Nuclear fuel

Nuclear fuel refers to any substance, typically fissile material, which is used by nuclear power stations or other nuclear devices to generate energy.

Oxide fuel

For fission reactors, the fuel 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

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 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 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 in the fuel absorbs excess neutrons and is transmuted into. rapidly decays into which in turn rapidly decays into. The small percentage of has a higher neutron cross section than. As the accumulates the chain reaction shifts from pure at initiation of the fuel use to a ratio of about 70% and 30% 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 to the enriched uranium feed for which most nuclear reactors were designed. MOX fuel is an alternative to low enriched uranium 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. 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 is 'burned' in the reactor, providing about one third of the total energy. It behaves like 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.

Metal fuel

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, 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

fuel is used in TRIGA 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. 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

fuels other than oxides have the advantage of high heat conductivities and melting points, but they are more prone to swelling than oxide fuels and are not understood as well.

Uranium nitride

is often the fuel of choice for reactor designs that NASA produces. One advantage is that uranium nitride has a better thermal conductivity than UO₂. Uranium nitride has a very high melting point. This fuel has the disadvantage that unless ¹⁵N was used, a large amount of ¹⁴C would be generated from the nitrogen by the reaction.
As the nitrogen needed for such a fuel would be so expensive it is likely that the fuel would require pyroprocessing to enable recovery of the ¹⁵N. It is likely that if the fuel was processed and dissolved in nitric acid that the nitrogen enriched with ¹⁵N would be diluted with the common ¹⁴N. Fluoride volatility is a method of reprocessing that does not rely on nitric acid, but it has only been demonstrated in relatively small scale installations whereas the established PUREX process is used commercially for about a third of all spent nuclear fuel.
All nitrogen-fluoride compounds are volatile or gaseous at room temperature and could be fractionally distilled from the other gaseous products to recover the initially used nitrogen. If the fuel could be processed in such a way as to ensure low contamination with non-radioactive carbon then fluoride volatility could be used to separate the produced by producing carbon tetrafluoride. is proposed for use in particularly long lived low power nuclear batteries called diamond batteries.

Uranium carbide

Much of what is known about uranium carbide is in the form of pin-type fuel elements for liquid metal fast reactors during their intense study in the 1960s and 1970s. Recently there has been a revived interest in uranium carbide in the form of plate fuel and most notably, micro fuel particles.
The high thermal conductivity and high melting point makes uranium carbide an attractive fuel. In addition, because of the absence of oxygen in this fuel as well as the ability to complement a ceramic coating, uranium carbide could be the ideal fuel candidate for certain Generation IV reactors such as the gas-cooled fast reactor. While the neutron cross section of carbon is low, during years of burnup, the predominantly will undergo neutron capture to produce stable as well as radioactive. Unlike the produced by using uranium nitrate, the will make up only a small isotopic impurity in the overall carbon content and thus make the entirety of the carbon content unsuitable for non-nuclear uses but the concentration will be too low for use in nuclear batteries without enrichment. Nuclear graphite discharged from reactors where it was used as a moderator presents the same issue.

Liquid fuels

Liquid fuels contain dissolved nuclear fuel and have been shown to offer numerous operational advantages compared to traditional solid fuel approaches. Liquid-fuel reactors offer significant safety advantages due to their inherently stable "self-adjusting" reactor dynamics. This provides two major benefits: virtually eliminating the possibility of a runaway reactor meltdown, and providing an automatic load-following capability which is well suited to electricity generation and high-temperature industrial heat applications.
In some liquid core designs, the fuel can be drained rapidly into a passively safe dump-tank. This advantage was conclusively demonstrated repeatedly as part of a weekly shutdown procedure during the highly successful Molten-Salt Reactor Experiment from 1965 to 1969.
A liquid core is able to release xenon gas, which normally acts as a neutron absorber and causes structural occlusions in solid fuel elements. In contrast, molten-salt reactors are capable of retaining the fuel mixture for significantly extended periods, which increases fuel efficiency dramatically and incinerates the vast majority of its own waste as part of the normal operational characteristics. A downside to letting the escape instead of allowing it to capture neutrons converting it to the basically stable and chemically inert, is that it will quickly decay to the highly chemically reactive, long lived radioactive, which behaves similar to other alkali metals and can be taken up by organisms in their metabolism.