Conceptual design of small modular reactor (SMR) driven by natural circulation and study of design characteristics using CFD Code

Abstract

A detailed computational fluid dynamics (CFD) simulation analysis model was developed using ANSYS CFX 16.1 and analyzed to simulate the basic design and internal flow characteristics of a 180 MW small modular reactor (SMR) with a natural circulation flow system. To analyze the natural circulation phenomena without a pump for the initial flow generation inside the reactor, the flow characteristics were evaluated for each output assuming various initial powers relative to the critical condition. The eddy phenomenon and the flow imbalance phenomenon at each output were confirmed, and a flow leveling structure under the core was proposed for an optimization of the internal natural circulation flow. In the steady-state analysis, the temperature distribution and heat transfer speed at each position considering an increase in the output power of the core were calculated, and the conceptual design of the SMR had a sufficient thermal margin (31.4 K). A transient model with the output ranging from 0% to 100% was analyzed, and the obtained values were close to the $T_{hot}$ and $T_{cold}$ temperature difference value estimated in the conceptual design of the SMR. In low power operations (1, 2, 4, 7%), recirculation at the top of the core occurred due to “Eddy” at the beginning of the flow, but disappeared when the internal flow stabilization step took place. In addition, the core temperature increased by 3 ~ 4$^\circ C$ compared to the porous media model during low power operations, but it was confirmed that there were no abnormalities in the system’s safety (DNB, etc.). The thermal and hydraulic characteristics of the models that applied porous media to the core & steam generators and the models that embodied the entire detail shape were compared and analyzed. Although there were differences in the ability to analyze detailed flow characteristics at some low powers, it was confirmed that there was no significant difference in the thermal hydraulic characteristics’ analysis of the SMR system’s conceptual design. Therefore, it is proposed that the CFD model adopting the porous media can be analyzed to reflect the internal structure changes, core output variations, and power up operation conditions of the SMR’s conceptual design.