Fast-neutron reactor

From top, left to right

1. Superphénix, the largest fast reactor ever
2. Cross-section of Clementine, the first fast reactor
3. EBR-I, the first fast breeder reactor
4. RORSAT Soviet space probe, extensively using the BES-5 reactor
5. BN-800, the largest operating fast reactor
6. Fast Flux Test Facility, a large research fast reactor at the Hanford Site

A fast-neutron reactor (FNR) or fast-spectrum reactor or simply a fast reactor is a category of nuclear reactor in which the fission chain reaction is sustained by fast neutrons (carrying energies above 1 MeV, on average), as opposed to slow thermal neutrons used in thermal-neutron reactors. Such a fast reactor needs no neutron moderator, but requires fuel that is comparatively rich in fissile material.

The fast spectrum is key to breeder reactors, which convert highly abundant uranium-238 into fissile plutonium-239, without requiring enrichment. It also leads to high burnup: many transuranic isotopes, such as of americium and curium, accumulate in thermal reactor spent fuel; in fast reactors they undergo fast fission, reducing total nuclear waste. As a strong fast-spectrum neutron source, they can also be used to transmute existing nuclear waste into manageable or non-radioactive isotopes.

These characteristics also cause fast reactors to be judged a higher nuclear proliferation risk, especially as breeder reactors require nuclear reprocessing, which can be redirected to produce weapons-grade plutonium.

As of 2025^[update], every fast reactor has used a liquid metal coolant, typically sodium-cooled or lead-cooled.^[a] This allows high thermal efficiency, without pressurization systems, however it also contributes to historical high costs and operational difficulties.

In total, 13 fast breeder reactors have been constructed for commercial nuclear power,^[1] alongside 65 fast-spectrum research reactors of various configurations.^[2] The first fast reactor was Los Alamos Laboratory's Clementine, operated from 1946. The largest was Superphénix, in France, designed to deliver 1,242 MWe. In the GEN IV initiative, about two thirds of the proposed reactors for the future use a fast spectrum.^[3]

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