Review of debris bed cooling in the TMI-2 accident

Still today after decades of severe accident research it is not well understood why the molten corium did not attack or even penetrate the lower head vessel wall in the TMI2 accident. The findings can only be explained by additional assumptions which have been proposed by various authors. This paper is also looking for an explanation by examining the role of the debris cooling in the TMI lower head. The present knowledge of debris cooling is based on small-scale experiments with simulant debris. It is argued that the experiments have been stopped to early and therefore do not reveal the potential of debris bed coolability in case of a corium debris bed. It is also argued that in TMI the debris bed was in a state of turbulence and fluidization such that the coolabilty was much higher than in the small-scale experiments. Basically this paper hypothesizes that the unknown phenomenon in the TMI 2 accident is the strong interaction of the debris particles of a wide range of grain sizes and the strong turbulent motion of the cooling fluid and vapor mixture in conjunction with a "virtual gap" at the vessel wall. (c) 2006 Published by Elsevier B.V.