1. ASTEC validation on PANDA tests A. BENTAIB, A. BLEYER Institut de Radioprotection et de Sûreté Nucléaire BP 17, 92262 Fontenay aux Roses Cedex, FRANCE B. ATANASOVA INRNE-BAS Tzarigradsko shossee 72, 1784 Sofia, BULGARIA

2. Nesseber september 2010 2/26 Motivation Conclusions & perspectives Outline PANDA T9, T9bis and T25 analysis

3. Nesseber september 2010 3/26 3 barriers between radioactive materials and environment (« Defense in depth » principle) : Combustible clads Primary Circuit Containment Volume : 50000 to 80000 m3 depending of the type Length scale : some cm to several meters Double concrete containment or simple concrete containment with liner (steel or composite) H2 risk issues: Pressurized Water Reactors

4. Nesseber september 2010 4/26 Severe Accident: A severe accident is characterized by a reactor core uncovery leading to core degradation and Fission Products release into the containment atmosphere (loss of the two first barriers) H2 Risk Issues: Pressurized Water Reactors Four main Phases : Loss of Fuel coolant phase in the primary circuit Core uncovery and core degradation phases Core melt-throughand reactor core vessel failure Core concrete interaction and base mat penetration

5. Nesseber september 2010 5/26 the composition of the gaseous mixture inside the containment at each location and at each time (Distribution) the effect of the mitigation means as spray, catalytic recombiners (Mitigation) an estimation of the possible ignition of the gaseous mixture (Flammability limits) an evaluation the propensity of a premixed flame to propagate inside the containment (Flame acceleration criteria) the pressure and temperature loads due to combustion events inside the containment (Combustion) Hydrogen Risk evaluation for PWR: Needs

6. Nesseber september 2010 6/26 IRSN/GRS cooperation since 1996 for development of an integral code for LWR (present/future PWR, BWR, VVER) source term severe accident (SA) calculation  Main objectives: oApplications to PSA2, including uncertainty analysis, oAccident management studies, oInvestigations of NPP behaviour in SA cond., including source term evaluation, oSupport and interpretation of experiments, oBasis for a better understanding of SA physical phenomena.  Main requirements: oComprehensive coverage of phenomena, account for their interactions. o"Reasonable" calculation time (fast-running code)  < 12h CPU for one day of accident simulated, oAccounting for safety systems and their availability (SAM), oHigh level of model validation, oModularity, flexibility, user-friendliness, easy model incorporation. => ASTEC aim : becoming the european reference code for Severe Accidents ASTEC : Accident Source Term Evaluation Code

7. Nesseber september 2010 7/26 TOSQAN (IRSN) ThAI (BT) MISTRA (CEA) PANDA (PSI) Example of experimental programs in support of ASTEC validation Addressed phenomena : Condensation, Gas and Thermal stratification, stratification break-up, Spray effect, Scaling effect 7 m 3 60 m 3 100 m 3 480 m 3

8. Nesseber september 2010 8/26 PANDA facility SETH configuration : Dimensions : Height 8 m, diameter 3.957 m, volume about 180 m³ Materials: Steel walls instrumentation: more than 275 TC and 47 sampling points for MS measurements Operated by Paul Scherrer Institute (Switzerland)

9. Nesseber september 2010 9/26 Experiments Main addressed phenomena - Steam and non-condensable gases mixing behavior - Thermal stratification phenomena - Characteristic of gas transportation between compartments - Steam condensation on the walls - Injection location effect

10. Nesseber september 2010 10/26 PANDA nodalization near wall injection configuration  50 zones  10 vertical levels  Zones connected by flowpaths with area according geometry  Structures: heat exchange with atmosphere, condensation, heat capacity, heat losses to environment

11. Nesseber september 2010 11/26  In the early phase the amount of steam is for both tests comparable over the Vessel 1 height Test 9 and Test 9bis analysis  Later on, due to the on-set of condensation in Test 9bis the steam concentration at the top of vessel increases faster in Test 9  Due to the evaporation of condensate water, steam concentration in lower becomes higher in test9bis

12. Nesseber september 2010 12/26 Test 9bis: steam concentration (DW1) the condensate water drained to the lower part of DW1 leads to a sharp decrease of gas temperature at 4000 seconds. Afterwards and due to the difference between gas and wall temperature, steam evaporation occurs and generates an increase of gas temperature and steam concentration in the bottom of DW1

13. Nesseber september 2010 13/26 Thermal stratification in DW1 Until 3000 s, the increase of gas temperature is comparable for both tests For test 9bis and after 3000 s, steam condensation induces a strong heat transfer and an increase of wall and gas temperature

14. Nesseber september 2010 14/26 Thermal stratification in DW2

15. Nesseber september 2010 15/26 Steam transport in IP steam is transported mainly in the top of the interconnecting pipe The on-set of condensation in Test 9bis determines similar observation as in Vessel 1

16. Nesseber september 2010 16/26 Steam transport to DW2  Sharp steam stratification is observed during the overall test period, between the regions above and under the interconnecting pipe height  The on-set of condensation in Test 9bis determines similar observation as in Vessel 1

17. Nesseber september 2010 17/26 Steam at the venting location Until 3000 s, steam concentration at venting location is compared for both tests After 3000 s, The on-set of condensation in Test 9bis determines similar observation as in Vessel 1, Vessel 2 and IP.

18. Nesseber september 2010 18/26 PANDA nodalization central injection configuration ww  53 zones with 12 vertical levels  Zones connected by flowpaths with area according geometry  Structures: heat exchange with atmosphere, condensation, heat capacity, heat losses to environment

19. Nesseber september 2010 19/26 Predicted pressure, gas composition and temperature in the vent are in good agreement with data Test 25 analysis : pressure time evolution

20. Nesseber september 2010 20/26 Test 25 analysis: 1 st Phase (time <815s)

21. Nesseber september 2010 21/26 Test 25 analysis 1 st Phase (2215<time <2815s)

22. Nesseber september 2010 22/26 Test 25 analysis 1 st Phase (4415<time <7015s)

23. Nesseber september 2010 23/26 Test 25 analysis 2 nd Phase (7415<time <14215s)

24. Nesseber september 2010 24/26 Conclusions Results on gas temperature and gas concentration obtained with the ASTEC code for both tests T9, T9bis and Test25 are in good agreement with the experimental data:  gas mixing and stratification above and under the height of the DW interconnecting pipe have been well reproduced by the code.  the steam condensation effect on the thermal and the concentration front propagation in the two drywells and the interconnecting pipe have been well predicted by the calculation

25. Nesseber september 2010 25/26 Conclusions The Astec validation process will continue by considering well instrumented experiments to check the effect of facility scale and to prepare rules and recommendations to best use of LP codes for reactor applications

26. Nesseber september 2010 26/26 Thank you for your attention