Neutron temperature

Neutron Temperature

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Neutron temperature

The neutron temperature, also called the neutron energy, indicates a free neutron's kinetic energy

Kinetic energy

The kinetic energy of an object is the extra energy which it possesses due to its motion. It is defined as the mechanical work needed to accelerate a body of a given mass from rest to its current velocity....
, usually given in electron volts. The term temperature is used, since hot, thermal and cold neutrons are moderated

Neutron moderator

In nuclear engineering, a neutron moderator is a medium which reduces the speed of fast neutrons, thereby turning them into thermal neutrons capable of sustaining a nuclear chain reaction involving uranium-235....
in a medium with a certain temperature.

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Encyclopedia

The neutron temperature, also called the neutron energy, indicates a free neutron's kinetic energy

Kinetic energy

Neutron moderator

Maxwell–Boltzmann distribution

The Maxwell?Boltzmann distribution is a probability distribution with applications in physics and chemistry. The most common application is in the field of statistical mechanics....
known for thermal motion. Qualitatively, the higher the temperature, the higher the kinetic energy

Kinetic energy

Speed

Speed is the rate of Motion , or equivalently the rate of change of distance.Speed is a Scalar quantity with dimensions length/time; the equivalent Vector quantity to speed is velocity....
and wavelength

Wavelength

In physics, wavelength is the distance between repeating units of a propagating wave of a given frequency. It is commonly designated by the Greek language letter lambda ....
of the neutron are related through the De Broglie relation

De Broglie hypothesis

In physics, the matter wave, aka de Broglie wave , is the wave-like nature of all matter . The de Broglie relations show that the wavelength is inversely proportional to the momentum of a particle and that the frequency is directly proportional to the particle's kinetic energy....
.

Neutron energy distribution ranges

Moderated and other, non-thermal neutron energy distributions or ranges are listed in the table below:

Fast neutrons have an energy greater than 1 eV, 0.1 MeV or approximately 1 MeV, depending on the definition.
Slow neutrons have an energy less than or equal 0.4 eV.
Epithermal neutrons have an energy from 0.025 to 1 eV.
Hot neutrons have an energy of about .2 eV.
Thermal neutrons have an energy of about 0.025 eV.
Cold neutrons have an energy from 5x10^-5 eV to 0.025 eV.
Very cold neutrons have an energy from 3x10^-7 eV to 5x10^-5 eV.
Ultra cold neutrons
Ultracold neutrons

Ultracold neutrons are free neutrons which can be stored in traps made from certain materials. The storage is based on the reflection of UCN by such materials under any angle of incidence....
have an energy less than 3x10^-7 eV.
Continuum region neutrons have an energy from 0.01 MeV to 25 MeV.
Resonance region neutrons have an energy from 1 eV to 0.01 MeV.
Low energy region neutrons have an energy less than 1 eV.

Fast neutrons

A fast neutron is a free neutron with a kinetic energy level close to 1 M

Mega

mega is an SI prefix in the SI system of Units of measurements denoting a factor of 1 E6, 1,000,000 .For example, 1 MW = 1,000,000 watts = 1,000 kilowatts....
eV

Electronvolt

In physics, the electron volt is a unit of energy. By definition, it is equal to the amount of kinetic energy gained by a single unbound electron when it accelerates through an Electrostatics potential difference of one volt....
(100 T

Tera

tera- is a SI prefix in the SI system of units denoting 1012, or 1,000,000,000,000 .Confirmed in 1960, it comes from the Greek language wikt:t??a?, meaning monster....
J

Joule

The joule is the SI derived unit of energy in the International System of Units. It is defined as:One joule is the amount of energy required to perform the following actions:...
/kg

Kilogram

The kilogram or kilogrammeThe spelling kilogram is used by the International Committee for Weights and Measures and the U.S....
), hence a speed of 14,000 km/s

Second

The second , sometimes abbreviated sec., is the name of a units of measurement of time, and is the International System of Units SI base unit of time....
. They are named fast neutron

Neutron

The neutron is a subatomic particle with no net electric charge and a mass slightly larger than that of a proton.Neutrons are usually found in atomic nucleus....
s to distinguish them from lower-energy thermal neutrons, and high-energy neutrons produced in cosmic showers or accelerators. Fast neutrons are produced by nuclear processes such as nuclear fission

Nuclear fission

In nuclear physics and nuclear chemistry, nuclear fission is a nuclear reaction in which the atomic nucleus of an atom splits into smaller parts, often producing free neutrons and lighter atomic nucleus, which may eventually produce photons ....
.

Neutrons from fusion reactions

Nuclear fusion

In nuclear physics and nuclear chemistry, nuclear fusion is the process by which multiple like-charged atomic nuclei join together to form a heavier nucleus....
are usually considerably more energetic than 1 MeV; the extreme case is deuterium

Deuterium

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 ....
-tritium

Tritium

Tritium is a radioactive isotope of hydrogen. The atomic nucleus of tritium contains one proton and two neutrons, whereas the nucleus of Hydrogen atom contains one proton and no neutrons....
fusion which produces 14.1 MeV neutrons (1400 TJ/kg, moving at 52,000 km/s, 17.3% of the speed of light

Speed of light

The speed of light in an free space is an important physical constant usually written as c, with a value of 299,792,458 metres per second....
) that can easily fission uranium-238

Uranium-238

Fissile

In nuclear engineering, a fissile material is one that is capable of sustaining a chain reaction of nuclear fission.All known fissile materials are capable of sustaining a chain reaction in which either thermal or slow neutrons or fast neutrons predominate....
actinides.

Fast neutrons can be made into thermal neutrons via a process called moderation. This is done with a neutron moderator

Neutron moderator

Heavy water

Graphite

The mineral graphite is one of the allotropes of carbon. It was named by Abraham Gottlob Werner in 1789 from the Greek language ??afe?? : "to draw/write", for its use in pencils, where it is commonly called lead, as distinguished from the actual metallic element lead....
are used to moderate neutrons.

Thermal neutrons

A thermal neutron is a free neutron with a kinetic energy of about 0.025 eV (approx. 4.0×10^-21 J

Joule

The joule is the SI derived unit of energy in the International System of Units. It is defined as:One joule is the amount of energy required to perform the following actions:...
; 2.4 MJ/kg, hence a speed of 2.2 km/s) which is the most probable energy at a temperature of 290 K (17°C or 62°F), the mode

Mode (statistics)

In statistics, the mode is the value that occurs the most frequently in a data set or a probability distribution. In some fields, notably education, sample data are often called scores, and the sample mode is known as the modal score....
of the Maxwell–Boltzmann distribution

Maxwell–Boltzmann distribution

The Maxwell?Boltzmann distribution is a probability distribution with applications in physics and chemistry. The most common application is in the field of statistical mechanics....
for this temperature.
The most probable energy is different from the mean energy, which as in any Maxwell–Boltzmann distribution is 50% greater than the mode. After a number of collisions with nuclei (scattering

Scattering

Scattering is a general physical process where some forms of radiation, such as light, sound, or moving particles,are forced to deviate from a straight trajectory by one or more localized non-uniformities in the medium through which they pass....
) in a medium (neutron moderator

Neutron moderator

Neutron

The neutron is a subatomic particle with no net electric charge and a mass slightly larger than that of a proton.Neutrons are usually found in atomic nucleus....
s arrive at about this energy level, provided that they are not absorbed.

Thermal neutrons have a different and often much larger effective neutron absorption cross-section

Neutron cross-section

The total neutron cross-section of an isotope of a chemical element is the effective cross section area that an atom of that isotope presents to neutron scattering and neutron absorption....
for a given nuclide

Nuclide

A nuclide is a species of atom characterized by the constitution of its Atomic nucleus and hence by the number of protons, the number of neutrons, and the energy content of the nucleus....
than fast neutrons, and can therefore often be absorbed more easily by an atomic nucleus

Atomic nucleus

The nucleus of an atom is the very dense region, consisting of nucleons , at the center of an atom. Although the size of the nucleus varies considerably according to the mass of the atom, the size of the entire atom is comparatively constant....
, creating a heavier - and often unstable - isotope

Isotope

Isotopes are any of the different types of atoms of the same chemical element, each having a different atomic mass . Isotopes of an element have atomic nucleus with the same number of protons but different numbers of neutron....
of the chemical element

Chemical element

A chemical element is a type of atom that is distinguished by its atomic number; that is, by the number of protons in its atomic nucleus. The term is also used to refer to a pure chemical Chemical substance composed of atoms with the same number of protons....
as a result. (neutron activation

Neutron activation

Neutron activation is the process in which neutron radiation induces radioactivity in materials, and occurs when Atomic nucleus capture free neutrons, becoming heavier and entering excited states....
)

Fast reactor and thermal reactor compared

Most fission reactor

Nuclear reactor

A nuclear reactor is a device in which nuclear chain reactions are initiated, controlled, and sustained at a steady rate, as opposed to a nuclear bomb, in which the chain reaction occurs in a fraction of a second and is uncontrolled causing an explosion....
s are thermal reactor

Thermal reactor

A thermal reactor uses slow or thermal neutrons. Most power reactors are of this type. These type of reactors use a neutron moderator to slow neutrons until they approach the average kinetic energy of the surrounding particles, that is, to reduce the speed of the neutrons to low velocity thermal neutrons....
s that use a neutron moderator

Neutron moderator

Nuclear fission

Cross section

Cross section can refer to:* Cross section , a three-piece rock band from the Mornington Peninsula, Australia* Cross section , the intersection of a 3-dimensional body with a plane...
for fissile

Fissile

Uranium-235

Uranium-235 is an Isotopes of uranium that differs from the element's other common isotope, uranium-238, by its ability to cause a rapidly expanding nuclear fission chain reaction, i.e., it is fissile....
or plutonium-239

Plutonium-239

Plutonium-239 is an isotope of plutonium. Plutonium-239 is the primary fissile isotope used for the production of nuclear weapons, although uranium-235 has also been used and is currently the secondary isotope....
. In addition, Uranium-238

Uranium-238

Uranium-238 , is the most common Isotopes of uranium of uranium found in nature. When hit by a neutron, it becomes uranium-239 , an unstable isotope which radioactive decay into neptunium-239 , which then itself decays, with a half-life of 2.355 days, into plutonium-239 ....
also has a much lower capture cross section for thermal neutrons, allowing more neutrons to cause fission of fissile nuclei and continue the chain reaction, rather than be captured by ²³⁸U. The combination of these effects, with the first being more important, allows thermal reactors to use lower-enriched uranium

Enriched uranium

Enriched uranium is a kind of uranium in which the percent composition of uranium-235 has been increased through the process of isotope separation....
or even natural uranium

Natural uranium

Natural uranium refers to refined uranium with the same isotopic ratio as found in nature. It contains 0.7 % uranium-235, 99.3 % uranium-238, and a trace of uranium-234 by weight....
, so long as the moderator is an efficient one like heavy water

Heavy water

Heavy water is water that contains a higher proportion than normal of the isotope deuterium, as deuterium oxide, D2O or ?H2O, or as deuterium protium oxide, HDO or ?H?HO....
or graphite, which means that it does not tend to capture neutrons.

An increase in fuel temperature also raises U-238's neutron absorption by Doppler broadening

Doppler broadening

In atomic physics, Doppler broadening is the broadening of spectral lines due to the Doppler effect in which the heat movement of atoms or molecules shifts the apparent frequency of each emitter....
, providing negative feedback

Negative feedback

Negative feedback feeds part of a system's output, inverted, into the system's input; generally with the result that fluctuations are attenuated....
to help control the reactor. Also, when the moderator is also a circulating coolant (light water or heavy water), boiling of the coolant will reduce the moderator density and provide negative feedback (a negative void coefficient

Void coefficient

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 Neutron moderator or coolant....
).

Intermediate-energy neutrons have poorer fission/capture ratios than either fast or thermal neutrons for most fuels. A possible exception is the uranium-233

Uranium-233

Uranium-233 is a fissile artificial Isotopes of uranium , which has been used in a few nuclear reactors and has been proposed for much wider use as a nuclear fuel....
of the thorium cycle.

Fast reactors use unmoderated fast neutrons to sustain the reaction and require the fuel to contain a higher concentration of fissile

Fissile

In nuclear engineering, a fissile material is one that is capable of sustaining a chain reaction of nuclear fission.All known fissile materials are capable of sustaining a chain reaction in which either thermal or slow neutrons or fast neutrons predominate....
material. However, fast neutrons have a better fission/capture ratio for many nuclides, and each fast fission releases a larger number of neutrons, so a fast breeder reactor

Fast breeder reactor

The fast breeder or fast breeder reactor is a fast neutron reactor designed to breed fuel by producing more fissile material than it consumes....
can potentially "breed" more fissile fuel than it consumes.

Breeder reacter control is more difficult because of decreased Doppler broadening and lack of negative void coefficient from a moderator. Once expected to be the wave of the future, fast breeder development has been nearly dormant with only a handful of reactors built in the decades since the Chernobyl accident (and because of low prices in the uranium market

Uranium market

The uranium market, like all commodity markets, has a history of volatility, moving not only with the standard forces of supply and demand, but also to whims of geopolitics....
)-- although there is now a revival with several Asian countries planning to complete larger prototype fast reactors in the next few years.

Neutron temperature

Neutron energy distribution ranges

Fast neutrons

Thermal neutrons

Fast reactor and thermal reactor compared

See also

External links