Rudolf Schulten —professor at RWTH Aachen University—was the main developer of the pebble bed reactor design, invented by Farrington Daniels. Schulten's concept compacts silicon carbide-coated uranium granules into hard, billiard-ball-like graphite spheres to be used as fuel for a new high temperature, helium-cooled type of nuclear reactor; the idea took root and in due course a 46 MWth experimental pebble bed reactor was built at the Jülich Research Centre in Jülich, West Germany. It was shut down in the wake of Chernobyl; some of the last pebble fuel tested in the AVR was for a low enriched uranium fuel cycle anticipated for use in the HTR-MODUL project design by Interatom/SIEMENS. Based on the AVR South Africa along with international partners developed an updated version called the PBMR; the TRISO fuel elements could use either U-235 in the form of LEU as fuel. The project was cancelled in 2010 due to lack of investment though the technology has been developed; the technology is being developed in China who operate a 10 MW test reactor of this type.
The Chinese are, as of 2015, building a commercial pebble-bed reactor: HTR-PM. Kirchner, Ulrich: Der Hochtemperaturreaktor - Konflikte, Entscheidungen, Campus Verlag, Frankfurt/New York, Campus Forschung Vol. 667, 1991. ISBN 3593345382
Uranium dioxide or uranium oxide known as urania or uranous oxide, is an oxide of uranium, is a black, crystalline powder that occurs in the mineral uraninite. It is used in nuclear fuel rods in nuclear reactors. A mixture of uranium and plutonium dioxides is used as MOX fuel. Prior to 1960, it was used as black color in ceramic glazes and glass. Uranium dioxide is produced by reducing uranium trioxide with hydrogen. UO3 + H2 → UO2 + H2O at 700 °C This reaction plays an important part in the creation of nuclear fuel through nuclear reprocessing and uranium enrichment; the solid is isostructural with fluorite, where each U is surrounded by eight O nearest neighbors in a cubic arrangement. In addition, the dioxides of cerium and neptunium have the same structures. No other elemental dioxides have the fluorite structure. Upon melting, the measured average U-O coordination reduces from 8 in the crystalline solid, down to 6.7±0.5 in the melt. Models consistent with these measurements show the melt to consist of UO6 and UO7 polyhedral units, where 2⁄3 of the connections between polyhedra are corner sharing and 1⁄3 are edge sharing.
Uranium dioxide is oxidized in contact with oxygen to the triuranium octaoxide. 3 UO2 + O2 → U3O8 at 700 °C The electrochemistry of uranium dioxide has been investigated in detail as the galvanic corrosion of uranium dioxide controls the rate at which used nuclear fuel dissolves. See spent nuclear fuel for further details. Water increases the oxidation rate of uranium metals. Uranium dioxide is carbonized in contact with carbon, forming carbon monoxide. UO2 + 4 C → UC2 + 2 CO This process must be done under an inert gas as uranium carbide is oxidized back into uranium oxide. UO2 is used as nuclear fuel as UO2 or as a mixture of UO2 and PuO2 called a mixed oxide, in the form of fuel rods in nuclear reactors. Note that the thermal conductivity of uranium dioxide is low when compared with uranium, uranium nitride, uranium carbide and zirconium cladding material; this low thermal conductivity can result in localised overheating in the centres of fuel pellets. The graph below shows the different temperature gradients in different fuel compounds.
For these fuels, the thermal power density is the same and the diameter of all the pellets are the same. Uranium oxide was used to color glass and ceramics prior to World War II, until the applications of radioactivity were discovered this was its main use. In 1958 the military in both the USA and Europe allowed its commercial use again as depleted uranium, its use began again on a more limited scale. Urania-based ceramic glazes are black when fired in a reduction or when UO2 is used. Orange-colored Fiestaware is a well-known example of a product with a urania-colored glaze. Uranium glass is pale green to yellow and has strong fluorescent properties. Urania has been used in formulations of enamel and porcelain, it is possible to determine with a Geiger counter if a glaze or glass produced before 1958 contains urania. Prior to the realisation of the harmfulness of radiation, uranium was included in false teeth and dentures, as its slight fluorescence made the dentures appear more like real teeth in a variety of lighting conditions.
Depleted UO2 can be used as a material for radiation shielding. For example, DUCRETE is a "heavy concrete" material where gravel is replaced with uranium dioxide aggregate. Casks can be made of DUO2-steel cermet, a composite material made of an aggregate of uranium dioxide serving as radiation shielding, graphite and/or silicon carbide serving as neutron radiation absorber and moderator, steel as the matrix, whose high thermal conductivity allows easy removal of decay heat. Depleted uranium dioxide can be used as a catalyst, e.g. for degradation of volatile organic compounds in gaseous phase, oxidation of methane to methanol, removal of sulfur from petroleum. It has high efficiency and long-term stability when used to destroy VOCs when compared with some of the commercial catalysts, such as precious metals, TiO2, Co3O4 catalysts. Much research is being done in this area, DU being favoured for the uranium component due to its low radioactivity; the use of uranium dioxide as a material for rechargeable batteries is being investigated.
The batteries could have high power potential of 4.7 V per cell. Another investigated application is in photoelectrochemical cells for solar-assisted hydrogen production where UO2 is used as a photoanode. In earlier times, uranium dioxide was used as heat conductor for current limitation, the first use of its semiconductor properties. Uranium dioxide is the strongest known piezomagnetic in the antiferromagnetic state observed at cryogenic temperatures below 30 kelvins. UO2 displays a linear magnetostriction that changes sign with the sign of the applied magnetic field, magnetoelastic memory switching at magnetic fields near 180,000 Oe; the band gap of uranium dioxide is comparable to these of silicon and gallium arsenide, near the optimum for efficiency vs band gap curve for absorption of solar radiation, suggesting its possible use for efficient solar cells based on Schottky diode structure. Its intrinsic conductivity at room temperature is about the same as of single crystal silicon; the dielectric constant of uranium dioxide is abou
Peach Bottom Nuclear Generating Station
Peach Bottom Atomic Power Station, a nuclear power plant, is located 50 miles southeast of Harrisburg in Peach Bottom Township, York County, Pennsylvania, on the Susquehanna River three miles north of the Maryland border. The Philadelphia Electric Company became one of the pioneers in the commercial nuclear industry when it ordered Peach Bottom 1 in 1958; the U. S.'s first nuclear power plant had gone on line a year earlier. Peach Bottom Unit 1 was an experimental graphite-moderated reactor, it operated from 1966 to 1974. The other two units, General Electric boiling water reactors, placed on-line in 1974, are still in operation on the 620-acre site. Both Units 2 and 3 rated at 3,514 megawatts thermal, equivalent to about 1,180 megawatts of electricity each, were uprated to 4,016 megawatts thermal, equivalent to about 1,382 megawatts net of electricity each in 2018, their licenses run until 2033 and 2034. Peach Bottom is operated by the Exelon and is jointly owned by Exelon and Public Service Enterprise Group Power LLC.
Peach Bottom was one of the plants analyzed in the NUREG-1150 safety analysis study. The Nuclear Regulatory Commission defines two emergency planning zones around nuclear power plants: a plume exposure pathway zone with a radius of 10 miles, concerned with exposure to, inhalation of, airborne radioactive contamination, an ingestion pathway zone of about 50 miles, concerned with ingestion of food and liquid contaminated by radioactivity; the 2010 U. S. population within 10 miles of Peach Bottom was 46,536, an increase of 7.2 percent in a decade, according to an analysis of U. S. Census data for msnbc.com. The 2010 U. S. population within 50 miles was 5,526,343, an increase of 10.6 percent since 2000. Cities within 50 miles include Baltimore. In 1987, PECO was ordered by the Nuclear Regulatory Commission to indefinitely shutdown Peach Bottom-2 and -3 on March 31 due to operator misconduct, corporate malfeasance and blatant disregard for the health and safety of the area. Infamously, operators were found sleeping on the job, playing video games, engaging in rubber band and paper ball fights, reading unauthorized material.
Among the incidents cited by the NRC: security guards were overworked, one guard was found asleep on the job, 36,000 gallons of "mildly radioactive water" leaked into the Susquehanna River, PECO mislaid data on radioactive waste classification causing misclassification of a waste shipment, a major fire occurred in the maintenance cage of the Unit 3 turbine building on March 4, 1987. Blame was not placed on the operators. "Latent organizational weakness" was targeted by industry regulators alike. INPO President Zack Pate came to the conclusion that “Major changes in the corporate culture at PECO are required.” In September 1988, NRC Chairman Lando Zech told senior management officials of PECO, "Your operators made mistakes, no question about that. Your corporate management problems are just as serious." A culture characterized by low morale and apathy prevailed. By April 1988, this emphasis on mismanagement contributed to the President of PECO resigning as well as to the retirement of the CEO.
Robert P. Crosby became the primary Organization Development influence during the PECO Nuclear turnaround following the Peach Bottom shut down, he used The Interpersonal Gap model by John L. Wallen along with a unique T-group method known as Conflict Management to speed culture change, applied his own version of Daryl Conner's Sponsor Agent Target model to improving and shortening outage management. By 1996, both Limerick and Peach Bottom were designated excellent by INPO, given strong Systematic Assessment of Licensee Performance ratings by the NRC. In 1999, PECO Nuclear eliminated their Organization Development positions as part of cost cutting initiative. Trouble arose again in September 2007, when former employee Kerry Beal videotaped Peach Bottom security guards sleeping on the job. Beal had tried to notify supervisors at Wackenhut Corp. and the US Nuclear Regulatory Commission. He was fired during the Exelon security transition, a decision which made a list of the 101 "dumbest moments in business" in the January 16, 2008 issue of Fortune.
The Nuclear Regulatory Commission's estimate of the risk each year of an earthquake intense enough to cause core damage to the reactor at Peach Bottom was 1 in 41,667. Exelon Nuclear reactor accidents in the United States Nuclear safety in the United States G4S Secure Solutions List of the largest nuclear power stations in the United States Specific Generalhttps://www.nrc.gov/info-finder/decommissioning/power-reactor/peach-bottom-atomic-power-station-unit.html "Peach Bottom Atomic Power Station, Pennsylvania". Energy Information Administration, U. S. Department of Energy. October 3, 2008. Retrieved 2008-11-18. "Peach Bottom 2 Boiling Water Reactor". Operating Nuclear Power Reactors. U. S. Nuclear Regulatory Commission. February 14, 2008. Retrieved 2008-11-18. "Peach Bottom 3 Boiling Water Reactor". Operating Nuclear Power Reactors. NRC. February 14, 2008. Retrieved 2008-11-18."The Peach Bottom Atomic Energy Station". PSEG Nuclear LLC. 2008. Archived from the original on 2008-06-07. Retrieved 2008-11-18
A nuclear reactor known as an atomic pile, is a device used to initiate and control a self-sustained nuclear chain reaction. Nuclear reactors are used at nuclear power plants for electricity generation and in propulsion of ships. Heat from nuclear fission is passed to a working fluid; these either turn electrical generators' shafts. Nuclear generated steam in principle can be used for industrial process heat or for district heating; some reactors are used to produce isotopes for medical and industrial use, or for production of weapons-grade plutonium. Some are run only for research; as of early 2019, the IAEA reports there are 454 nuclear power reactors and 226 nuclear research reactors in operation around the world. Just as conventional power-stations generate electricity by harnessing the thermal energy released from burning fossil fuels, nuclear reactors convert the energy released by controlled nuclear fission into thermal energy for further conversion to mechanical or electrical forms; when a large fissile atomic nucleus such as uranium-235 or plutonium-239 absorbs a neutron, it may undergo nuclear fission.
The heavy nucleus splits into two or more lighter nuclei, releasing kinetic energy, gamma radiation, free neutrons. A portion of these neutrons may be absorbed by other fissile atoms and trigger further fission events, which release more neutrons, so on; this is known as a nuclear chain reaction. To control such a nuclear chain reaction, neutron poisons and neutron moderators can change the portion of neutrons that will go on to cause more fission. Nuclear reactors have automatic and manual systems to shut the fission reaction down if monitoring detects unsafe conditions. Used moderators include regular water, solid graphite and heavy water; some experimental types of reactor have used beryllium, hydrocarbons have been suggested as another possibility. The reactor core generates heat in a number of ways: The kinetic energy of fission products is converted to thermal energy when these nuclei collide with nearby atoms; the reactor absorbs some of the gamma rays produced during fission and converts their energy into heat.
Heat is produced by the radioactive decay of fission products and materials that have been activated by neutron absorption. This decay heat-source will remain for some time after the reactor is shut down. A kilogram of uranium-235 converted via nuclear processes releases three million times more energy than a kilogram of coal burned conventionally. A nuclear reactor coolant — water but sometimes a gas or a liquid metal or molten salt — is circulated past the reactor core to absorb the heat that it generates; the heat is carried away from the reactor and is used to generate steam. Most reactor systems employ a cooling system, physically separated from the water that will be boiled to produce pressurized steam for the turbines, like the pressurized water reactor. However, in some reactors the water for the steam turbines is boiled directly by the reactor core; the rate of fission reactions within a reactor core can be adjusted by controlling the quantity of neutrons that are able to induce further fission events.
Nuclear reactors employ several methods of neutron control to adjust the reactor's power output. Some of these methods arising from the physics of radioactive decay and are accounted for during the reactor's operation, while others are mechanisms engineered into the reactor design for a distinct purpose; the fastest method for adjusting levels of fission-inducing neutrons in a reactor is via movement of the control rods. Control rods therefore tend to absorb neutrons; when a control rod is inserted deeper into the reactor, it absorbs more neutrons than the material it displaces—often the moderator. This action results in fewer neutrons available to cause fission and reduces the reactor's power output. Conversely, extracting the control rod will result in an increase in the rate of fission events and an increase in power; the physics of radioactive decay affects neutron populations in a reactor. One such process is delayed neutron emission by a number of neutron-rich fission isotopes; these delayed neutrons account for about 0.65% of the total neutrons produced in fission, with the remainder released upon fission.
The fission products which produce delayed neutrons have half lives for their decay by neutron emission that range from milliseconds to as long as several minutes, so considerable time is required to determine when a reactor reaches the critical point. Keeping the reactor in the zone of chain-reactivity where delayed neutrons are necessary to achieve a critical mass state allows mechanical devices or human operators to control a chain reaction in "real time"; this last stage, where delayed neutrons are no longer required to maintain criticality, is known as the prompt critical point. There is a scale for describing criticality in numerical form, in which bare criticality is known as zero dollars and the prompt critical point is one dollar, other points in the process interpolated in cents. In some reactors, the coolant acts as a neutron moderator. A moderator increases the power of the reactor by causin
Nuclear and radiation accidents and incidents
A nuclear and radiation accident is defined by the International Atomic Energy Agency as "an event that has led to significant consequences to people, the environment or the facility." Examples include lethal effects to individuals, radioactive isotope to the environment, or reactor core melt." The prime example of a "major nuclear accident" is one in which a reactor core is damaged and significant amounts of radioactive isotopes are released, such as in the Chernobyl disaster in 1986. The impact of nuclear accidents has been a topic of debate since the first nuclear reactors were constructed in 1954, has been a key factor in public concern about nuclear facilities. Technical measures to reduce the risk of accidents or to minimize the amount of radioactivity released to the environment have been adopted, however human error remains, "there have been many accidents with varying impacts as well near misses and incidents"; as of 2014, there have been more than 100 serious nuclear accidents and incidents from the use of nuclear power.
Fifty-seven accidents have occurred since the Chernobyl disaster, about 60% of all nuclear-related accidents have occurred in the USA. Serious nuclear power plant accidents include the Fukushima Daiichi nuclear disaster, Chernobyl disaster, Three Mile Island accident, the SL-1 accident. Nuclear power accidents can involve large monetary costs for remediation work. Nuclear-powered submarine accidents include the K-19, K-11, K-27, K-140, K-429, K-222, K-431. Serious radiation incidents/accidents include the Kyshtym disaster, Windscale fire, radiotherapy accident in Costa Rica, radiotherapy accident in Zaragoza, radiation accident in Morocco, Goiania accident, radiation accident in Mexico City, radiotherapy unit accident in Thailand, the Mayapuri radiological accident in India; the IAEA maintains a website reporting recent accidents. One of the worst nuclear accidents to date was the Chernobyl disaster which occurred in 1986 in Ukraine; the accident killed 31 people directly and damaged $7 billion of property.
A study published in 2005 estimates that there will be up to 4,000 additional cancer deaths related to the accident among those exposed to significant radiation levels. Radioactive fallout from the accident was concentrated in areas of Belarus and Russia. Other studies have estimated as many as over a million eventual cancer deaths from Chernobyl. Estimates of eventual deaths from cancer are contested. Industry, UN and DOE agencies claim low numbers of provable cancer deaths will be traceable to the disaster; the UN, DOE and industry agencies all use the limits of the epidemiological resolvable deaths as the cutoff below which they cannot be proven to come from the disaster. Independent studies statistically calculate fatal cancers from dose and population though the number of additional cancers will be below the epidemiological threshold of measurement of around 1%; these are two different concepts and lead to the huge variations in estimates. Both are reasonable projections with different meanings.
350,000 people were forcibly resettled away from these areas soon after the accident. Social scientist and energy policy expert, Benjamin K. Sovacool has reported that worldwide there have been 99 accidents at nuclear power plants from 1952 to 2009, totaling US$20.5 billion in property damages. Fifty-seven accidents have occurred since the Chernobyl disaster, two-thirds of all nuclear-related accidents have occurred in the US. There have been comparatively few fatalities associated with nuclear power plant accidents; the vulnerability of nuclear plants to deliberate attack is of concern in the area of nuclear safety and security. Nuclear power plants, civilian research reactors, certain naval fuel facilities, uranium enrichment plants, fuel fabrication plants, potentially uranium mines are vulnerable to attacks which could lead to widespread radioactive contamination; the attack threat is of several general types: commando-like ground-based attacks on equipment which if disabled could lead to a reactor core meltdown or widespread dispersal of radioactivity.
The United States 9/11 Commission found that nuclear power plants were potential targets considered for the September 11, 2001 attacks. If terrorist groups could sufficiently damage safety systems to cause a core meltdown at a nuclear power plant, and/or sufficiently damage spent fuel pools, such an attack could lead to widespread radioactive contamination; the Federation of American Scientists have said that if nuclear power use is to expand nuclear facilities will have to be made safe from attacks that could release massive quantities of radioactivity into the community. New reactor designs have features of passive nuclear safety. In the United States, the NRC carries out "Force on Force" exercises at all Nuclear Power Plant sites at least once every three years. Nuclear reactors become preferred targets during military conflict and, over the past three decades, have been attacked during military air strikes, occupations and campaigns. Various acts of civil disobedience since 1980 by the peace group Plowshares have shown how nuclear weapons facilities can be penetrated, the group's actions represent extraordinary breaches of security at nuclear weapons plants in the United States.
Molten salt reactor
A molten salt reactor is a class of nuclear fission reactor in which the primary nuclear reactor coolant and/or the fuel is a molten salt mixture. MSRs offer multiple advantages over conventional nuclear power plants, although for historical reasons, they have not been deployed; the concept was first established in the 1950s. The early Aircraft Reactor Experiment was motivated by the small size that the technique offered, while the Molten-Salt Reactor Experiment was a prototype for a thorium fuel cycle breeder nuclear power plant; the increased research into Generation IV reactor designs renewed interest in the technology. MSR research started with the U. S. Aircraft Reactor Experiment in support of the U. S. Aircraft Nuclear Propulsion program. ARE was a 2.5 MWth nuclear reactor experiment designed to attain a high energy density for use as an engine in a nuclear-powered bomber. The project included experiments, including high temperature and engine tests collectively called the Heat Transfer Reactor Experiments: HTRE-1, HTRE-2 and HTRE-3 at the National Reactor Test Station as well as an experimental high-temperature molten salt reactor at Oak Ridge National Laboratory—the ARE.
ARE used molten fluoride salt NaF-ZrF4-UF4 as fuel, moderated by beryllium oxide. Liquid sodium was a secondary coolant; the experiment had a peak temperature of 860 °C. It produced 100 MWh over nine days in 1954; this experiment used Inconel 600 alloy for the metal piping. An MSR was operated at the Critical Experiments Facility of the Oak Ridge National Laboratory in 1957, it was part of the circulating-fuel reactor program of the Whitney Aircraft Company. This was called Pratt and Whitney Aircraft Reactor-1; the experiment was run for a few weeks and at zero power, although it reached criticality. The operating temperature was held constant at 675 °C; the PWAR-1 used NaF-ZrF4-UF4 as coolant. It was one of three critical MSRs built. Oak Ridge National Laboratory took the lead in researching MSRs through the 1960s. Much of their work culminated with the Molten-Salt Reactor Experiment. MSRE was a 7.4 MWth test reactor simulating the neutronic "kernel" of a type of epithermal thorium molten salt breeder reactor called the liquid fluoride thorium reactor.
The large breeding blanket of thorium salt was omitted in favor of neutron measurements. MSRE's piping, core vat and structural components were made from Hastelloy-N, moderated by pyrolytic graphite, it ran for four years. Its fuel was LiF-BeF2-ZrF4-UF4; the graphite core moderated it. Its secondary coolant was FLiBe, it reached temperatures as high as 650 °C and achieved the equivalent of about 1.5 years of full power operation. The culmination of the ORNL research during the 1970–1976 timeframe resulted in a molten salt breeder reactor design. Fuel was to be LiF-BeF2-ThF4-UF4 with graphite moderator; the secondary coolant was to be NaF-NaBF4. Its peak operating temperature was to be 705 °C, it would follow a 4-year replacement schedule. The MSR program closed down in the early 1970s in favor of the liquid metal fast-breeder reactor, after which research stagnated in the United States; as of 2011, ARE and MSRE remained the only molten-salt reactors operated. The MSBR project received funding until 1976.
Inflation-adjusted to 1991 dollars, the project received $38.9 million from 1968 to 1976. The program was cancelled because: The political and technical support for the program in the United States was too thin geographically. Within the United States the technology was well understood only in Oak Ridge; the MSR program was in competition with the fast breeder program at the time, which got an early start and had copious government development funds with contracts that benefited many parts of the country. When the MSR development program had progressed far enough to justify an expanded program leading to commercial development, the United States Atomic Energy Commission could not justify the diversion of substantial funds from the LMFBR to a competing program. In 1980, the engineering technology division at Oak Ridge National Laboratory published a paper entitled "Conceptual Design Characteristics of a Denatured Molten-Salt Reactor with Once-Through Fueling." In it, the authors "examine the conceptual feasibility of a molten-salt power reactor fueled with denatured uranium-235 and operated with a minimum of chemical processing."
The main priority behind the design characteristics was proliferation resistance. Although the DMSR can theoretically be fueled by thorium or plutonium, fueling with low enriched uranium helps maximize proliferation resistance. Other important goals of the DMSR were to maximize feasibility; the Generation IV international Forum includes "salt processing" as a technology gap for molten salt reactors. The DMSR requires minimal chemical processing. Both reactors built at ORNL were burner designs. In addition, the choices to use graphite for neutron moderation and enhanced Hastelloy-N for piping simplified the design and reduced R&D; the UK's Atomic Energy Research Establishment were developing an alternative MSR design across its National Laboratories at Harwell, Culham and Winfrith. AERE opted to focus on a lead-cooled 2.5 GWe Molten Salt Fast Reactor concept using a chloride. They researched helium gas as a coolant; the UK MSFR would be fuelled by plutonium, a fuel considered to be'free' by the program's research scientists, because of the UK's plutonium stockpile.
Despite their differe
Nuclear fuel is material used in nuclear power stations to produce heat to power turbines. Heat is created. Most nuclear fuels contain heavy fissile actinide elements that are capable of undergoing and sustaining nuclear fission; the three most relevant fissile isotopes are Uranium-233, Uranium-235 and Plutonium-239. When the unstable nuclei of these atoms are hit by a slow-moving neutron, they split, creating two daughter nuclei and two or three more neutrons; these neutrons go on to split more nuclei. This creates a self-sustaining chain reaction, controlled in a nuclear reactor, or uncontrolled in a nuclear weapon; the processes involved in mining, purifying and disposing of nuclear fuel are collectively known as the nuclear fuel cycle. Not all types of nuclear fuels create power from nuclear fission. Nuclear fuel has the highest energy density of all practical fuel sources. For fission reactors, the fuel is based on the metal oxide. Uranium dioxide is a black semiconducting solid, it can be made by reacting uranyl nitrate with a base to form a solid.
It is heated to form U3O8 that can be converted by heating in an argon / hydrogen mixture to form UO2. The UO2 is mixed with an organic binder and pressed into pellets, these pellets are fired at a much higher temperature to sinter the solid; the aim is to form a dense solid. The thermal conductivity of uranium dioxide is low compared with that of zirconium metal, 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. Mixed oxide, or MOX fuel, is a blend of plutonium and natural or depleted uranium which behaves 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 itself 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, to a lesser extent in Russia 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 of the other reprocessing nations have long had nuclear weapons from military-focused "research"-reactor fuels except for Japan. With the fuel being changed every three years or so, about half of the Pu-239 is'burned' in the reactor, providing about one third of the total energy, it behaves like U-235 and its fission releases a similar amount of energy. The higher the burn-up, the more plutonium in the spent fuel, but the lower the fraction of fissile plutonium.
About one percent of the used fuel discharged from a reactor is plutonium, some two thirds of this is fissile. Worldwide, some 70 tonnes of plutonium contained in used fuel is removed when refueling reactors each year. Metal fuels have the advantage of a much higher heat conductivity than oxide fuels but cannot survive 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 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. 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. TRIGA 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 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 designed to use 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. 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 an alloy of zirconium, pluto