Nuclear fuel refers to any substance, typically fissile material, which is used by nuclear power stations or other nuclear devices to generate energy.
94-400: Mixed oxide fuel , commonly referred to as MOX fuel , is nuclear fuel that contains more than one oxide of fissile material , usually consisting of plutonium blended with natural uranium , reprocessed uranium , or depleted uranium . MOX fuel is an alternative to the low-enriched uranium fuel used in the light-water reactors that predominate nuclear power generation. For example,
188-436: A press and converted into pellets. The pellets can then be sintered into mixed uranium and plutonium oxide. Plutonium from reprocessed fuel is usually fabricated into MOX within less than five years of its production to avoid problems resulting from impurities produced by the decay of short-lived isotopes of plutonium. In particular, plutonium-241 decays to americium-241 with a 14-year half-life. Because americium-241
282-436: A Norwegian study, "the coolant void reactivity of the thorium-plutonium fuel is negative for plutonium contents up to 21%, whereas the transition lies at 16% for MOX fuel." The authors concluded, "Thorium-plutonium fuel seems to offer some advantages over MOX fuel with regards to control rod and boron worths, CVR and plutonium consumption." Nuclear fuel For fission reactors, the fuel (typically based on uranium )
376-406: 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,
470-432: A fuel would be so expensive it is likely that the fuel would require pyroprocessing to enable recovery of the N. It is likely that if the fuel was processed and dissolved in nitric acid that the nitrogen enriched with N would be diluted with the common N. 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
564-453: A kernel of UO X fuel (sometimes UC or UCO), which has been coated with four layers of three isotropic materials deposited through fluidized chemical vapor deposition (FCVD). The four layers are a porous buffer layer made of carbon that absorbs fission product recoils, followed by a dense inner layer of protective pyrolytic carbon (PyC), followed by a ceramic layer of SiC to retain fission products at elevated temperatures and to give
658-630: A lesser extent in Russia , India and Japan . In the UK THORP operated from 1994 to 2018. China plans to develop fast breeder reactors and reprocessing. Reprocessing of spent commercial-reactor nuclear fuel is not permitted in the United States due to nonproliferation considerations. Germany had plans for a reprocessing plant at Wackersdorf but as this failed to materialize, it instead relied on French nuclear reprocessing capabilities until legally outlawing
752-400: A mixture of 7% plutonium and 93% natural uranium reacts similarly, although not identically, to low-enriched uranium fuel (3 to 5% uranium-235). MOX usually consists of two phases, UO 2 and PuO 2 , and/or a single phase solid solution (U,Pu)O 2 . The content of PuO 2 may vary from 1.5 wt.% to 25–30 wt.% depending on the type of nuclear reactor. One attraction of MOX fuel is that it is
846-408: A new reactor with a complete fuel loading of MOX. As 2011, of the total nuclear fuel used, MOX provides about 2%. Licensing and safety issues of using MOX fuel include: About 30% of the plutonium originally loaded into MOX fuel is consumed by use in a thermal reactor. In theory, if one third of the core fuel load is MOX and two-thirds uranium fuel, there is zero net change in the mass of plutonium in
940-613: A properly designed reactor. Two such reactor designs are the prismatic-block gas-cooled reactor (such as the GT-MHR ) and the pebble-bed reactor (PBR). Both of these reactor designs are high temperature gas reactors (HTGRs). These are also the basic reactor designs of very-high-temperature reactors (VHTRs), one of the six classes of reactor designs in the Generation IV initiative that is attempting to reach even higher HTGR outlet temperatures. TRISO fuel particles were originally developed in
1034-762: A reactor is plutonium, and some two thirds of this is fissile (c. 50% Pu , 15% Pu ). 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
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#17328546175501128-438: A secondary sodium pump with an expansion tank located upstream, and an emergency pressure discharge tank. These feed a steam generator , which in turn supplies a condensing turbine that turns the generator. There is much international interest in the fast-breeder reactor at Beloyarsk. Japan has its own prototype fast-breeder reactors. The operation of the reactor is an international study in progress; Russia, France, Japan, and
1222-859: A similar design to the CANDU but built by German KWU was originally designed for non-enriched fuel but since switched to slightly enriched fuel with a U content about 0.1 percentage points higher than in natural uranium. Various other nuclear fuel forms find use in specific applications, but lack the widespread use of those found in BWRs, PWRs, and CANDU power plants. Many of these fuel forms are only found in research reactors, or have military applications. Magnox (magnesium non-oxidising) reactors are pressurised, carbon dioxide –cooled, graphite - moderated reactors using natural uranium (i.e. unenriched) as fuel and Magnox alloy as fuel cladding. Working pressure varies from 6.9 to 19.35 bars (100.1 to 280.6 psi) for
1316-449: A small percentage of the U in the fuel absorbs excess neutrons and is transmuted into U . U rapidly decays into Np which in turn rapidly decays into Pu . The small percentage of Pu has a higher neutron cross section than U . As the Pu accumulates the chain reaction shifts from pure U at initiation of the fuel use to a ratio of about 70% U and 30% Pu at the end of
1410-409: A solid called ammonium diuranate , (NH 4 ) 2 U 2 O 7 . This is then heated ( calcined ) to form UO 3 and U 3 O 8 which is then converted by heating with hydrogen or ammonia to form UO 2 . The UO 2 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
1504-498: A spent fuel would be difficult to reprocess for further reuse (burning) of plutonium. Regular reprocessing of biphasic spent MOX is difficult because of the low solubility of PuO 2 in nitric acid. As of 2015, the only demonstration of twice-recycled, high-burnup fuel occurred in the Phénix fast reactor. Reprocessing of commercial nuclear fuel to make MOX is performed in France and to
1598-411: A three-circuit coolant arrangement; sodium coolant circulates in both the primary and secondary circuits. Water and steam flow in the third circuit. The sodium is heated to a maximum of 550 °C (1,022 °F) in the reactor during normal operations. This heat is transferred from the reactor core via three independent circulation loops. Each has a primary sodium pump, two intermediate heat exchangers,
1692-476: A typical core loading is on the order of 4500–6500 bundles, depending on the design. Modern types typically have 37 identical fuel pins radially arranged about the long axis of the bundle, but in the past several different configurations and numbers of pins have been used. The CANFLEX bundle has 43 fuel elements, with two element sizes. It is also about 10 cm (4 inches) in diameter, 0.5 m (20 in) long and weighs about 20 kg (44 lb) and replaces
1786-399: A typical spent fuel assembly still exceeds 10,000 rem/hour, resulting in a fatal dose in just minutes. Two main modes of release exist, the fission products can be vaporised or small particles of the fuel can be dispersed. Post-Irradiation Examination (PIE) is the study of used nuclear materials such as nuclear fuel. It has several purposes. It is known that by examination of used fuel that
1880-404: A way as to ensure low contamination with non-radioactive carbon (not a common fission product and absent in nuclear reactors that don't use it as a moderator ) then fluoride volatility could be used to separate the C produced by producing carbon tetrafluoride . C is proposed for use in particularly long lived low power nuclear batteries called diamond batteries . Much of what
1974-478: A way of utilizing surplus weapons-grade plutonium, an alternative to storage of surplus plutonium, which would need to be secured against the risk of theft for use in nuclear weapons . On the other hand, some studies warned that normalizing the global commercial use of MOX fuel and the associated expansion of nuclear reprocessing would increase, rather than reduce, the risk of nuclear proliferation , by encouraging increased separation of plutonium from spent fuel in
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#17328546175502068-436: Is a gamma ray emitter, its presence is a potential occupational health hazard. It is possible, however, to remove the americium from the plutonium by a chemical separation process. Even under the worst conditions, the americium/plutonium mixture is less radioactive than a spent-fuel dissolution liquor, so it should be relatively straightforward to recover the plutonium by PUREX or another aqueous reprocessing method. It
2162-700: 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
2256-475: Is able to release xenon gas, which normally acts as a neutron absorber ( Xe is the strongest known neutron poison and is produced both directly and as a decay product of I as a fission product ) and causes structural occlusions in solid fuel elements (leading to the early replacement of solid fuel rods with over 98% of the nuclear fuel unburned, including many long-lived actinides). In contrast, molten-salt reactors are capable of retaining
2350-432: Is between 3–4% U. The control and scram system is composed of 27 reactivity control elements including 19 shimming rods, two automatic control rods, and six automatic emergency shut-down rods. On-power refueling equipment allows for charging the core with fresh fuel assemblies, repositioning and turning the fuel assemblies within the reactor, and changing control and scram system elements remotely. The unit employs
2444-489: Is commonly composed of enriched uranium sandwiched between metal cladding. Plate-type fuel is used in several research reactors where a high neutron flux is desired, for uses such as material irradiation studies or isotope production, without the high temperatures seen in ceramic, cylindrical fuel. It is currently used in the Advanced Test Reactor (ATR) at Idaho National Laboratory , and the nuclear research reactor at
2538-410: Is compacted to cylindrical pellets and sintered at high temperatures to produce ceramic nuclear fuel pellets with a high density and well defined physical properties and chemical composition. A grinding process is used to achieve a uniform cylindrical geometry with narrow tolerances. Such fuel pellets are then stacked and filled into the metallic tubes. The metal used for the tubes depends on the design of
2632-590: Is done is the ITU which is the EU centre for the study of highly radioactive materials. Materials in a high-radiation environment (such as a reactor) can undergo unique behaviors such as swelling and non-thermal creep. If there are nuclear reactions within the material (such as what happens in the fuel), the stoichiometry will also change slowly over time. These behaviors can lead to new material properties, cracking, and fission gas release. The thermal conductivity of uranium dioxide
2726-566: Is formed into pellets and inserted into Zircaloy tubes that are bundled together. The Zircaloy tubes are about 1 centimetre (0.4 in) in diameter, and the fuel cladding gap is filled with helium gas to improve heat conduction from the fuel to the cladding. There are about 179–264 fuel rods per fuel bundle and about 121 to 193 fuel bundles are loaded into a reactor core. Generally, the fuel bundles consist of fuel rods bundled 14×14 to 17×17. PWR fuel bundles are about 4 m (13 ft) long. In PWR fuel bundles, control rods are inserted through
2820-450: 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 (such as tristructural-isotropic particles). The high thermal conductivity and high melting point makes uranium carbide an attractive fuel. In addition, because of
2914-413: Is low; it is affected by porosity and burn-up. The burn-up results in fission products being dissolved in the lattice (such as lanthanides ), the precipitation of fission products such as palladium , the formation of fission gas bubbles due to fission products such as xenon and krypton and radiation damage of the lattice. The low thermal conductivity can lead to overheating of the center part of
MOX fuel - Misplaced Pages Continue
3008-455: Is possible that both americium and curium could be added to a U/Pu MOX fuel before it is loaded into a fast reactor or a subcritical reactor run in "Actinide burner mode". This is one means of transmutation. Work with curium is much harder than americium because curium is a neutron emitter, the MOX production line would need to be shielded with both lead and water to protect the workers. Also,
3102-433: Is significant – greater than 50% of the initial plutonium loading. However, during the burning of MOX the ratio of fissile (odd numbered) isotopes to non-fissile (even) drops from around 65% to 20%, depending on burn up. This makes any attempt to recover the fissile isotopes difficult and any bulk Pu recovered would require such a high fraction of Pu in any second generation MOX that it would be impractical. This means that such
3196-508: 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 UO 2 . This is then converted by heating with hydrogen to form UO 2 . It can be made from enriched uranium hexafluoride by reacting with ammonia to form
3290-529: The C 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 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
3384-630: The Beloyarsk Nuclear Power Station , in Zarechny, Sverdlovsk Oblast , Russia . It has a 600 MWe gross capacity and a 560 MWe net capacity, provided to the Middle Urals power grid . It has been in operation since 1980 and represents an improvement to the preceding BN-350 reactor . In 2014, its larger sister reactor, the BN-800 reactor , began operation. The plant is a pool type LMFBR , where
3478-556: The University of Massachusetts Lowell Radiation Laboratory . Sodium-bonded fuel consists of fuel that has liquid sodium in the gap between the fuel slug (or pellet) and the cladding. This fuel type is often used for sodium-cooled liquid metal fast reactors. It has been used in EBR-I, EBR-II, and the FFTF. The fuel slug may be metallic or ceramic. The sodium bonding is used to reduce the temperature of
3572-460: The actinides , including 92 U , fast reactors could use all of them for fuel. All actinides can undergo neutron induced fission with unmoderated or fast neutrons. A fast reactor is therefore more efficient than a thermal reactor for using plutonium and higher actinides as fuel. These fast reactors are better suited for the transmutation of other actinides than thermal reactors. Because thermal reactors use slow or moderated neutrons,
3666-404: The liquid fluoride thorium reactor (LFTR), this fuel salt is also the coolant; in other designs, such as the stable salt reactor , the fuel salt is contained in fuel pins and the coolant is a separate, non-radioactive salt. There is a further category of molten salt-cooled reactors in which the fuel is not in molten salt form, but a molten salt is used for cooling. Molten salt fuels were used in
3760-472: The spent fuel and the cycle could be repeated; however, there remains multiple difficulties in reprocessing spent MOX fuel. As of 2010, plutonium is only recycled once in thermal reactors, and spent MOX fuel is separated from the rest of the spent fuel to be stored as waste. All plutonium isotopes are either fissile or fertile, although plutonium-242 needs to absorb 3 neutrons before becoming fissile curium -245; in thermal reactors isotopic degradation limits
3854-509: The 18 to 24 month fuel exposure period. 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
MOX fuel - Misplaced Pages Continue
3948-499: The 37-pin standard bundle. It has been designed specifically to increase fuel performance by utilizing two different pin diameters. Current CANDU designs do not need enriched uranium to achieve criticality (due to the lower neutron absorption in their heavy water moderator compared to light water), however, some newer concepts call for low enrichment to help reduce the size of the reactors. The Atucha nuclear power plant in Argentina,
4042-521: The LFTR known as the Molten Salt Reactor Experiment, as well as other liquid core reactor experiments. The liquid fuel for the molten salt reactor was a mixture of lithium, beryllium, thorium and uranium fluorides: LiF-BeF 2 -ThF 4 -UF 4 (72-16-12-0.4 mol%). It had a peak operating temperature of 705 °C in the experiment, but could have operated at much higher temperatures since
4136-475: The TRISO particle more structural integrity, followed by a dense outer layer of PyC. TRISO particles are then encapsulated into cylindrical or spherical graphite pellets. TRISO fuel particles are designed not to crack due to the stresses from processes (such as differential thermal expansion or fission gas pressure) at temperatures up to 1600 °C, and therefore can contain the fuel in the worst of accident scenarios in
4230-523: The US and an additional 35 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 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, alloyed with other metals to lower its melting point and encapsulated in tantalum ,
4324-821: The United Kingdom as part of the Dragon reactor project. The inclusion of the SiC as diffusion barrier was first suggested by D. T. Livey. The first nuclear reactor to use TRISO fuels was the Dragon reactor and the first powerplant was the THTR-300 . Currently, TRISO fuel compacts are being used in some experimental reactors, such as the HTR-10 in China and the high-temperature engineering test reactor in Japan. In
4418-452: The United States, spherical fuel elements utilizing a TRISO particle with a UO 2 and UC solid solution kernel are being used in the Xe-100 , and Kairos Power is developing a 140 MWE nuclear reactor that uses TRISO. In QUADRISO particles a burnable neutron poison ( europium oxide or erbium oxide or carbide ) layer surrounds the fuel kernel of ordinary TRISO particles to better manage
4512-401: The absence of oxygen in this fuel (during the course of irradiation, excess gas pressure can build from the formation of O 2 or other gases) as well as the ability to complement a ceramic coating (a ceramic-ceramic interface has structural and chemical advantages), uranium carbide could be the ideal fuel candidate for certain Generation IV reactors such as the gas-cooled fast reactor . While
4606-494: The actinides that are not fissionable with thermal neutrons tend to absorb the neutrons instead of fissioning. This leads to buildup of heavier actinides and lowers the number of thermal neutrons available to continue the chain reaction. A subcritical reactor with an external neutron source could either be run in the fast neutron spectrum (without the need for highly enriched fuels as otherwise common in fast reactors) or use thermal neutrons to breed fissile materials, compensating
4700-515: 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 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
4794-424: The application of the new fuel-cladding material systems for various types of ATF materials. The aim of the research is to develop nuclear fuels that can tolerate loss of active cooling for a considerably longer period than the existing fuel designs and prevent or delay the release of radionuclides during an accident. This research is focused on reconsidering the design of fuel pellets and cladding, as well as
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#17328546175504888-420: The boiling point of the molten salt was in excess of 1400 °C. The aqueous homogeneous reactors (AHRs) use a solution of uranyl sulfate or other uranium salt in water. Historically, AHRs have all been small research reactors, not large power reactors. The dual fluid reactor (DFR) has a variant DFR/m which works with eutectic liquid metal alloys, e.g. U-Cr or U-Fe. Uranium dioxide (UO 2 ) powder
4982-420: The chain-reaction. This mechanism compensates for the accumulation of undesirable neutron poisons which are an unavoidable part of the fission products, as well as normal fissile fuel "burn up" or depletion. In the generalized QUADRISO fuel concept the poison can eventually be mixed with the fuel kernel or the outer pyrocarbon. The QUADRISO concept was conceived at Argonne National Laboratory . RBMK reactor fuel
5076-821: The civil nuclear fuel cycle . In every uranium-based nuclear reactor core there is both fission of uranium isotopes such as uranium-235 , and the formation of new, heavier isotopes due to neutron capture , primarily by uranium-238 . Most of the fuel mass in a reactor is uranium-238. By neutron capture and two successive beta decays , uranium-238 becomes plutonium-239 , which, by successive neutron capture, becomes plutonium-240 , plutonium-241 , plutonium-242 , and (after further beta decays) other transuranic or actinide nuclides. Plutonium-239 and plutonium-241 are fissile , like uranium-235. Small quantities of uranium-236 , neptunium-237 and plutonium-238 are formed similarly from uranium-235. Normally, with low-enriched uranium fuel being changed every five years or so, most of
5170-510: The coolant and contaminating it. Besides the prevention of radioactive leaks this also serves to keep the coolant as non-corrosive as feasible and to prevent reactions between chemically aggressive fission products and the coolant. For example, the highly reactive alkali metal caesium which reacts strongly with water, producing hydrogen, and which is among the more common fission products. Pressurized water reactor (PWR) fuel consists of cylindrical rods put into bundles. A uranium oxide ceramic
5264-424: The disadvantage of forming much radioactive dust. A mixture of uranyl nitrate and plutonium nitrate in nitric acid is converted by treatment with a base such as ammonia to form a mixture of ammonium diuranate and plutonium hydroxide. After heating in a mixture of 5% hydrogen and 95% argon will form a mixture of uranium dioxide and plutonium dioxide . Using a base , the resulting powder can be run through
5358-434: The established PUREX process is used commercially for about a third of all spent nuclear fuel (the rest being largely subject to a "once through fuel cycle"). All nitrogen-fluoride compounds are volatile or gaseous at room temperature and could be fractionally distilled from the other gaseous products (including recovered uranium hexafluoride ) to recover the initially used nitrogen. If the fuel could be processed in such
5452-491: The excess of reactivity. If the core is equipped both with TRISO and QUADRISO fuels, at beginning of life neutrons do not reach the fuel of the QUADRISO particles because they are stopped by the burnable poison. During reactor operation, neutron irradiation of the poison causes it to "burn up" or progressively transmute to non-poison isotopes, depleting this poison effect and leaving progressively more neutrons available for sustaining
5546-407: The fact that the used fuel can be cracked, it is very insoluble in water, and is able to retain the vast majority of the actinides and fission products within the uranium dioxide crystal lattice . The radiation hazard from spent nuclear fuel declines as its radioactive components decay, but remains high for many years. For example 10 years after removal from a reactor, the surface dose rate for
5640-412: The failure modes which occur during normal use (and the manner in which the fuel will behave during an accident) can be studied. In addition information is gained which enables the users of fuel to assure themselves of its quality and it also assists in the development of new fuels. After major accidents the core (or what is left of it) is normally subject to PIE to find out what happened. One site where PIE
5734-497: The first time. According to Atomic Energy of Canada Limited (AECL), CANDU reactors could use 100% MOX cores without physical modification. AECL reported to the United States National Academy of Sciences committee on plutonium disposition that it has extensive experience in testing the use of MOX fuel containing from 0.5 to 3% plutonium. The content of un-burnt plutonium in spent MOX fuel from thermal reactors
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#17328546175505828-403: The fuel being changed every three years or so, about half of the Pu is 'burned' in the reactor, providing about one third of the total energy. It behaves like 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
5922-403: The fuel is similar to PWR fuel except that the bundles are "canned". That is, there is a thin tube surrounding each bundle. This is primarily done to prevent local density variations from affecting neutronics and thermal hydraulics of the reactor core. In modern BWR fuel bundles, there are either 91, 92, or 96 fuel rods per assembly depending on the manufacturer. A range between 368 assemblies for
6016-850: 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 Xe escape instead of allowing it to capture neutrons converting it to the basically stable and chemically inert Xe , is that it will quickly decay to the highly chemically reactive, long lived radioactive Cs , which behaves similar to other alkali metals and can be taken up by organisms in their metabolism. Molten salt fuels are mixtures of actinide salts (e.g. thorium/uranium fluoride/chloride) with other salts, used in liquid form above their typical melting points of several hundred degrees C. In some molten salt-fueled reactor designs, such as
6110-408: The fuel of choice for reactor designs that NASA produces. One advantage is that uranium nitride has a better thermal conductivity than UO 2 . Uranium nitride has a very high melting point. This fuel has the disadvantage that unless N was used (in place of the more common N ), a large amount of C would be generated from the nitrogen by the (n,p) reaction . As the nitrogen needed for such
6204-406: The fuel rods, standing between the coolant and the nuclear fuel. It is made of a corrosion -resistant material with low absorption cross section for thermal neutrons , usually Zircaloy or steel in modern constructions, or magnesium with small amount of aluminium and other metals for the now-obsolete Magnox reactors . Cladding prevents radioactive fission fragments from escaping the fuel into
6298-532: The fuel. Accident tolerant fuels (ATF) are a series of new nuclear fuel concepts, researched in order to improve fuel performance under accident conditions, such as loss-of-coolant accident (LOCA) or reaction-initiated accidents (RIA). These concerns became more prominent after the Fukushima Daiichi nuclear disaster in Japan, in particular regarding light-water reactor (LWR) fuels performance under accident conditions. Neutronics analyses were performed for
6392-468: The interactions between the two. Used nuclear fuel is a complex mixture of the fission products , uranium , plutonium , and the transplutonium metals . In fuel which has been used at high temperature in power reactors it is common for the fuel to be heterogeneous ; often the fuel will contain nanoparticles of platinum group metals such as palladium . Also the fuel may well have cracked, swollen, and been heated close to its melting point. Despite
6486-465: The introduction of additional absorbers. CerMet fuel consists of ceramic fuel particles (usually uranium oxide) embedded in a metal matrix. It is hypothesized that this type of fuel is what is used in United States Navy reactors. This fuel has high heat transport characteristics and can withstand a large amount of expansion. Plate-type fuel has fallen out of favor over the years. Plate-type fuel
6580-683: The life of the core, so adding some plutonium oxide to the fuel at manufacture is not in principle a very radical step. About 30 thermal reactors in Europe (Belgium, the Netherlands, Switzerland, Germany and France) are using MOX and an additional 20 have been licensed to do so. Most reactors use it as about one third of their core, but some will accept up to 50% MOX assemblies. In France, EDF aims to have all its 900 MWe series of reactors running with at least one-third MOX. Japan aimed to have one third of its reactors using MOX by 2010, and has approved construction of
6674-475: The loss of neutrons by increasing the flux from the neutron source. The first step is separating the plutonium from the remaining uranium (about 96% of the spent fuel) and the fission products with other wastes (together about 3%) using the PUREX process. MOX fuel can be made by grinding together uranium oxide (UO 2 ) and plutonium oxide (PuO 2 ) before the mixed oxide is pressed into pellets, but this process has
6768-420: The neutron cross section of carbon is low, during years of burnup, the predominantly C will undergo neutron capture to produce stable C as well as radioactive C . Unlike the C produced by using uranium nitrate, the C 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
6862-476: The neutron irradiation of curium generates the higher actinides , such as californium , which increase the neutron dose associated with the used nuclear fuel ; this has the potential to pollute the fuel cycle with strong neutron emitters. As a result, it is likely that curium will be excluded from most MOX fuels. A subcritical reactor such as the Accelerator Driven System could "burn" such fuels if
6956-509: The nuclear accident at Fukushima Daiichi . In May 2018, the Department of Energy reported that the plant would require another $ 48 billion to complete, on top of the $ 7.6 billion already spent. Construction was cancelled. Most modern thermal reactors using high burn up uranium oxide fuel produce a quite significant proportion of their output towards the end of core life from fission of plutonium produced by neutron capture in uranium 238 earlier in
7050-447: The operating characteristics of a reactor, and the plant must be designed or adapted slightly to take it; for example, more control rods are needed. Often only a third to half of the fuel load is switched to MOX, but for more than 50% MOX loading, significant changes are necessary and a reactor needs to be designed accordingly. The System 80 reactor design deployed at the U.S. Palo Verde Nuclear Generating Station near Phoenix, Arizona
7144-495: The pellets during use. The porosity results in a decrease in both the thermal conductivity of the fuel and the swelling which occurs during use. According to the International Nuclear Safety Center the thermal conductivity of uranium dioxide can be predicted under different conditions by a series of equations. BN-600 reactor The BN-600 reactor is a sodium-cooled fast breeder reactor , built at
7238-450: The plutonium into usable fuel increases the energy derived from the original uranium by some 12%, and if the uranium-235 is also recycled by re-enrichment, this becomes about 20%. Plutonium is only reprocessed and used once as MOX fuel; spent MOX fuel, with a high proportion of minor actinides and plutonium isotopes, is stored as waste. Existing nuclear reactors must be re-licensed before MOX fuel can be introduced because using it changes
7332-455: The plutonium recycle potential. About 1% of spent nuclear fuel from current LWRs is plutonium, with approximate isotopic composition 52% 94 Pu , 24% 94 Pu , 15% 94 Pu , 6% 94 Pu and 2% 94 Pu when the fuel is first removed from the reactor. Because the fission-to-capture ratio of high energy or fast neutrons changes to favour fission for almost all of
7426-416: The plutonium-239 is "burned" in the reactor. It behaves like uranium-235, with a slightly higher cross section for fission, and its fission releases a similar amount of energy . Typically, about one percent of the spent fuel discharged from a reactor is plutonium , and some two-thirds of the plutonium is plutonium-239. Worldwide, almost 100 tonnes of plutonium in spent fuel arises each year. Reprocessing
7520-486: 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
7614-404: The problems associated with their handling and transportation are solved. However, to avoid power excursions due to unintended criticality, the neutronics must be known precisely at any given point in time, including the effect of build-up or consumption of neutron emitting nuclides as well as neutron poisons. MOX fuel containing thorium and plutonium oxides is also being tested. According to
7708-517: The reactor, coolant pumps, intermediate heat exchangers and associated piping are all located in a common liquid sodium pool. The reactor system is housed in a concrete rectilinear building and provided with filtration and gas containment. In the first 24 years of operations, there have been 12 water-into-sodium leaks in the steam generators , routinely addressed by isolating the faulty module with gate valves. These incidents did not have off-site impact, did not generate radioactive material (sodium in
7802-410: The reactor. Stainless steel was used in the past, but most reactors now use a zirconium alloy which, in addition to being highly corrosion-resistant, has low neutron absorption. The tubes containing the fuel pellets are sealed: these tubes are called fuel rods . The finished fuel rods are grouped into fuel assemblies that are used to build up the core of a power reactor. Cladding is the outer layer of
7896-570: The secondary circuit is not neutron-activated) and were not reported to IAEA, since they were deemed to have no impact on safety. As of 2022, the cumulative "energy Availability factor " recorded by the IAEA was 76.3%. The reactor core is 1.03 metres (3 ft 5 in) tall with a diameter of 2.05 metres (6 ft 9 in). It has 369 fuel assemblies , mounted vertically; each consists of 127 fuel rods enriched to between 17–26% U . In comparison, normal enrichment in other Russian reactors
7990-496: The smallest and 800 assemblies for the largest BWR in the U.S. form the reactor core. Each BWR fuel rod is backfilled with helium to a pressure of about 3 standard atmospheres (300 kPa). Canada deuterium uranium fuel (CANDU) fuel bundles are about 0.5 metres (20 in) long and 10 centimetres (4 in) in diameter. They consist of sintered (UO 2 ) pellets in zirconium alloy tubes, welded to zirconium alloy end plates. Each bundle weighs roughly 20 kilograms (44 lb), and
8084-587: The steel pressure vessels, and the two reinforced concrete designs operated at 24.8 and 27 bars (24.5 and 26.6 atm). Magnox alloy consists mainly of magnesium with small amounts of aluminium and other metals—used in cladding unenriched uranium metal fuel with a non-oxidising covering to contain fission products. This material has the advantage of a low neutron capture cross-section, but has two major disadvantages: Magnox fuel incorporated cooling fins to provide maximum heat transfer despite low operating temperatures, making it expensive to produce. While
8178-560: 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
8272-449: The top directly into the fuel bundle. The fuel bundles usually are enriched several percent in U. The uranium oxide is dried before inserting into the tubes to try to eliminate moisture in the ceramic fuel that can lead to corrosion and hydrogen embrittlement . The Zircaloy tubes are pressurized with helium to try to minimize pellet-cladding interaction which can lead to fuel rod failure over long periods. In boiling water reactors (BWR),
8366-677: The transport of German spent fuel for reprocessing in 2005. The United States was building a MOX fuel plant at the Savannah River Site in South Carolina. Although the Tennessee Valley Authority (TVA) and Duke Energy expressed interest in using MOX reactor fuel from the conversion of weapons-grade plutonium, TVA (the most likely customer) said in April 2011 that it would delay a decision until it could see how MOX fuel performed in
8460-407: The use of uranium metal rather than oxide made nuclear reprocessing more straightforward and therefore cheaper, the need to reprocess fuel a short time after removal from the reactor meant that the fission product hazard was severe. Expensive remote handling facilities were required to address this issue. Tristructural-isotropic (TRISO) fuel is a type of micro-particle fuel. A particle consists of
8554-560: 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
8648-459: Was designed for 100% MOX core compatibility, but so far has always operated on fresh low enriched uranium. In theory, the three Palo Verde reactors could use the MOX arising from seven conventionally fueled reactors each year and would no longer require fresh uranium fuel. Fast neutron BN-600 and BN-800 reactors are designed for 100% MOX loading. In 2022, the BN-800 was fully loaded with MOX fuel for
8742-400: 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. Ceramic 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
8836-548: Was used in Soviet -designed and built RBMK -type reactors. This is a low-enriched uranium oxide fuel. The fuel elements in an RBMK are 3 m long each, and two of these sit back-to-back on each fuel channel, pressure tube. Reprocessed uranium from Russian VVER reactor spent fuel is used to fabricate RBMK fuel. Following the Chernobyl accident, the enrichment of fuel was changed from 2.0% to 2.4%, to compensate for control rod modifications and
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