Delayed neutron
Encyclopedia
In nuclear engineering
, a delayed neutron is a neutron
emitted after a nuclear fission
event by one of the fission product
s anytime from a few milliseconds to a few minutes later.
In a nuclear reactor
large nuclides fission in two neutron-rich fission products i.e. unstable nuclides. Many fission products decay
but only a few do so while simultaneously emitting a (delayed) neutron. The moment of decay of these so called precursor-nuclides - who are the precursors of the delayed neutrons - happens orders of magnitude later compared to the emission of the prompt neutrons
. Hence the neutron that origins from the precursor's decay
is termed delayed neutron.
Delayed neutrons play an important roll in nuclear reactor safety analysis.
as an example, this nucleus absorbs thermal neutrons, and the immediate mass products of a fission event are two large fission fragments, which are remnants of the formed U-236 nucleus. These fragments emit, on average, two or three free neutrons (in average 2.47), called "prompt" neutrons
. A subsequent fission fragment occasionally undergoes a stage of radioactive decay (which is a beta minus decay
) that yields a new nucleus (the precursor nucleus) in an excited state that emits an additional neutron, called a "delayed" neutron, to get to ground state. These neutron-emitting fission fragments are called delayed neutron precursor atoms.
Delayed neutrons are associated with the beta decay
of the fission products. After prompt fission neutron emission the residual fragments are still neutron rich and undergo a beta decay chain. The more neutron rich the fragment, the more energetic and faster the beta decay. In some cases the available energy in the beta decay is high enough to leave the residual nucleus in such a highly excited state that neutron emission instead of gamma emission
occurs.
Delayed Neutron Data for Thermal Fission in U-235
. This effect of prompt neutron emission does not permit to obtain primary primary mass and kinetic distribution which is important to study fission dynamics from saddle to scission point.
happened to be prompt critical
- even very slightly - the number of neutrons would increase exponentially at a high rate, and very quickly the reactor would become uncontrollable by means of cybernetics. The control of the power rise would then be left to its intrinsic physical stability factors, like the thermal dilatation of the core, or the increased resonance absorptions of neutrons, that usually tend to decrease the reactor's reactivity when temperature rises; but the reactor would run the risk of being damaged or destroyed by heat.
However, thanks to the delayed neutrons, it is possible to leave the reactor in a subcritical state as far as only prompt neutrons are concerned: the delayed neutrons come a moment later, just in time to sustain the chain reaction when it is going to die out. In that regime, neutron production overall still grows exponentially, but on a time scale that is governed by the delayed neutron production, which is slow enough to be controlled (just as an otherwise unstable bicycle can be balanced because human reflexes are quick enough on the time scale of its instability). Thus, by widening the margins of non-operation and supercriticality and allowing more time to regulate the reactor, the delayed neutrons are essential to inherent reactor safety
and even in reactors requiring active control.
and it is equal to 0.0064 for U-235.
The delayed neutron fraction (DNF) is defined as:
These two factors, β and DNF, are not the same thing in case of a rapid change in the number of neutrons in the reactor.
Another concept, is the effective fraction of delayed neutrons, which is the fraction of delayed neutrons weighted (over space, energy, and angle) on the adjoint neutron flux. This concept arizes because delayed neutrons are emitted with an energy spectrum more thermalized relative to prompt neutrons. For low enriched uranium fuel working on a thermal neutron spectrum, the differerence between the average and effective delayed neutron fractions can reach 50 pcm.
Nuclear engineering
Nuclear engineering is the branch of engineering concerned with the application of the breakdown as well as the fusion of atomic nuclei and/or the application of other sub-atomic physics, based on the principles of nuclear physics...
, a delayed neutron is a neutron
Neutron
The neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...
emitted after a nuclear fission
Nuclear fission
In nuclear physics and nuclear chemistry, nuclear fission is a nuclear reaction in which the nucleus of an atom splits into smaller parts , often producing free neutrons and photons , and releasing a tremendous amount of energy...
event by one of the fission product
Fission product
Nuclear fission products are the atomic fragments left after a large atomic nucleus fissions. Typically, a large nucleus like that of uranium fissions by splitting into two smaller nuclei, along with a few neutrons and a large release of energy in the form of heat , gamma rays and neutrinos. The...
s anytime from a few milliseconds to a few minutes later.
In a nuclear reactor
Nuclear reactor
A nuclear reactor is a device to initiate and control a sustained nuclear chain reaction. Most commonly they are used for generating electricity and for the propulsion of ships. Usually heat from nuclear fission is passed to a working fluid , which runs through turbines that power either ship's...
large nuclides fission in two neutron-rich fission products i.e. unstable nuclides. Many fission products decay
Radioactive decay
Radioactive decay is the process by which an atomic nucleus of an unstable atom loses energy by emitting ionizing particles . The emission is spontaneous, in that the atom decays without any physical interaction with another particle from outside the atom...
but only a few do so while simultaneously emitting a (delayed) neutron. The moment of decay of these so called precursor-nuclides - who are the precursors of the delayed neutrons - happens orders of magnitude later compared to the emission of the prompt neutrons
Prompt neutron
In nuclear engineering, a prompt neutron is a neutron immediately emitted by a nuclear fission event, as opposed to a delayed neutron decay which can occur within the same context, emitted by one of the fission products anytime from a few milliseconds to a few minutes later.-Principle:Using U-235...
. Hence the neutron that origins from the precursor's decay
Neutron emission
Neutron emission is a type of radioactive decay of atoms containing excess neutrons, in which a neutron is simply ejected from the nucleus. Two examples of isotopes which emit neutrons are helium-5 and beryllium-13...
is termed delayed neutron.
Delayed neutrons play an important roll in nuclear reactor safety analysis.
Principle
Using U-235Uranium-235
- References :* .* DOE Fundamentals handbook: Nuclear Physics and Reactor theory , .* A piece of U-235 the size of a grain of rice can produce energy equal to that contained in three tons of coal or fourteen barrels of oil. -External links:* * * one of the earliest articles on U-235 for the...
as an example, this nucleus absorbs thermal neutrons, and the immediate mass products of a fission event are two large fission fragments, which are remnants of the formed U-236 nucleus. These fragments emit, on average, two or three free neutrons (in average 2.47), called "prompt" neutrons
Prompt neutron
In nuclear engineering, a prompt neutron is a neutron immediately emitted by a nuclear fission event, as opposed to a delayed neutron decay which can occur within the same context, emitted by one of the fission products anytime from a few milliseconds to a few minutes later.-Principle:Using U-235...
. A subsequent fission fragment occasionally undergoes a stage of radioactive decay (which is a beta minus decay
Beta decay
In nuclear physics, beta decay is a type of radioactive decay in which a beta particle is emitted from an atom. There are two types of beta decay: beta minus and beta plus. In the case of beta decay that produces an electron emission, it is referred to as beta minus , while in the case of a...
) that yields a new nucleus (the precursor nucleus) in an excited state that emits an additional neutron, called a "delayed" neutron, to get to ground state. These neutron-emitting fission fragments are called delayed neutron precursor atoms.
Delayed neutrons are associated with the beta decay
Beta decay
In nuclear physics, beta decay is a type of radioactive decay in which a beta particle is emitted from an atom. There are two types of beta decay: beta minus and beta plus. In the case of beta decay that produces an electron emission, it is referred to as beta minus , while in the case of a...
of the fission products. After prompt fission neutron emission the residual fragments are still neutron rich and undergo a beta decay chain. The more neutron rich the fragment, the more energetic and faster the beta decay. In some cases the available energy in the beta decay is high enough to leave the residual nucleus in such a highly excited state that neutron emission instead of gamma emission
Gamma ray
Gamma radiation, also known as gamma rays or hyphenated as gamma-rays and denoted as γ, is electromagnetic radiation of high frequency . Gamma rays are usually naturally produced on Earth by decay of high energy states in atomic nuclei...
occurs.
Delayed Neutron Data for Thermal Fission in U-235
| Group | Half-Life (s) | Decay Constant (s-1) | Energy (keV) | Yield, Neutrons per Fission | Fraction |
|---|---|---|---|---|---|
| 1 | 55.72 | 0.0124 | 250 | 0.00052 | 0.000215 |
| 2 | 22.72 | 0.0305 | 560 | 0.00546 | 0.001424 |
| 3 | 6.22 | 0.111 | 405 | 0.00310 | 0.001274 |
| 4 | 2.30 | 0.301 | 450 | 0.00624 | 0.002568 |
| 5 | 0.614 | 1.14 | - | 0.00182 | 0.000748 |
| 6 | 0.230 | 3.01 | - | 0.00066 | 0.000273 |
Importance in nuclear fission basic research
The standard deviation of the final kinetic energy distribution as a function of mass of final fragments from low energy fission of uranium 234 and uranium 236, presents a peak around light fragment masses region and another on heavy fragment masses region. Simulation by Monte Carlo method of these experiments suggests that that those peaks are produced by prompt neutron emission. This effect of prompt neutron emission does not permit to obtain primary primary mass and kinetic distribution which is important to study fission dynamics from saddle to scission point.
Importance in nuclear reactors
If a nuclear reactorNuclear reactor
A nuclear reactor is a device to initiate and control a sustained nuclear chain reaction. Most commonly they are used for generating electricity and for the propulsion of ships. Usually heat from nuclear fission is passed to a working fluid , which runs through turbines that power either ship's...
happened to be prompt critical
Prompt critical
In nuclear engineering, an assembly is prompt critical if for each nuclear fission event, one or more of the immediate or prompt neutrons released causes an additional fission event. This causes a rapid, exponential increase in the number of fission events...
- even very slightly - the number of neutrons would increase exponentially at a high rate, and very quickly the reactor would become uncontrollable by means of cybernetics. The control of the power rise would then be left to its intrinsic physical stability factors, like the thermal dilatation of the core, or the increased resonance absorptions of neutrons, that usually tend to decrease the reactor's reactivity when temperature rises; but the reactor would run the risk of being damaged or destroyed by heat.
However, thanks to the delayed neutrons, it is possible to leave the reactor in a subcritical state as far as only prompt neutrons are concerned: the delayed neutrons come a moment later, just in time to sustain the chain reaction when it is going to die out. In that regime, neutron production overall still grows exponentially, but on a time scale that is governed by the delayed neutron production, which is slow enough to be controlled (just as an otherwise unstable bicycle can be balanced because human reflexes are quick enough on the time scale of its instability). Thus, by widening the margins of non-operation and supercriticality and allowing more time to regulate the reactor, the delayed neutrons are essential to inherent reactor safety
Passive nuclear safety
Passive nuclear safety is a safety feature of a nuclear reactor that does not require operator actions or electronic feedback in order to shut down safely in the event of a particular type of emergency...
and even in reactors requiring active control.
Fraction definitions
The factor β is defined as:and it is equal to 0.0064 for U-235.
The delayed neutron fraction (DNF) is defined as:
These two factors, β and DNF, are not the same thing in case of a rapid change in the number of neutrons in the reactor.
Another concept, is the effective fraction of delayed neutrons, which is the fraction of delayed neutrons weighted (over space, energy, and angle) on the adjoint neutron flux. This concept arizes because delayed neutrons are emitted with an energy spectrum more thermalized relative to prompt neutrons. For low enriched uranium fuel working on a thermal neutron spectrum, the differerence between the average and effective delayed neutron fractions can reach 50 pcm.
See also
- prompt criticalPrompt criticalIn nuclear engineering, an assembly is prompt critical if for each nuclear fission event, one or more of the immediate or prompt neutrons released causes an additional fission event. This causes a rapid, exponential increase in the number of fission events...
- critical mass
- nuclear chain reactionNuclear chain reactionA 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. The specific nuclear reaction may be the fission of heavy isotopes or the fusion of light isotopes...