Nuclear Safety
NEA/CSNI/R(2018)6
March 2020www.oecd-nea.org

Improving Evaluations of Source
Terms for Severe Accidents at
Nuclear Installations

Final Report of the Source
Term Evaluation and
Mitigation Project (STEM)

Nuclear Energy Agency

NEA/CSNI/R(2018)6
Unclassified

English text only
17 March 2020

NUCLEAR ENERGY AGENCYCOMMITTEE ON THE SAFETY OF NUCLEAR INSTALLATIONS

Improving Evaluations of Source Terms for Severe Accidents at Nuclear
Installations
Final Report of the Source Term Evaluation and Mitigation Project (STEM)

This file is available as PDF only.

JT03458869
OFDE

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2 │ NEA/CSNI/R(2018)6
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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
This report was written by C. Mun, J. Colombani, M.N. Ohnet and O. Leroy of the Institut
de Radioprotection et de Sûreté Nucléaire (IRSN). The IRSN performed the experiments
for the Nuclear Energy (NEA) Agency Source Term Evaluation and Mitigation (STEM)
project. The IRSN acknowledges the support of the NEA in hosting the project.
Furthermore, the IRSN is grateful for the support and guidance from all the STEM partners:
Electricité de France (EDF), Canadian Nuclear Laboratories (CNL), the Teknologian
tutkimuskeskus (VTT, Finland), the Nuclear Research Institute (NRI, Czech Republic), theGesellschaft für Anlagen– und Reaktorsicherheit (GRS, Germany), the Korea Atomic
Energy Research Institute (KAERI), the Korea Institute for Nuclear Safety (KINS) and the
US Nuclear Regulatory Commission (US NRC).

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Table of contents
List of abbreviations and acronyms ..................................................................................................... 6
Executive summary ............................................................................................................................... 7
1. Introduction ....................................................................................................................................... 9
2. Main outcomes of STEM/IODINE part ........................................................................................ 10
3. Iodine-paint interaction .................................................................................................................. 15
4. Main outcomes of STEM/Ru part .................................................................................................. 16
5. Conclusions ...................................................................................................................................... 19
6. References ........................................................................................................................................ 20

FiguresFigure 1. General trend of the release kinetics under radiation of I2 (blue curve) and RI (redcurve) from epoxy painted coupons pre-loaded with I2 .................................................... 11
Figure 2. Scheme of the START test facility ..................................................................................... 16

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List of abbreviations and acronyms
CNL

Canadian Nuclear Laboratories

CSNI

Committee on the Safety of Nuclear Installations (NEA)

EDF

Electricité de France

IRSN

Institut de Radioprotection et de Sûreté Nucléaire (Institute of radiation protection andnuclear safety, France)

KAERI

Korea Atomic Energy Research Institute

KINS

Korea Institute for Nuclear Safety

MBManagement Board

NEA

Nuclear Energy Agency

NRI

Nuclear Research Institute (Czech Republic)

PRG

Programme Review Group

RCS

Reactor cooling system

SA

Severe accident

ST

Source term

STEM

Source Term Evaluation and Mitigation (NEA project)

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Executive summary
The Source Term Evaluation and Mitigation (STEM) project, hosted by the Nuclear
Energy Agency (NEA) and operated by the Institut de Radioprotection et de Sûreté
Nucléaire (French Institute of radiation protection and nuclear safety, IRSN), was launchedin mid-2011 in order to improve the evaluation of source term (ST) for a severe accident(SA) at a nuclear power plant (NPP) and to reduce uncertainties concerning specificphenomena related to the chemistry of two major fission products: iodine and ruthenium(Ru). Three main issues were considered for these two radionuclides:
1. medium-term iodine releases, with specific attention to the stability of iodine aerosol
particles under radiation (decomposition induced by radiation producing gaseousiodine species);
2. short-term and medium-term iodine-paint interactions under radiation;
3. ruthenium transport chemistry in order to determine the speciation of Ru, in particular
the partition between gaseous and condensed forms during its transport through thereactor cooling system (RCS).
The STEM/IODINE results significantly contributed to increasing knowledge in this area,
and it has led to improvements of the modelling of key phenomena concerning iodine
chemistry in the different calculation tools of all the involved partners. For example, in the
ASTEC code, these improvements mainly concern: 1) the interaction of I2 and CH3I withpaint under irradiation; 2) the formation and radiolytic decomposition of gaseous anddeposited iodine oxide aerosols; 3) The radiolytic conversion of gaseous I2 into CH3I and;4) the interaction of I2 with steel and aerosols. Concerning STEM/RUTHENIUM results,
in the same way, a preliminary Ru transport model through the RCS was implemented in
the ASTEC code, for example, allowing a first attempt to integrate a possible Ru source
term in probabilistic safety assessment level 2 (PSA-2) tools. From this, the source term
evaluation performed with PSA-2 tools concluded that ruthenium radiologicalconsequences could be quite significant.
These STEM results have helped to precisely identify some of the major remaining issues,
which are proposed for investigation during the second phase of the project. Indeed,according to the discussions during last STEM Programme Review Group (PRG) andManagement Board (MB) meetings in June 2015 and the conclusions from the
NEA/NUGENIA-SARNET International Workshop on the Progress in Iodine Behaviour
for NPP Accident Analysis and Management (March 2015), a second phase of the project,called STEM-2, has been launched within the project framework of the NEA. This fouryear follow-up project is focused on medium- and long-term releases and with four main
items, as prioritised by the STEM PRG:
1. The effect of the ageing of paints on iodine behaviour (both under normal operation
conditions and under accidental conditions): i.e. irradiation tests (medium- and longterm irradiation periods) on paints aged in representative reactor conditions andloaded with iodine, to confirm iodine release processes observed with un-aged paints(including adsorption/desorption kinetics);
2. The study of the stability of iodine oxide aerosols:i.

Iodine oxides radiolytic decomposition;

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ii.
Iodine oxides decomposition by carbon monoxide and/or hydrogen;3. Radiolytic oxidation of multi-component iodine aerosols;
4. Complementary tests based on ruthenium revaporisation processes withrepresentative oxidative conditions (atm. radiolysis simulants) with the presence ofgaseous pollutants representative of RCS conditions.
It should also be noted that an analytical working group will be set up to promote a final
comprehensive analysis of the results for reactor case source term evaluation, including
mitigation aspects. Finally, parts of the experimental investigations will be discussed andshared with the Canadian Nuclear Laboratories (CNL), the operating agent for theforthcoming NEA Behaviour of Iodine Project phase 3 [BIP-3].

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1. Introduction
The Source Term Evaluation and Mitigation (STEM) project [1], hosted by the Nuclear
Energy Agency (NEA) and operated by the Institut de Radioprotection et de Sûreté
Nucléaire (French Institute of radiation protection and nuclear safety, IRSN), was launchedin mid-2011 in order to improve the evaluation of source term (ST) for a severe accident(SA) on a nuclear power plant (NPP) and to reduce uncertainties on specific phenomena
related to the chemistry of two major fission products: iodine and ruthenium (Ru). Threemain issues were addressed concerning these two radionuclides:
1. medium-term iodine releases with specific attention to the stability of iodine aerosol
particles under radiation (decomposition induced by radiation producing gaseousiodine species);
2. short-term and medium-term iodine-paint interactions under radiation;
3. ruthenium transport chemistry in order to determine the speciation of Ru, in particular
the partition between gaseous and condensed forms, during its transport through thereactor cooling system (RCS).
The present report summarises the main outcomes of the STEM project. The first chapter
deals with the main outcomes concerning the iodine part (EPICUR facility), while thesecond chapter focuses on the main outcomes related to the ruthenium part (START bench).

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2. Main outcomes of STEM/IODINE part
The Source Term Evaluation and Mitigation (STEM)/IODINE experiments, carried out inthe EPICUR facility, were focused on the releases of volatile molecular iodine (I2) andorganic iodides (RI) from representative painted coupons loaded with molecular iodine [2]or iodine aerosol species (CsI, CdI2, IOx 1) 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 of iodine from epoxy painted coupons loaded with I2 (“LD tests” series). Eighttests were performed to study the releases of iodine from CsI (& CdI2) aerosols depositedon epoxy painted coupons, but also deposited on quartz and stainless steel coupons to makethe data interpretation easier by preventing any interaction with paint [3]. Four tests were
performed with IOx deposited on epoxy painted or quartz substrates. For all these tests
series, the effect of the parameters of main interest (temperature, relative humidity, initialconcentration of iodine deposited on the sample surface, etc.) on the releases of I2 and RI(and on the global volatilisation, defined as the difference between the initial quantity
deposited on the coupon before irradiation and the quantity of iodine remaining on the
coupon after irradiation) are presented and discussed below. Further details are given in“Main findings of the IRSN experimental programs performed on iodine chemistry insevere accident conditions” [4].

2.1. Releases from epoxy painted coupons loaded with I2
The experimental results of the LD1 to six tests obtained in the STEM/EPICUR programme
identify trends on the effects of the investigated parameters on the RI, I2 and global releases
from epoxy paint loaded with iodine exposed to radiations for more than 30 hours:•1

A typical profile of iodine species releases from the epoxy paint was observed forthis set of tests: RI and I2 releases are fast and usually significant during the firsthours of the irradiation phase (< 10 hours) whereas the release kinetics for both
species are significantly lower after that time (see Figure 1).

The ratio of RI released (RI released expressed in percentage of the initial activity loadedon the coupon, i.e. before irradiation) is stable for initial concentration of iodine on thepaint lower than ~10-3 mol(I).m-2, and then this release ratio decreases for higher initialconcentration of iodine. This phenomenon is probably due to the different (andpreferential) chemical interaction sites for iodine-paint interactions depending on theinitial concentration of iodine. The RI release is not significantly affected when thetemperature increases from 80°C to 120°C or when the dose rate increases from 1.2 to
3.8 kGy.h-1. The RI release decreases as the relative humidity (RH) increases from 20 to
60%. In conclusion, regarding the conversion rate of iodine deposited on the paint into
RI: the radiolytic conversion rate is mainly affected by the RH present in the irradiation
vessel, an increase of the RH from 20 to 60% leads to a decrease of the conversion rateby a factor of about 3. This phenomenon may be explained by the fact that iodine diffuses
more deeply in the epoxy matrix with a higher amount of steam, and then reacts
chemically with the epoxy paint on its way to the deep layers. It might be more complex

IOx (or IxOy) stands for iodine oxides.

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and difficult to get it back into the gaseous phase leading to a decrease of the RI releaseat a higher RH.
Figure 1. General trend of the release kinetics under radiation of I2 (blue curve) andRI (red curve) from epoxy painted coupons pre-loaded with I2

Source: IRSN.

•

The ratio of I2 released increases as the initial concentration of iodine on the paintincreases; this phenomenon is probably due to the different (and preferential)
chemical interaction sites for iodine-paint interactions depending on the initialconcentration of iodine. The I2 release decreases drastically as the temperature
increases from 80°C to 120°C or as the dose rate increases from 1.2 to 3.8 kGy.h1. The I2 release decreases more moderately as the RH increases from 20 to 60%.
In conclusion, the release of I2 is mainly affected by the temperature and the dose
rate: an increase of these parameters leads to a decrease of the release of I2 by afactor higher than 5. A possible explanation of the temperature effect is that iodineinteraction with paint could be enhanced at a higher temperature, leading to better
iodine bounding to the paint and thus to a decrease in the I2 release. A possible
explanation of the dose rate effect is that the increase of this parameter leads to anincrease of the concentration of radiation-induced radicals in the paint, which could
promote the chemical reactions between iodine and paint-radicals and/or a better
diffusion of steam and iodine between the polymer chains that leads to a more
efficient iodine trapping in the deep layers of the paint. These two phenomena leadto a decrease of the I2 release.

•

The global release is linked to the RI and I2 releases and is thus affected by the mostinfluent parameters on the RI and I2 releases.

2.2. Releases from coupons loaded with CsI (& CdI2) under radiation
Previously, the importance of this process was suspected following analysis of the
PHEBUS-FP tests, but it has not been quantified up until now, which is why tests
investigating the effects of irradiation on deposited iodine aerosols (like CsI or CdI2) were
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performed in STEM. The experimental results of the AER-CsI test series (AER-1 to 6 andAER-11) obtained in the STEM/EPICUR programme determined the effects of theinvestigated parameters on the aerosols I2, RI and global releases from CsI aerosols
deposited on several surfaces and exposed to radiation for 30 hours under temperature andhumid air.
The global release is almost total from CsI aerosols deposited on quartz and stainless steel
surfaces and irradiated under air at 50% RH, whereas this global release is incomplete fromCsI aerosols deposited on epoxy paint surfaces (from 15 to 77% of global release dependingon the dose rate and the initial concentration of CsI on the epoxy paint surface). Thisphenomenon can be explained by the CsI interactions enhanced in the case of epoxy paintand/or to the I2 affinity for the epoxy paint leading to a “re-adsorption” process. The global
release is linked to the aerosols, as well as I2 and RI releases, and is thus affected by themost influent parameters on these releases.
•

The aerosol release is important under certain irradiation conditions. These released
aerosols are assumed to be iodine oxide species; they may be formed by reactionof I2 (produced from CsI aerosols irradiation) and oxidative products formed by air
radiolysis during the irradiation phase. The aerosol release decreases with epoxy
paint surfaces, compared to quartz and stainless steel surfaces, because of theaffinity of I2 for epoxy paint surfaces (the reactions between I2 and air radiolysis
products are disadvantaged in favour of a phenomena of “re-adsorption” of I2 on
the epoxy paint). The aerosol release decreases at higher RH and high temperature,
due to the thermal instability at high temperature of the iodine oxide species andtheir reactions of decomposition under humid conditions, which leads to I2
production, also explaining the effect of several parameters on the I2 release. The
aerosol release increases at high dose rate, probably due to an increase of theconcentration of air radiolysis products promoting the formation of iodine oxide
species. This release decreases when the CsI concentration (and the particle size)
decreases, probably due to the decrease of the I2 release at low concentrations.

•Two typical profiles of I2 release from CsI aerosols are observed, depending on thenature of the surface. The release of I2 from CsI aerosols deposited on quartz or
stainless steel surfaces starts in the early hours of the irradiation phase and presents
two kinetics: the release is fast and important during the first hours of the irradiation
phase, whereas the release kinetics are significantly lower after that time. In
contrast, the release from CsI aerosols deposited on epoxy painted surfaces starts
later and displays only one kinetic rate during the irradiation phase. Moreover, thefinal total release of I2 from CsI aerosol deposited on epoxy paint surfaces is lessthan that from CsI deposited on quartz and stainless steel surfaces. This differenceof behaviour may be due to the affinity of I2 with the epoxy paint. Indeed, theradiolysis of CsI aerosols may produce I2 that is released from quartz or stainlesssteel surfaces, whereas it may also be re-adsorbed in case of epoxy painted surfaces.
Otherwise, the I2 release increases at high RH (50% at 80°C), probably because thedeliquescence point of CsI is almost reached (~57% at 80°C) and/or thedecomposition of iodine oxide species into I2 is enhanced at high RH. This releasedecreases at a high dose rate as a result of an increase of the concentration ofradiation-induced radicals in the epoxy paint promoting the chemical reactions
between I2 and radio-induced paint-radicals and/or an increase of the concentrationof air radiolysis products promoting the formation of iodine oxide species. The I2
release decreases moderately as the CsI concentration (and the particle size)

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decreases, probably due to CsI aerosol interactions with the epoxy surface enhanced
at low concentrations.
•

The RI release was low in most of the tests. The RI release observed in the test
performed with quartz or stainless steel coupons can only be attributed to thepresence of organic contaminants in the loop, in the irradiation vessel or on the
coupon or to contamination sources of the carrier gas.

The experimental results of the specific AER-12 test – aimed at studying the formation andrelease of volatile iodine species from CdI2 aerosols under irradiation in “standard”
conditions (irradiation during 30 hours in air with steam at 50% of relative humidity at
80°C) – have shown that the radiolysis of CdI2 aerosols deposited on a quartz coupon leads
to an almost total global release (~98%). The main species volatilised are molecular iodine(63.0% at the end of the test) and species suspected to be aerosols (13.4% at the end of thetest). The molecular iodine volatilisation is attributed to the decomposition under radiationof CdI2 aerosols deposited on the quartz coupon. The species trapped on the quartz fibre
filter might be aerosol species; these aerosols produced from CdI2 radiolysis are assumed
to be iodine oxide aerosols. The results of this AER-12 test are thus consistent withthose obtained with CsI aerosols deposited on a quartz coupon and irradiated insimilar conditions.

2.3. Releases from coupons loaded with iodine oxide Aerosols (IxOy)
Air radiolysis leads to the formation of air radiolytic products like NO2, O3 or HNO3 [5] aswell as shorter-lived radical intermediates that can oxidise iodine and lead to the formationof iodine oxides (IOx) particles that settle down onto surfaces [6]. IOx are small aerosol
particles, whose composition and chemical behaviour, particularly under irradiation, are
not well known. A specific device to generate this kind of aerosols has been designed, andthen experiments were performed in the EPICUR facility. Thus, four dedicated tests: AER7, AER-8, AER-9 and AER-10 tests, have been carried out in order to evaluate the influenceof the temperature and the influence of the nature of the coupon surface on thevolatilisations of iodine species. The AER-10 test was performed in the same conditions as
the reference test AER-7 in order to evaluate the reproducibility of the data obtained. The
main outcome is that the instability (at least partially) under radiation of such iodine oxides
species is now clearly experimentally demonstrated. More precisely, the results of thesetests have shown that:•

Regarding the tests performed with iodine oxide aerosols deposited on a quartz
coupon and irradiated at 80°C, the values of global volatilisation and moleculariodine release are reproducible. The aerosol fraction is very low, and the loworganic iodide species release is attributed to the presence of organic pollutions.

•

The temperature increase from 80 to 120° leads to an increase of molecular iodine
release from oxide aerosols deposited on the irradiated quartz coupon. The
molecular iodine release is significant during the first sweeping phase at 120°C(without radiation). These two results, coupled with literature data concerning thedecomposition temperatures of IxOy species, might indicate that the iodine oxide
species produced during the loading phase and deposited onto the quartz coupon
would be I2O4 rather than I4O9 or HIO3.

•

The irradiation at 80°C of iodine oxide aerosols deposited on an epoxy painted
coupon leads to a higher global volatilisation by a factor of four than with iodineoxides deposited on a quartz coupon. This result is probably due to a more

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significant production of aerosols (more than 10%) and to an increase of the releaseof I2 (by a factor of four). In spite of the presence of potential organic radical
sources in the case of the epoxy coupon, the release of organic iodides species
remains of the order of magnitude of the measured values corresponding topollutions. The increase of I2 release with an epoxy painted coupon might be linked
to an increase of the residual quantity of I2 fixed on the epoxy painted coupon
during the loading phase or to species deposited on the epoxy painted coupon withdifferent speciation being less stable under radiation.
These experimental results of the STEM/IODINE part allow a better understandingof the iodine-paint interaction with containment surfaces and will be used to improvethe models currently implemented in calculation tools of all involved partners, suchas the ASTEC code [7], or other calculation tools like COCOSYS-AIM, KICHE, and
RAIM.

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3. Iodine-paint interaction
A specific deliverable of the Source Term Evaluation and Mitigation (STEM) project
concerned a literature survey report dealing with the status of current knowledge andunderstanding on iodine-paint interaction issue. This report provides an overall status on
the paints composition, insights of their resistance towards different stresses(temperature, irradiation, humidity) and their ageing chemical processes.
It also shows the likely chemical reactions between iodine and (aged) epoxy paint underirradiation and that, according to one article published in the literature [8] and some NEA
Behaviour of Iodine Project (BIP) and BIP-2 test results, a pre-irradiation dose on a painted
epoxy coupon (before the iodine loading) can reduce significantly the I2 adsorption kinetics
on the paint which could in turn lead to an increase of the gaseous iodine concentration,
especially in the short term (<20 h). So, irradiation could have a significant influence on
gaseous iodine concentration for doses received by the paint above 100 kGy as it could
reduce I2 adsorption rate and thus increase gaseous I2 and probably gaseous CH3I by I2
radiolytic conversion reaction with organic compounds in the gaseous phase. As paints andmore generally polymers can be exposed to several MGy in case of SA, it appears relevant
to investigate the effect of paint ageing on: i) iodine adsorption kinetics and ii) iodine
release kinetics from paints, both in normal operating conditions and during a SA by a highirradiation field composed of beta and/or gamma rays.
These identified issues are integrated in the STEM-2 description of work proposal (seeChapter 5 “Conclusions”).

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4. Main outcomes of STEM/Ru partAs already demonstrated by previous international or domestic experimental programmes[9][10][11][12], ruthenium species released from the core in case of a severe accident (SA)with oxidising conditions can reach the containment and form volatile ruthenium tetroxide.
The transport of Ru species in the reactor cooling system (RCS) is much less understood.
That is why, in the frame of the Source Term Evaluation and Mitigation(STEM)/RUTHENIUM programme aimed at ruthenium transport studies [13], a series of
22 vaporisation and revaporisation experiments have been performed in the START test
facility (see Figure 2). Ru is vaporised from a crucible containing RuO2 powder in a furnace
at 1 200°C, and then transported through a thermal gradient tube, with an outlet temperatureof 150°C to 250°C depending on the tests. The main studied parameters were the carriergas which was a steam-air mixture at different ratios, the type of thermal gradient (“abrupt”
or “smooth”) in the tube, and the tube material (quartz or stainless steel).
Figure 2. Scheme of the START test facility

Quartz tube

RuO2(c)Argon
SG

Furnace
1200°C max

150°C

Injection lines :Steam + air

150°C/250°C
1.20m/0.70m

Aerosol
FilterFurnaceLiquid traps
NaOH 1M

Source: IRSN.

The Ru vaporisation kinetics in the furnace with oxidising conditions was characterised,with a good reproducibility, for different carrier gas mixtures (H2O/air) and can beconsidered constant and steam content dependent. The dry air condition is the mostfavourable condition for the vaporisation of RuO2 from the crucible at 1 200°C.
The long duration tests (7 hours) show the existence of transient phenomena during the 1sthour (vaporisation phase), confirming thus the necessity of studying the ruthenium
chemistry for several hours to observe the precise phenomenology. Whatever the gaseous
mixture, most of the vaporised Ru is deposited in the quartz tube [i.e. >95% of the released
initial inventory (i.i.)]. The total Ru transported downstream of this tube (gas and aerosols)
represents a few percent of the Ru mass vaporised for long duration tests, and is mainly ingaseous form, which confirms the potential important impact of Ru on the source term inoxidising conditions. The main conclusions of the long duration vaporisation tests can besummarised as follows:•

In long duration vaporisation tests, an “abrupt” thermal profile promoted thetransport of Ru at the tube outlet (x4 compared to a “smooth” one), but it favours
also RuO2 particle formation (aerosols) in the gaseous phase. On the contrary, there
is only Ru gaseous species with a “smooth” gradient profile.
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No clear effect of steam on the total Ru transported (gas + aerosol) at the tube outlet
was observed in the vaporisation tests. Nevertheless, whatever the tube material
and the gradient thermal profile, a carrier gas of 60 wt% steam and 40 wt% air
promotes the quantity of gaseous Ru transported.

•

The tube material (quartz or stainless steel) has no significant effect on the total Ru
transported at the tube outlet during the vaporisation phase, whatever the gaseous
mixture. However, it has an impact on the gas/aerosol partition.

Two-thirds of the tests have been performed with a revaporisation phase, consisting of
sweeping the steam-air mixture into the tube containing Ru deposits (without generating
ruthenium from the crucible). Contrary to the tests studying vaporisation, no transientphenomenon was observed during this revaporisation phase. The main conclusions of therevaporisation tests can be summarised as follows:•

Whatever the parameters studied (material of the tube: quartz or stainless steel,thermal profile: “abrupt” or “smooth” and gaseous mixture: with or without steam),
the ending of revaporisation phenomenon could not be reached after 29h ofexperiment in START test conditions. So, the given values are not a maximum.
During all these revaporisation tests, the Ru is transported mainly in gaseous form
to the tube outlet. However, there is no Ru transport (lower than the detection limit)
in absence of air (H2O/Ar mixture).

•

Whatever the thermal profile, it was noticed that in the transport tube the Ru deposit
profile changed between the vaporisation and revaporisation tests. The Ru deposit
shifted from the higher to lower temperature zones during the revaporisation test.
With regard to the total Ru transported at the tube outlet (mainly in gaseous form),
it remains in the same order of magnitude (≤ 1% i.i.), so it can be said that thethermal profile (abrupt or smooth) has no significant effect.

•

There is a gaseous mixture composition effect on the Ru transported at the tube
outlet. With a quartz tube and an abrupt or a smooth thermal profile, the gaseous
Ru transport is enhanced with an excess of air. On the contrary, for the tests withstainless steel tube and an abrupt thermal profile, the gaseous Ru transport is
promoted with an excess of steam. To conclude: with an abrupt profile, there is aneffect of the tube material on the Ru transport during the revaporisation phase (incondition with a steam/air ratio of 60/40 wt%).

•

The gaseous Ru amount transported at the tube outlet seems to be promoted withthe stainless steel tube.

The transport of ruthenium in conditions similar to those prevailing in the RCS,
during a PWR SA (stainless steel, abrupt thermal profile, steam in excess and an
outlet temperature at about 170°C), during both vaporisation and revaporisation
phases lead to comparable gaseous Ru releases at the transport tube outlet (~2% i.i.),in START experimental test conditions. However, this is not the maximum reachable
value because the revaporisation phase was not finished at the end of the experiment(only 12 hours of revaporisation).
The STEM/Ruthenium results allowed the participants to gain knowledge on the key
parameters impacting the transport kinetics, the partition between gaseous RuO4 andparticles of RuO2 and the extent of the revaporisation process of deposited species inthe thermal gradient tube. A preliminary Ru transport model through the RCS was

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18 │ NEA/CSNI/R(2018)6
implemented in the ASTEC code [7], for instance, and a similar approach can betaken for other fission products release and transport codes.

SOURCE TERM EVALUATION AND MITIGATION PROJECT (STEM): FINAL REPORTNEA/CSNI/R(2018)6

5. Conclusions
The Source Term Evaluation and Mitigation Project (STEM)/IODINE results significantly
contributed to increasing knowledge and led to improvements of the modelling of key
phenomena concerning iodine chemistry in the different calculation tools of all involved
partners. For example, in the ASTEC code [7], these improvements mainly concern: i) theinteraction of I2 and CH3I with paint under irradiation; ii) the formation and radiolytic
decomposition of gaseous and deposited iodine oxide aerosols; iii) The radiolytic
conversion of gaseous I2 into CH3I and; iv) the interaction of I2 with steel and aerosols.
Concerning STEM/RUTHENIUM results, in the same way, a preliminary Ru transportmodel through the RCS was implemented in the ASTEC code [7], for example allowing afirst attempt to integrate possible Ru source term in PSA-2 tools. From this, the source term
evaluation performed with PSA-2 tools concluded that ruthenium radiologicalconsequences could be quite significant.
These STEM results have helped to precisely identify the major remaining issues that areproposed to be investigated during the second phase of the project. Indeed, according to thediscussions during the last STEM Programme Review Group & Management Board
meetings (June 2015) and the conclusions from the International NEA/NUGENIASARNET Workshop on the Progress in Iodine Behaviour for NPP Accident Analysis andManagement (March 2015), a second phase of the project, named STEM-2, has beenlaunched in the /NEA framework. This four-year follow-up project is focused on mediumand long-term releases and with four main items, as prioritised by the STEM Programme
Review Group:●

The effect of the ageing of paints on iodine behaviour (both during normal operation
conditions and during accidental conditions): i.e. irradiation tests (medium- and long-term
irradiation periods) on paints aged in representative reactor conditions and loaded withiodine to confirm iodine release processes observed with un-aged paints (incl.
adsorption/desorption kinetics);●

The study of iodine oxide aerosols stability:●

o Iodine oxides radiolytic decomposition;o Iodine oxides decomposition by carbon monoxide and/or hydrogen;Radiolytic oxidation of multi-components iodine aerosols;●

Complementary tests based on Ruthenium revaporisation processes with representativeoxidative conditions (atm. Radiolysis simulants) + presence of gaseous pollutants
representative of RCS conditions.
It should also be noted that an analytical working group has been set up to promote a final,
comprehensive analysis of the results for reactor case source term evaluation, including
mitigation aspects. Finally, parts of the experimental investigation conditions have been
discussed and shared with the Canadian Nuclear Laboratories (CNL) (Operating Agent of
the forthcoming third phase of the NEA Behaviour of Iodine Project [BIP-3][14]).

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20 │ NEA/CSNI/R(2018)6

6. References[1] B. Clement and B. Simondi-Teisseire, “STEM: An IRSN project on source term evaluation andmitigation”, Transactions of the American Nuclear Society - 2010 ANS Annual Meeting; Las Vegas,
NV, U.S.; November 7-11th, 103, 475-476, (2010).
[2] J. Colombani et al., “Experimental study of organic iodide volatilization from painted surfaces
present in the containment during a severe accident”, Proceedings of the 6th European Review
Meeting on Severe Accident Research, Avignon, 2-4 October, France (2013).
[3] L. Bosland et al., “Towards a mechanistic interpretation of the iodine – paint interactions”,
Proceedings of the 4th European Review Meeting on Severe Accident Research, Bologna, 11-12 May,
Italy (2010).
[4] J. Colombani, A.C. Grégoire, S. Morin, “Main findings of the IRSN experimental programs
performed on iodine chemistry in severe accident conditions”, Workshop on the Prog. in Iodine
Behaviour for NPP Acc. Anal. and Manag. - March 30, April 1 - Marseille (France), (2015).
[5] F. Funke, S. Gupta, and al., “Interaction of gaseous I2 with painted surfaces and aerosols in largescale THAI tests”, Proc. of the Int. NEA/NUGENIA-SARNET Workshop on the Prog. in Iodine
Behaviour for NPP Acc. Anal. and Manag. - March 30, April 1 - Marseille (France), (2015).
[6] L. Bosland, F. Funke, and al., “PARIS project: Radiolytic oxidation of molecular iodine incontainment during a nuclear reactor severe accident: Part 2: Formation and destruction of iodineoxides compounds under irradiation – experimental results modelling”, Nucl. Eng. Des., 241, (9), p.
4026-4044, (2011).
[7] L. Cantrel, F. Cousin, and al., “ASTECV2 severe accident integral code: Fission product modellingand validation", Nucl. Eng. & Des., 272, p. 195-206, (2014).
[8] A.B. Aleksandrov, N.I. Ampelogova, M.A. Karaseva; V.G. Kritskii and N.G. Petrik, “The effect ofphysicochemical conditions on the absorption of molecular iodine 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.
[9] Y. Pontillon, G. Ducros, “Behaviour of fission 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).
[10] N. Vér, L. Matus, M. Kunstár, J. Osán, Z. Hózer, A. Pintér, “Influence of fission products onruthenium oxidation and transport in air ingress nuclear accidents”, Journal of Nuclear Materials.,
396, pp. 208-217 (2010).
[11] C. Mun, L. Cantrel, C. Madic, “Radiolytic oxidation of ruthenium oxide deposits”, Nuclear
Technology 164, pp. 245-254 (2008).
[12] T. Kärkelä, Ruthenium databook – version 1.0, SARNET2-ST-D8.7 report (EU Severe Accident
Research Network of Excellence 2) (2013).
[13] O. Leroy, M.N. Ohnet, S. Planteur-Kieffer, “Study of the Ruthenium transport under prevailingconditions in the primary circuit in case of a PWR severe accident”, ERMSAR 2015 conference,
Marseille (France).

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Proc. of the Int. NEA/NUGENIA-SARNET Workshop on the Prog. in Iodine Behaviour for NPP
Acc. Anal. and Manag., 30 March, 1 April, Marseille (France), (2015).

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