The present paper mainly summarizes the operational and safety characteristics found by the neutronics and thermal-hydraulics coupling analysis for molten-salt fast reactors (MFR). Analysis methods for MFRs have been developed using system codes and Computational Fluid Dynamics (CFD) codes. Traditionally, the analysis has been limited to single-phase flow of the molten fuel salt, but recently analysis has been conducted under twophase flow conditions with a small amount of helium gas injected to remove fission products (FP). In the MSFR developed by the EU, the void fraction due to two-phase flow is locally distributed, and a method for analyzing this by coupling advanced neutronics calculation methods with a CFD code has been proposed. On the other hand, a method that models all heat transport systems using a system code with reactor point kinetic (PRK) models have also been proposed. In this case, a CFD code is also used to calculate the precise behavior of flow scheme in the reactor. Since MFRs generally do not have control rods, the method of starting up the reactor must be different from that of light water reactors (LWR). Various methods have been proposed, but this paper introduces a startup method that takes advantage of the characteristics of two-phase flow and its negative reactivity. In the analysis of load-following operation, several methods have been proposed to perform time-order load-following operation by actively varying the reactor power, as in conventional LWRs. Recently, a passive load-following operation method has been proposed that fully exploits the characteristics of MFR. In this passive method, the reactor is operated to respond naturally to temperature changes in the heat transport system caused by flow rate changes corresponding to load variations in the energy conversion system, without controlling the fuel and intermediate circuits. This operation method introduces a heat storage tank in the intermediate circuit, which allows load-following operation in all modes from long to short duration. With regard to safety, the paper presents examples of system code and CFD code analysis of the behavior with a safety protection system bypassed for various transients in two-phase flow conditions. These analyses show that molten salt fast reactors always transition to the safe operating range after transients.
