Iodine pit
Encyclopedia
Iodine pit, also called iodine hole and xenon pit, is a temporary disabling of a nuclear reactor due to buildup of short-lived
nuclear poison
s in the core of a nuclear reactor
. The main isotope responsible is xenon-135
, mainly produced by natural decay
of iodine-135. Iodine-135 is a weak neutron absorber, while xenon-135 is the most powerful known neutron absorber. When xenon-135 builds up in the fuel rods of a reactor, it significantly lowers their reactivity, by absorbing a significant amount of the neutrons which provide the nuclear reaction.
The presence of iodine-135 and xenon-135
in the reactor is one of the main reasons for its power fluctuations in reaction to change of control rod
positions.
The buildup of short-lived fission product
s acting as nuclear poisons is called reactor poisoning, or xenon poisoning. Buildup of stable or long-lived neutron poisons is called reactor slagging.
s is tellurium-135, which undergoes beta decay
with half-life
of 19 seconds to iodine-135. Iodine-135 itself is a weak neutron absorber. It builds up in the reactor in the rate proportional to the rate of fission, which is proportional to the reactor thermal power. Iodine-135 undergoes beta decay with half-life of 6.57 hours to xenon-135
. The yield of 135Xe for uranium fission is 6.3%; about 95% of xenon-135 originates from decay of iodine-135.
135Xe has a huge cross section
for thermal neutrons, 2.6×106 barns
, so it acts as a neutron absorber or "poison
" that can slow or stop the chain reaction
after a period of operation. This was discovered in the earliest nuclear reactors built by the American Manhattan Project
for plutonium
production. Fortunately the designers had made provisions in the design to increase the reactor's reactivity (the number of neutrons per fission that go on to fission other atoms of nuclear fuel
).
135Xe reactor poisoning played a major role in the Chernobyl disaster.
Xenon-135 is the most powerful known neutron absorber. Its buildup in the fuel rods significantly lowers reactivity of the reactor core. By a neutron capture
, Xe-135 is transformed ("burned") to xenon-136, which is stable and does not significantly absorb neutrons. The burn rate is proportional to the neutron flux
, which is proportional to the reactor power; a reactor running on twice the power will have twice the xenon burn rate.
Xenon-135 beta-decays with half-life of 9.2 hours to caesium-135; a poisoned core will spontaneously recover after several half-lives. For some reactors, the 135Xe concentration will be equal to its equilibrium concentration at full power. After about 3 days of shutdown, the core can be assumed to be free of 135Xe, without it introducing errors into the reactivity calculations.
The increase of the 135Xe concentration during lowering the reactor power can lower the reactivity enough to effectively shut down the reactor. As there are not enough neutrons to offset their absorption by 135Xe, nor to burn the built-up xenon, the reactor has to be kept in shutdown state for 1–2 days until enough of 135Xe decays. The inability to the reactor to be restarted in such state is called xenon precluded start up or dropping into an iodine pit; the duration of this situation is known as xenon dead time, poison outage, or iodine pit depth. Due to the risk of such situations, in the early Soviet nuclear industry, many servicing operations were performed on running reactors, as downtimes longer than an hour led to xenon buildup that could keep the reactor offline for significant time, lower the production of valuable weapon plutonium-239
, and cause an investigation by a committee and punishment of the operators.
of reactivity causes damping
of these oscillations, and is a desired reactor design feature.
before the shutdown. Iodine pit behavior is not observed in reactors with neutron flux density below 5×1016 neutrons/(m2·s), as the 135Xe is primarily removed by decay instead of neutron capture. As the core reactivity reserve is usually limited to 10% of Dk/k, thermal power reactors tend to use neutron flux at most about 5×1017 neutrons/(m2·s) to avoid restart problems after shutdown.
The concentration changes of 135Xe in the reactor core after its shutdown
is determined by the short-term power history
of the reactor (which determines the initial concentrations of 135I and 135Xe), and then by the half-life differences of the isotopes governing the rates of its production and removal; if the activity of 135I is higher than activity of 135Xe, the concentration of 135Xe will rise, and vice-versa.
During reactor operation at a given power level, a secular equilibrium
is established within 40–50 hours, when the production rate of iodine-135, its decay to xenon-135, and its burning to xenon-136 and decay to caesium-135 are keeping the xenon-135 amount in the reactor constant at a given power level.
The equilibrium concentration of 135I is proportional to the neutron flux φ. The equilibrium concentration of 135Xe however depends very little on neutron flux for φ>1017 neutrons/(m2·s).
Increase of the reactor power, and the increase of neutron flux, causes raise in production of 135I and consumption of 135Xe. At first, the concentration of xenon decreases, then slowly increases again to a new equilibrium level as now excess 135I decays. During typical power increases from 50 to 100%, the 135Xe concentration falls for about 3 hours.
Decrease of the reactor power lowers production of new 135I, but also lowers the burn rate of 135Xe. For a while 135Xe builds up, governed by the amount of available 135I, then its concentration decreases again to an equilibrium for the given reactor power level. The peak concentration of 135Xe occurs after about 11.1 hours after power decrease, and the equilibrium is reached after about 50 hours. A total shutdown of the reactor is an extreme case of power decrease.
s are extracted and criticality
is reached, neutron flux
increases many orders of magnitude and the 135Xe begins to absorb neutrons and be transmuted to 136Xe. The reactor burns off the nuclear poison. As this happens, the reactivity increases and the control rods must be gradually re-inserted or reactor power will increase. The time constant for this burn-off transient depends on the reactor design, power level history of the reactor for the past several days (therefore the 135Xe and 135I concentrations present), and the new power setting. For a typical step up from 50% power to 100% power, 135Xe concentration falls for about 3 hours.
Reactors with large physical dimensions, e.g. the RBMK
type, can develop significant nonuniformities of xenon concentration through the core. Control of such nonhomogeneously poisoned core, especially at low power, is a challenging problem. The Chernobyl disaster
resulted from an attempt to get the reactor from a nonuniformly poisoned state.
The iodine pit effect has to be taken in account for reactor designs. High values of power density
, leading to high production rates of fission products and therefore higher iodine concentrations, require higher amount and enrichment of the nuclear fuel
used to compensate. Without this reactivity reserve, a reactor shutdown would preclude its restart for several tens of hours until 135I/135Xe sufficiently decays, especially shortly before replacement of spent fuel (with high burnup
and accumulated nuclear poison
s) with fresh one.
Fluid fuel reactors cannot develop xenon inhomogeneity because the fuel is free to mix. Also, the Molten Salt Reactor Experiment demonstrated that spraying the liquid fuel as droplets through a gas space during recirculation can allow xenon and krypton to leave the fuel salts. However, removing xenon-135 from neutron exposure also means that the reactor will produce more of the long-lived fission product
caesium-135.
Half-life
Half-life, abbreviated t½, is the period of time it takes for the amount of a substance undergoing decay to decrease by half. The name was originally used to describe a characteristic of unstable atoms , but it may apply to any quantity which follows a set-rate decay.The original term, dating to...
nuclear poison
Nuclear poison
A neutron poison is a substance with a large neutron absorption cross-section in applications, such as nuclear reactors. In such applications, absorbing neutrons is normally an undesirable effect...
s in the core of 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...
. The main isotope responsible is xenon-135
Xenon-135
Xenon-135 is an unstable isotope of xenon with a half-life of about 9.2 hours. 135Xe is a fission product of uranium and Xe-135 is the most powerful known neutron-absorbing nuclear poison , with a significant effect on nuclear reactor operation...
, mainly produced by natural 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 iodine-135. Iodine-135 is a weak neutron absorber, while xenon-135 is the most powerful known neutron absorber. When xenon-135 builds up in the fuel rods of a reactor, it significantly lowers their reactivity, by absorbing a significant amount of the neutrons which provide the nuclear reaction.
The presence of iodine-135 and xenon-135
Xenon-135
Xenon-135 is an unstable isotope of xenon with a half-life of about 9.2 hours. 135Xe is a fission product of uranium and Xe-135 is the most powerful known neutron-absorbing nuclear poison , with a significant effect on nuclear reactor operation...
in the reactor is one of the main reasons for its power fluctuations in reaction to change of control rod
Control rod
A control rod is a rod made of chemical elements capable of absorbing many neutrons without fissioning themselves. They are used in nuclear reactors to control the rate of fission of uranium and plutonium...
positions.
The buildup of short-lived 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 acting as nuclear poisons is called reactor poisoning, or xenon poisoning. Buildup of stable or long-lived neutron poisons is called reactor slagging.
Fission products decay and burnup
One of the common fission productFission 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 is tellurium-135, which undergoes 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...
with half-life
Half-life
Half-life, abbreviated t½, is the period of time it takes for the amount of a substance undergoing decay to decrease by half. The name was originally used to describe a characteristic of unstable atoms , but it may apply to any quantity which follows a set-rate decay.The original term, dating to...
of 19 seconds to iodine-135. Iodine-135 itself is a weak neutron absorber. It builds up in the reactor in the rate proportional to the rate of fission, which is proportional to the reactor thermal power. Iodine-135 undergoes beta decay with half-life of 6.57 hours to xenon-135
Xenon-135
Xenon-135 is an unstable isotope of xenon with a half-life of about 9.2 hours. 135Xe is a fission product of uranium and Xe-135 is the most powerful known neutron-absorbing nuclear poison , with a significant effect on nuclear reactor operation...
. The yield of 135Xe for uranium fission is 6.3%; about 95% of xenon-135 originates from decay of iodine-135.
135Xe has a huge cross section
Neutron cross-section
In nuclear and particle physics, the concept of a neutron cross section is used to express the likelihood of interaction between an incident neutron and a target nucleus. In conjunction with the neutron flux, it enables the calculation of the reaction rate, for example to derive the thermal power...
for thermal neutrons, 2.6×106 barns
Barn (unit)
A barn is a unit of area. Originally used in nuclear physics for expressing the cross sectional area of nuclei and nuclear reactions, today it is used in all fields of high energy physics to express the cross sections of any scattering process, and is best understood as a measure of the...
, so it acts as a neutron absorber or "poison
Nuclear poison
A neutron poison is a substance with a large neutron absorption cross-section in applications, such as nuclear reactors. In such applications, absorbing neutrons is normally an undesirable effect...
" that can slow or stop the chain reaction
Chain reaction
A chain reaction is a sequence of reactions where a reactive product or by-product causes additional reactions to take place. In a chain reaction, positive feedback leads to a self-amplifying chain of events....
after a period of operation. This was discovered in the earliest nuclear reactors built by the American Manhattan Project
Manhattan Project
The Manhattan Project was a research and development program, led by the United States with participation from the United Kingdom and Canada, that produced the first atomic bomb during World War II. From 1942 to 1946, the project was under the direction of Major General Leslie Groves of the US Army...
for plutonium
Plutonium
Plutonium is a transuranic radioactive chemical element with the chemical symbol Pu and atomic number 94. It is an actinide metal of silvery-gray appearance that tarnishes when exposed to air, forming a dull coating when oxidized. The element normally exhibits six allotropes and four oxidation...
production. Fortunately the designers had made provisions in the design to increase the reactor's reactivity (the number of neutrons per fission that go on to fission other atoms of nuclear fuel
Nuclear fuel
Nuclear fuel is a material that can be 'consumed' by fission or fusion to derive nuclear energy. Nuclear fuels are the most dense sources of energy available...
).
135Xe reactor poisoning played a major role in the Chernobyl disaster.
Xenon-135 is the most powerful known neutron absorber. Its buildup in the fuel rods significantly lowers reactivity of the reactor core. By a neutron capture
Neutron capture
Neutron capture is a kind of nuclear reaction in which an atomic nucleus collides with one or more neutrons and they merge to form a heavier nucleus. Since neutrons have no electric charge they can enter a nucleus more easily than positively charged protons, which are repelled...
, Xe-135 is transformed ("burned") to xenon-136, which is stable and does not significantly absorb neutrons. The burn rate is proportional to the neutron flux
Neutron flux
The neutron flux is a quantity used in reactor physics corresponding to the total length travelled by all neutrons per unit time and volume . The neutron fluence is defined as the neutron flux integrated over a certain time period....
, which is proportional to the reactor power; a reactor running on twice the power will have twice the xenon burn rate.
Xenon-135 beta-decays with half-life of 9.2 hours to caesium-135; a poisoned core will spontaneously recover after several half-lives. For some reactors, the 135Xe concentration will be equal to its equilibrium concentration at full power. After about 3 days of shutdown, the core can be assumed to be free of 135Xe, without it introducing errors into the reactivity calculations.
The increase of the 135Xe concentration during lowering the reactor power can lower the reactivity enough to effectively shut down the reactor. As there are not enough neutrons to offset their absorption by 135Xe, nor to burn the built-up xenon, the reactor has to be kept in shutdown state for 1–2 days until enough of 135Xe decays. The inability to the reactor to be restarted in such state is called xenon precluded start up or dropping into an iodine pit; the duration of this situation is known as xenon dead time, poison outage, or iodine pit depth. Due to the risk of such situations, in the early Soviet nuclear industry, many servicing operations were performed on running reactors, as downtimes longer than an hour led to xenon buildup that could keep the reactor offline for significant time, lower the production of valuable weapon 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. Plutonium-239 is also one of the three main isotopes demonstrated usable as fuel in...
, and cause an investigation by a committee and punishment of the operators.
Xenon-135 oscillations
The interdependence of 135Xe buildup and the neutron flux can lead to periodic power fluctuations. In large reactors, with little neutron flux coupling between their regions, flux nonuniformities can lead to formation of xenon oscillations, periodic local variations of reactor power moving through the core with the period of about 15 hours. A local variation of neutron flux causes increased burnup of 135Xe and production of 135I, depletion of 135Xe increases the reactivity in the core region. The local power density can change by factor of three or more, while the average power of the reactor stays more or less unchanged. Strong negative temperature coefficientTemperature coefficient
The temperature coefficient is the relative change of a physical property when the temperature is changed by 1 K.In the following formula, let R be the physical property to be measured and T be the temperature at which the property is measured. T0 is the reference temperature, and ΔT is the...
of reactivity causes damping
Damping
In physics, damping is any effect that tends to reduce the amplitude of oscillations in an oscillatory system, particularly the harmonic oscillator.In mechanics, friction is one such damping effect...
of these oscillations, and is a desired reactor design feature.
Iodine pit behavior
The reactivity of the reactor after the shutdown first decreases, then increases again, having a shape of a pit; this gave the "iodine pit" its name. The degree of poisoning, and the depth of the pit and the corresponding duration of the outage, depends on the neutron fluxNeutron flux
The neutron flux is a quantity used in reactor physics corresponding to the total length travelled by all neutrons per unit time and volume . The neutron fluence is defined as the neutron flux integrated over a certain time period....
before the shutdown. Iodine pit behavior is not observed in reactors with neutron flux density below 5×1016 neutrons/(m2·s), as the 135Xe is primarily removed by decay instead of neutron capture. As the core reactivity reserve is usually limited to 10% of Dk/k, thermal power reactors tend to use neutron flux at most about 5×1017 neutrons/(m2·s) to avoid restart problems after shutdown.
The concentration changes of 135Xe in the reactor core after its shutdown
Shutdown (nuclear reactor)
In a nuclear reactor, shutdown refers to the state of the reactor when it is subcritical by at least a margin defined in the reactor's technical specifications...
is determined by the short-term power history
Power history
Power History refers to the power of a nuclear reactor over an extended period of time. Power history is important for calculations and operations that involve decay heat and fission product poisons and to avoid the iodine pit during reactor shutdowns....
of the reactor (which determines the initial concentrations of 135I and 135Xe), and then by the half-life differences of the isotopes governing the rates of its production and removal; if the activity of 135I is higher than activity of 135Xe, the concentration of 135Xe will rise, and vice-versa.
During reactor operation at a given power level, a secular equilibrium
Secular equilibrium
In nuclear physics, secular equilibrium is a situation in which the quantity of a radioactive isotope remains constant because its production rate is equal to its decay rate.-Secular equilibrium in radioactive decay:...
is established within 40–50 hours, when the production rate of iodine-135, its decay to xenon-135, and its burning to xenon-136 and decay to caesium-135 are keeping the xenon-135 amount in the reactor constant at a given power level.
The equilibrium concentration of 135I is proportional to the neutron flux φ. The equilibrium concentration of 135Xe however depends very little on neutron flux for φ>1017 neutrons/(m2·s).
Increase of the reactor power, and the increase of neutron flux, causes raise in production of 135I and consumption of 135Xe. At first, the concentration of xenon decreases, then slowly increases again to a new equilibrium level as now excess 135I decays. During typical power increases from 50 to 100%, the 135Xe concentration falls for about 3 hours.
Decrease of the reactor power lowers production of new 135I, but also lowers the burn rate of 135Xe. For a while 135Xe builds up, governed by the amount of available 135I, then its concentration decreases again to an equilibrium for the given reactor power level. The peak concentration of 135Xe occurs after about 11.1 hours after power decrease, and the equilibrium is reached after about 50 hours. A total shutdown of the reactor is an extreme case of power decrease.
Design precautions
If sufficient reactivity control authority is available, the reactor can be restarted, but a xenon burn-out transient must be carefully managed. As the control rodControl rod
A control rod is a rod made of chemical elements capable of absorbing many neutrons without fissioning themselves. They are used in nuclear reactors to control the rate of fission of uranium and plutonium...
s are extracted and criticality
Critical mass
A critical mass is the smallest amount of fissile material needed for a sustained nuclear chain reaction. The critical mass of a fissionable material depends upon its nuclear properties A critical mass is the smallest amount of fissile material needed for a sustained nuclear chain reaction. The...
is reached, neutron flux
Neutron flux
The neutron flux is a quantity used in reactor physics corresponding to the total length travelled by all neutrons per unit time and volume . The neutron fluence is defined as the neutron flux integrated over a certain time period....
increases many orders of magnitude and the 135Xe begins to absorb neutrons and be transmuted to 136Xe. The reactor burns off the nuclear poison. As this happens, the reactivity increases and the control rods must be gradually re-inserted or reactor power will increase. The time constant for this burn-off transient depends on the reactor design, power level history of the reactor for the past several days (therefore the 135Xe and 135I concentrations present), and the new power setting. For a typical step up from 50% power to 100% power, 135Xe concentration falls for about 3 hours.
Reactors with large physical dimensions, e.g. the RBMK
RBMK
RBMK is an initialism for the Russian reaktor bolshoy moshchnosti kanalniy which means "High Power Channel-type Reactor", and describes a class of graphite-moderated nuclear power reactor which was built in the Soviet Union. The RBMK reactor was the type involved in the Chernobyl disaster...
type, can develop significant nonuniformities of xenon concentration through the core. Control of such nonhomogeneously poisoned core, especially at low power, is a challenging problem. The Chernobyl disaster
Chernobyl disaster
The Chernobyl disaster was a nuclear accident that occurred on 26 April 1986 at the Chernobyl Nuclear Power Plant in Ukraine , which was under the direct jurisdiction of the central authorities in Moscow...
resulted from an attempt to get the reactor from a nonuniformly poisoned state.
The iodine pit effect has to be taken in account for reactor designs. High values of power density
Power density
Power density is the amount of power per unit volume....
, leading to high production rates of fission products and therefore higher iodine concentrations, require higher amount and enrichment of the nuclear fuel
Nuclear fuel
Nuclear fuel is a material that can be 'consumed' by fission or fusion to derive nuclear energy. Nuclear fuels are the most dense sources of energy available...
used to compensate. Without this reactivity reserve, a reactor shutdown would preclude its restart for several tens of hours until 135I/135Xe sufficiently decays, especially shortly before replacement of spent fuel (with high burnup
Burnup
In nuclear power technology, burnup is a measure of how much energy is extracted from a primary nuclear fuel source...
and accumulated nuclear poison
Nuclear poison
A neutron poison is a substance with a large neutron absorption cross-section in applications, such as nuclear reactors. In such applications, absorbing neutrons is normally an undesirable effect...
s) with fresh one.
Fluid fuel reactors cannot develop xenon inhomogeneity because the fuel is free to mix. Also, the Molten Salt Reactor Experiment demonstrated that spraying the liquid fuel as droplets through a gas space during recirculation can allow xenon and krypton to leave the fuel salts. However, removing xenon-135 from neutron exposure also means that the reactor will produce more of the long-lived fission product
Long-lived fission product
Long-lived fission products are radioactive materials with a long half-life produced by nuclear fission.-Evolution of radioactivity in nuclear waste:...
caesium-135.