No Convection, No Problem: Neutron Economy and Heat Rejection for Space, Lunar, and Martian Fission Power

Nuclear reactors operated in orbit or on airless/near-airless bodies must simultaneously optimize neutron economy and purely radiative heat rejection, two constraints that tightly couple spectrum, reactivity margins, and temperature feedbacks. In this work, I present a multi-physics model of compact fission systems for (i) earth-orbital shadow-shielded power (50–200 kWe), (ii) lunar surface units (40–100 kWe), and (iii) Mars surface power (100–500 kWe). Modeling neutron and photon transport using OpenMC/Serpent, quantifying leakage, reflector efficacy (Be, BeO, graphite, ZrHₓ), and γ-heating; a thermal-radiation network with view-factor geometry models heat-pipe/radiator fields with temperature-dependent emissivity, dust-induced degradation (Moon), and CO₂ frost scenarios (Mars). Tight neutronics-thermal coupling propagates Doppler and moderator temperature coefficients into power/temperature equilibria and startup/scram transients, including decay-heat removal in vacuum. Results map achievable k_eff, spectrum, and component temperatures versus radiator area, shadow-shield mass, and regolith shielding thickness, yielding design charts that trade specific mass against dose and thermal margins. The framework identifies regime boundaries where hydride moderators lose advantage, radiator fouling drives derating, or γ-heating dominates thermal bottlenecks, providing first-order design rules for orbital, lunar, and martian reactors