Nuclear Electric Propulsion with Low Enriched Uranium
Most space missions have used monopropellant or fuel-oxidiser propulsion systems to provide the required delta-v for orbital transfers and other in-space manoeuvres. Electric propulsion offers an alternative means of providing thrust that makes much more economical use of propellant due to its very high specific impulse. For higher payload missions, especially for travelling to Jupiter and beyond, the only technology capable of supplying the high levels of power required with a reasonable mass and size in the medium term is nuclear fission. However, most proposed space fission systems' designs have opted for the technically desirable but politically problematic fuel choice of highly enriched uranium (HEU). Using low enriched uranium (LEU, 20% U-235 by mass) avoids many of the security costs and public perception issues associated with HEU but has implications for the power system design. The core will require more fuel and/or a moderator, increasing its size and mass. The use of a hydrogenous moderator also reduces allowable core temperatures and brings a reduction in thermodynamic efficiency. The impact on overall system mass of choosing an LEU over a HEU fuel is examined in the context of a prismatic, gas-cooled reactor using a tri-structural isotropic (TRISO) fuel form, in conjunction with a direct Brayton conversion system producing an electrical power of 1MW. This is a closely coupled system with many potential trade-offs. A deterministic neutronics route is established within WIMS allowing fast analysis of many possible core designs. Core thermal limits are assessed using a finite element method within a thermal hydraulics solver. Models are constructed for estimating the mass of the power conversion system components: turbomachinery, alternator, recuperator and waste heat radiator. A surrogate function optimisation method is adopted to seek minimum mass power system designs in the cases of LEU and HEU fuels in order to make a fair comparison between them. A zirconium hydride moderator is found to be sub-optimal for an LEU system, with a larger but higher-temperature graphite-based core leading to an overall 40% lighter power system. This system was found to have a mass 50% greater than the best HEU-based system. It was confirmed that a 40g/mol helium-xenon working fluid is preferable to pure helium. The design space search revealed that diverse systems with significant differences in the allocation of mass across components can have similar overall masses.
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