrating
uncertainties. These uncertainties are related tomodeling error, lack of knowledge about
physical phenomena, or scenario uncertainties. In
L2 PSA, the uncertain parameters can begathered in four groups:

In a L2 PSA, the gaseous form of ruthenium(ruthenium tetroxide) is considered as a nonfiltered species.

·

The following species are taken into account:
·
·
·
·
·
·
·
·
·
·
·

·

noble gases;
volatile aerosols;
semi volatile aerosols;
ruthenium aerosols;
ruthenium oxide (RuO2);
ruthenium tetroxide (RuO4);
iodine aerosols (aerosols containing I suchas CsI);
molecular iodine I2;
organic iodine ICH3;
iodine oxide IOx,
iodide ion, molecular iodine I2, AgI andorganic reactive compounds contained in thepaint in sumps.

·

·

·
·

concerning the accidental scenario (mainly
the containment break size for certain types ofcontainment failures, the duration of in-vesselcore degradation phase and molten corium
concrete interaction phase, etc.);
concerning the aerosol behavior (mainly
releases from fuel, retention in circuits,
fraction of wall-deposited aerosols that
subsequently undergo resuspension, etc.);
concerning the iodine behavior (mainly thefraction of gaseous iodine released at the RCS
break, the iodine adsorption/desorption onto
paints in gaseous phase, the production oforganic iodide from paints etc.);
concerning the ruthenium behavior (mainly
releases from fuel, retention in circuits, etc.).

The evaluation of their impact is assessed using aprobabilistic Monte Carlo method based on Latin
Hypercube Sampling. For each of thesenumerical
results,
statistical
distribution
percentiles are evaluated: 5%, 50%, and 95%.
The average value is also calculated.

The outputs of the assessment are the following:·fission products retained mass in the filtersof ventilation systems;
dose equivalent and thyroid dose at given
points (in the axis of the plume) versus time(corresponding to total release and eachpathways);deposited activities 2 weeks from the
beginning of the accident, among which
137Cs deposited activity (corresponding tototal release and each pathways);data linked to emergency preparedness: thetime when counter-measure levels arereached. At a given time, the geographicalextent of each countermeasure;the calculation of the INES level of theaccidental sequence.

environmental release (mass or activity) for
each fission products group, for each phaseof the accident and each release pathway;suspended activity/mass in the buildings;
fission products deposits in the buildings;

III.2. Validation process
The MER validation process includes three steps.
To illustrate the validation process, the iodine

397

NEA/CSNI/R(2016)5
behavior into the containment (liquid and gas
phases) is given thereafter.
Before performing an ASTEC/MER comparison
for different sequences, the unitary validation of
MER models is realized first by comparison with
ASTEC models and a specific assessment. The
unitary validation corresponds to a comparisonof the results of a chemical model only in theliquid or gaseous phase. Figure 8 illustrates thisstep by presenting the comparison of theradiolytic oxidation of iodide into moleculariodine between MER and ASTEC.

behavior of fission products in the containment,in particular the different iodine species, as
illustrated by Figure 11.

Figure 8: radiolytic oxidation of iodide intomolecular iodine, unitary validation
After comparing each elementary reaction, an
overall validation is performed for each phase(gas and liquid). The MER model considers alimited number of reactions representative of themain reactions occurring in the liquid phase.
Figure 9 illustrates the impact of these
simplifications.
Figure 9: iodine behaviour in liquid phase overall validation

As mentioned before, the last stage of MER
validation process uses ASTEC code fordifferent accident categories (LOCA, SGTR,
blackout situation etc.). The validation consistsin comparing the ST kinetics and amplitude interms of released activities and masses as shownin Figure 10. It is thus necessary to compare the

The validation of iodine behaviour can bedetailed with the comparison of chemical
reaction kinetics allowing the identification of
preponderant reactions.

398

NEA/CSNI/R(2016)5
developed and is operationally used to meet theneeds of the emergency responder experts. Eachtool of the SESAME suite is devoted to one
specific field of expertise. Among these
8 specific tools, two thermo-hydraulic softwares
are devoted to the determination of the break size
and the delay before the core may be uncovered.
The information given by these tools is then used
as input for the two different ST calculation
tools, one devoted to Steam Generator Tube
Rupture (SGTR) situations the other made forLOss of Coolant Accident situations (LOCA).
Figure 10: ASTEC/MER comparison: elements
and isotopes activities.

The fast-running Program for Evaluation of
Release during a Severe Accident on a Nuclear
reactor, PERSAN, assesses the releases to theatmosphere for LOCA situations. This
operational software has been designed anddeveloped to be used by experts during
emergency situations. One of the main constrain
which has been imposed throughout theconception and the development of the software
is a very short computing time; in order to beable to assess iteratively some key parameters of
the plant during the diagnosis stage of theexpertise. Furthermore, some sensitivity studies
are also needed to estimate the influence of someuncertainties on the atmospheric releases(i.e. equipment availability), require that thesoftware to run rapidly. The computing time of
PERSAN is lower than one minute to calculate
24 hours of release. The accident progression andphysical phenomena occurred in the reactor does
not change the computing time.

Finally, after performing the validation with
ASTEC, a comparison can be realized withPERSAN, although model assumptions aredifferent. MER performs realistic calculation
with uncertainties evaluation which corresponds
to the R&D knowledge, while PERSAN
performs more consolidated and conservative
calculation. The 95 percentile of MER
calculations could be compared to the PERSAN
result while the median could be compared to the
ASTEC calculation.
III.2 An example source term calculation using
MER
To illustrate an example of MER use, we havechosen to present the results of the French
900 MWe PWR PSA level 2 study. Figure 12
details the global risk of the French 900 MWe
PWR. For this, the release into environment, theradiological consequences and the INES level
were calculated with MER.

Getting reliable information on the plantcondition is complex and uncertain during thedevelopment of the accident. Consequently, thelack of information must not prevent theestimation of STs. The Fukushima accident
confirms this point where evaluations ofradiological consequences where mandatory toprotect the surrounding population but also togive the world a general understanding of thedeveloping situation. Moreover, limiting thenumber of hypothesis as input of expertise
emergency response software is a paramountconcept to insure a proper expertise process.

At this step, for this study, around one thousand
scenarios of release are found, associated with anenvironmental release for each phase of theaccident and with radiological consequences.
IV.
EMERGENCY
SOURCEASSESSMENT: PERSAN

TERM

IV.1. Presentation
For the technical emergency center staff a suite
named Software for Emergency Situations andAssessment Methodology, SESAME, has been

The main input assumptions in PERSAN code
are: buildings leak rates, fuel damage,399

NEA/CSNI/R(2016)5
containment pressure evolution, long term
availability of containment spray system and ofventilations systems in the auxiliary buildings.
The measured buildings leak rates obtained
during containment building tests are used as
default input assumptions. STs calculated by
PERSAN are realistic but are believed to beconservative in term of quantities released or
chemical form produced in regards with theradiological consequences. The uncertainties
which still exist on many physical phenomena
arising during a core melt require, for emergency
response, to use a conservative approach for theselection of model parameters as, for example,
for the release rate of some chemical element inprimary circuit.

an important source of molecular iodine forFrench 1300 MWe and 1450 MWe PWRs. The
iodine aerosols emitted in the building aredeposited on walls, sumps, etc. The deposition
repartition between all elements in buildings is
hard to determine and requires a detailed
modeling. PERSAN uses a simplified modelingwhich insures an upper bound evaluation of the
ST. The deposition model considers that all
iodine aerosols are deposited in sumps.
The liquid phase model is based on ASTEC
model. The amount of molecular iodine releaseby sumps depends of the liquid volume, the pH,
the temperature, the silver concentration and thedose rate. All these parameters, except the silver
concentration, are to be set before the start the ST
calculation. The silver emission from control
rods is not modeled in PERSAN. A
representative silver concentration is imposed in
the model.

The PERSAN models are based on fission
products mass balances. The FP concentration insuspension in each building depends of the massemitted in the building, the mass deposit and thebuilding leak rate. Several correlations are used
for the aerosols deposition. These correlations
depend mainly on the building type, the mass insuspension and the thermohydraulic conditions.
The deposition laws in containment building
vary during FP emission, after the emission, withthe use of spray system and during concreteinteraction with melt core. Especially, themolecular iodine is deposited in the containmentbuilding on painted wall and by the containment
spray system. All deposition correlations havebeen calculated with an ASTEC module. The
validation has been realized with ASTEC with alarge representative sample of accident scenarios.

In the case of 900 MWe French PWR, the silver
concentration in sumps is considered sufficiently
high to trap all iodide. No molecular iodine isreleased from sumps for this power plant in
PERSAN.
The recent research advances on iodine
chemistry in containment building alreadypresent in ASTEC and MER codes are not yet
integrated in PERSAN. This work is underprogress. The integration of a new model in crisis
tools follows a lengthy validation process andrequires secure and well established results from
R&D. Indeed, the ST evaluations from PERSAN
are used to make recommendations to the French
public authorities relative to the population
protection during an emergency situation basedon calculated doses to the public. The
government uses also the calculated protected
doses from different type of accident to define
preparedness strategies and the management of asevere nuclear accident in France. Thus, theimpact of any new model of a process related to
ST released into the atmosphere must be
carefully evaluated in term of impact on theradiological consequences and on the population
protection strategies.

The current iodine chemistry model in liquid andgaseous phases has been developed by IRSN
R&D and emergency experts few years ago.
In the gaseous phase, a part of the moleculariodine is adsorbed by painted walls. The
adsorption rate is given by empiric formulawhich depends of wall temperature. The
production of organic iodine by paints is
simplified. The molecular iodine adsorbed bypaints is immediately transformed into organic
iodine on the basis of a conversion coefficient.
The iodine behaviour in the liquid phase is more
complex. The sumps of containment building are400

NEA/CSNI/R(2016)5
The last produced research on iodine chemistry[32]] showed a large production of oxide iodine
in containment building. This specie was notconsidered in previous models. Uncertainties
associated to this new model must be listed aswell as the parameters not reachable inemergency situation.

analyzed before to being implemented for the ST
calculation in emergency situation.
Concerning iodine chemistry in the reactor
building, results of the PHEBUS program havebeen taken into account for the modeling of therelease rate of molecular iodine in the primarycircuit. The uncertainty on this parameter isimportant. Therefore, in order to be reasonably
conservative, 25% has been added to the
maximum experimental measured rate. The restof iodine is emitted in aerosol form.
The fission products retention in the primarycircuit is an input hypothesis and can be
modifiedbythecontainment
expert.
Nevertheless, the localization of the break during
an emergency response being difficult, thisretention rate is set to zero by default, which isequivalent to setting a default location of the
break on the hot leg, ensuring conservatism.

Figure 11: ASTEC/MER comparison: iodine mass
speciation at 1 day and 2 days (kg).

The realization of all these steps requires timeand explains the modeling difference between
PERSAN and ASTEC on iodine model.

IV.2. An example of use: the Fukushima accident
source term assessment
During the Fukushima accident, several
evaluations of radiological consequences havebeen realized by IRSN technical emergency
centre with STs provided by PERSAN. These
calculations require the evolution of the releaseover the time. Around 40 STs have been
calculated during the first two weeks of activityof the IRSN technical emergency center, fromMarch, 11. An example of Fukushima ST
published on 18 March 2011 is represented infigure 13. The calculation starts on 11 March2011 and finishes on 24 March 2011. Each peakcorresponds to a containment depressurization
which occurred on the damaged reactors.

Figure 12: Overall results (IRSN L2 PSA 900
MWe PWR) – Frequency-Consequence diagram– Ines Scale
Afterwards, a simplified modeling shall be
developed in accordance with the uncertainties
identified and the constraints for an operationaluse. The model must be validated with ASTEC
comparisons. In parallel, a study must be done on
thyroid dose associated to this specie (actually nodose coefficient is known for this chemicalform). Finally, the consequences of this new
iodine chemical model must be studied and

V. CONCLUSION
This paper presented various source term
assessment tools used at IRSN on different
purposes: from the validation of phenomenology
comprehension and capitalization, ASTEC is an401

NEA/CSNI/R(2016)5
integrated tool used for predictive calculations,representative of the knowledge status at a given
time.

EPR
FP
GRS

European Pressurized water Reactor
Fission productGesellschaftfür Anlagen und
Reaktorsicherheit mbH
INES International Nuclear Event Scale
IRSN Institut de Radioprotection et deSûreté Nucléaire
L2 PSA Level 2 Probabilistic Safety
Assessment
LOCA LOss of Coolant Accident
MCCI Molten-Core-Concrete Interaction
PERSA Program for Evaluation of ReleaseNduring a Severe Accident on a
Nuclear reactor
PWR
Pressurized Water Reactor
RCS
Reactor Coolant System
SESAM Software for Emergency Situations
E& Assessment Methodology named
SGTR Steam Generator Tube RuptureST
Source Term
SIC
Silver Indium Cadmium
VVER Vodo-Vodianoï Energuetitcheski
ReaktorFast running tool MER, based on a fitted topurpose precision modeling provides the ability
to produce a global insight regarding severe
accident related probabilistic safety assessment.
The MER validation is based partly on
ASTEC/MER comparison.
Finally, conservative modeling for emergency
fast source term assessment tool PERSAN, andthe cautious incorporation of research anddevelopment results provides French publicauthorities a relevant decision making tool.
For ASTEC and MER, possible uncertainties
have to be considered. These are currently under
re-assessment at IRSN, for instance integrating
the results of R&D programs like OCDE STEM
program. Models related to iodine behavior insumps are now considered mature; a reasonably
conservative model, based on remaining
uncertainties, can therefore be elaborated for fastrunning tool PERSAN.

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404

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IN WHAT EXTENT THE IODINE REACTIVITY IN THE ATMOSPHERE CAN IMPACT
THE RADIOLOGICAL CONSEQUENCES
J. Trincal(1,3)*, L. Cantrel(1,3), F. Cousin(1,3), V. Fèvre-Nollet(2,3), P. Lebègue(2,3)(1)

Institut de Radioprotection et de Sûreté Nucléaire, Saint-Paul Lez Durance, France, (2)Laboratoire dePhysicoChimie des Processus de Combustion et de l'Atmosphère (PC2A), UMR 8522 CNRS/Lille 1,
France, (3)Laboratoire de Recherche Commun IRSN-CNRS-Lille1 "Cinétique Chimique, Combustion,
Réactivité" (C3R), Saint-Paul Lez Durance, France*Corresponding author, tel: (+33) 4.42.19.91.87, Email: julien.trincal@irsn.fr

Abstract – Following a severe accident on a nuclear reactor, the main radiological consequences are due tothe quantity of deposited radionucleide, and more specifically iodine. This quantities can be evaluated byseveral simulation codes, through deposition velocities of each involved species.
However, significant differences between measurements and modeling of iodine radiation levels resulting
from the Fukushima nuclear accident have been observed. The gap could be at least partially explained by thefact that the IRSN dispersion tool consider no chemical transformation during transport in the atmosphere,
which can appear as a rough assumption because some parameters like the deposition velocity or the doseeffect factor depend on iodine chemical speciation as well as physical forms (gas or particle).
In order to quantify the behaviour of the iodine reactivity in the atmosphere, a chemical mechanism of
246 gas phase reactions has been implemented in 0D simulation software. For simulations, iodine is injected asgaseous I2 and/or CH3I, which are some iodine species potentially released during a nuclear incident. The
calculations are made on 10 days in representative atmospheric conditions, and the influence of many
parameters are evaluated, such as the pollution levels (O3, NOx, VOC), the photolysis conditions, the form andthe rate of the injected iodine and the reaction mechanism (inorganic, organic).
Concerning the inorganic simulations, result analysis revealed that depending on the simulation parameters,
the iodine speciation could be very different. Indeed, iodine is distributed between HI and HIO during the day
and between I2 and IONO2 the night, which highlight the important impact of day/night cycle. The modificationof the photolysis rates (calculated for an other date) indicates that the iodinated species photolysis is afundamental element of the mechanism, leading to the formation of I and IO radical, which are the central
species of the iodine reactivity.
Moreover, I2O5 is product all along the calculation from iodine reactions with O3, pointing out the pollutant
role in iodine reactivity, especially the O3/NOx ratio, O3 being produced from the NOx (Chapman cycle).
The iodine injection rate variation also showed that iodine reactions with NOx constitute a high reactive
cycle, involving I, I2, IO, INO2 and IONO2. With a low injection rate (10-4 ppt.s-1), the iodine is mainly under
IONO2 form, I2O5 become predominant for higher rate (1 ppt.s-1).
Regarding the organic simulations, results were very differents. In this cases, the iodine reacts with the VOC
to form iodocarbons. However, the organic mechanism is still not very well known, several species can beproduced without any destruction reaction like photolysis process, leadind to an accumulation in thesimulations. The results reavealed that the main iodine forms are iodo-cyclopentane, iodo-methylbenzene andsome other iodo-alcanes.
Radioactive iodine speciation in the atmosphere appears to be very dependent on the conditions, and inparticular the photolysis process, the pollutant levels and the iodine injection rate. The species form being
important in the determination of the radiological consequences, it is relevant to include the iodine reactivity inthe dispersion tools. In parallel, the organic mechanism have to be improved to reduce the lack of knowledge
around the iodo-carbons.

405

NEA/CSNI/R(2016)5
I. INTRODUCTION

II. REACTIONNAL MECHANISM OF IODINE
IN THE ATMOSPHERE

Following a severe accident on a nuclear
reactor, radionuclides produced by the fuelfission may be released to the environment, andthen are carried in the atmosphere or deposited.
Among the fission products, iodine is the mostdangerous at short term because of its existence
in gaseous and aerosol forms, and its ability to
fix on the thyroid, increasing the effects ofradiation on the human body.

The strategy used to constitute the reactional
mechanism of iodine was to collect all the gas
phase reactions involving iodine through theliterature. The kinetic databases of NIST and
IUPAC has been used, as well as the IRSN
knowledge on the nuclear context.
After performing the literature review, akinetic scheme of 246 reactions has been built,
including 13 photolysis reactions. The chemicalsystem complements those used in the literature(Vogt & al. [4], McFiggans & al. [5], Calvert& al. [6], Mahajan & al. [7], Saiz-Lopez & al.
[8], Ordonez & al. [9], Sommariva & al. [10]), it
is schematized in Figures 1 and 2.

After the Fukushima accident, the IRSN
dispersion tools have been used to predict theradiological consequences. The simulations havebeen compared to the field measurements,
highlighting significant under-estimation ofiodine radiation levels on the ground. Concerning
the caesium, an other fission product of interest,
it is mainly released in aerosol form and has a
low reactivity. Its deposit levels obtained bysimulation were in good agreement with themeasurements.

These figures show that the mechanism remains
to complete because of the 25 reactions without
identified products and the 11 non-destroyed
species. More, they are only 13 photolysis
reactions for 81 iodine species.

For iodine gap, one possible explanation may
be the absence of chemical iodine reactivity inthe transport simulation software. In order toevaluate the impact of this reactivity on theiodine deposition, a chemical mechanism of
iodine in the atmosphere has been constituted,
with 246 gas phase reactions including
13 photolysis reactions, in the {I-H-O-N-C}
elementary system (described in section 2). Thismechanism has been coupled to the RACM
mechanism [1], which define the atmosphericpollutant reactivity.

So there are many lacks of knowledge around theiodine mechanism, which could be reduced byexperiment or ab-initio calculations. In the next
paragraphs, this mechanism will be used in the
0D and 3D simulations, to investigate iodine
behaviour in the atmosphere.

In a first step, the iodine mechanism has been
implemented in a 0D box-model simulation code,and studied under representative atmosphericconditions. Parametric studies have then been led
to evaluate their influence on the iodine
behaviour (described in section 3).

Fig. 1: Iodine Mechanism, inorganic partIn a second step, the mechanism has been
implemented in the 3D chemical-transport code
(Polair3D [2], Polyphemus platform [3]), in
order to study the iodine reactivity under realistic
conditions, adding the dispersion, pollutant
emissions, meteorological data, initial andboundary conditions. The results of the 3D
simulations will be presented in section 4.
Fig. 2: Iodine Mechanism, organic part406

NEA/CSNI/R(2016)5
III. 0D SIMULATIONS

The results (reported in Table 2) show thatiodine, injected as I2, rapidly reacts with theatmospheric species NOx and O3 to form IONO2
during the night and HI during the day.
Moreover, I2O5 accumulates all along thesimulation, mainly produced during the day
period. A second observation concerns theimportance of the day/night cycle, linked to thephotolysis reaction kinetics.

1. Inorganic Base Case
In a first step, only the inorganic mechanism
has been used ({I-H-O-N} elementary system) to
study the impact of the initial pollutant levels(O3, NOx), the photolysis conditions and theiodine injection rate on iodine chemical
speciation.
The Inorganic Base Case is a reference
simulation for atmospheric iodine behaviour, theparameters are described in Table 1. In this case,iodine is injected as I2, a gaseous species which
may be released outside during a nuclear
accident. The injection rate of 0.01 ppt.s-1 isarbitrary, but the total injected iodine over the
10 days (1g) is in the order of magnitude of an
expected potential release in case of severe
accident. The molecular iodine reacts with
pollutant, which are not injected during thesimulation.
To preserve the atmospheric pollution
conditions, all the non-iodine species quantities
are fixed. For this base case, the O3 and NO2
quantities are set to 25 ppb, which roughtly
corresponds to measured background levels.
Photolysis
location

The analysis of the reactional paths show that
this behaviour can explained by the formation of
the I and IO radicals by photolysis, these radicals
being very reactive.
2. Inorganic Parameters Sensitivity Studies
From the Base Case, several parameters havebeen studied, to define their impact on the iodine
speciation. First, the pollutant ratio [O3]/[NO2]have been modified from 1.0 to 1/500 and 500, in
order to represent typical conditions (rural or
urban with wind). Secondly, the photolysis
reactions have been moderated (rates calculated
for January or completely removed). Finally, theinjection rates of I2 has been either multiplied or
divided by 100. The results of the different cases
are gathered in Table 2.

Gravelines (2.11°E, 51.0°N)

Base[O3]/[NO2]

Photolysis

Injection

Case

1/500

500

Jan.

No

/100

x100

6.66

13.7

0.01

17.2

5.75

0.64

20.0

9.45

48.7

0.41

10.5

5.84

0.30

15.9

30.8

82.2

0.65

33.5

0.01

14.6

7.34

2.66

12.0

0.07

0.86

0.01

0.95

0.95

0.42

0.53

0.12

2.24

4.48

0.11

0.80

7.69

36.8

1.15

8.64

4.55

0.29

12.2

3.41

0.03

0.14

3.48

88.4

83.8

0.01

22.3

2.25

1.23

37.5

88.1

98.4

0.03

56.2

0.01

96.6

40.6

0.55

0.05

43.5

56.7

0.01

94.7

41.2

0.61

0.05

42.7

0.46

-

1.37

0.24

-

-

27.9

0.50

-

1.39

0.26

-

-

28.2

Spe.
Beginning2013-08-08, 00:00

Duration

10 days
I2

Pressure

105 PaTemperature

298 K
HI

VolumeInitialquantities

104 m3
O3

25

NO2

25

H

2.5x10-10H2

5.5x10+02HO

2.5x10-04

H2O2

1.0

HNO2

0.25

HNO3

0.25

H2O

73.4 % RH

HO2

2.5x10-02

HOI

IONO2

(ppb)

Injection Emission
Non-iodine
species

I2O5

I2, 0.01 ppt.s-1
I4O10
Fixed quantities

Table 2: Iodine distribution (%) at the last day of simulation(10d.), at 12 am (white) and 12 pm (gray)

Table 1: Box-Model parameters, Inorganic Base Case

407

NEA/CSNI/R(2016)5
It has been observed that the iodine speciation
can widely vary, depending on the atmosphericconditions (pollution, photolysis, injection). The
most important impact concerns the repartition
between the iodine oxides (IxOy), the iodine
nitroxides (INOx) and the molecular forms (HI,
HOI, I2). Indeed, when there are no photolysis
reactions, the iodine predominant form is IONO2.
With photolysis, and for [O3][NO2], the IxOy
forms become predominant. On the contrary, for[O3][NO2], the iodine is mainly under HI, HOI
and I2 forms.

ALD

1.50

HC5

1.20

OLI

5.10-4

SO2

3.50

CSL

0.01

HC8

1.00

OLT

1.5.10-2

CO

80.0

ETE

0.12

HCHO

4.00

TOL

0.25

HC3

8.00

KET

10.0

XYL

0.50

Table 3: Initial VOC concentrations (ppb) for the
Global Base Case

The iodocarbons are formed by several
reactions of I and I2 with the VOC (HC3, HC5,
ALD, TOL, ...). However it remains an importantlack of knowledge concerning the mechanism,with 25 unknown products, 11 non-destroyed
species and only 13 photolysis reactions.
Possible consequence would be to enhance thepersistence of iodocarbons in the atmosphere.

With a low iodine injection rate, the iodine isunder IONO2 form, which turns to IxOy withhigher injection, highlighting a reaction cycle
between iodine and the NOx .
Assuming that the iodine oxides andnitroxides could nucleate to form aerosols (withdifferent deposition rates), the iodine reactivity inthe atmosphere could influence the iodine
predicted deposits.

3.4. Global Parameters Sensibility Studies
In order to study the impact of the VOC on
the iodine speciation, several cases have been
examined with VOC initial concentrations only
(IC only) and with VOC emission rates only (Emonly). The iodine inlet as I2 has also beenchanged to CH3I (0.02 ppt.s-1 ) to get the iodine
inlet form importance, CH3I being an otherpotential realeased gaseous species. The last day
repartitions of iodine for this cases are presented
in Table 4.

3.3. Global Mechanism Base Case
The inorganic mechanism was completedwith some organic reactions and coupled to
RACM (Regional Atmospheric Chemical
Mechanism, [1]), to be more realistic of theoverall iodine reactivity.
The parameters of the Global Base Case are
identical to the inorganic one (Table 1), to whichwere added volatil organic compounds (VOC)
emission rates (Kuhn & al. [11]) and VOC initialconcentrations. The pollutant reactivity being
included in RACM, the non-iodine species
concentrations are not fixed but calculated.
RACM regroup the species in 72 categories, so
the non-iodine species have been converted to
the RACM chemical categories. The initial VOC
concentrations have been obtained with asimulation in the same conditions without iodine,
including the VOC emissions rates from Kuhn &al. [11], by averaging the last day concentrations,the resulting values are detailed in Table 3. For
this case, the iodine speciation is mainly underiodocarbon forms, such as C2H4I, cC5H9I(iodocyclopentane) or benzMI (iodomethylbenzene), that highlights the predominance of theorganic part of the mechanism.

408

NEA/CSNI/R(2016)5

Species Reactions

Base
Case

CH2CH2I
andCH3CHI

VOC

3.5. Conclusions on the 0D Simulations

Injection

Em Only

IC Only

CH3I

21.5

20.2

8.17

20.1

21.6

20.3

7.79

20.2

11.5

12.3

0.54

16.5

11.3

12.1

0.52

15.8

9.17

10.1

0.68

10.1

8.88

9.79

0.65

9.84

6.48

5.75

3.01

6.78

6.47

5.81

2.86

6.72

6.82

6.81

0.24

6.08

6.20

6.20

0.20

5.38

4.78

3.97

10.8

5.30

4.77

3.98

10.3

5.08

0.26

0.30

-

2.55

-

-

-

0.11

2.44

1.65

68.4

3.61

3.98

3.25

70.0

8.66

Through the 0D simulations, iodine reactivity
in atmospheric conditions is high and appears to
be influenced by several parameters, such as thepollutant levels (O3, NOx, VOC), the photolysis
rates (location, season, day/night cycle) and theiodine level. Concerning the inorganic behaviour,
iodine is able to form iodine oxides (IxOy), iodine
nitroxide (IONOx) or molecular species (I2, HI,
HOI), depending on the conditions. When VOC
are added to the box-model, iodocarbon forms
become the dominant iodine species, which canbe interpreted as a high reactivity of iodine with
VOC. The dose-effect factor being different fororganic than for inorganic compounds, theiodocarbon formation could be a relevant point.

cC5H9I

benzMI

IO +HC3P*
products

CH3CH2I

However the lack of reactional paths for
iodocarbons could also lead to theiraccumulation, which can bias the simulation
results. Future theoretical works and experiments
in atmospheric chamber has been planned to
improve this iodine mechanism.

CH3COI

CH3I

InorganicSum

IV. 3D SIMULATIONSIn order to study the iodine reactivity in more
relevant conditions, the 3D-chemical-transport
code Polair3D-Chemistry [2] has been used,supported by the Polyphemus platform [3]. The
selected 3D simulation domain is presented in
Figure 3. The simulation are based on the same
parameters than for the box-model studies, withthe global iodine mechanism, a continuous inletof CH3I at Gravelines location in France, during
the 10 days duration of the simulation. The
pollutant concentrations come from MOZART(www2.acd.ucar.edu), the emission rates from
EMEP (www.emep.int), and the meteorologicalfields are calculated with WRF (wrfmodel.org/index.php).

Table 4: Iodine distribution (%) at the last day of simulation(10d.), at 12 am (white) and 12 pm (gray). HC3P : RACM
family (peroxides with 3 carbons or less)With VOC initial concentrations only, iodine
species react to form iodocarbons until
consuming all the VOC inventory. The pollutant
reactivity (O3, NOx, VOC) lead to the formationof inorganic species like I2O5 and IONO2, whichaccumulates after the VOC consumption. With
only VOC emission rates, the final iodine
speciation is similar to the global base case. In
the case with CH3I injection, the final speciationof iodine is also under iodocarbon forms. It is
worth noticing the lower reactivity of CH3I with
VOC compare to I2, due to a lower photolysis
oxidation reaction.

The Figure 4 illustrate the dispersion
calculated by the code. The analysis of the time
concentrations shows that the iodocarbon forms
are predominant, the inorganic fraction being
insignificant (1.5 % the night). The calculateddifferences between day and night highlight theimportance of the photolysis reactions, especiallyfor the highest vertical levels where thephotolysis rates are more consequent. The 3D
simulations give the same trend than 0D ones.

The relevant impact is an increase of the
inorganic sum fractions (I2O5, IONO2) andcC5H9I (iodocyclopentane), and a decreasing of
the CH2CH2I and CH3CHI fraction.

409

NEA/CSNI/R(2016)5
However, the observed iodocarbons are notexactly the same, the differences being due to themore realistic VOC emissions used. It thenappears in particular a relevant concentration ofiC3H5nI (CH2CHCH2I), coming from the reactionof I or I2 with toluene.

drastically changed during the iodine transport in
the atmosphere.
In the ideal case without organic species,
from an initial I2 ST, the final formation of iodineoxides IxOy and IONOx has been obtained, with arepartition depending on the conditions such as
the pollutant levels (O3, NOx), the photolysis
rates (location, season, day/night cycle) and theiodine injection rate. Theses iodine oxides andnitroxides can nucleate by heterogeneous
processes to form aerosols with different
transport properties. For the inorganic part, thestability of iodine oxides in the atmosphere has
to be deep because some possible reactions, forinstance reduction of IxOy by CO into I2, should
be included but kinetic parameters are still
unknown.

Because of the high reactivity of the iodine in
the atmosphere, the speciation can be very
different at a short distance from the accident
location (few tens of kilometers), depending on
the wind speed essentially. s

However, in the more realistic case withorganic species, the iodine speciation becomesvery different with the predominance of theiodocarbon forms. With the current knowledge,
the results show an accumulation of the organic
iodine, for which there remains a lot of
uncertainties, such as 25 unknown reaction
products, 11 non-destroyed species, and only
13 photolysis reactions. It means some works
remains to do to get a firm conclusion.

Fig. 3: 3D simulation domain. Gravelines nuclear
powerplant : iodine injection poin

To sum-up, the chemical mechanism may befurther improved by a combination ofexperimental data and theoretical chemistry
works. A last, the development of an aerosol
modeling module has to be considered, to treat
first radioactive iodine aerosol behaviour in thedispersion softwares as well as interactions
between gaseous iodine and atmospheric aerosols
knowing that I2(g) is adsorbed onto aerosol
surfaces.
ACKNOWLEDGMENTS

This work is realised in the C3R common
research laboratory (IRSN/CNRS/Lille 1), andis co-funded by the Bel-V institute.
Fig. 4: Concentration of CH3I, after 12 hours of
release, altitude : 0-50 m

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V. CONCLUSIONS
The 0D and 3D simulation results show thatwhen the iodine reactivity is activated, theaccident source term (ST) speciation is410

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