Effects of Inhomogeneous Power Distribution on Nuclear Thermal Rocket Propulsion

Nuclear thermal propulsion is a promising technology for enabling faster and more flexible crewed Earth-Mars transfers. Although chemical propulsion remains the baseline for such missions, its low specific impulse results in long transit times and complex mission architectures. These limitations, combined with strict Earth-Mars alignment constraints, drive the exploration of higher performance alternatives. Solid-core nuclear thermal propulsion systems offer a compelling solution, providing approximately twice the specific impulse of chemical engines while maintaining comparable thrust. Although this technology was highly developed during the 1960s in the US, modern implementations must shift from high-enriched uranium to high-assay low-enriched uranium to address proliferation concerns. This transition leads to a move from homogeneous to heterogeneous reactor architectures by introducing moderator elements into the core to sustain criticality at lower enrichment levels. This study investigates the neutronic and thermal implications of such a shift. Neutronic analyses were performed using the OpenMC code, while conjugate heat transfer was modeled using in-house tools. The results show significant differences in power distribution and thermal behavior, highlighting key design challenges for systems based on low uranium enrichment levels.