Nuclear Safety
NEA/CSNI/R(2020)4
April 2022
www.oecd-nea.org

Proceedings of the Workshop on

Source Term: A Report by the Committee
on the Safety of Nuclear Installations
Working Group on the Analysis
and Management of Accidents
21-22 January 2019
Boulogne-Billancourt

Nuclear Energy Agency

NEA/CSNI/R(2020)4
Unclassified

English text only
12 April 2022

NUCLEAR ENERGY AGENCYCOMMITTEE ON THE SAFETY OF NUCLEAR INSTALLATIONS

Proceedings of the Workshop on Source Term
A Report by the Committee on the Safety of Nuclear Installations Working Groupon the Analysis and Management of Accidents

21 and 22 January 2019
Boulogne-Billancourt

This document is available in PDF format only.

JT03493280
OFDE

This document, as well as any data and map included herein, are without prejudice to the status of or sovereignty over any territory,
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2  NEA/CSNI/R(2020)4
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COMMITTEE ON THE SAFETY OF NUCLEAR
INSTALLATIONSThe Committee on the Safety of Nuclear Installations (CSNI) is responsible for the Nuclear Energy Agency(NEA) programmes and activities that support maintaining and advancing the scientific and technicalknowledge base of the safety of nuclear installations.
The Committee constitutes a forum for the exchange of technical information and for collaboration
between organisations, which can contribute, from their respective backgrounds in research, development
and engineering, to its activities. It has regard to the exchange of information between member countries
and safety R&D programmes of various sizes in order to keep all member countries involved in and abreastof developments in technical safety matters.
The Committee reviews the state of knowledge on important topics of nuclear safety science andtechniques and of safety assessments, and ensures that operating experience is appropriately accounted forin its activities. It initiates and conducts programmes identified by these reviews and assessments in order
to confirm safety, overcome discrepancies, develop improvements and reach consensus on technical issuesof common interest. It promotes the co-ordination of work in different member countries that serve tomaintain and enhance competence in nuclear safety matters, including the establishment of joint
undertakings (e.g. joint research and data projects), and assists in the feedback of the results to participatingorganisations. The Committee ensures that valuable end-products of the technical reviews and analyses areprovided to members in a timely manner, and made publicly available when appropriate, to support broader
nuclear safety.
The Committee focuses primarily on the safety aspects of existing power reactors, other nuclear
installations and new power reactors; it also considers the safety implications of scientific and technicaldevelopments of future reactor technologies and designs. Further, the scope for the Committee includes
human and organisational research activities and technical developments that affect nuclear safety.

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Acknowledgements
The Nuclear Energy Agency wishes to express its sincere gratitude to Professor Luis E.
Herranz (Spain), who co-ordinated this report, and to the members of the Working Groupon the Analysis and Management of Accidents for their support of and active participationin the workshop.
Special thanks are also due to Richard Lee (United States), Luis E. Herranz (Spain),Mounia Berdai (France), Sanjeev Gupta (Germany), Christian Mun (France), Yu
Maruyama (Japan), and Didier Jacquemain (France) for preparing material andpresentations for the workshop.

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Table of contents
Executive summary ............................................................................................................................... 6
List of abbreviations and acronyms ..................................................................................................... 8
1. Introduction ..................................................................................................................................... 11
2. Bases for discussion ......................................................................................................................... 13
2.1. Session 1: Key Source Term background (Chaired by: Y. Maruyama, JAEA) .......................... 13
2.2. Session 2: Recent progress and current status (Chaired by: N. Mesmous, CNSC) .................... 15
2.3. Session 3: Concluding session (Chaired by: L.E. Herranz, CIEMAT; D. Jacquemain, IRSN) .. 17
3. Conclusions ...................................................................................................................................... 20
4. References ........................................................................................................................................ 22
Annex A. Workshop agenda ............................................................................................................... 23
Annex B. List of participants .............................................................................................................. 25

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Executive summary
The investigation conducted in the last 15 years on different aspects of Source Termworldwide, and particularly in the context of the Nuclear Energy Agency (NEA) in theform of projects and the Working Group on the Analysis and Management of Accidents(WGAMA) activities, has resulted in a significant enlargement of the available databaseand an enhancement of the modelling capabilities. Nonetheless, the Committee on the
Safety of Nuclear Installations (CSNI) WGAMA group considered it timely to assess thestatus on the matter by holding a workshop with a twofold purpose:

to identify the knowledge gaps and remaining uncertainties in the Source Term areathat might have a sizeable impact on safety; andto discuss the best way to address them with existing and/or future methods,
analytical tools, and facilities, if needed.

The scope of the international Workshop on Source Term, held on 21 and 22 January 2019at the NEA headquarters in Boulogne-Billancourt (France), was limited to water cooled
reactors and established types of fuels. The participants expect the major outcomes of theworkshop to contribute to the development of the necessary activities to address theremaining safety-significant gaps in Source Term (ST).
Generally speaking, a reasonable consensus was reached on “still open issues” worth
investigating. The participants highlighted four open issues that in previous investigations
had been hardly addressed or not at all:
1. Remobilisation phenomena (delayed releases); late events, either natural or causedby the accident management action, might result in the remobilisation of somefission products deposited in previous phases of the accident with the possibility ofan unexpected increase in the ST.
2. Fine particles (sources, behaviour and mitigation); the potential capability of
microscopic particles to remain airborne, interact with iodine, affect safety systems(and vice versa) as well as the lower efficiency of filtration systems to trap them,suggests further experimental investigations should be conducted after reviewing
the current knowledge and data available.
3. Organic iodides (completion of the existing database); there is evidence that thecurrent modelling of organic iodides is incomplete, despite the substantial
improvements made in recent years, to match observations from the PHEBUS-FP
tests. This being said, there is not complete agreement among iodine experts on thephenomena to investigate.
4. Fission product leaching (from the accident to decommissioning). After the
Fukushima Daiichi nuclear power plant accident, a strong focus has been on the socalled long-term releases that strongly depend on the core degradation phenomena
and then on the fuel leaching. These issues would need at least a review of existingmodels to assess the present predictive capability.

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It should be emphasised that, even if some new experimental projects are to be launched in
the short run, actions should be put in place to further and better exploit the data available,with the intention of maximising their usefulness for plant applications. In addition, any
experimental programme should pay due attention to aspects like the representativeness ofdata for realistic conditions and reactor-scale conditions.
The open issues mentioned above are not a comprehensive list. There are some others
presently under investigation, like high temperature chemistry and pool scrubbing. Also,
the potential ST that might come from accident-tolerant fuels (ATFs), particularly, thoseof first generation (evolutionary type), should be given some attention. Finally, it wasagreed that the leak paths during severe accidents should be in the ST research agenda.
It is foreseen that Best Estimate Plus Uncertainties methods, applied to severe accident
scenarios, particularly if ST variables are adopted as Figures of Merit, will bring essential
insight concerning the most necessary research in the mid-term. High expectations areplaced on the H2020 Management and Uncertainties in Severe Accidents (MUSA) project
and the International Atomic Energy Agency (IAEA) Coordinated Research Project (CRP)on Advancing the State-of-Practice in Uncertainty and Sensitivity Methodologies for
Severe Accident Analysis in Water Cooled Reactors. In fact, a good initiative would be toensure a close link between these projects and WGAMA during the coming years.

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List of abbreviations and acronyms

AC2

ATHLET, ATHLET-CD and COCOSYS (severe accident code)

ARC-F

Analysis of Information from Reactor Buildings and Containment Vesselsof Fukushima Daiichi Nuclear Power Station project (NEA)

ARTIST

Aerosol Trapping In Steam GeneraTor facility and project (PSI,
Switzerland)

ASTEC

Accident Source Term Evaluation Code (severe accident code)

ATF

Accident-tolerant fuel

BDBA

Beyond Design Basis Accident

BIP

Behaviour of Iodine Project (NEA)

BSAF

Benchmark Study of the Accident at the Fukushima Daiichi Nuclear
Power Plant project (NEA)BT

Becker Technologies (Germany)

CEA

French Alternative Energies and Atomic Energy Commission(Commissariat à l’énergie atomique et aux énergies alternatives)

CHIP

Experimental facility to investigate chemistry of iodine in the reactor
coolant system (IRSN, France)

CIEMAT

Centre for Energy, Environment and Technology (Spain)

CNL

Canadian Nuclear Laboratories (Canada)

CNSC

Canadian Nuclear Safety Commission (Canada)

CRP

Coordinated Research Project (IAEA)

CSNICommittee on the Safety of Nuclear Installations (NEA)EC

European Commission

EPICUR

Experimental facility to investigate iodine chemistry under radiation(IRSN, France)

EURSAFE

European expert network for the reduction of uncertainties in severe
accident safety issues (European Commission)

FASTNETFAST Nuclear Emergency Tools project (European Commission)

FASTRUNFast-running Software Tools Used to Model Releases During Nuclear
Accidents (benchmark) (NEA)

FOM

Figures of MeritFP

Fission product
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NEA/CSNI/R(2020)4  9

FPT

Fission product test

GRS

Gesellschaft fuer Anlagen und Reaktorsicherheit (Germany)

IAEA

International Atomic Energy Agency

IRSN

Institute for Radiological Protection and Nuclear Safety (Institut deRadioprotection et de Sûreté Nucléaire, France)

ISP

International Standard Problem

ISTP

International Source Term Programme (CEA, IRSN, France)

JAEA

Japan Atomic Energy Agency

LOCA

Loss-of-coolant accident

LTMNPPLong Term Management of Actions for a Severe Accident in a Nuclear
Power Plant

MAAP

Modular Accident Analysis Program (severe accident code)

MELCOR

Severe accident code (SNL, United States)

MOX

Mixed oxide

MUSA

Management and Uncertainties in Severe Accidents project (European
Commission)

NEA

Nuclear Energy Agency

NGNP

New Generation of Nuclear Power

NRC

Nuclear Regulatory Commission (United States)

IPRESCA

Integration of Pool scrubbing Research to Enhance Source-term
Calculations project (NUGENIA)

OECD

Organisation for Economic Co-operation and Development

OrgI

Organic iodidesPAR

Passive autocatalytic recombiners

PASSAM

Passive and Active Systems on Severe Accident Source Term Mitigation
project (European Commission)

PIRT

Phenomena Identification and Ranking Table

PreADES

Preparatory Study on Analysis of Fuel Debris project (NEA)

RCB

Reactor containment building

RCS

Reactor coolant system

RCV

Reactor containment vessel

SAMG

Severe Accident Management Guidelines

SARNET

European Severe Accident Research NETwork of excellence

SBO

Station Black-Out

SGTR

Steam generator tube rupture

SNL

Sandia National Laboratories (United States)

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SOAR

State-of-the-Art ReportST

Source Term

STEM

Source Term Evaluation and Mitigation project (NEA)

TCOFF

Thermodynamic Characterisation of Fuel Debris and Fission Products
Based on Scenario Analysis of Severe Accident Progression at Fukushima
Daiichi Nuclear Power Station project (NEA)

THAI

Thermal-hydraulics, Hydrogen, Aerosols, Iodine facility and project(Becker Technologies, Germany)

UaSA

Uncertainty and Sensitivity Analysis

UQ

Uncertainty Quantification

VERCORSA testing facility to investigate fission products release from nuclear fuelsin an accident (CEA, France)

VERDONA testing facility to investigate fission products release from nuclear fuelsin an accident, follow-up of VERCORS (CEA, France)

VTT

VTT Technical Research Centre of Finland (Teknologian Tutkimuskeskus
VTT Oy)WGAMAWorking Group on Analysis and Management of Accidents (NEA)

WGIAGE

Working Group on Integrity and Ageing of Components and Structures(NEA)

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1. Introduction
In the last 15 years, a good number of international projects have been carried out on Source
Term, both under the auspices of the Nuclear Energy Agency (NEA) Committee on the
Nuclear Safety of Nuclear Installations (CSNI) and beyond. As for the CSNI, in the lastyears, the Behaviour of Iodine Project (BIP), the Source Term Evaluation and Mitigation(STEM) and Thermal-hydraulics, Hydrogen, Aerosols, and Iodine (THAI) projects haveprovided important data on key aspects of Source Term, like iodine and ruthenium
behaviour in containment in both the short and long term, including revolatilisation fromsurfaces and pools or even the effect of mitigation systems (passive autocatalytic
recombiners [PARs], etc.). In addition, there have been a number of projects, like theBenchmark Study of the Accident at the Fukushima Daiichi Nuclear Power Plant (BSAF)
project and the more recent Thermodynamic Characterisation of Fuel Debris and Fission
Products Based on Scenario Analysis of Severe Accident Progression at Fukushima Daiichi
Nuclear Power Station (TCOFF), Preparatory Study on Analysis of Fuel Debris (PreADES)
and Analysis of Information from Reactor Buildings and Containment Vessels of
Fukushima Daiichi Nuclear Power Station (ARC-F) projects, which have been set up in thewake of the Fukushima Daiichi accident and are closely related to Source Term. No lessimportant, activities like those addressed within the NEA Working Group on Analysis andManagement of Accidents (WGAMA) group on Long Term Management of Actions for a
Severe Accident in a Nuclear Power Plant (LTMNPP) and the FASTRUN (Fast-running
Software Tools Used to Model Releases During Nuclear Accidents [benchmark], NEA)
exercise have provided and are providing elements for discussion in the area of Source
Term. It is worth emphasising that in Spring 2015 an international workshop on iodine wasorganised by WGAMA and yielded a set of recommendations, partially addressed in the
recently completed or in completion phase projects.
Beyond the CSNI, the European Severe Accident Research NETwork of excellence(SARNET) framework provided good progress on Source Term and framed a good numberof small and mid-scale experimental projects on the release and transport of fission
products. No less important, interpretation work of the key PHEBUS-FP experiments was
also conducted under the SARNET sponsorship. The so-called PHEBUS FPT3 benchmark
is worth mentioning, as it has been the last integral code benchmark with broad
participation and with the main focus on fission products and aerosols release and transport.
Other experiments also provided meaningful achievements in different aspects of Source
Term, including mitigation, like the ones performed in:

VERCORS (facility to investigate fission products release from nuclear fuels in an
accident);

EPICUR (facility to investigate iodine chemistry under radiation), VERDON(facility to investigate fission products release from nuclear fuels in an accident,
follow-up of VERCORS) and CHIP (facility to investigate chemistry of iodine in
the reactor coolant system) testing, as part of the International Source Term
Programme (ISTP).

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and projects, such as:

the Aerosol Trapping In Steam GeneraTor (ARTIST) project;the Passive and Active Systems on Severe Accident Source Term Mitigation(PASSAM) project, etc.

With the completion of such significant work and enlargement of the knowledge base, it
was timely to assess the current status and knowledge base for strengths and weaknesses
and, importantly, to establish the major gaps that should be investigated in the short- to
medium-term. Accordingly, the CSNI/WGAMA working group organised a workshop
with two main purposes:

to identify the knowledge gaps and remaining uncertainties in the Source Term areathat might have a sizeable impact on safety;

to discuss the best way to address them with the existing and/or future methods,
analytical tools, and facilities, if needed.

The Workshop was devised to provide insights into how to proceed with Source Term
research to effectively reduce primary uncertainties. All the possible inputs wereconsidered, from OECD-related projects and WGAMA activities to past and/or ongoing
SARNET (NUGENIA) and H2020 (FAST Nuclear Emergency Tools project [FASTNET])
projects. The scope was limited to water cooled reactors and established types of fuels.
The international Workshop on Source Term was held on 21-22 January 2019 at the NEA.
The outcomes will contribute to enhancing the prevention and/or mitigation of severe
accident consequences.
The workshop was structured in three sessions. The first session summarised the main
messages from the key pillars in the Source Term domain. The second session gathered the
major progress achieved and the main insights gained recently in Source Term throughexperimentation, model development, and from the Fukushima Daiichi accident. The third
session, which was based on the two previous ones, was an open and broad discussion of
the current status, the most important needs and the work ahead in the short- and mid-term.
The presentations were solicited by the Organising Committee from reputed organisationsin the field.
This report is a brief summary of the discussions with emphasis on the recommendations
given regarding Source Term research: experimental needs and analytical work to be
performed (development, validation and benchmarking).

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2. Bases for discussion
This section synthesises the main issues presented and discussed in each of the sessions. It
is not intended to compile every detail in each presentation, but to focus on the main points
addressed and to contribute to the final discussion.

2.1. Session 1: Key Source Term background (Chaired by: Y. Maruyama, JAEA)
The introductory session consisted of three presentations:“Recent key milestones (and findings): from PHEBUS-FP to present times”;

“Last decade priorities: the SOAR perspectives”;

“Use of PIRT in the assessment of Source Term (ST) phenomena”.

R. Lee (United States Nuclear Regulatory Commission [NRC]) presented “Recent key
milestones (and findings): from PHEBUS-FP to present times”. He explained the
motivation for NRC to get involved in Source Term research, from the major effort
launched after the Three Mile Island accident to the present times. He emphasised the
PHEBUS-FP project as a major opportunity to examine chemical and physical forms taken
by radionuclides during an accident. He also mentioned other projects, like ARTIST, which
tested containment by-pass sequences (i.e. steam generator tube rupture [SGTR] severe
accident sequences), Behaviour of Iodine Project (BIP) and Source Term Evaluation and
Mitigation project (STEM), mainly dealing with iodine chemistry, and VERDON, which
improved understanding of fission products released from fuel. Some key findings werehighlighted:
Releases from fuel were adequately predicted, except for tellurium (Te) andmolybdenum (Mo). (It should be recognised that this statement is not shared by all
Source Term workshop attendees.)Aerosol behaviour and transport was estimated reasonably, despite particles form
remaining uncertain.



Cs2MoO4 has been credited as the dominant chemical species transporting caesium
to the containment and beyond.

Long-run iodine concentration in the gas phase might not come from incontainment sumps, as previously assumed, and some other sources of organic
iodine should be considered.



Small particles might be formed from airborne iodine through oxidation in thecontainment gas phase.

To finish the presentation, Mr Lee listed the main areas where more research is considerednecessary: deeper analyses of VERDON tests; iodine-paint interactions under realistic
conditions (i.e. ageing, competition with other reactive species, doses); particles formed in
the primary circuit; iodine interactions with water contaminants (complications associated

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to contaminated waters have not been thoroughly addressed so far). He highlighted the
Fukushima Daiichi accident, for which forensic analyses have been conducted within the
Benchmark Study of the Accident at the Fukushima Daiichi Nuclear Power Plant (BSAF)
projects and are now conducted within the Analysis of Information from Reactor Buildings
and Containment Vessels of Fukushima Daiichi Nuclear Power Station project (ARC-F),
as a source of data that might help reduce model uncertainties.
L.E. Herranz (CIEMAT) presented “Last decade priorities: the SOAR perspectives”, basedon the State-of-the-Art Reports issued by the Committee on the Safety of Nuclear
Installations (CSNI) on Nuclear Aerosols (NEA, 2009) and Iodine Chemistry (NEA, 2007).
In both cases, he focused on the remaining issues.
In the case of nuclear aerosols, Mr Herranz highlighted a number of issues, including: theshape of aerosols, which might be investigated through uncertainty and sensitivity
analyses; aerosol resuspension, which might pose a threat in by-pass sequences; particle
break-up, for which no data is available for prototypical nuclear aerosols; deposit
revaporisation, which might have a potential impact in risk-dominant sequences; reentrainment, which in case of contaminated pools reaching saturation conditions might turninto an unexpected late source of radioactivity. Other issues were mentioned but they were
not discussed in the workshop’s final session.
In the case of iodine chemistry, some issues were also highlighted: the impact of chemical
kinetics in radionuclide transport in the primary circuit; the large uncertainties in key
conditions for iodine chemistry, like the sump pH; the adsorption/desorption of iodine
onto/from containment surfaces; the production of organic iodides from any substrate
and/or homogeneous reaction; and, the iodine chemical reactivity at the secondary side ofa steam generator undergoing an SGTR severe accident sequence.
Some of the above issues have been tackled in recent experimental programmes, like BIP,
STEM and Thermal-hydraulics, Hydrogen, Aerosols, Iodine (THAI), but others have yet
to be addressed. Mr Herranz also emphasised that after the Fukushima Daiichi accident,
the situation might have changed considerably and a longer time horizon should be bornein mind when discussing accidents, which might give rise to the so-called long-term effects.
M. Berdai (CNSC) presented “Use of PIRT in the assessment of Source Term (ST)phenomena” based on the PIRT-type studies conducted in Europe, Japan and the UnitedStates. She reviewed the bases of a PIRT exercise, emphasising its goal (identifying
relevant phenomena to rank them according to their safety importance and current state ofknowledge) and limitations (the interaction between phenomena is not thoroughly
considered).
The European PIRT was conducted in the context of the EC EURSAFE (European expert
network for the reduction of uncertainties in severe accident safety issues) project. It
addressed the entire domain of severe accidents, retaining about 100 phenomena andselecting around 20 areas requiring further research. Among them was the oxidising impact
on Source Term (particular reference to ruthenium volatilisation in the form of RuO4).
Updates on the PIRT have been provided periodically over recent years; this update did not
follow the same quantitative method but it relied on the votes of an expert panel created forthe purpose. In short, two major issues concerning Source Term were highlighted as
deserving further attention (other issues being investigated at the time in international
contexts like OECD projects were disregarded): fission product release and transport underoxidation conditions (in particular, air ingress for high burnup and MOX fuel with special

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attention to semi-volatile molybdenum and low volatile ruthenium) and high temperaturechemistry in the reactor coolant system (RCS) as a need for processes like revaporisation.
The high-level American PIRT focused on a Station BlackOut (SBO) accident and wasconsidered an integral phenomena assessment. The Figures of Merit (FOM) selected were
the radiological consequences (essentially doses) and system/component operability in the
BDBA (Beyond Design Basis Accident) domain. The scope was limited to three mainareas: in-vessel melt progression, emergency core cooling system equipment performanceand ex-vessel debris coolability and core-concrete interaction. Therefore, no major remark
on Source Term was made, but it was agreed that uncertainties related to core degradation
overshadow other sources of uncertainties. The study emphasised the need to perform
human performance assessments and focus attention on instrumentation for severe
accidents.
A specific Source Term PIRT based on findings during the Fukushima Daiichi accident
was developed by Japan (Atomic Energy Society of Japan). The authors split the accident
in three phases (early, middle and late) and set FOMs quite focused on Source Term:
radionuclide release fractions, radionuclide chemical forms and release timing. It wasdeveloped on qualitative bases, mostly on expert judgement. They ranked in-vessel releaserelated phenomena to Source Term and condensation, revaporisation and adsorption
phenomena as key mechanisms for radionuclide behaviour in the reactor pressure vessel,
the primary circuit and the steam line.
In the concluding remarks, besides summarising the main ideas of the presentation,
Ms Berdai offered recommendations, mainly related to the optimisation of the PIRT
methodology.

2.2. Session 2: Recent progress and current status (Chaired by: N. Mesmous, CNSC)
The second session was designed to update everybody on the latest achievements made andthe major findings from Fukushima related to Source Term. It consisted of threepresentations:“Main experimental findings of recent investigations”;“Current predictive capability of Source Term”;

“Key Source Term issues from Accident at Fukushima Daiichi Nuclear Power
Station”.

S. Gupta (Becker Technologies [BT], Germany) presented, in collaboration with otherorganisations responsible for the latest experimental projects run on Source Term (French
Alternative Energies and Atomic Energy Commission [CEA]; Canadian Nuclear
Laboratories [CNL]; Institute for Radiological Protection and Nuclear Safety [IRSN,France]; VTT Technical Research Centre of Finland [VTT, Finland]), the “mainexperimental findings of recent investigations”. After reminding the group of the
motivation behind Source Term experimental research and referring to the existing
infrastructures, particularly in Europe, he classified the main findings according to: release,chemistry and transport in RCS and RCB (reactor containment building) and mitigation.
Fission product (FP) release was split according to in-vessel and ex-vessel phases of theaccident. As for the in-vessel phase, the database (HEVA/VERCORS, PHEBUS-FP, etc.)
has been enlarged with ISTP/VERDON tests and a classification has been proposed:
volatile FPs (Noble Gases, Cs, I, Te and others), semi-volatile FPs (Mo, Ba, Rh, etc.), low

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volatile FPs (Sr, Ru, La and others), non-volatile FPs (Zr, Nd, Pr) and actinides (also nonvolatiles). The presentation highlighted some major observations: higher release kinetics
for volatile FPs for high burnup and MOX fuel in comparison to low burnup UO2 fuel,starting at a rather low temperature; substantial release in oxidising conditions for Mo(steam environment) and Ru (air ingress) and in reducing conditions for Ba. As for the exvessel phase, Ms Gupta stressed the potential relevance and incomplete knowledge ofaerosol releases from boiling sumps. Special mention was made of delayed releases due toremobilisation of a fraction of fission product deposits, due to revaporisation, resuspension
and/or revolatilisation. Very limited knowledge exists for these phenomena, as is the casefor long-term FP leaching. As for transport through RCS, there are still some uncertainties
affecting iodine and ruthenium speciation, related to the effect of various gaseous andaerosol species/impurities on the volatilisation of both elements. As for containment, in
addition to iodine volatilisation from boiling pools (mentioned above), a reduced number
of issues stand out over others (like FP interactions with PARs): formation of organic iodine
from sources other than transfer from pools and reactions with paints and production andbehaviour of small iodine oxide particles in the containment atmosphere. Finally, as for
mitigation, the performance of safety systems under realistic conditions would notably
strengthen their reliability under BDBA.
At the end of the presentation, Ms Gupta emphasised that given the time passed since thelast NEA Working Group on Analysis and Management of Accidents (WGAMA) SOARs
were issued it might be the right time for an update, or at least a technical report on key
aspects not previously addressed such as nature and amount of potential water poolcontaminants/impurities. Given the potential impact of this aspect on FP release andtransport of some key radionuclides, this suggestion was supported by the WGAMA Chair.
Ch. Mun (IRSN), in collaboration with other code developers SNL/NRC, GRS and CNSC,
prepared the presentation entitled “Current predictive capability of Source Term”. Once thebroad features of the severe accident codes Accident Source Term Evaluation Code(ASTEC), ATHLET, ATHLET-CD and COCOSYS (AC2), MELCOR (SNL, UnitedStates), and Modular Accident Analysis Program (MAAP) were introduced, the presenter
discussed in detail their approaches to FP releases, in-RCS and in-containment transportand chemistry and mitigation. Through the analysis of individual model approaches toreleases, prevailing conditions and importance of FPs and control rod materials (no models
available for structural materials in any of the codes), they identified some specific open
issues: late in-vessel releases, ex-vessel molten pool releases, fuel leaching and accidenttolerant fuels (ATFs), among others. For the area of in-RCS transport and chemistry,
remobilisation mechanisms and gaseous iodine generation mechanisms were considered toneed further development. In containment, the areas identified for further study were iodineoxides reactivity, the production of gaseous organic iodides through homogeneous
reactions, and natural containment leakages. Finally, the presenters emphasised that
Uncertainty and Sensitivity Analysis (UaSA) at plant scale would be necessary to assessthe dominating uncertainties and sensitivities of all the issues raised during thepresentation.
Y. Maruyama (JAEA) presented the “Key Source Term issues from Accident at Fukushima
Daiichi Nuclear Power Station”. After providing an overview of the implications of Source
Term estimates in terms of emergency preparedness and severe accident mitigation,
Maruyama noted it is important to keep enhancing predictive tools. The accidents unfolding
in units 1-3 were characterised through the pressure evolution. The presentation highlighted
the influence of upper structures on FP transport through the potential chemisorption ofsome species (CsOH) and later revaporisation. It also highlighted the effect of FP chemical
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NEA/CSNI/R(2020)4  17

speciation on transport and behaviour, stressing the importance of some boundary
conditions, like pH in the suppression pool. Concerning ex-vessel phenomena, Maruyama
said temperature and injection flow rates in suppression pools might have been higher thanestimated in previous studies and did not rule out the possibility of having iodine species
revolatilisation with the current knowledge of the accident scenarios, in which variables asimportant as suppression pool pH are not known. Another interesting point made was thestrong effect of leak paths definition between RCV and containment and between
containment and reactor building; both definition of flow-paths and phenomena governing
the transport and deposition of particles entail huge uncertainties. At the end of thepresentation, other issues were discussed, like the FP leakage through aquatic pathways
and what might happen during a severe accident in a spent fuel pool.

2.3. Session 3: Concluding session (Chaired by: L.E. Herranz, CIEMAT;
D. Jacquemain, IRSN)
The last session was devoted to the synthesis and discussion of the main issues raised
during the previous ones. Two presentations were given:“ Summary of discussions @ Workshop on Source Term”;

“Main open issues and possible ways forward”.

R. Lee (NRC) highlighted four major points concerning Source Term in “Summary of
discussions @ Workshop on Source Term” that are essential for NRC at the present time.
The first was the role of severe accident analysis (MELCOR) to assess the efficacy of
mitigation systems in the Defence-in-Depth approach and the siting and emergency
planning, the safety issue resolution, the risk-informed decision making and the testing of
Severe Accident Management Guidelines (SAMGs) effect. The second focused on iodine
chemistry and particularly on specific aspects of the iodine interaction with paints and theneed to understand how to extrapolate iodine research findings to actual plants. The thirdconcerned the modelling techniques used in aerosol transport in the RCS, the enhancementof numerics, the potential effect of “fractal” collision kernels or the actual nature of aerosols(can PHEBUS-FP observations be adopted as fully prototypical?). And fourth, radionuclide
releases and behaviour in New Generation of Nuclear Power (NGNP). In addition, theUncertainty Quantification (UQ) approach to be used in severe accident analysis,
particularly in the frame of new Source Term to be defined in high burnup and MOX fuels,
was highlighted as a key element in the forthcoming research on Source Term.
D. Jacquemain (IRSN) synthesised the “Main open issues and possible ways forward” inSource Term research according to the preceding presentations with the intention to spark
discussion on future investigations. The key points identified are:

Delayed FP releases by remobilisation phenomena. Observed in Fukushima andproved in Source Term experiments, this potential contribution to radiologicalreleases is worth investigating in multiple ways (revaporisation, revolatilisation,
reentrainment and resuspension). As a first step, a review of existing knowledge
and data is proposed before launching any experimental investigation.

Fine particle behaviour in containment. Several mechanisms can produce
microscopic particles, which have the potential to remain airborne, interact withiodine, affect (and be affected by) safety systems, and are more difficult for
filtration systems to retain. The group recommended conducting experimental
investigations after reviewing the current knowledge and data available.

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

Organic iodides (OrgI) in containment. Despite the efforts made in the latest
experimental programmes to identify the major sources of OrgI, there is evidence
that something is still missing in the current modelling to match observations madein the long-run in PHEBUS-FP tests. In this case, before launching any
experimental venture, it was recommended to review all the data available andcarefully set the right conditions to investigate other routes of OrgI formation. Itshould be noted that there is not a consensus among iodine experts on thephenomena to investigate, i.e. whether there is or is not a homogeneous reaction
between organics and iodine in the gas phase.

FP release. Even though there has been significant progress in research, there aresome areas worth further effort. This is the case of SGTR and LOCA sequences,which will be investigated in the H2020 R2CA project. What seems to be drawing
more attention after the Fukushima Daiichi accident are the so-called long-term
releases that strongly depend on the core degradation phenomena and fuel leaching.
These issues would need at least a review of existing models to assess the current
predictive capabilities. A different matter is the need to investigate releases fromthe ATFs, which should be articulated consistently with other ongoing initiatives
on ATF research.

Pool scrubbing. As highlighted in the European Commission Passive and Active
Systems on Severe Accident Source Term Mitigation project (EC PASSAM)
project, this issue lacks a systematic approach and a full-scope investigation
addressing conditions as yet explored, with a particular emphasis on saturated pools
and high injection velocities. In this research, a deep study of hydrodynamics isrecognised to be of vital importance. The NUGENIA Integration of Pool scrubbing
Research to Enhance Source-term Calculations project (IPRESCA) project iscurrently being conducted, so no specific recommendation was given at this time.

High temperature chemistry in RCS. This issue has been investigatedexperimentally and new models addressing control rod effect (Sg/In/Cd allow andB) on fission product transport are presently being developed. Given the current
status, no specific recommendation was made at this time.

Beyond the specific issues listed above the group highlighted a few more points toconsider in any ST research initiatives launched in the future:The need to address realistic conditions such as the presence of impurities in thesystem, the multicomponent nature of aerosols, the high temperature and doses
anticipated, the boiling conditions in sumps, and, more importantly, scaling theresults to nuclear power plants.

The key information expected from projects like H2020 Management andUncertainties in Severe Accidents (MUSA) project and IAEA CoordinatedResearch Project (CRP) on uncertainties. These two projects will bring insight into
the sources of uncertainties dominating Source Term estimates and, as aconsequence, they should be instrumental in defining research in the mid-term, as
the outcomes are expected no earlier than 2023.

The key role played by leak pathways when analysing accident sequences. System
failures and leaks play an outsized role in any Source Term prediction on theenvironment. Any reduction of uncertainties affecting leak path definition in asevere accident analysis would have a strong effect on the results. This issue might

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NEA/CSNI/R(2020)4  19

be worth bringing to the attention of the Working Group on Integrity and Ageingof Components and Structures (WGIAGE) for potential work together with the
WGAMA or even a joint project preparation in the context of the NEA.

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3. Conclusions

The structure of the Workshop on Source Term met its main goals: identifying theremaining safety-significant uncertainties on Source Term (ST) and proposing how theyshould be addressed. The invited lectures met the highest possible expectations. They werecomprehensive and included discussion elements that the attendees appreciated and paid
attention to, both at the end of each presentation and in the final discussion held in the last
Workshop session.
Generally speaking, a reasonable consensus was reached on the issues that are still “open”,
although discussions could have been more detailed on some specifics, like theinstrumentation that should be used to investigate experimentally the different issues. On
the other hand, the tools needed to approach the investigations (Nuclear Energy Agency[NEA] research projects, State-of-the-Art Reports [SOARs], Phenomena Identification andRanking Table [PIRTs], International Standard Problems [ISPs], and more) were discussed
extensively, which should be sufficient to properly address the issues identified. It should
be noted that despite the fact that some issues are still open, some attendees felt that there
was a need for a peer review of existing data. Some felt that the database could be further
exploited; others suggested that some of the data used for model development andvalidation might not be directly applicable.
The open issues requiring further investigation can be summarised as follows (a list
supported by most of the attendees):remobilisation phenomena (delayed releases);

fine particles (sources, behaviour and mitigation);organic iodides (completion of the existing database);

fission product leaching (from the accident to decommissioning).

It should be emphasised that even if some new experimental projects are launched in the
short run, actions should be put in place to better exploit the data available with the intentionof bringing them to their maximum usefulness for plant applications. In addition, any
experimental programme should pay due attention to aspects like the representativeness ofdata for realistic conditions (“dirty water” included) and scaling issues.
In addition to the open issues listed above, there are others of a certain interest, like hightemperature chemistry and pool scrubbing, that are being addressed in other contexts anddo not need further actions until the ongoing work is finished. Another emerging matterthat should be given some attention is the potential Source Term that might come out ofaccident-tolerant fuels (ATFs), particularly those of first generation that can be considereda development of the fuels currently used in nuclear power plants.
Even though without a specific formulation on which type of activity should be carried out,
but with the conviction that something should be put in place (maybe as a collaboration
between the NEA Working Group on Integrity and Ageing of Components and Structures[WGIAGE] and the NEA Working Group on Analysis and Management of Accidents

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NEA/CSNI/R(2020)4  21

[WGAMA]), it was generally agreed that leak paths during severe accidents should be in
the ST research agenda.
Finally, it was agreed that the application of Best Estimate Plus Uncertainties methods insevere accidents, particularly if Source Term variables are adopted as Figures of Merit, willbring essential insight concerning the most necessary research in the mid-term. High
expectations are placed on the H2020 Management and Uncertainties in Severe Accidents(MUSA) project and the Coordinated Research Project (CRP) of the IAEA. In fact, a goodinitiative would be to ensure a close link between these projects and the WGAMA during
the coming years.

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4. References
NEA (2009), “State-Of-the-Art Report (SOAR) on Nuclear Aerosols (NARSOAR)”, OECD Publishing,Paris, www.oecd-nea.org/jcms/pl_18750.
NEA (2007), “State of the Art Report (SOAR) on Iodine Chemistry”, OECD Publishing, Paris,
www.oecd-nea.org/jcms/pl_18434.

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NEA/CSNI/R(2020)4  23

Annex A. Workshop agenda
Monday 21 January 2019, Room BB2
9:00-9:10

Welcome & opening remarks(L. Herranz, CIEMAT; N. Sandberg, OECD/NEA)

Session 1

Key Source Term
Background
Session Chair: Y.
Maruyama (JAEA)

1.1

Recent key milestones(and findings): From
PHEBUS-FP to present
times(R. Lee, NRC)

9:30-10:00

1.2

Last decade priorities:
the SOAR perspectives(L. Herranz, CIEMAT)

10:00-10:30

1.3

Use of PIRT in theassessment of Source
Term (ST) phenomena(M. Berdai, CNSC)

11:30-12:00

1.4

Session 1 Discussion

Session 2

Recent Progress andCurrent Status
Session Chair: N.
Mesmous (CNSC)

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12:00-12:30

24  NEA/CSNI/R(2020)4
2.1

Main experimental
findings of recent
investigations(S. Gupta, BT)

13:45-15:00

2.2

Current predictive
capability of Source
Term(C. Mun, IRSN)

15:00-16:15

2.3

Key Source Termissues from Accident
at Fukushima Daiichi
Nuclear Power Station(Y. Maruyama, JAEA)

16:45-17:30

End of day 1

Tuesday 22 January 2019, Room BB2
Session 3

Concluding session
Session Chairs: L. Herranz (CIEMAT), D. Jacquemain (IRSN)

3.1

Summary of discussions@ Workshop on Source
Term(D. Jacquemain, IRSN)

9:00-9:30

3.2

Final discussion(all)

10:00-10:30

3.3

Main open issues andpossible ways forward(L. Herranz, CIEMAT)

11:00-12:00

End of the workshop

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NEA/CSNI/R(2020)4  25

Annex B. List of participants
BELGIUM
Dr Martina ADORNIBEL V (subsidiary of FANC), Brussels

Mr Philippe DEJARDIN

Tractebel (ENGIE), Brussels

Mr Thibaut HELMAN

Tractebel (ENGIE), Brussels

Mr Thibaut VAN ROMPUYBEL V (subsidiary of FANC), Brussels

CANADA
Mrs Mounia BERDAI

Canadian Nuclear Safety Commission, Ottawa, Ontario

Mr Glenn GLOWA

Canadian Nuclear Laboratories, Chalk River, Ontario

Mr Noreddine MESMOUS

Canadian Nuclear Safety Commission, Ottawa, Ontario

Dr Andrew C. MORREALE

Canadian Nuclear Laboratories, Chalk River, Ontario

CHINA
Mr Yansong WEI

China Institute of Atomic Energy, Beijing

Dr Liang ZHANG

State Nuclear Power Technology Corporation, Beijing

CZECH REPUBLICMr Petr VOKAC

Nuclear Research Inst., ÚJV Řež, a. s.

FINLAND
M. Teemu KARKELA

Valtion Teknillinen Tutkimuskeskus, Espoo

Mrs Ilona LINDHOLM

Valtion Teknillinen Tutkimuskeskus, Espoo

FRANCEMr Thierry ALBIOLIRSN Cadarache, Saint-Paul-lès-Durance

Dr Laurent CANTRELIRSN Cadarache, Saint-Paul-lès-Durance

Mr Frédéric COUSINIRSN Cadarache, Saint-Paul-lès-Durance

Mr Didier JACQUEMAIN

IRSN Cadarache, Saint-Paul-lès-Durance

Dr Martin KISSANE

IRSN Cadarache, Saint-Paul-lès-Durance

Dr Christophe JOURNEAU

CEA/CEN de Cadarache, Saint-Paul-lès-Durance

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26  NEA/CSNI/R(2020)4
Dr Christian MUNIRSN Cadarache, Saint-Paul-lès-Durance

Mr Hadrien PERRONElectricité de France, Centre des Renardières

Mr Marc PETITIRSN Cadarache, Saint-Paul-lès-Durance

Ms Sophie PIGNET

Institut de Radioprotection et de Sûreté NucléaireMr Yves PONTILLON

Centre d'Etudes Nucléaires de Cadarache,Saint-Paul-lès-Durance

Mr Bruno TOURNIAIRE

Electricité de France-SEPTEN, Villeurbanne

GERMANYMr Hans-Josef ALLELEINKFA Forschungszentrum Juelich GmbH, Juelich

Mrs Sara BECK

Gesellschaft fuer Anlagen und Reaktorsicherheit (GRS)Dr Sanjeev GUPTA

Becker Technologies GmbH, Eschborn

Mr Gerhard POSS

Becker Technologies GmbH, Eschborn

Mr Victor Hugo
SANCHEZ-ESPINOZA

Karlsruher Institut fuer Technologie, Karlsruhe

Dr Martin SONNENKALB

Gesellschaft fuer Anlagen- und Reaktorsicherheit (GRS)

Ms Livia TIBORCZ

Gesellschaft fuer Anlagen und Reaktorsicherheit (GRS)ITALY
Dr Fulvio MASCARI

Ente Nazionale Energie Alternative/CRE, Bologna

JAPAN
Dr (Paul) David W. BOTTOMLEY

Japan Atomic Energy Agency, Tokai-mura

Dr Hideo KAWAGUCHIJapan Nuclear Regulation Authority

Dr Masaki KURATA

Japan Atomic Energy Agency, Tokai-mura

Mr Masaaki KURIYAMAJapan Atomic Energy Agency, Tokai-mura

Dr Yu MARUYAMAJapan Atomic Energy Agency, Tokai-mura

Dr Shinya MIZOKAMI

Tokyo Electric Power Co. Inc., Yokohama

Mr Koichi NAKAMURA

Central Research Institute of Electric Power Industry

Dr Yasushi NAUCHI

Central Research Institute of Electric Power Industry

Mr Marco PELLEGRINI

Institute of Applied Energy, Tokyo

Dr Tomoyuki SUGIYAMAJapan Atomic Energy Agency, Tokai-mura

Dr Masami TAIRA

Japan Nuclear Regulation Authority

Dr Makoto TAKANOJapan Atomic Energy Agency, Tokai-mura

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NEA/CSNI/R(2020)4  27

KOREA
Mr Tae-Hoon KIM

Korea Institute of Nuclear Safety, Yuseong-gu, Daejeon

Dr Sung Il KIM

Korea Atomic Energy Research Institute, Daejeon

Mr Kyun-Tae KIM

Korea Institute of Nuclear Safety, Yuseong-gu, Daejeon

Dr Yoonhee LEE

Korea Institute of Nuclear Safety, Yuseong-gu, Daejeon

Dr Jin Ho SONG

Korea Atomic Energy Research Institute, Daejeon

SPAIN
Prof. Luis E. HERRANZ

Centro de Investigaciones Energeticas Medioambient

SWEDENProf. Sevostian BECHTA

Kungliga Tekniska Högskolan (ex-RIT), Stockholm

Dr Henrik GLANNESKOG

Miscellaneous in SWEDEN

Dr Pavel KUDINOV

Kungliga Tekniska Högskolan (ex-RIT), Stockholm

Dr Christian LINDE

Stralsakerhetsmyndigheten, Stockholm

SWITZERLAND
Dr Terttaliisa LINDPaul Scherrer Institute, Villigen PSI

Dr Sergii NICHENKOPaul Scherrer Institute, Villigen PSI

UNITED KINGDOMMr Chris BOYD

Office for Nuclear Regulation - Headquarters, Boot

Dr Shirley DICKINSON

National Nuclear Laboratory, Harwell

Mr Jeff GLOVER

Office for Nuclear Regulation - Headquarters, Boot

Mr Stephen MANNEDF Energy UK, Bristol

Ms Holly ROEBUCKNational Nuclear Laboratory (ex-Nexia Solutions),

UNITED STATESMr Lucas ALBRIGHT

University of Utah, Salt Lake City, UTDr Nathan ANDREWS

Sandia National Laboratories, Albuquerque, NM

Mr Nathan BIXLER

Sandia National Laboratories, Albuquerque, NM

Dr Randall O. GAUNTT

Sandia National Laboratories, Albuquerque, NM

Dr Larry HUMPHRIES

Sandia National Laboratories, Albuquerque, NM

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28  NEA/CSNI/R(2020)4
Dr Richard LEE

United States Nuclear
Washington, DC

Regulatory

Commission,

Mr Rick WACHOWIAKElectric Power Research Institute, Palo Alto, CA

International Organisations
Mr Kentaro FUNAKI

Nuclear Energy Agency, Boulogne-Billancourt

Dr Nils SANDBERG

Nuclear Energy Agency, Boulogne-Billancourt

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