<P> Metal fuels have the advantage of a much higher heat conductivity than oxide fuels but cannot survive equally high temperatures . Metal fuels have a long history of use, stretching from the Clementine reactor in 1946 to many test and research reactors . Metal fuels have the potential for the highest fissile atom density . Metal fuels are normally alloyed, but some metal fuels have been made with pure uranium metal . Uranium alloys that have been used include uranium aluminum, uranium zirconium, uranium silicon, uranium molybdenum, and uranium zirconium hydride . Any of the aforementioned fuels can be made with plutonium and other actinides as part of a closed nuclear fuel cycle . Metal fuels have been used in water reactors and liquid metal fast breeder reactors, such as EBR - II . </P> <P> TRIGA fuel is used in TRIGA (Training, Research, Isotopes, General Atomics) reactors . The TRIGA reactor uses UZrH fuel, which has a prompt negative fuel temperature coefficient of reactivity, meaning that as the temperature of the core increases, the reactivity decreases--so it is highly unlikely for a meltdown to occur . Most cores that use this fuel are "high leakage" cores where the excess leaked neutrons can be utilized for research . TRIGA fuel was originally designed to use highly enriched uranium, however in 1978 the U.S. Department of Energy launched its Reduced Enrichment for Research Test Reactors program, which promoted reactor conversion to low - enriched uranium fuel . A total of 35 TRIGA reactors have been installed at locations across the USA . A further 35 reactors have been installed in other countries . </P> <P> In a fast neutron reactor, the minor actinides produced by neutron capture of uranium and plutonium can be used as fuel . Metal actinide fuel is typically an alloy of zirconium, uranium, plutonium, and minor actinides . It can be made inherently safe as thermal expansion of the metal alloy will increase neutron leakage . </P> <P> Molten plutonium, alloyed with other metals to lower its melting point and encapsulated in tantalum, was tested in two experimental reactors, LAMPRE I and LAMPRE II, at LANL in the 1960s . "LAMPRE experienced three separate fuel failures during operation ." </P>

What is the fuel used at nuclear power plants