<Li> Lead - Cooled Fast Reactor (LFR) based on Soviet naval propulsion units . </Li> <Li> Supercritical Water Reactor (SCWR) based on existing LWR and supercritical boiler technology . </Li> <P> FBRs usually use a mixed oxide fuel core of up to 20% plutonium dioxide (PuO) and at least 80% uranium dioxide (UO). Another fuel option is metal alloys, typically a blend of uranium, plutonium, and zirconium (used because it is "transparent" to neutrons). Enriched uranium can also be used on its own . </P> <P> Many designs surround the core in a blanket of tubes that contain non-fissile uranium - 238, which, by capturing fast neutrons from the reaction in the core, converts to fissile plutonium - 239 (as is some of the uranium in the core), which is then reprocessed and used as nuclear fuel . Other FBR designs rely on the geometry of the fuel itself (which also contains uranium - 238), arranged to attain sufficient fast neutron capture . The plutonium - 239 (or the fissile uranium - 235) fission cross-section is much smaller in a fast spectrum than in a thermal spectrum, as is the ratio between the Pu / U fission cross-section and the U absorption cross-section . This increases the concentration of Pu / U needed to sustain a chain reaction, as well as the ratio of breeding to fission . </P>

Are there any breeder reactors in the us