European Sodium Fast Reactor: innovative design of reactor pit aiming at suppression of safety vessel

All existing sodium fast reactors, built or operated, have a safety vessel around the main reactor vessel. The safety vessel function is to contain the primary sodium in case of the main vessel leakage while avoiding the reduction of the primary sodium level below inlet windows of the intermediate heat exchangers and therefore interruption of the core cooling by natural convection. In this accidental situation, the reactor will be shut down and never start again and an unloading of the core will be necessary. This unloading however will take at least one year because it is necessary to wait until residual power reduces to an acceptable level. In this long-term situation with a safety vessel filled with sodium (and recalling a sodium leak occurred in the Superphenix drum vessel), the question of the safety authorities could be: what happens in case of a leakage of the safety vessel? In addition, it is also asked to have a reactor designed for severe accident mitigation. Under these conditions it becomes useful to have a reactor pit design able to receive a sodium leak and to fulfill the safety vessel functions. That’s why one of the new safety measures proposed in the EU ESFR-SMART project [1] is to design a reactor pit that can withstand sodium leakage from the main vessel taking therefore the confinement functions of the safety vessel. This option also has a number of advantages, among others a much more effective radiative decay heat removal by the pit cooling system. The purpose of this paper is to present this reactor pit design allowing the suppression of the safety vessel, with a certain number of pre-calculations and related sizing.