The construction of an International Lunar Research Station represents a key objective for the next phase of China's lunar exploration and development initiatives. Nuclear reactor power sources, capable of delivering continuous high-power supply, are well-suited for sustaining the operation of lunar bases. The TOPAZ-Ⅱ, a space nuclear reactor power source developed by the Soviet Union, exhibits a high technology readiness level and serves as a valuable reference for designing nuclear reactor power systems for lunar bases. However, its original design was tailored for low-Earth orbit applications with a relatively short operational life, rendering it unsuitable for direct deployment on lunar bases without significant modifications.

This study aims to improve the design life of TOPAZ-Ⅱ through design modifications to achieve seven years' operational life for the reactor core, addressing the long-term energy requirements of lunar bases.

The reinforced single-cell thermionic fuel elements was employed to enhance core design and increase both the moderator share and fuel loading to fulfill the reactivity demands for a 7-a operational lifespan. By incorporating fuel elements with an extended design life and adjusting parameters such as core dimensions, special attention was given to critical safety concerns, with a proposal to meet nuclear safety standards during launch by loading fuel on the lunar surface. Additionally, the reactor shutdown system was designed to include reflector slip-down as a secondary independent emergency shutdown mechanism, hence further enhancing reactor safety. Finally, neutronics characteristics of the optimized design were evaluated using the Monte Carlo code MCNP (Monte Carlo N-Particle Transport Code).

Evaluation results indicate that the height of the core active zone in the improved design is increased from 37.5 cm to 44 cm, while the ²³⁵U loading is risen from 22.87 kg to 26.84 kg, satisfying the reactivity requirements for a seven-year lifespan. Fuel loading on the lunar surface fulfills special critical safety criteria, and the reflector's properties enable it to function as a secondary shutdown system.

Through these enhancements in this study, the TOPAZ-Ⅱ can achieve a seven-year operational life while meeting nuclear safety objectives. The findings provide a valuable reference for selecting nuclear reactor power solutions for lunar research stations.