Thermal, physico-chemical and rheological boundary layers in multi-component oxidic melt spreads

Abstract

In the unlikely case of a severe accident in a nuclear reactor, the core material could melt and form a molten mixture named corium. The CEA has undertaken a large program to improve knowledge on corium behaviour. An experiment in which 17 kg of simulant corium — mainly made of hafnon, zircon, silica and wüstite — at more than 2 200 K, was spread in the VULCANO facility, is described. It is analysed in terms of thermal cooling by fitting the pyrometric measurements with outputs of a conduction code. This enables the estimation of temperature profiles inside the corium. From physicochemical modelling, the solid fraction, density and viscosity can be estimated. Thermal, physicochemical and rheological boundary layers are thus found. The rheological boundary layer is very steep (exponential decay parameter around 800 m −1) and could significantly affect the flow dynamics. A visible aspect of this boundary layer is the presence of folds on the corium surface, similar to those on ropy pahoehoe lavas.