Design of flashing-driven natural circulation reactor at low pressure

Abstract

Since a few decades ago, one of the major research focuses lies on developing small and medium reactors due to their modularity, small size, and various applications. In particular, it has been emphasized that the active systems in conventional reactors should be replaced by passive ones with the goal of more competitive design. Accordingly, natural circulation reactors have been researched to achieve the goal, and some of them adopt flashing phenomenon to obtain another passive characteristic, self-pressurization mechanism. Therefore, in line with this trend, design of flashing-driven reactor was studied modifying the existing reactor. The reference design is AHR400, which was proposed by KAIST for the purpose of nuclear desalination. In order to perform the design, the in-house code was developed. The Homogeneous Equilibrium Model is used to model two-phase flow, and the self-pressurizer modeling is based on the conventional two-region model. The code was validated with the CAPCN test, and the results indicates the adequacy of the developed code. Since the original design uses the reactor coolant pump, the design process starts from modifying the reactor to be operated by natural circulation. Considering the two limiting factors, pinch point temperature difference in the heat exchanger and stability, it was shown that the modified one can be operated under the conditions of power reduced by half and flashing boundary near the riser exit. Then, the transient analysis was conducted to investigate the passive safety of the system, and the results indicate that the reactor is safe under loss of heat sink accident condition due to its natural circulation and the self-pressurization mechanism.