Steam explosion resistance of an internal core catcher of a nuclear reactor pressure vessel

Abstract

This paper deals with a feasibility design of a core catcher structure that could mitigate the consequences of the core meltdown in a commercial light water reactor. This activity has been performed in the framework of the Concerted Action IVCRS (In Vessel Core Retention Strategies), financed by the European Union in order to assess the safety of the next generation of nuclear power plants. These plants will be designed to support the consequences deriving from a core meltdown accident and from the phenomena that accompany it. Steam explosion is considered as a potential risk in the hypothesis of a severe accident occurring in a Pressurised Water Reactor nuclear power plant. The loss of coolant, which can occur in the case of a pipe break, provokes the degradation of the core geometry, its coolability and then its melting. The molten core falls down in the vessel to the lower hemispherical part rapidly transferring its energy to the water remaining in the lower plenum, which vaporises. In order to mitigate the consequences of a severe accident, a core catcher device, able to contain and to cool the molten core, has been designed. The same structure is analysed here as energy dissipators to prevent the reactor pressure vessel lower head failing in the case of steam explosion. In this paper, emphasis is placed on the structural aspects of the problem. Thermal fluid dynamics is treated macroscopically. This conservative approach allows us to overcome the actual uncertainties in the heat transfers mechanism between molten core and water. The results of a simulation, conducted with a finite element code, shows that the implementation of an internal core catcher could prevent the Reactor Pressure Vessel lower head from failing.