Study on start-up of molten salt fast modular reactor from shutdown based on two-phase flow CFD analysis

In the present study, the effect of helium gas bubbles injected into a molten salt reactor on nuclear characteristics is analyzed. The analysis uses a computational fluid dynamics code and a system code to evaluate the effects of void fraction and core temperature on reactor power, which are affected by changes in fuel pump speed. Because molten salt reactors are equipped with a fission gas removal system such as Xe gas, the core of a molten salt fast reactor is strictly a two-phase flow and the analysis considers the effects of voids. In this system, the Xe gas is removed by injecting helium gas bubbles into the molten fuel salt. When the void fraction of the helium gas increases by 1%, the reactivity of approximately-360 pcm & delta;k/k is introduced. To understand the importance of two-phase flow analysis by the FLUENT code with a user defined function which incorporates the point kinetics model and void effect model, a fuel pump trip at rated reactor power with a void fraction of 0.4% in the core average was analyzed with FLUENT and compared with a conventional analysis that does not consider the effects of voids at all. The results show that the transient behavior cannot be properly evaluated without considering the effects of voids. A comparison of two-phase and single-phase flow calculations using FLUENT is also provided, and a method to evaluate the effects of voids in single-phase flow calculations is also proposed. Analysis using FLUENT under two-phase flow conditions shows that the reactor can increase power by increasing the fuel pump speed. Under single-phase flow assumption, the analysis was performed using RELAP5-3D and showed that the reactor can be started in a similar manner by considering the reactivity associated with changes in the void fraction. It was pointed out that the reactor could be controlled by adjusting the amount of helium injected. It was also noted that voids increase the reactor shutdown margin.