A Numerical Framework for Analyzing the Multidimensional Effects of Inhomogeneous Heating in Nuclear Thermal Propulsion Reactors

Nuclear thermal propulsion is a promising technology for interplanetary missions thanks 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 in the reactor core to enhance neutron thermalization. This design choice alters the neutronic behavior of the system, changing how power is distributed across the core. Whereas earlier nuclear thermal propulsion reactors exhibited nearly uniform power generation across the fuel-element cross section, modern low-enrichment concepts show significant power peaking near moderator regions. This non-uniformity challenges traditional designs based on evenly spaced propellant channels, potentially leading to localized hot spots and steep thermal gradients. Since 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, we present a numerical framework for the steady-state design and analysis of nuclear thermal propulsion reactors, integrating system-level requirements with neutronic calculations and detailed three-dimensional thermodynamic simulations. The methodology captures the strong physical coupling among these domains, which is essential for predicting realistic reactor-core behavior. To demonstrate its capabilities, the framework is applied to a representative low-enrichment reactor concept. The results highlight the importance of a multiphysics approach for designing next-generation nuclear thermal propulsion systems and explicitly reveal the inter-dependencies between individual solvers, providing insights into the approximations and limitations inherent to single-physics analyses.