Neutronics and Transient Analyses of a Supercritical CO₂-cooled Micro Modular Reactor (MMR)

This paper presents the neutronics and transient studies of a supercritical CO₂-cooled micro modular reactor (MMR). The suggested MMR is an extremely compact, integrated, and truck-transportable reactor with 36.2 MW_th power and a 20-year lifetime without refueling. A salient feature of the MMR is that all the components including the generator are contained in a small reactor vessel. For a minimum volume and maximum lifetime of the MMR core, a fast neutron spectrum is utilized. To improve the fuel-coolant compatibility and to maximize the fuel volume fraction, UC fuel is considered. A unique replaceable fixed absorber (RFA) is also introduced in order to maintain the excess reactivity at less than 1 dollar during the whole operational period. A drum-type reactivity control system is adopted in the MMR as the primary control system to prevent a control rod ejection accident and a single absorber rod located at the core center is used as the secondary ultimate shutdown system. The neutronics analysis is performed by using the continuous energy Monte Carlo code Serpent with the ENDF/B-VII.1 library. In addition, an unprotected loss of flow (ULOF) accident is simulated by the system analysis code GAMMA+ to show the passive safety of the MMR system.