[en] There is a class of hypothetical core-melt accidents in nuclear reactors in which molten core materials are deposited into the concrete reactor cavity, resulting in a vigorous thermal attack on the concrete. The temperature of the pool is maintained at a point where heat losses to concrete and to surroundings above the pool approximately balance heat gains from decay of fission products. During the early stages while the pool is fully molten, the dominant heat-transfer process within the pool is convection. At later times, this changes as temperatures fall and refreezing begins. The existing CORCON-MOD1 code, which does not allow these refreezing effects, is being modified to account for the changes in heat-transfer behavior as refreezing takes place