Demonstration of the heat removing capabilities of the EPR core catcher

The EPR implements an additional, fourth level of defense-in-depth that aims at limiting and restricting the consequences of a postulated severe accident with core melting to the immediate vicinity of the plant. As this requires an intact confinement, it is necessary, among others, to avoid an attack of the molten core on the basemat. For that purpose, the EPR includes a large ex-vessel core catcher. It increases the surface-to-volume ratio of the melt after its release from the reactor pressure vessel (RPV) and allows the effective quenching and stabilization of the melt before it can attack the structural concrete. The bottom and sides of the core catcher are cooled by a system of horizontal water-filled channels. The water is provided either passively, by overflow from an internal reservoir, or actively by the containment heat removal system (CHRS). To quantify the heat removing capability of the horizontal part of the proposed cooling structure, a set of experiments in a full-scale, horizontal, 5m long cooling channel have been performed. To simulate decay heat, the channel was electrically heated from the top. The experiment was integrated in the BENSON test rig, a highly flexible, separate-effect test facility operated by Framatome ANP. In accordance with the potential later modes of operation, both co-current and counter-current flow of the water/steam mixture have been investigated. The tests demonstrated the good-natured behavior of the system, even for induced heat fluxes that significantly exceed realistically expectable maximum values. Although, at high heat fluxes, a local dry-out occurred at the top of the channel, structural temperatures remained in a safe range. This excellent performance is attributed to the fact that heat can enter the water through both the horizontal and vertical surfaces of the cooling channel. As a result a high, effective critical heat flux (CHF) level is achieved. The performed tests yield a valuable contribution to the validation of the function of the EPR core catcher concept.