FLOW AND SOLIDIFICATION OF CORIUM IN THE VULCANO FACILITY

In the hypothetical case of a nuclear reactor severe accident, the reactor core could melt and form a mixture, called corium, of highly refractory oxides (UO2, ZrO2), steel in a metallic or oxidised form and of the substrate decomposition products, which could eventually flow out of the vessel. The VULCANO facility is dedicated to experiments with prototypic corium (including depleted UO2). This paper is devoted to corium spreading experiments, in which the effects of flow and solidification are studied. Due to the complex behavior of corium in the solidification range, an interdisciplinary approach has been used relating thermodynamics of multicomponent mixtures, rheological models of silicic semisolid materials, high temperature heat transfer, free-surface flow with temperature-dependant properties. This is coupled with the use of specific high temperature instrumentation requiring in-situ cross calibration. Eleven high temperature spreading tests have been performed and analyzed. The main experimental results are the good spreadability of corium, the effects of cooling rates on the microstructure, the occurrence of a large thermal contact resistance at the corium-substrate interface, the presence of a rheological boundary layer at the surface. Furthermore, the experiments showed the role of the gaseous inclusions in the melt. These tests with prototypic material have been used to validate spreading models and codes