A fast neutron spectrum refers to a reactor operating regime in which the nuclear fission chain reaction is sustained by neutrons at high kinetic energies (typically above 100 keV), without the use of a moderating material to slow them down. In the fast spectrum, the nuclear physics landscape differs dramatically from thermal reactors: fission cross-sections are lower (requiring higher fissile concentrations to achieve criticality), but the ratio of neutrons produced per fission is higher, and the probability of neutron capture in fertile isotopes like uranium-238 to produce fissile plutonium-239 is substantially increased. These physics enable fast reactors to achieve breeding ratios at or above unity, meaning they can produce as much or more fissile material than they consume.
The leading fast-spectrum SMR designs span two coolant technologies. Sodium-cooled fast reactors include TerraPower's Natrium (345 MWe, construction permit issued, Kemmerer, Wyoming), Oklo's Aurora (15-75 MWe, ground broken at INL September 2025), and ARC Clean Technology's ARC-100 (100 MWe, completed CNSC Phase 2 Vendor Design Review in July 2025). Lead-cooled fast reactors include Newcleo's LFR-AS-200 (200 MWe, building a non-nuclear PRECURSOR prototype in Italy), Blykalla's SEALER (55 MWe, test facility operational at Oskarshamn since Q3 2025), and Westinghouse's LFR (eight test facilities running in the UK since 2023). Elysium Industries is pursuing a Molten Chloride Salt Fast Reactor using chloride salts to achieve a fast spectrum without solid fuel elements. All these designs require HALEU or MOX fuel to provide the fissile concentration needed to sustain criticality in the fast spectrum.
The practical significance of the fast spectrum for the nuclear industry extends beyond fuel efficiency. Fast neutrons can fission minor actinides (americium, curium, neptunium) that are the primary long-lived radioactive constituents of nuclear waste, potentially reducing repository requirements by orders of magnitude in both volume and radiotoxicity duration. This waste transmutation capability is a key policy argument for fast reactor deployment and is embedded in the design philosophies of several developers. Fast-spectrum reactors also offer natural resistance to weapons-grade plutonium production, as the plutonium isotopic mix produced in a fast spectrum is less suitable for weapons than that from thermal reactors, enhancing nonproliferation characteristics when operating in a sustained burn mode.