Integrated Framework for Design and Analysis of Nuclear Thermal Propulsion Engines

Nuclear thermal propulsion is a promising technology for interplanetary missions due to its higher specific impulse and increased payload capacity relative to chemical propulsion. Earlier designs relied on highly enriched uranium, but proliferation concerns have shifted interest toward reactors fueled with high-assay low-enriched uranium. To maintain compact reactor configurations under these reduced enrichment levels, moderator elements are introduced into the reactor core to enhance neutron thermalization. This design choice alters the neutronic behavior of the system and modifies the spatial power distribution, leading to significant power peaking in the vicinity of the moderator regions. This nonuniformity challenges traditional designs based on evenly spaced propellant channels, which may lead to critical local temperatures and steep thermal gradients. Because overall engine performance is constrained by material temperature limits, achieving a uniform propellant temperature at the reactor outlet becomes a central design requirement. In this work, a numerical framework for the steady-state design and analysis of nuclear thermal propulsion reactors is presented, integrating system-level requirements with neutronic calculations and detailed three-dimensional conjugate heat transfer simulations. The framework is applied to a reactor concept targeting a specific impulse of 850 s embedded in an expander cycle engine. As compared to the system-level analysis, the three-dimensional coupled analysis allows one to identify localized hot spots, indicating substantially more severe thermal conditions that, if not considered in the design, could bring materials close to or beyond their allowable limits.