European Review Meeting on Severe Accident Research June 12-14, 2007 – Karlsruhe (Germany)

1. European Review Meeting on Severe Accident Research June 12-14, 2007 – Karlsruhe (Germany) • Preliminary analyses of the Phebus FPT3 experiment using Severe Accident Codes (ATHLET-CD, ICARE/CATHARE, MELCOR) • Georges Repetto, Olivier De Luze (IRSN), • Tilman Drath, Thorsten Hollands, Marco K. Koch (RUB-LEE), • Christine Bals, Klaus Trambauer, Henrique Austregesilo (GRS) • Jon Birchley (PSI),

2. OUTLINE Introduction Modelling and boundary conditions Code results Thermal evolution Hydrogen release Control rod rupture B4C oxidation and gas releases Bundle Degradation Conclusion

3. Introduction Pre test calculation were performed using several S. A codes such as ICARE2 (IRSN), ATHLET-CD (GRS) and MELCOR (PSI and JNES) This benchmark was published in the NUCLEAR TECHNOLOGY (Feb.07) This presentation is devoted to FPT3 post test calculations performed as a Joint Activity, by : IRSN (ICARE2) GRS RUB PSI (MELCOR) (ATHLET-CD) FPT3 in the framework of the Phebus Bundle Interpretation Circle and the SARNET CORIUM WP 9.2 : early phase and B4C effects

4. Part I Brief review of the codes modelling and the FPT3 boundary conditions

5. Codes and modelling options

6. Modelling options (Zry oxidation) Best fitted coming from the COLOSS project MELCOR PSI ICARE2 IRSN ATHLET-CD GRS-RUB

7. Codes and modelling options

8. Modelling options (B4C-SS interaction) ICARE2 IRSN ATHLET-CD RUB ATHLET-CD GRS domain

9. Codes and modelling options

10. Modelling options : B4C oxidation (1/2) ATHLET-CD GRS ATHLET-CD RUB

11. Modelling options : B4C oxidation (1/2) For ICARE2 (IRSN) the kinetics of solid B4C oxidation based on IRSN (VERDI) and FzK (BOX) experiments (mole/m2/s) Steinbrück et al. (FZKA 6979), for ATHLET-CD FzK VERDI (119 experiments) This work performed in the frame of the WP9.2 activities

12. FPT3 boundary conditions (versus FPT2) Steam flow rate Oxidation phase Heat up phase Calibration phase Similar Bundle power transient as FPT2 except for the P4 Power plateau and the maximum power (which is lower of about 27%) Constant steam mass flowrate (0.5 g/s) (as for FPT2)

13. FPT3 boundary conditions (Bundle Power transient) Test Code input deck P4 As for previous Phebus tests, to achieve good thermal results during the calibration phase and the P4 plateau, the bundle power has been reduced (-15 to -25%)

14. Part II Results of the simulations

15. Thermal results : temperatures evolution (1/3) Assuming the bundle power adjustment, rather good thermal results were obtained up to the end of the transient 0.5 m elevation Test (TUS) Test (Tc) bundle Test (Tc) Shroud

16. Thermal results : temperatures evolution (2/3) Rather good thermal results with however overestimation of temperature during oxidation °C 0.7 m elevation Code overestimation Test bundle Shroud

17. Thermal results : temperatures evolution (3/3) An other elevation shows the same behavior which seems to be linked to overestimation of the steam starvation duration given by the codes °C 2600 K 0.6 m elevation Code overestimation bundle Test Shroud

18. Hydrogen generation (1/2) H2 (g) Rather good results obtained with both code simulations regarding the total hydrogen generation The lower value produced with MELCOR is due to the under- estimation of the H2 during the main oxidation period (mainly coming from B4C) experiment ** + 6 g from the sample

19. Hydrogen generation (2/2) ~360 s The total H2 produced is correct Nevertheless, the rather short starvation period observed in the experiment is not simulated by both codes • the pure starvation period during the test was confirmed to be shorter than what was foreseen by pre-calculations (1200 +/-200 s)

20. Zry GT SS Clad B4C Control Rod rupture (1/2) ATHLET-CD ICARE2 B4C-SS interaction Rupture of the Control rod at t = 9490 s / 9370 s : values consistent with the experiment observation (9550 s) In the simulation, it happens when the SS clad is completely dissolved by interaction with B4C (in ICARE2 and ATHLET-CD) (no specific model for the Guide Tube rupture in both S.A codes)

21. Control Rod rupture (2/2) ATHLET-CD (GRS) 0.3 kg Control rod rupture Clad rupture Metallic melt and crust from control rod (B,C, SS, Zry) is relocated to lower elevations with ICARE2, whereas, it remained in place with ATHLET-CD code This should increase the oxidation of the Control rod materials in ATHLET-CD versusICARE2 results Example of mass amount involved in the Control rod degradation with ATHLET-CD code

22. B4C oxidation and Hydrogen generation ICARE2 ATHLET-CD (GRS) H2 70% 15% CR Rupture About 15 % solid B4C remained intact, which is consistent with the experiment

23. Gas release coming from B4C oxidation (test results) Carbonaceous species 0.64 mole of CO mainly during the starvation phase most of the release during the heat up phase at low H2 concentration 0.37 mole of CO2

24. Gas release : Comparison with the code simulations experiment 1 mole 1 mole CO (0.64 mole) B4C + 7H2O(g)  2B2O3 + CO(g) + 7H2(g) H2 1 mole 1 mole CO2 (0.37 mole) B4C + 8H2O(g)  2B2O3 + CO2(g) + 8H2(g) Taking into account the most probable reaction : 1 mole of Carbon produced during the experiment should come from 1 mole of oxidised B4C At the end of the test, carbon release by codes (0,95 to 1.11 mole) is consistent with the test result (roughly 1 mole) Nevertheless, kinetics have to be improved experiment MELCOR produces only 0.2 g of CO (part of the reacted B4C lead to elemental carbon not account for in the curve : equilibrium chemistry model in SOLGASMIX model MELCOR

25. Bundle degradation Clad fuel Early bundle degradation due to fuel rods / B,C,SS interaction is suspected Not accounted for by S.A codes ICARE2 ATHLET-CD (GRS) predicted bundle degradation due to higher temperature at the end of the transient ATHLET-CD 1.6 kg

26. Thermal results : temperatures evolution Slight temperature over estimation during the heat up phase, in the ATHLET-CD (GRS) case leading to fuel degradation at 2600 K °C 0.6 m elevation Code overestimation bundle Test Shroud

27. Conclusion • FPT3 post test calculations have been performed using 3 different codes (ICARE2 , ATHLET-CD and MELCOR) • Overall thermal behaviour well reproduced. • Cladding temperature during oxidation are overestimated. • Total amount of hydrogen production is correct but not the kinetics. • Starvation duration not well reproduced by both S.A codes. • Kinetics of solid B4C oxidation with a suitable correlation in the codes (ICARE2 and ATHLET-CD) and gas release are rather well predicted • Modelling effort is foreseen in MELCOR (only adapted for BWR) • No significant fuel degradation is predicted by those S.A codes • Early bundle degradation is suspected due to interaction between CR materials and neighbouring fuel rods • Modelling effort on this area has to be done in both S.A codes

28. Thank you for your attention ….. Regarding ATHLET-CD analyses performed by RUB and GRS, the authors give specific acknowledgement to the German Federal Ministry of Economics and Technology (BMWi ) for their sponsoring.