A fast-running computational model has been developed that deals with the nuclear steam supply system heat sink as a two-dimensional slice of steel with its inner and outer surfaces subjected to different thermal and material boundary conditions imposed by such surrounding media as core material, steel layer, water, and gas. This model is generally applicable to two- or one-dimensional heat sinks in the process of heatup and cooldown including liquefaction and resolidification. The numerical model and its solution technique were validated against a set of well-defined initial and boundary value problems. The computer model was applied to analyzing the temperature response of the lower head in a pressurized water reactor large-break loss of coolant accident (LOCA) with ex-vessel cooling. It was of importance to properly account for radiative heat transfer between the two exposed surfaces of the heat sink and the debris bed in the lower plenum, incorporating the physically based view factors, and to allow the heat sink to melt and relocate to the lower plenum. The model was also applied to analyzing the thermal behavior of the lower head in a boiling water reactor large-break LOCA without ex-vessel cooling. It was indicated that the vessel lower head could undergo a noticeable ablation due to the decay power generated from the debris bed in the absence of external cooling. The computer model was demonstrated to produce consistent results for the applications of practical interest in the severe accident arena.
