Key issues concerning in-vessel scenarios

摘要

The major question concerning severe in-vessel accidents is whether the melt can be retained and cooled inside the vessel. The only measure designed to reach this under all circumstances is external cooling of the vessel by ex-vessel flooding. In this concept, the whole core material is considered as molten and relocated to the lower head, forming a melt pool. This is treated as the worst case. However, there remain questions concerning the general applicability and assured functioning of the concept depending on the reactor type (especially power level and resulting amount of decay heat) and layering phenomena in the melt pool, especially the occurrence of metal layers. If they are not very thin and not thick enough, such layers may yield focussing of heat flux towards regions of the vessel wall, with resulting local values which cannot be removed by external cooling. This accident management measure has already been treated intensively in separate publications and will not specially be considered here.