Neutron moderator - Citizendia

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 involving uranium-235. Nuclear engineering is the application of the breakdown of atomic nuclei and/or other sub-atomic physics based on the principles of Nuclear physics. The neutron temperature, also called the neutron energy, indicates a free neutron's Kinetic energy, usually given in Electron volts The term The neutron temperature, also called the neutron energy, indicates a free neutron's Kinetic energy, usually given in Electron volts The term A nuclear chain reaction occurs when one Nuclear reaction causes an average of one or more nuclear reactions thus leading to a self-propagating number of these reactions

Commonly used moderators include regular (light) water (75% of the world's reactors), solid graphite (20% of reactors) and heavy water (5% of reactors). The Mineral graphite, as with Diamond and Fullerene, is one of the Allotropes of carbon. Heavy water is water which contains a higher proportion than normal of the Isotope Deuterium, as deuterium oxide, D2O or ²H2O ^[1] Beryllium has also been used in some experimental types, and hydrocarbons have been suggested as another possibility. Beryllium (bəˈrɪliəm is a Chemical element with the symbol Be and Atomic number 4 In Organic chemistry, a hydrocarbon is an Organic compound consisting entirely of Hydrogen and Carbon.

1 Explanation
2 Form and location
3 Moderator impurities
4 Non graphite moderators
5 Materials used
6 References
7 See also

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"). A thermal reactor has moderating materials to reduce the speed of Neutrons to low velocity Thermal neutrons so that Uranium-235 will be more likely Uranium (jʊˈreɪniəm is a silvery-gray Metallic Chemical element in the The neutron temperature, also called the neutron energy, indicates a free neutron's Kinetic energy, usually given in Electron volts The term Nuclear fission is the splitting of the nucleus of an atom into parts (lighter nuclei) often producing Free neutrons and other smaller nuclei which may Fission products are the atomic fragments left after a large nucleus fissions. 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. The neutron temperature, also called the neutron energy, indicates a free neutron's Kinetic energy, usually given in Electron volts The term In Physics and other Sciences energy (from the Greek grc ἐνέργεια - Energeia, "activity operation" from grc ἐνεργός 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. 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 to take place 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 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. A collision is an isolated event in which two or more bodies (colliding bodies exert relatively strong forces on each other for a relatively short time 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 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. The neutron temperature, also called the neutron energy, indicates a free neutron's Kinetic energy, usually given in Electron volts The term

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 reactor nearly all fission reactions are produced by thermal neutrons, while in a PWR a considerable portion of the fissions are produced by higher-energy neutrons. The CANDU reactor is a Canadian-invented Pressurized heavy water reactor developed initially in the late 1950s and 1960s by a partnership between Atomic Energy of Pressurized water reactor ( PWR s (also VVER if of Russian design are generation II nuclear power reactors that use ordinary Water In the proposed water-cooled SCWR, the proportion of fast fissions may exceed 50%, making it technically a fast neutron reactor. The Supercritical water reactor (SCWR is a Generation IV reactor concept that uses supercritical water as the working fluid 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

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. Corrosion means the breaking down of essential properties in a material due to Chemical reactions with its surroundings Ablation is defined as the removal of material from the surface of an object by Vaporization, Chipping, or other erosive processes In some materials, notably graphite, the impact of the neutrons with the moderator can cause the moderator to accumulate dangerous amounts of Wigner energy. The Mineral graphite, as with Diamond and Fullerene, is one of the Allotropes of carbon. The Wigner effect (named for its discoverer E P Wigner) also known as the discomposition effect, is the displacement of Atoms in a solid caused by At Windscale, this problem led to the infamous Windscale fire. On October 10, 1957, the Graphite core of a British Nuclear reactor at Windscale, Cumbria, caught fire releasing substantial amounts

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 pebble bed reactor ( PBR) is a graphite- moderated, gas-cooled Nuclear reactor. Pyrolytic carbon is a Material similar to Graphite, but with some Covalent bonding between its Graphene sheets as a result of imperfections A tennis ball is a ball designed for the sport of Tennis, approximately 2 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 Wigner effect (named for its discoverer E P Wigner) also known as the discomposition effect, is the displacement of Atoms in a solid caused by

Moderator impurities

Good moderators are also free of neutron-absorbing impurities such as boron. Boron (ˈbɔərɒn is a Chemical element with Atomic number 5 and the chemical symbol B. 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 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. Heavy water is water which contains a higher proportion than normal of the Isotope Deuterium, as deuterium oxide, D2O or ²H2O In the U. S. , Leo Szilard, a former chemical engineer, discovered the problem. Leó Szilárd (Szilárd Leó February 11, 1898 – May 30, 1964) was an Hungarian - American Physicist who

Non graphite moderators

Some moderators are quite expensive, for example beryllium, and reactor grade heavy water. Beryllium (bəˈrɪliəm is a Chemical element with the symbol Be and Atomic number 4 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. A chemical bond is the physical process responsible for the attractive interactions between Atoms and Molecules and which confers stability to diatomic and polyatomic

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. Enriched uranium is a kind of Uranium in which the percent composition of Uranium-235 has been increased through the process of Isotope separation. Nuclear reprocessing separates components of Spent nuclear fuel such as Reprocessed uranium Plutonium Minor 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. Reprocessing schemes that are more resistant to proliferation are currently under development.

The CANDU reactor's moderator doubles as a safety feature. The CANDU reactor is a Canadian-invented Pressurized heavy water reactor developed initially in the late 1950s and 1960s by a partnership between Atomic Energy of 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. A loss-of-coolant accident (LOCA is a mode of failure for a Nuclear reactor; if not managed effectively the results of a LOCA could result in reactor core damage 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".

Materials used

Hydrogen, as an ordinary water ("light water"), in light water reactors. Hydrogen (ˈhaɪdrədʒən is the Chemical element with Atomic number 1 Water ( H2[[oxygen O]] H OH) is the most abundant Molecule on Earth 's surface composing of about 70% of the Earth's surface as See also Nuclear power "LWR" redirects here See also LWR (disambiguation A light water reactor or LWR is The reactors require enriched uranium to operate. Enriched uranium is a kind of Uranium in which the percent composition of Uranium-235 has been increased through the process of Isotope separation.
Deuterium, in the form of heavy water, in heavy water reactors, eg. Deuterium, also called heavy hydrogen, is a Stable isotope of Hydrogen with a Natural abundance in the Oceans of Earth Heavy water is water which contains a higher proportion than normal of the Isotope Deuterium, as deuterium oxide, D2O or ²H2O Heavy water reactors use Heavy water as a Neutron moderator. Heavy water is Deuterium Oxide, D2O CANDU. The CANDU reactor is a Canadian-invented Pressurized heavy water reactor developed initially in the late 1950s and 1960s by a partnership between Atomic Energy of Reactors moderated with heavy water can use unenriched natural uranium. Natural uranium (NU refers to refined Uranium with the same Isotopic ratio as found in nature
Carbon, in the form of reactor-grade graphite or pyrolytic carbon, used in eg. Carbon (kɑɹbən is a Chemical element with the symbol C and its Atomic number is 6 The Mineral graphite, as with Diamond and Fullerene, is one of the Allotropes of carbon. Pyrolytic carbon is a Material similar to Graphite, but with some Covalent bonding between its Graphene sheets as a result of imperfections RBMK and pebble-bed reactors. RBMK is an acronym for the Russian reaktor bolshoy moshchnosti kanalniy (Реактор Большой Мощности Канальный which means "High Power Channel The pebble bed reactor ( PBR) is a graphite- moderated, gas-cooled Nuclear reactor. Lower-temperature reactors are susceptible to buildup of Wigner energy in the material. The Wigner effect (named for its discoverer E P Wigner) also known as the discomposition effect, is the displacement of Atoms in a solid caused by Like deuterium-moderated reactors, some of these reactors can use unenriched natural uranium.
Beryllium, in the form of metal. Beryllium (bəˈrɪliəm is a Chemical element with the symbol Be and Atomic number 4 Beryllium is expensive and toxic, so its use is limited.
Lithium-7, in the form of a fluoride salt, typically in conjunction with beryllium fluoride salt (FLiBe). Lithium (ˈlɪθiəm is a Chemical element with the symbol Li and Atomic number 3 This is the most common type of moderator in a Molten Salt Reactor. A molten salt reactor (MSR is a type of Nuclear reactor where the primary coolant is a Molten salt.

Other light-nuclei materials are unsuitable for various reasons. Helium is a gas and is not possible to achieve its sufficient density, lithium-6 and boron absorb neutrons. Helium ( He) is a colorless odorless tasteless non-toxic Inert Monatomic Chemical Lithium (ˈlɪθiəm is a Chemical element with the symbol Li and Atomic number 3 Boron (ˈbɔərɒn is a Chemical element with Atomic number 5 and the chemical symbol B.

References

^ Miller, Jr. , George Tyler (2002). Living in the Environment: Principles, Connections, and Solutions (12th Edition). Belmont: The Thomson Corporation, p. This article is primarily about Thomson prior to its 2008 merger with Reuters 345. ISBN 0-534-37697-5.

Contents