Thermal Neutron Spectrum

A thermal neutron spectrum refers to the operating regime of nuclear reactors in which the fission chain reaction is sustained by neutrons that have been slowed (moderated) to thermal equilibrium with their surroundings, corresponding to kinetic energies of approximately 0.025 eV at room temperature. At these low energies, the fission cross-section of uranium-235 is approximately 580 barns, roughly 500 times greater than at fast neutron energies, meaning thermal neutrons are far more likely to cause fission in U-235. This enormous cross-section advantage allows thermal reactors to achieve criticality with lower fuel enrichment (typically 3-5% U-235 for LEU), smaller fissile inventories, and smaller core volumes per unit of thermal output compared to fast reactors.

The vast majority of commercial nuclear reactors worldwide, and the majority of SMR designs, operate in the thermal spectrum. Pressurized water reactors (NuScale VOYGR, Rolls-Royce SMR, Holtec SMR-300, Westinghouse AP300, KHNP i-SMR, SMART100, Linglong One) and boiling water reactors (GE-Hitachi BWRX-300) use ordinary water as both moderator and coolant. High-temperature gas-cooled reactors (X-energy Xe-100, Radiant Kaleidos, HTR-PM, Nano Nuclear KRONOS, Ultra Safe MMR) use graphite as the moderator with helium gas coolant. Fluoride salt-cooled reactors (Kairos Power KP-FHR) use graphite moderation with molten salt coolant. Copenhagen Atomics employs heavy water moderation in their thorium molten salt reactor. Each moderator choice involves trade-offs in neutron economy, fuel enrichment requirements, achievable temperatures, and materials compatibility.

The thermal spectrum's primary limitation relative to fast reactors is fuel utilization efficiency. Thermal reactors consume only a small fraction of the energy potential in mined uranium, as they primarily fission U-235 (0.7% of natural uranium) with limited conversion of fertile U-238 to fissile Pu-239. Conversion ratios in thermal reactors typically range from 0.5 to 0.7, far below the breeding ratios achievable in fast-spectrum designs. However, thermal-spectrum SMRs benefit from decades of operating experience, established fuel supply chains using conventional LEU (avoiding HALEU constraints), well-understood materials behavior, and comprehensive regulatory frameworks. These advantages translate to lower licensing risk and faster time to deployment, which is why the nearest-term SMR projects, including the BWRX-300 at Darlington (under construction), Rolls-Royce SMR at Wylfa (site work beginning 2026), and Linglong One in China (commercial operation H1 2026), are all thermal-spectrum designs.