Ex-vessel corium spreading: results from the VULCANO spreading tests

摘要

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 (UO_2, ZrO_2) and metallic or oxidized steel, that could eventually flow out of the vessel and mix with the basemat decomposition products (generally oxides such as SiO_2, Al_2O_3, CaO, Fe_2O_3,...). For some years, the French Atomic Energy Commission (CEA) has launched an R&D program which aimed at providing the tools for improving the mastering of severe accidents. Within this program, the VULCANO experimental facility is operated to perform experiments with prototypic corium (corium of realistic chemical composition including depleted UO_2). This is coupled with the use of specific high-temperature instrumentation requiring in situ cross calibration. This paper is devoted to the "spreading experiments" performed in the VULCANO facility, 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 combining thermodynamics of multicomponent mixtures, rheological models of silicic semisolid materials, heat transfer at high temperatures, free-surface flow of a fluid with temperature-dependant properties. Twelve high-temperature spreading tests have been performed and analyzed. The main experimental results are the good spreadability of corium-concrete mixtures having large solidification ranges even with viscous silicic melts, the change of microstructure due to cooling rates, the occurrence of a large thermal contact resistance at the corium-substrate interface, the presence of a steep viscosity gradient at the surface, the transient concrete ablation. Furthermore, the experiments showed the presence of the gaseous inclusions in the melt even without concrete substrate. This gas release is linked to the local oxygen content in the melt which is function of the nature of the atmosphere, of the phases (FeO_x, UO_y,...) and of the substrate. These tests with prototypic material have improved our knowledge on corium and contributed to validate spreading models and codes which are used for the assessment of corium mastering concepts.