Practical considerations for shielding-mass constraints on mobile heat-pipe microreactors

Mobile nuclear microreactors are frequently proposed for remote power generation, resilient energy systems, and defense-related applications. Although many designs emphasize compactness and autonomy, radiation shielding requirements impose fundamental constraints on reactor mobility. Mobile microreactors are intended to be moved repeatedly during their operational lifetime, often immediately following shutdown with limited cooling. Transportable reactors may be relocated after extended cooling periods, whereas relocatable systems are typically deployed once and moved only at end of life. These distinctions have significant implications for reactor design. This study evaluates shielding mass and pre-shipment cooling times as a function of reactor power as a first order consideration to determine whether a truly mobile microreactor is viable for a representative heat pipe microreactor concept. A highly enriched uranium (HEU) tri-structural isotropic (TRISO)–fueled heat pipe concept was scaled to fit within a typical conveyance, and then operation was modeled at three power levels to generate a representative source term for shielding thickness calculations. Results show that, for the HEU TRISO heat-pipe cases considered here, shielding mass strongly constrains the feasible mobile power envelope and becomes particularly restrictive near the 5 MW thermal (MWth) scale. The 5 MWth case approaches routine mobility feasibility assuming an idealized reactor-adjacent shielding configuration. Notably, the analyses neglect thermal constraints and the mass of other necessary reactor features and is therefore only useful as a cursory example of the practical restrictions that must be addressed to achieve routine microreactor mobility. However, it demonstrates the feasibility of a truly mobile microreactor at a useful power level. Because the modeled microreactor used HEU, the applicability for civilian deployment is limited. The findings further suggest that compact high-assay low-enriched uranium metallic fuel forms may offer significant advantages for future mobile reactor concepts.