Multiphysics Analysis of Natural Circulation-Driven Operation of Passive Molten Salt Fast Reactor and Effect of Guide Structure

A new molten salt reactor (MSR) design has been developed aiming for long-term operation and high safety. In order to enhance the integrity and economy of the system during the long-term operation, pumps were removed from the primary system, and the fuel salt flow was developed by natural circulation. In terms of thermal–fluidic, the natural circulation operation without a pump increases the reactor safety and resistance to accidents. The normal operation feasibility of the reactor was evaluated with the multiphysics analysis conducted with the Generalized Nuclear Foam (GeN-Foam) code. It was shown that the reactor can maintain stable power under a fixed heat exchanger outlet temperature condition. To increase the natural circulation, the active core region was designed to have a simple cylindrical shape, which induced a stagnation zone with slow velocity near the side wall. Due to the slow velocity, the stagnation zone has a substantially high temperature, and a flow guide was introduced to mitigate the stagnation effect. The impact of the flow guide was evaluated, including the reactivity feedback and delayed neutron precursor drift effect. The results highlight the importance of analyzing the flow distribution within the core in an MSR and demonstrate the effectiveness of the guide structure in ensuring stable core flow.