The breeding ratio (BR), also called the conversion ratio when below 1.0, quantifies a nuclear reactor's ability to produce new fissile material from fertile isotopes through neutron capture and subsequent radioactive decay. In a reactor with a breeding ratio greater than 1.0, neutron capture in fertile uranium-238 produces plutonium-239 (or in thorium-232 produces uranium-233) faster than existing fissile material is consumed by fission, meaning the reactor generates more fuel than it burns. This capability is exclusive to fast-spectrum reactors, where the higher energy neutrons and more favorable nuclear cross-sections enable sufficient excess neutron production to achieve breeding. Thermal-spectrum reactors typically have conversion ratios of 0.5-0.7, meaning they convert some fertile material to fissile material but cannot achieve net fuel production.
Fast-spectrum SMR designs in development, including TerraPower's Natrium, Oklo's Aurora, ARC Clean Technology's ARC-100, and Newcleo's LFR-AS-200, all operate in regimes where the conversion ratio can approach or exceed unity, depending on core configuration and fuel management strategy. TerraPower's Natrium, for example, uses metallic HALEU fuel in a sodium-cooled fast-spectrum core that achieves high fuel utilization through conversion of U-238 to Pu-239. While these designs are not optimized as dedicated breeders (as some Generation IV concepts envision), their elevated conversion ratios significantly improve fuel utilization compared to thermal reactors, extracting more energy per unit of mined uranium and reducing the volume of high-level waste per unit of electricity generated.
The historical pursuit of breeding has shaped nuclear development for decades. France's Phenix and Superphenix sodium-fast reactors, Russia's BN-600 and BN-800, and the U.S. EBR-II all demonstrated breeding physics. The renewed interest in fast reactors for SMR applications has reignited discussions about the closed fuel cycle, where spent fuel is reprocessed to recover bred plutonium for recycling as MOX fuel. Newcleo's business model explicitly depends on this approach, using MOX fuel manufactured from reprocessed spent fuel in its lead-cooled fast reactor, which motivated its relocation from the UK to France after the UK decided to immobilize its plutonium stockpile. The breeding ratio's practical significance extends beyond fuel economics to nuclear waste policy, as fast-spectrum reactors with high conversion ratios can also transmute long-lived minor actinides into shorter-lived isotopes, potentially reducing the long-term radiotoxicity of nuclear waste by orders of magnitude.