Neutron economy

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Neutron economy is defined as the ratio of excess neutron production divided by the rate of fission. [1] [2] The numbers are a weighted average based primarily on the energies of the neutrons.

Nuclear fission is a process in which the nuclei of atoms are split apart. Among the various particles released in this process are high-energy neutrons with energies spread over the neutron spectrum. Those neutrons may cause other nuclei to undergo fission, leading to the possibility of a chain reaction. However, the neutrons can only cause another fission under certain conditions based on their energy; high-energy, or "relativistic", neutrons will often fly right through another nucleus without causing fission. The chance that a neutron will be captured increases greatly when its energy is about that of the target nucleus, which is known as a "thermal neutron". In order to maintain a chain reaction in a nuclear reactor, a neutron moderator is used to slow the neutrons down. This moderator is often used as the coolant that is used for energy extraction as well, and the most common moderator is water. The neutrons also slow due to elastic and inelastic collisions with fuel and other materials in the reactor.

A fission reactor is based on the idea of maintaining criticality, where every fission event leads to another fission event, no more and no less. As fission of uranium releases two or three neutrons, this means some of the neutrons must be removed as part of the overall process. Some will be lost purely due to geometry, those released travelling outward from the outer edge of the fuel mass will not have a chance to cause fission, for instance. Others will be absorbed through various processes in the mass, and still others will be deliberately absorbed by control rods or similar devices to maintain the correct overall balance. [3] The process of moderating the neutrons almost always leads to some of them being absorbed as well.

Neutron economy is a measure of the number of neutrons being released that can cause fission compared to the number needed to maintain the chain reaction. This is not simply an accounting of the total number of neutrons, as it also includes a weighting based on the energy. Thus, remaining high-energy neutrons are not a major part of the "overall economy" as they do not maintain the chain reaction. The quantity that indicates how much the neutron economy is out of balance is given the term reactivity. If a reactor is exactly critical—that is, the neutron production is exactly equal to neutron destruction—the reactivity is zero. If the reactivity is positive, the reactor is supercritical. If the reactivity is negative, the reactor is subcritical.

The term "neutron economy" is used not just for the instantaneous reactivity of a reactor, but also to describe the overall efficiency of a nuclear reactor design. Common reactor designs using conventional water as the coolant and moderator generally have poor relative neutron economies because the water will absorb some of the thermal neutrons, reducing the number available to keep the reaction going. In contrast, heavy water already has an extra neutron, and the same reaction generally causes it to be released, meaning that a reactor moderated with heavy water does not absorb neutrons and thus has a better neutron economy. [4] [5] Reactors with high neutron economies have more "leftover neutrons" which can be used for other purposes, like breeding additional fuel or causing sub-critical fission in nuclear waste to "burn off" some of the more radioactive components.

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<span class="mw-page-title-main">Nuclear fission</span> Nuclear reaction splitting an atom into multiple parts

Nuclear fission is a reaction in which the nucleus of an atom splits into two or more smaller nuclei. The fission process often produces gamma photons, and releases a very large amount of energy even by the energetic standards of radioactive decay.

<span class="mw-page-title-main">Nuclear chain reaction</span> When one nuclear reaction causes more

In nuclear physics, a nuclear chain reaction occurs when one single nuclear reaction causes an average of one or more subsequent nuclear reactions, thus leading to the possibility of a self-propagating series or "positive feedback loop" of these reactions. The specific nuclear reaction may be the fission of heavy isotopes. A nuclear chain reaction releases several million times more energy per reaction than any chemical reaction.

<span class="mw-page-title-main">Nuclear reactor</span> Device used to initiate and control a nuclear chain reaction

A nuclear reactor is a device used to initiate and control a fission nuclear chain reaction or nuclear fusion reactions. Nuclear reactors are used at nuclear power plants for electricity generation and in nuclear marine propulsion. Heat from nuclear fission is passed to a working fluid, which in turn runs through steam turbines. These either drive a ship's propellers or turn electrical generators' shafts. Nuclear generated steam in principle can be used for industrial process heat or for district heating. Some reactors are used to produce isotopes for medical and industrial use, or for production of weapons-grade plutonium. As of 2022, the International Atomic Energy Agency reports there are 422 nuclear power reactors and 223 nuclear research reactors in operation around the world.

<span class="mw-page-title-main">Pressurized water reactor</span> Type of nuclear reactor

A pressurized water reactor (PWR) is a type of light-water nuclear reactor. PWRs constitute the large majority of the world's nuclear power plants. In a PWR, the primary coolant (water) is pumped under high pressure to the reactor core where it is heated by the energy released by the fission of atoms. The heated, high pressure water then flows to a steam generator, where it transfers its thermal energy to lower pressure water of a secondary system where steam is generated. The steam then drives turbines, which spin an electric generator. In contrast to a boiling water reactor (BWR), pressure in the primary coolant loop prevents the water from boiling within the reactor. All light-water reactors use ordinary water as both coolant and neutron moderator. Most use anywhere from two to four vertically mounted steam generators; VVER reactors use horizontal steam generators.

<span class="mw-page-title-main">Neutron moderator</span> Substance that slows down particles with no electric charge

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 a nuclear chain reaction of uranium-235 or other fissile isotope by colliding with their atomic nucleus.

A thermal-neutron reactor is a nuclear reactor that uses slow or thermal neutrons.

<span class="mw-page-title-main">Fast-neutron reactor</span> Nuclear reactor where fast neutrons maintain a fission chain reaction

A fast-neutron reactor (FNR) or fast-spectrum reactor or simply a fast reactor is a category of nuclear reactor in which the fission chain reaction is sustained by fast neutrons, as opposed to slow thermal neutrons used in thermal-neutron reactors. Such a fast reactor needs no neutron moderator, but requires fuel that is relatively rich in fissile material when compared to that required for a thermal-neutron reactor. Around 20 land based fast reactors have been built, accumulating over 400 reactor years of operation globally. The largest of this was the Superphénix Sodium cooled fast reactor in France that was designed to deliver 1,242 MWe. Fast reactors have been intensely studied since the 1950s, as they provide certain advantages over the existing fleet of water cooled and water moderated reactors. These are:

In nuclear engineering, the void coefficient is a number that can be used to estimate how much the reactivity of a nuclear reactor changes as voids form in the reactor moderator or coolant. Net reactivity in a reactor is the sum total of multiple contributions, of which the void coefficient is but one. Reactors in which either the moderator or the coolant is a liquid typically will have a void coefficient value that is either negative or positive. Reactors in which neither the moderator nor the coolant is a liquid will have a void coefficient value equal to zero. It is unclear how the definition of "void" coefficient applies to reactors in which the moderator/coolant is neither liquid nor gas.

<span class="mw-page-title-main">Light-water reactor</span> Type of nuclear reactor that uses normal water

The light-water reactor (LWR) is a type of thermal-neutron reactor that uses normal water, as opposed to heavy water, as both its coolant and neutron moderator; furthermore a solid form of fissile elements is used as fuel. Thermal-neutron reactors are the most common type of nuclear reactor, and light-water reactors are the most common type of thermal-neutron reactor.

<span class="mw-page-title-main">Nuclear fuel</span> Material fuelling nuclear reactors

Nuclear fuel is material used in nuclear power stations to produce heat to power turbines. Heat is created when nuclear fuel undergoes nuclear fission.

<span class="mw-page-title-main">Neutron cross section</span> Measure of neutron interaction likelihood

In nuclear physics, the concept of a neutron cross section is used to express the likelihood of interaction between an incident neutron and a target nucleus. The neutron cross section σ can be defined as the area in cm2 for which the number of neutron-nuclei reactions taking place is equal to the product of the number of incident neutrons that would pass through the area and the number of target nuclei. In conjunction with the neutron flux, it enables the calculation of the reaction rate, for example to derive the thermal power of a nuclear power plant. The standard unit for measuring the cross section is the barn, which is equal to 10−28 m2 or 10−24 cm2. The larger the neutron cross section, the more likely a neutron will react with the nucleus.

<span class="mw-page-title-main">Nuclear reactor physics</span> Field of physics dealing with nuclear reactors

Nuclear reactor physics is the field of physics that studies and deals with the applied study and engineering applications of chain reaction to induce a controlled rate of fission in a nuclear reactor for the production of energy. Most nuclear reactors use a chain reaction to induce a controlled rate of nuclear fission in fissile material, releasing both energy and free neutrons. A reactor consists of an assembly of nuclear fuel, usually surrounded by a neutron moderator such as regular water, heavy water, graphite, or zirconium hydride, and fitted with mechanisms such as control rods which control the rate of the reaction.

In applications such as nuclear reactors, a neutron poison is a substance with a large neutron absorption cross-section. In such applications, absorbing neutrons is normally an undesirable effect. However, neutron-absorbing materials, also called poisons, are intentionally inserted into some types of reactors in order to lower the high reactivity of their initial fresh fuel load. Some of these poisons deplete as they absorb neutrons during reactor operation, while others remain relatively constant.

<span class="mw-page-title-main">Neutron temperature</span> The kinetic energy of an unbound 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 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 the large cross section.

The Whiteshell Reactor No. 1, or WR-1, was a Canadian research reactor located at AECL's Whiteshell Laboratories (WNRL) in Manitoba. Originally known as Organic-Cooled Deuterium-Reactor Experiment (OCDRE), it was built to test the concept of a CANDU-type reactor that replaced the heavy water coolant with an oil substance. This had a number of potential advantages in terms of cost and efficiency.

The Clean and Environmentally Safe Advanced Reactor (CAESAR) is a nuclear reactor concept created by Claudio Filippone, the Director of the Center for Advanced Energy Concepts at the University of Maryland, College Park and head of the ongoing CAESAR Project. The concept's key element is the use of steam as a moderator, making it a type of reduced moderation water reactor. Because the density of steam may be controlled very precisely, Filippone claims it can be used to fine-tune neutron fluxes to ensure that neutrons are moving with an optimal energy profile to split 238
92
U
nuclei – in other words, cause fission.

Shutdown is the state of a nuclear reactor when the fission reaction is slowed significantly or halted completely. Different nuclear reactor designs have different definitions for what "shutdown" means, but it typically means that the reactor is not producing a measurable amount of electricity or heat, and is in a stable condition with very low reactivity.

Hybrid nuclear fusion–fission is a proposed means of generating power by use of a combination of nuclear fusion and fission processes.

A pressurized heavy-water reactor (PHWR) is a nuclear reactor that uses heavy water (deuterium oxide D2O) as its coolant and neutron moderator. PHWRs frequently use natural uranium as fuel, but sometimes also use very low enriched uranium. The heavy water coolant is kept under pressure to avoid boiling, allowing it to reach higher temperature (mostly) without forming steam bubbles, exactly as for a pressurized water reactor. While heavy water is very expensive to isolate from ordinary water (often referred to as light water in contrast to heavy water), its low absorption of neutrons greatly increases the neutron economy of the reactor, avoiding the need for enriched fuel. The high cost of the heavy water is offset by the lowered cost of using natural uranium and/or alternative fuel cycles. As of the beginning of 2001, 31 PHWRs were in operation, having a total capacity of 16.5 GW(e), representing roughly 7.76% by number and 4.7% by generating capacity of all current operating reactors.

<span class="mw-page-title-main">Organic nuclear reactor</span> Nuclear reactor that uses organic liquids for cooling and neutron moderation

An organic nuclear reactor, or organic cooled reactor (OCR), is a type of nuclear reactor that uses some form of organic fluid, typically a hydrocarbon substance like polychlorinated biphenyl (PCB), for cooling and sometimes as a neutron moderator as well.

References

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