Mass exchange between light metal layer and oxidic layer in lower plenum corium pool for a high steel content condition

Abstract

In-vessel retention is a severe accident strategy adopted in several PWR designs. The success of this strategy is subject to whether the mass transfer between the light metal layer and oxidic layer is mutual or one-way. The one-way mass transfer, which is due to heavy metal generated at the metal/oxide interface, causes the light metal layer to continuously lose its mass. The thinning process may challenge the lower plenum integrity. A steady-state model has been developed and reported in this paper to evaluate which mode the mass transfer between the light metal and oxidic layers is. The objective of the model is to assess the potential for heavy metal generation at the metal/oxide interface. The model is based on the idealization of a thin stationary interface through which mass is transferred by convective-diffusion processes in the light metal layer and in the oxidic layer. That is, it is conservatively assumed that the crust formed between the light metal layer and the oxidic layer is sufficiently permeable that it offers no resistance to the interfacial mass transfer. The equilibrium compositions at both sides of the liquid-liquid interface (with the crust ignored) are assumed to follow a simplified phase diagram of U-O-Zr-steel at a high temperature (Salay and Fichot, 2004). Based on the model calculations, and even considering the uncertainty in the value of a key dimensionless parameter, it is concluded that heavy metal formation at the interface is unlikely in a lower plenum pool of high steel content.