For fission reactors, the fuel (typically based on
uranium) is usually based on the metal
oxide; the oxides are used rather than the metals themselves because the oxide
melting point is much higher than that of the metal and because it cannot burn, being already in the oxidized state.
Uranium dioxide Uranium dioxide is a black
semiconducting solid. It can be made by heating
uranyl nitrate to form . : This is then converted by heating with
hydrogen to form UO2. It can be made from
enriched uranium hexafluoride by reacting with
ammonia to form a solid called
ammonium diuranate, . This is then heated (
calcined) to form and U3O8 which is then converted by heating with hydrogen or ammonia to form UO2. The UO2 is mixed with an organic binder and pressed into pellets. The pellets are then fired at a much higher temperature (in hydrogen or argon) to
sinter the solid. The aim is to form a dense solid which has few pores. The
thermal conductivity of uranium dioxide is very low compared with that of
zirconium metal, and it goes down as the temperature goes up. Corrosion of uranium dioxide in water is controlled by similar
electrochemical processes to the
galvanic corrosion of a metal surface. While exposed to the
neutron flux during normal operation in the core environment, a small percentage of the {{chem2|^{238}U}} in the fuel absorbs excess neutrons and is transmuted into {{chem2|^{239}U}}. {{chem2|^{239}U}} rapidly decays into {{chem2|^{239}Np}} which in turn rapidly decays into {{chem2|^{239}Pu}}. The small percentage of {{chem2|^{239}Pu}} has a higher neutron cross section than {{chem2|^{235}U}}. As the {{chem2|^{239}Pu}} accumulates the chain reaction shifts from pure {{chem2|^{235}U}} at initiation of the fuel use to a ratio of about 70% {{chem2|^{235}U}} and 30% {{chem2|^{239}Pu}} at the end of the 18 to 24 month fuel exposure period.
MOX Mixed oxide, or
MOX fuel, is a blend of
plutonium and
natural or
depleted uranium which behaves similarly (though not identically) to the enriched uranium feed for which most nuclear reactors were designed. MOX fuel is an alternative to low enriched uranium (LEU) fuel used in the
light water reactors which predominate
nuclear power generation. Some concern has been expressed that used MOX cores will introduce new disposal challenges, though MOX is a means to dispose of surplus plutonium by
transmutation. Reprocessing of commercial nuclear fuel to make MOX was done in the
Sellafield MOX Plant (England). As of 2015, MOX fuel is made in France at the
Marcoule Nuclear Site, and to a lesser extent in Russia at the
Mining and Chemical Combine, India and Japan. China
plans to develop fast breeder reactors and reprocessing. The
Global Nuclear Energy Partnership was a U.S. proposal in the
George W. Bush administration to form an international partnership to see
spent nuclear fuel reprocessed in a way that renders the plutonium in it usable for nuclear fuel but not for nuclear weapons. Reprocessing of spent commercial-reactor nuclear fuel has not been permitted in the United States due to
nonproliferation considerations. All other reprocessing nations have long had nuclear weapons from military-focused
research reactor fuels except for Japan. Normally, with the fuel being changed every three years or so, about half of the {{chem2|^{239}Pu}} is 'burned' in the reactor, providing about one third of the total energy. It behaves like {{chem2|^{235}U}} and its fission releases a similar amount of energy. The higher the
burnup, the more plutonium is present in the spent fuel, but the available fissile plutonium is lower. Typically about one percent of the used fuel discharged from a reactor is plutonium, and some two thirds of this is fissile (c. 50% {{chem2|^{239}Pu}}, 15% {{chem2|^{241}Pu}}). ==Metal fuel== at the
Institut Laue-Langevin 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,
uranium zirconium hydride (UZrH), and uranium zirconium carbonitride. 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
light-water reactors and liquid metal
fast breeder reactors, such as
Experimental Breeder Reactor II.
TRIGA fuel 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. Most cores that use this fuel are "high leakage" cores where the excess leaked neutrons can be utilized for research. That is, they can be used as a
neutron source. 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. There are 35 TRIGA reactors in the US and an additional 35 in other countries.
Actinide fuel 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.
Molten plutonium 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
Los Alamos National Laboratory in the 1960s. LAMPRE experienced three separate fuel failures during operation. ==Non-oxide ceramic fuels==