The article presents an analysis of the effect the aspect number A (the ratio of the melt layer height to the reactor pressure vessel inner diameter) and the temperature conditions at the boundaries of the cylindrical steel melt layer heated at its lower surface have on the heat flux focusing on its lateral surface. Such layer corresponds to the metallic layer in the stratified corium melt pool that is produced in the nuclear reactor pressure vessel during a severe accident, and the heat flux of which affects the pressure vessel, causing its heating and partial melting. The pressure vessel ability to retain high-temperature melt inside of it in the course of a severe accident in using external cooling of the vessel depends on the heat fluxes acting on it from the high-temperature corium side. If the metallic melt is stratified, a heat flux focusing effect emerges in the area of melt upper layer, which determines the most dangerous and thermally stressed place, the heat flux in which may exceed 1.5 MW/m2. The possibility to promptly and adequately estimate the influence of heat fluxes on the reactor pressure vessel is an important objective in elaborating severe accident management measures. The article proposes a procedure for determining the heat flux focusing factor on the steel melt layer lateral surface, which is based on applying the previously obtained modified formulas for calculating the Nusselt number and the melt layer bulk temperature. The newly developed procedure was used to perform a parametric analysis of the influence of the layer geometrical parameters and the temperature conditions at its boundary surfaces on the focusing effect. The analysis results have shown that the heat flux focusing effect depends on the temperature conditions at the layer boundaries and on the layer thickness. Thus, variations of differences between the temperatures of layer lower and upper surfaces, and also between the temperatures of its lateral and lower surfaces may entail a several-time increase/decrease of heat flux intensities acting at the melt layer surface and on the reactor pressure vessel during a severe accident. The heat flux focusing factor depends most essentially on the temperature conditions at the layer boundaries at the aspect ratio A values not larger than 0.4, which should be taken into account in elaborating severe accident management strategies. In the subsequent, it is worthwhile to perform experimental studies to confirm the validity of the newly developed procedure for determining the heat flux focusing effect in the relevant scenarios and under real severe accident progression conditions, as well as with model media (melts) having properties close to those of the materials forming the corium melt bath in nuclear power facilities during accidents of a similar class.
Loktionov et al. (Mon,) studied this question.