An experimental study of the freeze valve opening time with considering wall effect

Nuclear power has a lot of merits including low carbon-dioxide emission and economic advantage. However, it possesses fear factors such as severe accident and radioactive waste. Currently, several new-type reactors are developed to overcome those issues. Molten Salt Reactor (MSR) is one of the generation IV nuclear reactors. To enhance safety, it adopts a passive safety system called freeze valve. Freeze valve is located between the reactor and drain tanks. The salt in the freeze valve is kept in the solid state by forced cooling during normal operation, and melts passively in the occasion of station blackout to discharge the liquid fuel in the reactor vessel to the drain tanks. Obviously, accurate prediction of opening time of freeze valve is important to assure the safety of MSR. In the freeze valve, salt is contained in a metal tube. Since thermal conductivity of the tube wall material is much higher than that of salt, heat conduction within the tube wall may have great impact on the opening time of the freeze valve. Thus, in this study, the wall effect on the opening time of the freeze valve was experimentally investigated. To investigate the wall effect, a metal stick was placed vertically in the frozen HTS (heat transfer salt), and liquid HTS was poured on the frozen HTS. The heat in the liquid HTS was mainly conveyed in the stick by thermal conduction to transferred to the solid HTS. In consequence, melting of solid HTS started around the metal stick rather than the solid-liquid interface. Since tension was applied to the stick in the upward direction, the stick moved upward when the melt interface reached the bottom end of stick. This result suggested that opening of the freeze valve is established along the tube wall in the real MSR plant. To provide experimental data to validate the numerical simulation results, parametric trends of the onset time of stick movement on the several important parameters were investigated. These data can also be used as the guideline to determine the wall material, wall thickness, length of freezing region, and the initial temperature of the frozen salt.