A new concept of a small and medium sized light water reactor, named the DMS (double MS: modular simplified & medium small reactor) has been developed. The DMS features significantly simplified plant systems realized by adoption of a natural circulation system of coolant and a free surface separation (FSS) system that is based on the gravitational separation of steam and water. With these systems, reactor internal pumps and steam separators are not needed, reducing plant cost. In this study, the DMS plant performance under transient conditions has been evaluated using TRACG code. TRACG code, which can treat multi-dimensional hydrodynamic calculations in a reactor pressure vessel (RPV), is well suited for evaluating DMS reactor transient performance because it can evaluate the void fraction in the chimney and therefore evaluate the natural circulation flow. As critical transient cases, generator load rejection with total turbine bypass failure (LRNBP) and loss of feedwater heating (LFWH) were chosen to evaluate. LRNBP and LFWH are the most severely recognized events as a pressure increase event and a thermal power increase event, respectively. In case of LRNBP, heat flux increased to about 110% of rated power, and the natural circulation flow barely changed, resulting in a lower ΔMCPR than that of LFWH case. The reason that heat flux only increased to 110% was because the RPV of the DMS has a large steam region volume in the chimney region compared to the thermal power. As a result, the change in the void fraction with a pressure increase in the core was small. In case of LFWH, the maximum heat flux, calculated using the neutron flux, was 121% of rated power when a scram occurred, and ΔMCPR was 0.14, almost the same as for current ABWRs. Since the analysis conditions were set conservatively, these results show that the DMS performs as well for transient events as conventional BWRs.

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