In nuclear engineering, a neutron moderator is a medium which reduces the velocity of fast neutrons, thereby turning them into thermal neutrons capable of sustaining a nuclear chain reaction. Commonly used moderators include deuterium (as heavy water), hydrogen (as ordinary or light water) and graphite. Beryllium has also been used in some experimental types, and hydrocarbons have been suggested as another possibility. Nuclear engineering is the practical application of the atomic nucleus gleaned from principles of nuclear physics and the interaction and maintenance of nuclear fission systems and components, specifically, nuclear reactors, nuclear power plants and/or nuclear weapons. ... A fast neutron is a free neutron with a kinetic energy level close to 1 MeV (10 TJ/kg, hence a speed of 14,000 km/s. ... This article does not cite its references or sources. ... A schematic nuclear fission chain reaction. ... Deuterium, also called heavy hydrogen, is a stable isotope of hydrogen with a natural abundance in the oceans of Earth of approximately one atom in 6500 of hydrogen (~154 PPM). ... Heavy water is dideuterium oxide, or D2O or 2H2O. It is chemically the same as normal water, H2O, but the hydrogen atoms are of the heavy isotope deuterium, in which the nucleus contains a neutron in addition to the proton found in the nucleus of any hydrogen atom. ... Graphite (named by Abraham Gottlob Werner in 1789 from the Greek γραφειν (graphein): to draw/write, for its use in pencils) is one of the allotropes of carbon. ... General Name, Symbol, Number beryllium, Be, 4 Chemical series alkaline earth metals Group, Period, Block 2, 2, s Appearance white-gray metallic Standard atomic weight 9. ... Oil refineries are key to obtaining hydrocarbons; crude oil is processed through several stages to form desirable hydrocarbons, used in fuel and other commercial products. ...

Explanation

In a thermal nuclear reactor, the nucleus of a heavy fuel element such as uranium absorbs a slow-moving free neutron, becomes unstable, and then splits ("fissions") into two smaller atoms ("fission products"). The fission process for uranium atoms yields two fission products, two to three fast-moving free neutrons, plus an amount of energy primarily manifested in the kinetic energy of the recoiling fission products. Because more free neutrons are released from a uranium fission event than are required to initiate the event, the reaction can become self sustaining — a chain reaction — under controlled conditions, thus liberating a tremendous amount of energy. However, the probability of further fission events occurring is dependent upon the speed (energy) of the incident neutrons. Faster neutrons are much less likely to cause further fission. (Note: It is not impossible for fast neutrons to cause fission, just much less likely.) 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 kilometers per second, if they are to be likely to cause further fission in neighbouring uranium nuclei and hence continue the chain reaction. A thermal reactor is the most common category of nuclear reactor. ... General Name, Symbol, Number uranium, U, 92 Chemical series actinides Group, Period, Block n/a, 7, f Appearance silvery gray metallic; corrodes to a spalling black oxide coat in air Standard atomic weight 238. ... This article does not cite its references or sources. ... For the generation of electrical power by fission, see Nuclear power plant An induced nuclear fission event. ... Fission products are the residues of fission processes. ... A fast neutron is a free neutron with a kinetic energy level close to 1 MeV (10 TJ/kg, hence a speed of 14,000 km/s. ... A free neutron is a neutron that exists outside of an atomic nucleus. ... A chain reaction is a sequence of reactions where a reactive product or by-product causes additional reactions. ... A line showing the speed of light on a scale model of Earth and the Moon The speed of light in a vacuum is an important physical constant denoted by the letter c for constant or the Latin word celeritas meaning swiftness.[1] It is the speed of all electromagnetic...

A good neutron moderator is a material full of atoms with light nuclei which do not easily absorb neutrons. The neutrons strike the nuclei and bounce off. In this process, some energy is transferred between the nucleus and the neutron. More energy is transferred per collision if the nucleus is lighter; see elastic collision. After sufficiently many such impacts, the velocity of the neutron will be comparable to the thermal velocities of the nuclei; this neutron is then called a thermal neutron. For other uses, see Collision (disambiguation). ... An elastic collision is a collision in which the total kinetic energy of the colliding bodies after collision is equal to their total kinetic energy before collision. ... This article does not cite its references or sources. ...

A fast reactor uses no moderator, but relies on fission produced by unmoderated fast neutrons to sustain the chain reaction. A fast neutron reactor or simply a fast reactor is a category of nuclear reactor in which the fission chain reaction is sustained by fast neutrons. ...

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 nearly all fissions are produced by thermal neutrons, while in a PWR a considerable portion of the fissions are produced by higher-energy neutrons. In the proposed water-cooled SCWR, the proportion of fast fissions may exceed 50%, making it technically a fast neutron reactor. Supercritical water reactor scheme. ... Shevchenko BN350 nuclear fast reactor and desalination plant situated on the shore of the Caspian Sea. ...

Form and location

The form and location of the moderator can greatly influence the cost and safety of a reactor. Classically, moderators were precision-machined blocks with embedded ducting to carry away heat. Also, they were in the hottest part of the reactor, and therefore subject to corrosion and ablation. In some materials, notably graphite, the impact of the neutrons with the moderator can cause the moderator to accumulate dangerous amounts of Wigner energy. At Windscale, this problem led to the infamous Windscale fire. See corrosive for the hazard. ... Ablation is defined as the removal of material from the surface of an object by vaporization, chipping, or other erosive processes. ... Graphite (named by Abraham Gottlob Werner in 1789 from the Greek γραφειν (graphein): to draw/write, for its use in pencils) is one of the allotropes of carbon. ... Wigner energy is created inside nuclear reactors that use graphite, a form of carbon, as neutron moderator. ... On October 10, 1957, the graphite core of a British nuclear reactor at Windscale, Cumbria, caught fire, releasing substantial amounts of radioactive contamination into the surrounding area. ...

Some pebble-bed reactor's moderators are not only simple, but also inexpensive: the nuclear fuel is embedded in spheres of reactor-grade pyrolytic carbon, roughly of the size of tennis balls. The spaces between the balls serve as ducting. The reactor is operated above the Wigner annealing temperature so that the graphite does not accumulate dangerous amounts of Wigner energy. The Pebble Bed Reactor is an advanced nuclear reactor design. ... Pyrolytic carbon is a material similar to graphite, but with some covalent bonding between its graphene sheets. ... A Penn tennis ball. ... Wigner energy is created inside nuclear reactors that use graphite, a form of carbon, as neutron moderator. ...

Moderator impurities

Good moderators are also 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 is then changed by the operators by adding boric acid or by diluting with water to manipulate reactor power. The German World-War II nuclear program suffered a substantial setback when its inexpensive graphite moderators failed to work. At that time, most graphites were deposited on 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. In the U.S., Leo Szilard, a former chemical engineer, discovered the problem. General Name, Symbol, Number boron, B, 5 Chemical series metalloids Group, Period, Block 13, 2, p Appearance black/brown Standard atomic weight 10. ... Heavy water is dideuterium oxide, or D2O or 2H2O. It is chemically the same as normal water, H2O, but the hydrogen atoms are of the heavy isotope deuterium, in which the nucleus contains a neutron in addition to the proton found in the nucleus of any hydrogen atom. ... Leó Szilárd (right) working with Albert Einstein. ...

Non graphite moderators

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 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. General Name, Symbol, Number beryllium, Be, 4 Chemical series alkaline earth metals Group, Period, Block 2, 2, s Appearance white-gray metallic Standard atomic weight 9. ... A chemical bond is the physical process responsible for the attractive interactions between atoms and molecules, and that which confers stability to diatomic and polyatomic chemical compounds. ...

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. More proliferation resistant reprocessing schemes are under development however. Enriched uranium is uranium whose uranium-235 content has been increased through the process of isotope separation. ... // Nuclear reprocessing separates any usable elements (e. ...

The CANDU reactor's moderator doubles as 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". The CANDU reactor is a pressurized-heavy water, natural-uranium power reactor designed in the 1960s by a partnership between Atomic Energy of Canada Limited and the Hydro-Electric Power Commission of Ontario as well as several private industry participants. ... A Loss of Coolant Accident (LOCA) is a mode of failure for a nuclear reactor; in a nuclear reactor, the results of a LOCA could be catastrophic to the reactor, the facility that houses it, and the immediate vicinity around the reactor. ...

Materials used

• Hydrogen, as an ordinary water ("light water"), in light water reactors. The reactors require enriched uranium to operate.
• Deuterium, in the form of heavy water, in heavy water reactors, eg. CANDU. Reactors moderated with heavy water can use unenriched natural uranium.
• Carbon, in the form of reactor-grade graphite or pyrolytic carbon, used in eg. RBMK and pebble-bed reactors. Lower-temperature reactors are susceptible to buildup of Wigner energy in the material. Like deuterium-moderated reactors, some of these reactors can use unenriched natural uranium.
• Beryllium, in the form of metal. Beryllium is expensive and toxic, so its use is limited.

Other light-nuclei materials are unsuitable for various reasons. Helium is a gas and is not possible to achieve its sufficient density, lithium and boron absorb neutrons. General Name, Symbol, Number hydrogen, H, 1 Chemical series nonmetals Group, Period, Block 1, 1, s Appearance colorless Atomic mass 1. ... Light water, in the terminology of nuclear reactors, is ordinary water. ... A light water reactor or LWR is a thermal nuclear reactor that uses ordinary water, also called light water, as its neutron moderator. ... These pie-graphs showing the relative proportions of uranium-238 (blue) and uranium-235 (red) at different levels of enrichment. ... Deuterium, also called heavy hydrogen, is a stable isotope of hydrogen with a natural abundance in the oceans of Earth of approximately one atom in 6500 of hydrogen (~154 PPM). ... Heavy water is dideuterium oxide, or D2O or 2H2O. It is chemically the same as normal water, H2O, but the hydrogen atoms are of the heavy isotope deuterium, in which the nucleus contains a neutron in addition to the proton found in the nucleus of any hydrogen atom. ... Heavy water reactors use heavy water as a neutron moderator. ... The CANDU reactor is a pressurized-heavy water, natural-uranium power reactor designed in the 1960s by a partnership between Atomic Energy of Canada Limited and the Hydro-Electric Power Commission of Ontario as well as several private industry participants. ... Natural uranium (NU) refers to refined uranium with the same isotopic ratios as found in nature. ... General Name, Symbol, Number carbon, C, 6 Chemical series nonmetals Group, Period, Block 14, 2, p Appearance black (graphite) colorless (diamond) Standard atomic weight 12. ... Graphite (named by Abraham Gottlob Werner in 1789 from the Greek γραφειν (graphein): to draw/write, for its use in pencils) is one of the allotropes of carbon. ... Pyrolytic carbon is a material similar to graphite, but with some covalent bonding between its graphene sheets. ... RBMK is an acronym for the Russian reaktor bolshoy moshchnosti kanalniy (Russian: Реактор Большой Мощности Канальный) which means reactor (of) high power (of the) channel (type), and describes a now obsolete class of graphite-moderated nuclear power reactor which was built only in the Soviet Union. ... The Pebble Bed Reactor is an advanced nuclear reactor design. ... Wigner energy is created inside nuclear reactors that use graphite, a form of carbon, as neutron moderator. ... General Name, Symbol, Number beryllium, Be, 4 Chemical series alkaline earth metals Group, Period, Block 2, 2, s Appearance white-gray metallic Standard atomic weight 9. ... General Name, Symbol, Number helium, He, 2 Chemical series noble gases Group, Period, Block 18, 1, s Appearance colorless Standard atomic weight 4. ... General Name, Symbol, Number lithium, Li, 3 Chemical series alkali metals Group, Period, Block 1, 2, s Appearance silvery white/grey Standard atomic weight 6. ... General Name, Symbol, Number boron, B, 5 Chemical series metalloids Group, Period, Block 13, 2, p Appearance black/brown Standard atomic weight 10. ...

See also

Nuclear cross section The nuclear cross section of a nucleus is used to characterize the probability that a nuclear reaction will occur. ...