T. Haste 1 , P. Giordano 2 , L. Herranz 3

1. Achievements and Status of Research Activities in the Source Term areaEuropean Review Meeting on Severe Accident Research(ERMSAR-2007)Forschungszentrum Karlsruhe, Germany, 12-14 June 2007 T. Haste1, P. Giordano2, L. Herranz3 1PSI Villigen, Switzerland,2IRSN Cadarache, France,3CIEMAT Madrid, Spain

2. OBJECTIVES SOURCE TERM AIMS • WP14.1 OXIDEN: effect of air ingress - under such conditions, the fuel and its fission products (FPs) may oxidise. Some (especially the very radio-toxic ruthenium) may form highly volatile oxide species which may be released to the environment; • WP14.2 HITEMP: iodine volatility in the Reactor Coolant System (RCS) - the impact of high temperature on FP behaviour is investigated to improve the predictability of iodine species exiting the RCS; the effect of silver-indium-cadmium release on the speciation is also considered; • WP15 AEROB: aerosol behaviour – quantification of the source term following steam generator tube rupture, which leads to containment by-pass; aerosol leakages through containment concrete wall cracks, revaporisation phenomena are also studied; interaction of deposited aerosols with the RCS substrate may be considered later; deposition/resuspension included specifically in JPA3; • WP16 CONTCHEM: iodine behaviour in-containment - to improve the predictability of the various chemical and physical processes which control the iodine behaviour in both the gas and water phases; FP heating/PARs, Ru behaviour in containment added in JPA3. These aims are addressed in Technical Circles, 14 in all, composed of experts working in the areas concerned

3. Model proposals : Reactor scenario calculations : ASTEC, ICARE/CATHARE (IRSN) SATURNE, MAAP4 (EDF) SCDAP/RELAP5, ICARE/CATHARE (INR) UO2 oxidation kinetics Ru release kinetics WORK PROGRAMME 1- FP release under highly oxidizing conditions (WP14-1) : Experimental means : RUSET (AEKI), Ru speciation (VTT)MERARG (CEA), analytical tests (UCL)FIPRED (INR), AECL facilities (AECL)future VERDON (CEA) Modelling work Experim. review Jointly-executed research : IRSN – AEKI –CEA – EDF ENEA - FZK – GRSINR – VTT– AECL Interpretation

4. Experimental means : PHEBUS FPT2 (IRSN), CHIP (IRSN), VERCORS HT (CEA) EMAIC (CEA), QUENCH (FZK) Model proposals : Iodine speciation in RCS SIC release kinetics, and coupling with control rod degradation WORK PROGRAMME 2- Transport :High temperature gas phase chemistry in RCS (WP14-2) : Modelling work Experim. review Jointly-executed research : IRSN – AEKI - EDF – FZK GRS – JRC/IE – PSI UJV – VTT - AECL Interpretation

5. Vane Dryer Panels of Steam Dryer Exit of Steam Dryer Turbulence Grid Swirl Vane of DropletSeparator Tube Bundle Support Plate Tube Sheet WORK PROGRAMME 3- Aerosol behaviour impact on Source Term (WP15) : Experimental means : PHEBUS FP (IRSN), REVAP (JRC/ITU), RADSOL (UCL), PECA/SGTR (CIEMAT) ARTIST/FP5 (PSI), STORM (JRC Ispra), PSAERO/HORIZON (FORTUM/VTT), HCE (AECL), IRSN crack tests, possible COLIMA (CEA) Modelling work Experim. review Jointly-executed research : IRSN – CEA – CESI – CIEMAT Demokritos – EDF – Fortum GRS - JRC/ITU – JRC/IE – PSI UJV – VTT – AECL - UNEW Interpretation Model proposals : Aerosol retention in cracks Aerosol retention in SG Deposit remobilization

6. Modelling work Experim. review Jointly-executed research : IRSN- WMT – CEA – Chalmers CIEMAT – Demokritos – EDF AREVA – GRS- PSITUS – VTT - AECL Interpretation WORK PROGRAMME 4- Containment chemistry (WP16) : Experimental means : Chalmers tests (Univ.), CAIMAN (CEA), PARIS (AREVA), SISYPHE (IRSN) PHEBUS FP (IRSN), EPICUR (IRSN)ThAI-Iod9 (GRS/Becker)RECI (IRSN) Model proposals : Adsorption/desorption Liquid-gas mass transfer Radiolytic oxidation of iodine Ruthenium chemistry Iodine data book : WMT, PSI, AECL

7. RESULTS RUTHENIUM BEHAVIOUR • Reactor calculations have indicated that UO2 may be exposed to air thus enhancing Ru release potential and have specified prevailing conditions • Ruthenium release occurs in oxide form after an incubation period during which full oxidation of fuel and cladding occurs (RUSET and AECL tests), further data from a MERARG test is pending • Models have been developed for fuel oxidation and Ru release applicable to oxidising conditions • Further data are required on Ru release in air to validate these models independently (future VERDON programme, ISTC/VERONIKA?) • Oxide forms can stay volatile enough at lower temperatures to be transported to the reactor containment (RUSET and VTT tests) • The potential Ru release into the containment and its demonstrated persistence there in volatile RuO4 form have stimulated investigations under containment conditions - experimental and modelling studies at IRSN and Chalmers University are addressing these issues

8. RESULTS IODINE BEHAVIOUR - CIRCUIT • Knowledge of iodine transport through the RCS is essential to define the source term to the containment or to ‘environment’ (bypass sequences) • Analysis of Phebus FP and VERCORS-HT data, with equilibrium ASTEC/SOPHAEROS calculations have demonstrated the influence of Mo and structural materials on I vapour chemistry • Consensus reached on the close connection amongst ‘Cs, Mo, I and Cd’ and on the next steps to improve understanding • The CHIP facility is now producing the first-ever data on chemical kinetics effects, that will enable advanced models accounting for non-equilibrium effects to be developed • Other chemical elements released from degrading AIC control rods (Ag, In and Cd) would affect the physico-chemical environment of iodine transport • The AIC release models developed from separate-effects data (EMAIC) will be tested in near-prototypical conditions in the upcoming QUENCH-13 integral test, aiming to assess better existing uncertainties (e.g. for Cd burst release) • Active collaboration on Q-13 experimental and modelling support under way

9. RESULTS IODINE BEHAVIOUR - CONTAINMENT • Current knowledge on iodine chemical behaviour in the containment is concisely encapsulated in the recent Iodine Data Book • Nonetheless, further experimental data are required on iodine oxidation in gas and aqueous phases, liquid-mass iodine transfer, RI formation/destruction, etc. The EPICUR, CAIMAN, PARIS and SISYPHE tests are producing/have produced the data required to understand better and then to model all those aspects. These are further supported by new availability of AECL data • Current code capabilities in an integral containment geometry are being assessed through the ThAI-Iod9 benchmark and the Phebus FPT2experiment interpretation, the latter resulting in a consensus on gaseous iodine evolution and inorganic/organic iodine distribution in the gas phase • Potential conversion of particulate iodine into gaseous iodine species due to iodine interaction with in-containment safety systems (i.e., PARs) was shown by RECI small-scale tests, and theoretical assessments have shown that this could be quantitative. However, these results need to be confirmed at a larger scale to fully assess actual consequences

10. RESULTS AEROSOL BEHAVIOUR - SGTR • Aerosol leakages to the environment either through containment cracks or directly from the primary circuit (bypass sequences), can contribute to the source term • In the case of SGTR sequences some aerosol retention could occur in the secondary side, as experimentally demonstrated in the SGTR project of the 5th EU FWP of EURATOM • Even if the secondary side of the steam generator is dry, data seem to indicate that some decontamination should be foreseen (this being much larger if water is present) • Interpretation of available data, as well as development of models for dry and wet conditions has enhanced understanding and predictability, although there is some way to go • Resuspension is a key phenomenon in the SGTR scenario and it is presently being revisited based on STORM, PSAERO data – that from thick deposits in particular needs better understanding

11. RESULTS AEROSOL BEHAVIOUR – CONTAINMENT CRACKS • Loss of containment integrity by cracking of the concrete could also affect the source term if radioactive particles can leak through • Aerosol transport models have been developed and compared satisfactorily to available data (SIMIBE tests) - they seem to indicate a high decontamination factor in the cracks, particularly in the presence of steam • nonetheless, prototypical experiments are missing and scoping tests in the COLIMA facility under the EU PLINIUS transnational access programme have been proposed REVAPORISATION • Revaporisation of previously deposited radionuclides (as evidenced by PHEBUS) could lead to a late source term. Data from the REVAP facility together with those from AECL are being analyzed to improve modelling capability in integral situations.

12. HIGHLIGHTS OF FORWARD PROGRAMME FORWARD PROGRAMME • WP14-1: joint interpretation of new Ru data from AECL tests (in joint selection), RUSET (AEKI) continued experiments on Ru release from alloys, VTT Ru tests • WP14-2: CHIP data on iodine speciation in the circuit becoming available for both analytical and phenomenological lines, good cooperation on design and construction, possibilities for joint analysis; QUENCH-13, strong cooperation on test conduct (especially SIC aerosol measurements) and pre-test calculational support; new VTT facility on revaporisation and speciation (link with WP15) • WP15: new revaporisation data from AECL, new effort on resuspension/deposition modelling including turbulent deposition (use of newly-acquired STORM data is being encouraged); possible COLIMA experiment (s) under PLINIUS • WP16: availability of AECL RTF data with possibility of joint interpretation, EPICUR tests and interpretation, ThAI-Iod9 benchmark, continuing experiments on PARs/FP heating by IRSN (RECI successor) and on Ru behaviour in-containment at IRSN and Chalmers.

13. COOPERATION AND DISSEMINATION COOPERATION • In SARNET, with the Corium, ASTEC, Research Priorities, Database and Spreading of Excellence areas (here through staff mobilities and contribution to the severe accident book and to training courses • Outside SARNET, with PHEBUS FP and ISTP (especially chemistry areas, joint technical meetings); ISTC, though review of proposals (EVAN – now a project, VERONIKA); watching brief kept on CSNI GAMA-related matters such as ThAI continuation and BIP; FZK QUENCH through support to QUENCH-13 DISSEMINATION • In SARNET, via ACT, where meetings are organised and documents stored (almost 60 technical papers so far) • Outside SARNET, in journals (4 papers so far) and conference papers (19 so far, plus contributions to overall SARNET reports)

14. CONCLUSIONS CONCLUDING REMARKS • The Source Term area has continued to make good overall progress in increasing understanding in its technical areas through a combination of experimental work, interpretation of data and modelling directed towards ASTEC • Good cooperation has been established internally within other SARNET work areas such as Corium, IED, Spreading of Excellence and Research Priorities, and externally with Phebus FP, ISTP, ISTC and QUENCH, etc. • The work of the SARNET area has been reported fully in the open literature and in journals; more publications are expected as the project matures • A process of reflection is starting on possible follow-on projects in the 7th FW (2007-2013), taking account of the recommendations of the Research Priorities Group The authors thank the many technical contributors within SARNET organisations, too numerous to name, and the European Commission for funding SARNET, in the 6th Framework Programme area “Nuclear Fission: Safety of Existing Nuclear Installations”, under contract number FI6O-CT-2004-509065.