Safety analysis and development of control logic of KAIST Micro Modular Reactor with GAMMA+ code

Abstract

To supply distributed power system to remote region, a concept of small modular reactor with super-critical $CO_2 (S-CO_2)$ cooled direct Brayton cycle called KAIST Micro Modular Reactor, MMR in short, has been developed. The MMR has a small reactor core with power output of 36MWth, power conversion system and passive decay heat removal system in a double wall containment whose weight is approximately 150 tons and dimensions are 7.0 m in length and 3.8 m in diameter. The dimension and weight are sized so that the sin-gle modular of whole MMR can be transported by a ship or a truck. Design parameters and configuration of the reactor core, power conversion system and passive decay heat removal system have been optimized by the KAIST-research team. Until now, only the on-design performances were obtained. The nuclear system design can be finalized after transient conditions are simulated to guarantee its economy and safety. Before the transient simulation, thermal-hydraulic feedback mechanisms are imple-mented to check suitability of the design in terms of the fuel temperature distribution. The $S-CO_2$ system code platform is prepared to simulate various transient conditions of the $S-CO_2$ Brayton cycle. GAMMA+ code, which is developed by KAERI for gas-cooled reactor system analysis, is modified for the $S-CO_2$ system analy-sis. With this modified GAMMA+ code, the load following nature of the MMR is first simulated. Mass invento-ry controller, core bypass controller and turbine throttling controller are used for adjusting demand of the load. Each automatic controller is designed by Ziegler-Nichols method and reactor power is autonomously controlled by a strong negative feedback coefficient. Design basis accidents of MMR are also simulated to assure whether its integrity is maintained during the accidents. Among various design basis accidents, loss of external load is first modeled because MMR will be operating in a remote region where grid infrastructure isn’t well developed. Furthermore, since MMR has a high pressure boundary, loss of coolant accidents with different break sizes are also analyzed. Lastly, the antic-ipated transient without scram scenario is analyzed to check the inherent safety feature of MMR reactor core which has a strong negative feedback coefficient. It is concluded that the current design of MMR has an abil-ity to keep its integrity for the analyzed accident scenarios.