Analysis of the Phebus FPT-0, FPT-1 and FPT-2 experiments with the Icare/Cathare code

G. Repetto, O. De Luze, C. Jamond, M. Barrachin (1), S. Ederli (2), J. Fontanet (3). 5th PHÉBUS FP Technical Seminar Aix-en-Provence, June 24-26, 2003

This paper describes the results and the current status of the analysis of the core degradation aspects for the FPT-0 FPT-1 and FPT-2 tests, using the mechanistic ICARE/CATHARE code system developed by IRSN “Institut de Radioprotection et de Sûreté Nucléaire”. The objective of those experiments, in the framework of the International Phebus Fission Product program, was to get a significant FP release induced by fuel rod degradation and fuel melting in a prototypical way using real materials as present in a PWR fuel assembly. During the transient, most of the phenomena that could occur in-vessel during a PWR severe accident (thermo mechanical fuel rod rupture, absorber rod degradation, steam-zircaloy chemical reaction and hydrogen generation, fuel dissolution and molten pool formation) have been observed. They are quite well simulated with the ICARE2 V3mod1 code version, in particular the hydrogen generation. The experiments showed core degradation far beyond any other integral experiment (PBF SFD, Phebus-SFD, CORA, FLHT, and LOFT-FP-2). The severe damage observed in the bundle seems to be due to significant material interactions, initiated by structural materials possibly enhanced by the fuel swelling and fragmentation, and its changes in stoichiometry. The fuel burn-up and the oxygen potential during cladding oxidation are probably important factors. It was observed that fuel liquefaction and transition from rod like geometry to a molten pool could occur at temperatures (2600+/-200K) far below the actual melting point of the pure UO2 (3110K). Though the detailed modelling of such interactions has still to be improved, the ICARE2 code simulates the observed fuel degradation fairly well. The Phebus FP series of integral experiments have proved the capability of the program to significantly improve our understanding of the source term issue and important aspects of core degradation.

(1) : IRSN
(2) : ENEA
(3) : CSN