A type of high-temperature nuclear reactor that has no moderator and uses a liquid metal coolant such as sodium. They are used as either fast breeder reactors in which fissile material such as uranium enriched with plutonium-239 is used as the fuel and is able to produce more plutonium-239 at a rate faster than it uses; or as converter reactors in which fertile material such as uranium-238 is converted into plutonium-239 by the absorption of a neutron to form uranium-238 and which then decays to plutonium-239. Fertile material is a nuclide that can absorb a neutron to form fissile material.