EFFECT OF EXTERNAL COOLING ON THE THERMAL-BEHAVIOR OF A BOILING WATER-REACTOR VESSEL LOWER HEAD

Flooding of the drywell of a boiling water reactor (BWR) is one of man y accident management strategies being proposed to manage severe accid ents in light water reactors. The effect of external cooling on the th ermal behavior of the BWR vessel lower head containing molten core mat erial has been numerically investigated using a two-dimensional implic it finite difference scheme. Results have been obtained for the vessel shell temperature, the molten pool temperature, and the crust thickne ss for steady-state conditions. For each equilibrium state, the therma l behavior of the vessel lower head has been investigated by parametri cally changing the emissivity of the pool free surface, the vessel wal l, the baffle plate, the core shroud, and the upper structure; and the temperature of the upper structure. For a certain set of parameters, nucleate boiling on the outer surface of the vessel wall is found to b e effective in lowering the temperature of the inner wall of the vesse l below the melting temperature of the steel. For most cases, failure of the BWR baffle plate would occur.