Preliminary assessment of the freeze-plug melting behavior in the Molten Salt Fast Reactor

This paper focuses on the freeze-plug, a key safety component of the Molten Salt Fast Reactor, one of the six Generation IV nuclear reactors that must excel in safety, reliability, and sustainability. The freeze-plug is a valve made of frozen fuel salt, designed to melt when an event requiring the core drainage occurs. It must melt passively, relying on the decay heat, and before the reactor incurs structural damages. This work aims at preliminarily investigating the freeze-plug melting behavior, assessing the influence of various design parameters (e.g., sub-cooling temperature, number of plugs, height of cavity above the plug). An apparent heat capacity method available within COMSOL Multiphysics (R) was adopted for the simulations. Results showed that the single-plug designs generally outperform the multi-plug ones, where melting is inhibited by the formation of a frozen layer, whose thickness is strongly dependent on the sub-cooling temperature and the cavity height, on top of the metal grate. The P/D ratio negligibly influences melting and, therefore, should be chosen to minimize the draining time. Due to the absence of significant mixing in the draining cavity, acceptable melting times (i.e., below 1000 s) were observed only for cavity heights up to 0.1 m. Such distance from the core is considered not sufficient to host all the cooling equipment on the outside of the draining pipe and to protect the plug from possible large temperature oscillations in the core. Hence, it is concluded that a freeze-plug design based only on decay heat to melt is likely unfeasible. A suggested design improvement, preserving passivity, consists in enhancing melting via heat stored in metal structures adjacent to the draining pipe.