Exploratory Studies of Reactor Physics Characteristics of a Simplified 100-kWe-Class Space Atomic Pile

Abstract Reliable electrical power is needed for space exploration and development, including supporting human outposts on the surface of the Moon, Mars, asteroid mining, and other applications, and this power could be provided by micronuclear reactors designed to operate for a decade or more in such remote locations and extreme environments. Based on anticipated near-term needs for a Moon outpost, it is expected that the design and performance targets for a space microreactor will be to provide 100-kWe for 10 years, and to have a total mass (core + reflector + shield) that is below 3000 kg, to accommodate the additional mass of secondary systems (power converter, radiators, and other components), and limitations for launch and Moon landing vehicles. Exploratory scoping studies have been carried out using the serpent reactor physics analysis code for a simplified homogeneous model to evaluate the performance characteristics of a potential thermal-spectrum space microreactor concept with a nominal power level of 500 kWth/100 kWe, using the early system nuclear auxiliary power (SNAP)-8 space reactor design as a guide. It is anticipated that such a space reactor would provide power for human outposts on the Moon or Mars. A reference design concept (50-cm diameter, 100-cm height) has been developed, and permutations and modifications to that concept have been tested, using different combinations of fuel, coolant, moderator, and reflector materials. As a simplifying approximation, the core materials are homogenized for the computational reactor physics modeling with serpent to evaluate impacts of design permutations on the core operation life and total system mass (core + reflector + shield). A reactor concept that closely meets the fuel/core lifetime requirement of 10 years has been developed, although its mass exceeds the target 3000 kg by over 400 kg. This concept is based on modifications to the historical SNAP-8 reactor concept with low-enriched uranium (LEU) fuel. The reference design concept uses U-75Zr fuel, ZrH1.8 moderator, Na coolant, and a BeO reflector. The clad for the fuel and other components is INOR-8/Hastelloy-N, while the outer shield is LiH. Alternative materials for the fuel, moderator, coolant, and reflector were also tested, while adjusting the fuel and moderator volume fractions to ensure a constant H/U-235 = 79 atom ratio. It is anticipated that further optimization studies with different combinations of materials could potentially reduce the core, reflector, and shield sizes to achieve performance objectives.