SFR core-catcher safety issue: Experimentation and simulation of its ablation by an impinging jet

Abstract

These coupled experimental and numerical studies support the safety demonstration of dedicated mitigation features in Sodium Fast Reactors (SFR). In order to limit potential power excursion in SFR core in the case of hypothetical severe accident, the implementation of the mitigation devices, called transfer tubes, was considered into actual French SFR core design. More specifically, they connect the core to the lower plenum and enable the early discharge of the corium towards the core-catcher. However, these devices, which effectively reduce the probability of reactivity excursions in the core, emphasize the issue of core-catcher thermomechanical resistance. Contrary to previous reactor core designs, the presence of these tubes could lead to coherent corium jets impinging the core-catcher surface. In the past, numerous studies dedicated to thermal ablation of a solid by a jet have been carried out to characterize the maximum ablated depth. Although these experiments are very valuable at a macroscopic scale, they do not give information about the local physical phenomena governing heat transfer and melting. Keeping this final goal in mind, this study followed a R&D methodology based on an experimental situation using simulant materials enabling direct visualization. In this paper, the ablation of an ice block by an immersed water jet and a free surface jet are investigated in conditions shown to be close to the reactor case. The physical analysis of the heat transfer at jet impingement and in its vicinity is characterized for the immersed jet conditions. Associated to the experimental part, CFD simulations of these tests are performed to validate the calculation methodology (mesh, turbulence models and interface melting…). In the future, simulations will be used in particular to characterize steel/ice transposition biases.