SYSTEM THERMALHYDRAULICS DURING PWR SEVERE ACCIDENTS

The final objective of all severe accident evaluations is to obtain th e magnitude of the potential Fission Product (FP) release in the envir onment. Consequently, severe accident experts have focused, very often mainly, and sometimes exclusively, on FP physical and chemical behavi our. In many evaluations, the thermal hydraulic transient was calculat ed by very simplified models. But the situation is evolving. By carryi ng out more frequent physical analyses to improve plant evaluation or to prepare experiments (such as Phebus FP), a growing number of therma lhydraulics problems have been raised and recognized as being signific ant for FP prediction. The paper therefore reviews the main thermalhyd raulic phenomena which have to be taken into account and especially ex amines their influence on severe accident initiation and scenario, cor e degradation, vessel failure and more generally RCS failure, FP relea se, transport, deposition, revaporization. The needs for model improve ment and for specific assessment are listed. They cover not only the d escription of special thermalhydraulic phenomena, but also the handlin g of complex coupling with other physical modules. Prospective researc h directions are proposed to answer these needs.