Load following characteristics of a molten salt fast reactor under two-phase flow conditions

Reactor responses are studied analytically with respect to load-following characteristic of a molten salt fast reactor. Load-following operation is intended to respond to hourly and minute/second fluctuations that simulate actual load changes. In this study, the system analysis code RELAP5-3D and the CFD code FLUENT is used to simulate the system from the reactor to the power conversion system with the neutronics and thermal-hydraulics coupling analysis method. The analysis assumes two-phase flow, taking into account void fraction changes due to helium bubbles injected into the core for fission product removal. First, the case of actively varying the reactor power of a molten salt fast reactor by varying the fuel pump flow rate was considered, just as conventional light water nuclear power plants use control rods to vary reactor power and achieve the daily load-following operation. The result clearly shows that reducing the fuel pump flow rate to about 40 % from the rating reduces the nuclear power to about 50 %. When considering this large temporal output fluctuation simultaneously with even shorter time variations, it is clear that temperature fluctuations caused by flow fluctuations in the power conversion system do not affect the fuel circuit when the storage tank capacity is set to about 3,000 m 3 . In addition, an analysis was conducted to determine whether the system could handle simultaneous daily load-following, load - frequency-control, and governor-free operations by simply changing the flow rate of the power conversion system without controlling the fuel circuit and/or intermediate circuit. As a result of this passive method, the reactor does not follow small fluctuations, but follows large load fluctuations gradually, and the range of reactor power change is smaller than when the reactor power is forced to change. If the load is suddenly lost and the safety protection system is bypassed, the reactor can still maintain a safe temperature range for an extended period of time without operator intervention.