Simulations on natural convection of stratified melt pools with volumetric heat generation

In-vessel retention (IVR) has been approved as part of the severe accident management strategy for pressurized water reactors. So far, extensive studies have been done on single-layer melt pool behaviors while the melt pool may evolve to two or even three-layer configurations due to the different densities between corium compositions. In this paper, through large eddy simulations, numerical research was conducted to help understand the behaviors of two-layer melt pools. A melt pool geometrically similar to the VVER-type lower head was used for the simulations, which is a two-dimensional semi-ellipsoidal slice, measuring a span of 1.15m, a total pool depth of 0.32m and a thickness of 0.2m. For better representation of the typical two-layer configuration of the melt pool, two immiscible fluids, i.e. ethanol-water solution (ethanol of 50wt%) and fluorinert liquid FC-70, are selected as simulant materials for metal layer and molten oxide layer, respectively. In addition, to evaluate the effects induced by the possible crust formation between the layers in real scenario, which could physically separate the melt pool into two parts, simulations were also done with a steel sheet (3mm thick) placed in the interface of the two immiscible fluids selected. The results show that similar to single-layer configuration of the melt pool, focusing effect is also observed near the free surface region under the stratified situations, but the maximum heat flux transferred to the vessel varies with time due to interface instability between the layers. When there's crust (a solid steel sheet in simulations) between the two layers, a higher maximum temperature inside the melt pool and a lower maximum heat flux toward the vessel are observed, which indicates an obvious influence of the possible crust formation on the heat transfer characteristics in the two-layer configuration of the melt pool.