Preliminary conceptual design of a micro supercritical CO2-cooled pressure-tube reactor for unmanned underwater vehicles

Nuclear reactors possess extremely high energy density and can continuously deliver high power within a compact, sealed system over long periods. Utilizing nuclear power as the main power source for Unmanned Underwater Vehicles (UUVs) is an ideal solution to overcome endurance limitations and enable long-term deep-sea missions. This paper proposes a preliminary conceptual design of a micro supercritical carbon dioxide (S-CO 2 )-cooled pressure-tube reactor for UUVs, and conducts core physics calculations and inherent safety characteristic analyses under extreme accident conditions to validate the safety features of this reactor. The reactor has a thermal power of 3.2 MW and a design life of 10 years. It uses heavy water moderation with UN fuel (25% enrichment), with reactivity controlled by control drums and Gd 2 O 3 burnable poison. Core neutronic calculations based on OpenMC show that the core exhibits negative feedback characteristics (fuel temperature coefficient of −1.18 pcm/K core, void reactivity of −1302 pcm), and the control drums provide sufficient shutdown margin (keff < 0.95). Heavy water moderation shifts the neutron energy spectrum towards the epithermal region, with a radial power peaking factor of only 1.044. Steady-state thermal–hydraulic analysis conducted using FLUENT indicates that under normal operation, the maximum cladding temperature is 1063.7 K and the maximum fuel pellet temperature is 1769.0 K, meeting design requirements. Studies on the extreme condition of a large-break loss-of-coolant accident with loss of emergency core cooling system (LBLOCA/LOECC) resulting in complete coolant loss show that the reactor can transfer decay heat from the fuel rods to the moderator solely by radiation and conduction, and then effectively remove the decay heat through convective heat transfer in the moderator. Under such extreme accident conditions, the maximum cladding temperature is 1578.0 K, which is lower than the temperature limit of 12YWT-type ODS steel. The results demonstrate that this design meets the requirements for compactness, long life, and inherent safety, presenting a promising core design concept for UUVs.