A neutron moderator is a material that decelerates fast neutrons (born from fission at energies around 2 MeV) to thermal energies (approximately 0.025 eV at room temperature) through elastic and inelastic scattering collisions. The moderation process is essential in thermal-spectrum reactors because the fission cross-section of uranium-235, the probability that a neutron will cause fission, is roughly 500 times greater for thermal neutrons than for fast neutrons. Effective moderators are materials with low atomic mass (to maximize energy transfer per collision), low neutron absorption cross-section (to avoid parasitically capturing neutrons), and high scattering cross-section. The three principal moderator materials in commercial use are ordinary (light) water, heavy water (deuterium oxide), and graphite.
Light water (H2O) is the most widely used moderator, serving double duty as both moderator and coolant in PWRs and BWRs. This includes the majority of SMR designs: NuScale's VOYGR, GE-Hitachi's BWRX-300, Rolls-Royce SMR, Holtec SMR-300, Westinghouse AP300, and China's Linglong One all use light water as the moderator-coolant. Light water is an effective moderator due to hydrogen's low atomic mass, but it also absorbs neutrons at a higher rate than heavy water or graphite, which is why light-water reactors require enriched uranium fuel (3-5% U-235 for LEU) to compensate for this parasitic absorption. Canada's CANDU reactor fleet uses heavy water (D2O) as the moderator, where deuterium's extremely low neutron absorption enables the use of natural uranium fuel without enrichment.
Graphite serves as the moderator in high-temperature gas-cooled reactors such as X-energy's Xe-100, Radiant's Kaleidos, China's HTR-PM, and Nano Nuclear's KRONOS. In these designs, graphite provides structural support and moderation while helium gas handles heat removal. Copenhagen Atomics' thorium MSR uses heavy water as the moderator with molten salt as the coolant and fuel carrier. Fast-spectrum reactors, including TerraPower's Natrium, Oklo's Aurora, ARC Clean Technology's ARC-100, and Newcleo's LFR-AS-200, deliberately avoid using a moderator, operating with unslowed fast neutrons to achieve superior fuel utilization, breeding capability, and actinide transmutation. The choice of moderator, or the absence of one, is among the most fundamental design decisions in reactor physics, determining the neutron energy spectrum, fuel enrichment requirements, core size, and ultimately the reactor's capabilities and applications.