fastspectrum

Fast spectrum refers to a neutron energy distribution in a nuclear reactor that is dominated by fast neutrons, typically with energies above about 0.1 MeV, rather than thermal neutrons near 0.025 eV. In a fast spectrum reactor, little or no neutron moderator is used, allowing the reactor to operate with fast neutrons produced directly by fission. The neutron economy relies on fast fission of fissile or fertile isotopes such as U-235, Pu-239, and Pu-241; breeding of Pu-239 from U-238 is a central feature.

Because fast neutrons have different reaction cross sections from thermal neutrons, fast-spectrum reactors have different fuel-cycle requirements. They can be configured to breed more fuel than they consume (breeding ratio greater than one) and to burn actinides, including plutonium and minor actinides, offering potential waste-management benefits. However, the lack of moderator makes reactor physics more challenging and typically requires robust, high-temperature, chemically compatible coolants such as liquid sodium or lead, along with structural materials tolerant of high fast-neutron flux. This leads to engineering challenges, including materials irradiation, corrosion, and chemical handling safety issues.

Historically, fast reactors were pursued as potential breeders during the 1950s through the 1980s, with prototypes and plants such as EBR-I/II in the United States, Phénix and Superphénix in France, and BN-series reactors in Russia. Modern interest continues in experimental and demonstration fast reactors and in accelerator-driven systems as alternatives to traditional thermal reactors. In summary, fast spectrum denotes neutron populations with minimal moderation, enabling fast-neutron fission, potential breeding, and actinide burning, at the cost of greater materials and safety challenges.