Application of mechanistic criteria of cladding oxide shell failure to the analysis of core degradation simulated in bundle meltdown tests

In this paper, the strong influence of the clad failure criteria on the calculated scenario of bundle degradation was demonstrated on the basis of comparative analysis of Phebus FP and PBF–SFD tests using the SVECHA/QUENCH (S/Q) code. The main peculiarity of the proposed mechanistic criteria is that they are based on the actual geometry of degraded fuel rods and not on a constant, user-defined failure temperature. This allows better modelling concerning the formation of local molten pool(s) in the upper part of bundle and the timing of subsequent materials relocations from the local molten pool(s). In general, the mechanistic criteria developed allow one to reasonably calculate the bundle degradation progression in both tests. Therefore, it is recommended to use these new, mechanistic cladding failure criteria in the severe accident system codes as well.