The final objective of all severe accident evaluations is to obtain the magnitude of the potential Fission Product (FP) release in the environment. Consequently, severe accident experts have focused, very often mainly, and sometimes exclusively, on FP physical and chemical behaviour. In many evaluations, the thermal hydraulic transient was calculated by very simplified models. But the situation is evolving. By carrying out more frequent physical analyses to improve plant evaluation or to prepare experiments (such as Phebus FP), a growing number of thermalhydraulics problems have been raised and recognized as being significant for FP prediction. The paper therefore reviews the main thermalhydraulic phenomena which have to be taken into account and especially examines their influence on severe accident initiation and scenario, core degradation, vessel failure and more generally RCS failure, FP release, transport, deposition, revaporization. The needs for model improvement and for specific assessment are listed. They cover not only the description of special thermalhydraulic phenomena, but also the handling of complex coupling with other physical modules. Prospective research directions are proposed to answer these needs. © 1993.
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