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54-581: A conceptual design was produced by 1997, and it was hoped to have a final design by 2005, and a prototype plant commissioning by 2010. The core consists of a graphite cylinder with a radius of 4 metres (13 ft) and a height of 10 metres (33 ft) which includes 1 metre (3 ft 3 in) axial reflectors at top and bottom. The cylinder allocates three or four concentric rings, each of 36 hexagonal blocks with an interstitial gap of 0.2 centimetres (0.079 in). Each hexagonal block contains 108 helium coolant channels and 216 fuel pins. Each fuel pin contains

108-500: A {\displaystyle \Sigma _{a}} : i.e., ξ Σ s Σ a {\displaystyle {\frac {\xi \Sigma _{s}}{\Sigma _{a}}}} . For a compound moderator composed of more than one element, such as light or heavy water, it is necessary to take into account the moderating and absorbing effect of both the hydrogen isotope and oxygen atom to calculate ξ {\displaystyle \xi } . To bring

162-414: A half-life of 10 minutes and 11 seconds . The release of neutrons from the nucleus requires exceeding the binding energy of the neutron, which is typically 7-9 MeV for most isotopes . Neutron sources generate free neutrons by a variety of nuclear reactions, including nuclear fission and nuclear fusion . Whatever the source of neutrons, they are released with energies of several MeV. According to

216-865: A neutron moderator to slow down (" thermalize ") the neutrons produced by nuclear fission . Moderation substantially increases the fission cross section for fissile nuclei such as uranium-235 or plutonium-239 . In addition, uranium-238 has a much lower capture cross section for thermal neutrons, allowing more neutrons to cause fission of fissile nuclei and propagate the chain reaction, rather than being captured by U. The combination of these effects allows light water reactors to use low-enriched uranium . Heavy water reactors and graphite-moderated reactors can even use natural uranium as these moderators have much lower neutron capture cross sections than light water. An increase in fuel temperature also raises uranium-238's thermal neutron absorption by Doppler broadening , providing negative feedback to help control

270-591: A nuclear chain reaction of uranium-235 or other fissile isotope by colliding with their atomic nucleus . Water (sometimes called "light water" in this context) is the most commonly used moderator (roughly 75% of the world's reactors). Solid graphite (20% of reactors) and heavy water (5% of reactors) are the main alternatives. Beryllium has also been used in some experimental types, and hydrocarbons have been suggested as another possibility. Neutrons are normally bound into an atomic nucleus and do not exist free for long in nature. The unbound neutron has

324-443: A "hydride" primary, the degree of compression would not make deuterium to fuse, but the design could be subjected to boosting, raising the yield considerably. The cores consisted of a mix of uranium deuteride (UD 3 ), and deuterated polyethylene. The core tested in Ray used uranium low enriched in U , and in both shots deuterium acted as the neutron moderator. The predicted yield

378-550: A challenging one. In August 1945, when information of the atomic bombing of Hiroshima was relayed to the scientists of the German nuclear program who were interred at Farm Hall in England, chief scientist Werner Heisenberg hypothesized that the device must have been "something like a nuclear reactor, with the neutrons slowed by many collisions with a moderator". The German program, which had been much less advanced, had never even considered

432-431: A far higher Σ a {\displaystyle \Sigma _{a}} , so that the moderating efficiency is nearly 80 times higher for heavy water than for light water. The ideal moderator is of low mass, high scattering cross section, and low absorption cross section . After sufficient impacts, the speed of the neutron will be comparable to the speed of the nuclei given by thermal motion; this neutron

486-640: A handful of reactors built in the decades since the Chernobyl accident due to low prices in the uranium market , although there is now a revival with several Asian countries planning to complete larger prototype fast reactors in the next few years. Neutron moderator In nuclear engineering , a neutron moderator is a medium that reduces the speed of fast neutrons , ideally without capturing any, leaving them as thermal neutrons with only minimal (thermal) kinetic energy . These thermal neutrons are immensely more susceptible than fast neutrons to propagate

540-407: A heavier, often unstable isotope of the chemical element as a result. This event is called neutron activation . Fast neutrons are produced by nuclear processes: Fast neutrons are usually undesirable in a steady-state nuclear reactor because most fissile fuel has a higher reaction rate with thermal neutrons. Fast neutrons can be rapidly changed into thermal neutrons via

594-406: A higher degree of uranium enrichment in their fuel. Good moderators are free of neutron-absorbing impurities such as boron . In commercial nuclear power plants the moderator typically contains dissolved boron. The boron concentration of the reactor coolant can be changed by the operators by adding boric acid or by diluting with water to manipulate reactor power. The Nazi Nuclear Program suffered

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648-553: A large amount of fissile material moderated by a neutron moderator, similar in structure to a nuclear reactor or "pile". Only the Manhattan Project embraced the idea of a chain reaction of fast neutrons in pure metallic uranium or plutonium. Other moderated designs were also considered by the Americans; proposals included using uranium deuteride as the fissile material. In 1943 Robert Oppenheimer and Niels Bohr considered

702-425: A larger number of neutrons, so a fast breeder reactor can potentially "breed" more fissile fuel than it consumes. Fast reactor control cannot depend solely on Doppler broadening or on negative void coefficient from a moderator. However, thermal expansion of the fuel itself can provide quick negative feedback. Perennially expected to be the wave of the future, fast reactor development has been nearly dormant with only

756-560: A light water reactor where adding water to the core in an accident might provide enough moderation to make a subcritical assembly go critical again, heavy water reactors will decrease their reactivity if light water is added to the core, which provides another important safety feature in the case of certain accident scenarios. However, any heavy water that becomes mixed with the emergency coolant light water will become too diluted to be useful without isotope separation. Early speculation about nuclear weapons assumed that an "atom bomb" would be

810-415: A material called a moderator . The probability of scattering of a neutron from a nucleus is given by the scattering cross section . The first few collisions with the moderator may be of sufficiently high energy to excite the nucleus of the moderator. Such a collision is inelastic , since some of the kinetic energy is transformed to potential energy by exciting some of the internal degrees of freedom of

864-476: A neutron from the fission energy of E 0 {\displaystyle E_{0}} 2 MeV to an E {\displaystyle E} of 1 eV takes an expected n {\displaystyle n} of 16 and 29 collisions for H 2 O and D 2 O, respectively. Therefore, neutrons are more rapidly moderated by light water, as H has a far higher Σ s {\displaystyle \Sigma _{s}} . However, it also has

918-567: A new reactor that utilizes the power conversion features of the GT-MHR, the Energy Multiplier Module (EM2). The EM2 uses fast neutrons and is a gas-cooled fast reactor , enabling it to reduce nuclear waste considerably by transmutation . Thermal neutron The neutron detection temperature , also called the neutron energy , indicates a free neutron 's kinetic energy , usually given in electron volts . The term temperature

972-521: A process called moderation. This is done through numerous collisions with (in general) slower-moving and thus lower-temperature particles like atomic nuclei and other neutrons. These collisions will generally speed up the other particle and slow down the neutron and scatter it. Ideally, a room temperature neutron moderator is used for this process. In reactors, heavy water , light water , or graphite are typically used to moderate neutrons. Most fission reactors are thermal-neutron reactors that use

1026-407: A random lattice of TRISO particles dispersed into a graphite matrix. The reactor exhibits a thermal spectrum with a peak neutron energy located at about 0.2 eV . The TRISO fuel concept allows the reactor to be inherently safe. The reactor and containment structure is located below grade and in contact with the ground, which serves as a passive safety measure to conduct heat away from the reactor in

1080-415: A safety feature. A large tank of low-temperature, low-pressure heavy water moderates the neutrons and also acts as a heat sink in extreme loss-of-coolant accident conditions. It is separated from the fuel rods that actually generate the heat. Heavy water is very effective at slowing down (moderating) neutrons, giving CANDU reactors their important and defining characteristic of high " neutron economy ". Unlike

1134-590: A substantial setback when its inexpensive graphite moderators failed to function. At that time, most graphites were deposited onto boron electrodes, and the German commercial graphite contained too much boron. Since the war-time German program never discovered this problem, they were forced to use far more expensive heavy water moderators. This problem was discovered by physicist Leó Szilárd . Some moderators are quite expensive, for example beryllium , and reactor-grade heavy water. Reactor-grade heavy water must be 99.75% pure to enable reactions with unenriched uranium. This

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1188-468: Is by choosing a moderating nucleus that has near identical mass. A collision of a neutron which has mass of 1 with a H nucleus (a proton ) could result in the neutron losing virtually all of its energy in a single head-on collision. More generally, it is necessary to take into account both glancing and head-on collisions. The mean logarithmic reduction of neutron energy per collision , ξ {\displaystyle \xi } , depends only on

1242-555: Is determined by the fission cross section, which is dependent upon the speed (energy) of the incident neutrons. For thermal reactors, high-energy neutrons in the MeV-range are much less likely (though not unable) to cause further fission. The newly released fast neutrons, moving at roughly 10% of the speed of light , must be slowed down or "moderated", typically to speeds of a few kilometres per second, if they are to be likely to cause further fission in neighbouring U nuclei and hence continue

1296-715: Is difficult to prepare because heavy water and regular water form the same chemical bonds in almost the same ways, at only slightly different speeds . The much cheaper light water moderator (essentially very pure regular water) absorbs too many neutrons to be used with unenriched natural uranium, and therefore uranium enrichment or nuclear reprocessing becomes necessary to operate such reactors, increasing overall costs. Both enrichment and reprocessing are expensive and technologically challenging processes, and additionally both enrichment and several types of reprocessing can be used to create weapons-usable material, causing proliferation concerns. The CANDU reactor's moderator doubles as

1350-559: Is not fissile at all with thermal neutrons. Moderators are also used in non-reactor neutron sources , such as plutonium - beryllium (using the Be ( α ,n) C reaction) and spallation sources (using ( p ,xn) reactions with neutron rich heavy elements as targets). The form and location of the moderator can greatly influence the cost and safety of a reactor. Classically, moderators were precision-machined blocks of high-purity graphite with embedded ducting to carry away heat. They were in

1404-483: Is operated above the Wigner annealing temperature so that the graphite does not accumulate dangerous amounts of Wigner energy. In CANDU and PWR reactors, the moderator is liquid water (heavy water for CANDU, light water for PWR). In the event of a loss-of-coolant accident in a PWR, the moderator is also lost and the reaction will stop. This negative void coefficient is an important safety feature of these reactors. In CANDU

1458-508: Is that the time between subsequent neutron generations is increased, slowing down the reaction. This makes the containment of the explosion a problem; the inertia that is used to confine implosion type bombs will not be able to confine the reaction. The result may be a fizzle. The explosive power of a fully moderated explosion is thus limited; at worst it may be equal to a chemical explosive of similar mass. According to Heisenberg: "One can never make an explosive with slow neutrons, not even with

1512-423: Is the uranium-233 of the thorium cycle , which has a good fission/capture ratio at all neutron energies. Fast-neutron reactors use unmoderated fast neutrons to sustain the reaction, and require the fuel to contain a higher concentration of fissile material relative to fertile material (uranium-238). However, fast neutrons have a better fission/capture ratio for many nuclides, and each fast fission releases

1566-503: Is the average squared neutron speed, and k B {\displaystyle k_{B}} is the Boltzmann constant . The characteristic neutron temperature of several-MeV neutrons is several tens of billions kelvin . Moderation is the process of the reduction of the initial high speed (high kinetic energy) of the free neutron. Since energy is conserved, this reduction of the neutron speed takes place by transfer of energy to

1620-532: Is then called a thermal neutron , and the process may also be termed thermalization . Once at equilibrium at a given temperature the distribution of speeds (energies) expected of rigid spheres scattering elastically is given by the Maxwell–Boltzmann distribution . This is only slightly modified in a real moderator due to the speed (energy) dependence of the absorption cross-section of most materials, so that low-speed neutrons are preferentially absorbed, so that

1674-491: Is used, since hot, thermal and cold neutrons are moderated in a medium with a certain temperature. The neutron energy distribution is then adapted to the Maxwell distribution known for thermal motion. Qualitatively, the higher the temperature, the higher the kinetic energy of the free neutrons. The momentum and wavelength of the neutron are related through the de Broglie relation . The long wavelength of slow neutrons allows for

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1728-488: The Boudouard reaction needs to be taken into account. This is also the case if fuel elements have an outer layer of carbon—as in some TRISO fuels—or if an inner carbon layer becomes exposed by failure of one or several outer layers. In pebble-bed reactors , the nuclear fuel is embedded in spheres of reactor-grade pyrolytic carbon , roughly of the size of pebbles . The spaces between the spheres serve as ducting. The reactor

1782-413: The equipartition theorem , the average kinetic energy , E ¯ {\displaystyle {\bar {E}}} , can be related to temperature , T {\displaystyle T} , via: where m n {\displaystyle m_{n}} is the neutron mass, ⟨ v 2 ⟩ {\displaystyle \langle v^{2}\rangle }

1836-406: The flux . Therefore, a further criterion for an efficient moderator is one for which this parameter is small. The moderating efficiency gives the ratio of the macroscopic cross sections of scattering, Σ s {\displaystyle \Sigma _{s}} , weighted by ξ {\displaystyle \xi } divided by that of absorption, Σ

1890-507: The mode of the Maxwell–Boltzmann distribution for this temperature, E peak = k T. After a number of collisions with nuclei ( scattering ) in a medium ( neutron moderator ) at this temperature, those neutrons which are not absorbed reach about this energy level. Thermal neutrons have a different and sometimes much larger effective neutron absorption cross-section for a given nuclide than fast neutrons, and can therefore often be absorbed more easily by an atomic nucleus , creating

1944-518: The atomic mass, A {\displaystyle A} , of the nucleus and is given by: ξ = ln ⁡ E 0 E = 1 − ( A − 1 ) 2 2 A ln ⁡ ( A + 1 A − 1 ) {\displaystyle \xi =\ln {\frac {E_{0}}{E}}=1-{\frac {(A-1)^{2}}{2A}}\ln \left({\frac {A+1}{A-1}}\right)} . This can be reasonably approximated to

1998-422: The chain reaction. This speed occurs at temperatures in the few hundred Celsius range. In all moderated reactors, some neutrons of all energy levels will produce fission, including fast neutrons. Some reactors are more fully thermalised than others; for example, in a CANDU reactor nearly all fission reactions are produced by thermal neutrons, while in a pressurized water reactor (PWR) a considerable portion of

2052-526: The event of a coolant failure. The Gas Turbine Modular Helium Reactor utilizes the Brayton cycle turbine arrangement, which gives it an efficiency of up to 48% – higher than any other reactor, as of 1995. Commercial light water reactors (LWRs) generally use the Rankine cycle , which is what coal-fired power plants use. Commercial LWRs average 32% efficiency, again as of 1995. In 2010 General Atomics conceptualized

2106-463: The fissions are produced by higher-energy neutrons. In the proposed water-cooled supercritical water reactor , the proportion of fast fissions may exceed 50%, making it technically a fast-neutron reactor . A fast reactor uses no moderator but relies on fission produced by unmoderated fast neutrons to sustain the chain reaction. In some fast reactor designs, up to 20% of fissions can come from direct fast neutron fission of uranium-238 , an isotope which

2160-477: The heavy water machine, as then the neutrons only go with thermal speed, with the result that the reaction is so slow that the thing explodes sooner, before the reaction is complete." While a nuclear bomb working on thermal neutrons may be impractical, modern weapons designs may still benefit from some level of moderation. A beryllium tamper used as a neutron reflector will act as a moderator. Other light-nuclei materials are unsuitable for various reasons. Helium

2214-550: The heavy water will increase reactivity until so much is removed that too little moderation is provided to keep the reaction going. This design gives CANDU reactors a positive void coefficient, although the slower neutron kinetics of heavy-water moderated systems compensates for this, leading to comparable safety with PWRs. In the light-water-cooled, graphite-moderated RBMK , a reactor type originally envisioned to allow both production of weapons grade plutonium and large amounts of usable heat while using natural uranium and foregoing

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2268-608: The hottest part of the reactor and therefore subject to corrosion and ablation . In some materials, including graphite, the impact of the neutrons with the moderator can cause the moderator to accumulate dangerous amounts of Wigner energy . This problem led to the infamous Windscale fire at the Windscale Piles, a nuclear reactor complex in the United Kingdom, in 1957. In a carbon dioxide cooled graphite moderated reactor where coolant and moderator are in contact with one another,

2322-435: The kinetic energy of the recoiling fission products. The free neutrons are emitted with a kinetic energy of ~2 MeV each. Because more free neutrons are released from a uranium fission event than thermal neutrons are required to initiate the event, the reaction can become a self-sustaining nuclear chain reaction under controlled conditions, thus liberating a tremendous amount of energy. The probability of further fission events

2376-411: The large cross section. But different ranges with different names are observed in other sources. The following is a detailed classification: A thermal neutron is a free neutron with a kinetic energy of about 0.025 eV (about 4.0×10 J or 2.4 MJ/kg, hence a speed of 2.19 km/s), which is the energy corresponding to the most probable speed at a temperature of 290 K (17 °C or 62 °F),

2430-466: The moderator is located in a separate heavy-water circuit, surrounding the pressurized heavy-water coolant channels. The heavy water will slow down a significant portion of neutrons to the resonance integral of U increasing the neutron capture in this isotope that makes up over 99% of the uranium in CANDU fuel thus decreasing the amount of neutrons available for fission. As a consequence, removing some of

2484-405: The nucleus to form an excited state . As the energy of the neutron is lowered, the collisions become predominantly elastic , i.e., the total kinetic energy and momentum of the system (that of the neutron and the nucleus) is conserved. Given the mathematics of elastic collisions , as neutrons are very light compared to most nuclei, the most efficient way of removing kinetic energy from the neutron

2538-573: The plutonium option and did not discover a feasible method of large scale isotope separation in uranium. After the success of the Manhattan Project, all major nuclear weapons programs have relied on fast neutrons in their weapons designs. The notable exception is the Ruth and Ray test explosions of Operation Upshot–Knothole . The aim of the University of California Radiation Laboratory (UCRL) designs

2592-404: The possibility of using a "pile" as a weapon. The motivation was that with a graphite moderator it would be possible to achieve the chain reaction without the use of any isotope separation. However, plutonium can be produced (" bred ") sufficiently isotopically pure as to be usable in a bomb and then has to be "only" separated chemically, a much easier processes than isotope separation, albeit still

2646-467: The reactor. When the coolant is a liquid that also contributes to moderation and absorption (light water or heavy water), boiling of the coolant will reduce the moderator density, which can provide positive or negative feedback (a positive or negative void coefficient ), depending on whether the reactor is under- or over-moderated. Intermediate-energy neutrons have poorer fission/capture ratios than either fast or thermal neutrons for most fuels. An exception

2700-448: The true neutron velocity distribution in the core would be slightly hotter than predicted. In a thermal-neutron reactor , the nucleus of a heavy fuel element such as uranium absorbs a slow-moving free neutron, becomes unstable, and then splits into two smaller atoms ( fission products ). The fission process for U nuclei yields two fission products, two to three fast-moving free neutrons, plus an amount of energy primarily manifested in

2754-415: The use of heavy water, the light water coolant acts primarily as a neutron absorber and thus its removal in a loss-of-coolant accident or by conversion of water into steam will increase the amount of thermal neutrons available for fission. Following the Chernobyl nuclear accident the issue was remedied so that all still operating RBMK type reactors have a slightly negative void coefficient, but they require

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2808-605: The very simple form ξ ≃ 2 A + 2 / 3 {\displaystyle \xi \simeq {\frac {2}{A+2/3}}} . From this one can deduce n {\displaystyle n} , the expected number of collisions of the neutron with nuclei of a given type that is required to reduce the kinetic energy of a neutron from E 0 {\displaystyle E_{0}} to E 1 {\displaystyle E_{1}} Some nuclei have larger absorption cross sections than others, which removes free neutrons from

2862-400: Was 1.5 to 3 kt for Ruth (with a maximum potential yield of 20 kt ) and 0.5-1 kt for Ray . The tests produced yields of 200 tons of TNT each; both tests were considered to be fizzles . A side effect of using a moderator in a nuclear explosive is that as the chain reaction progresses, the moderator will be heated, thus losing its ability to cool the neutrons. Another effect of moderation

2916-412: Was the exploration of deuterated polyethylene charge containing uranium as a candidate thermonuclear fuel, hoping that deuterium would fuse (becoming an active medium) if compressed appropriately. If successful, the devices could also lead to a compact primary containing minimal amount of fissile material, and powerful enough to ignite RAMROD a thermonuclear weapon designed by UCRL at the time. For

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