Xenon-135 (
135Xe) is an unstable
isotope
Isotopes are two or more types of atoms that have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), and that differ in nucleon numbers (mass numbers) ...
of
xenon
Xenon is a chemical element with the symbol Xe and atomic number 54. It is a dense, colorless, odorless noble gas found in Earth's atmosphere in trace amounts. Although generally unreactive, it can undergo a few chemical reactions such as the ...
with a
half-life
Half-life (symbol ) is the time required for a quantity (of substance) to reduce to half of its initial value. The term is commonly used in nuclear physics to describe how quickly unstable atoms undergo radioactive decay or how long stable ato ...
of about 9.2 hours.
135Xe is a
fission product
Nuclear fission products are the atomic fragments left after a large atomic nucleus undergoes nuclear fission. Typically, a large nucleus like that of uranium fissions by splitting into two smaller nuclei, along with a few neutrons, the release ...
of
uranium
Uranium is a chemical element with the symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Uranium is weak ...
and it is the most powerful known
neutron
The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the nuclei of atoms. Since protons and neutrons beh ...
-absorbing
nuclear poison
In applications such as nuclear reactors, a neutron poison (also called a neutron absorber or a nuclear poison) is a substance with a large neutron absorption cross-section. In such applications, absorbing neutrons is normally an undesirable eff ...
(2 million
barns
A barn is an agricultural building usually on farms and used for various purposes. In North America, a barn refers to structures that house livestock, including cattle and horses, as well as equipment and fodder, and often grain.Allen G. N ...
;
up to 3 million
barns
A barn is an agricultural building usually on farms and used for various purposes. In North America, a barn refers to structures that house livestock, including cattle and horses, as well as equipment and fodder, and often grain.Allen G. N ...
under reactor conditions), with a significant effect on
nuclear reactor
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 nu ...
operation. The ultimate yield of xenon-135 from fission is 6.3%, though most of this is from fission-produced
tellurium-135
There are 39 known isotopes and 17 nuclear isomers of tellurium (52Te), with atomic masses that range from 104 to 142. These are listed in the table below.
Naturally-occurring tellurium on Earth consists of eight isotopes. Two of these have been ...
and
iodine-135
There are 37 known isotopes of iodine (53I) from 108I to 144I; all undergo radioactive decay except 127I, which is stable. Iodine is thus a monoisotopic element.
Its longest-lived radioactive isotope, 129I, has a half-life of 15.7 million year ...
.
135Xe effects on reactor restart
In a typical nuclear reactor fueled with
uranium-235
Uranium-235 (235U or U-235) is an isotope of uranium making up about 0.72% of natural uranium. Unlike the predominant isotope uranium-238, it is fissile, i.e., it can sustain a nuclear chain reaction. It is the only fissile isotope that exis ...
, the presence of
135Xe as a fission product presents designers and operators with problems due to its large
neutron cross section
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 ...
for absorption. Because absorbing neutrons can detrimentally affect a nuclear reactor's ability to increase power, reactors are designed to mitigate this effect; operators are trained to properly anticipate and react to these transients. In fact, during World War II,
Enrico Fermi
Enrico Fermi (; 29 September 1901 – 28 November 1954) was an Italian (later naturalized American) physicist and the creator of the world's first nuclear reactor, the Chicago Pile-1. He has been called the "architect of the nuclear age" and ...
suspected the effect of Xe-135, and followed the advice of
Emilio Segrè
Emilio Gino Segrè (1 February 1905 – 22 April 1989) was an Italian-American physicist and Nobel laureate, who discovered the elements technetium and astatine, and the antiproton, a subatomic antiparticle, for which he was awarded the Nobel ...
in contacting his student
Chien-Shiung Wu
)
, spouse =
, residence =
, nationality = ChineseAmerican
, field = Physics
, work_institutions = Institute of Physics, Academia SinicaUniversity of California at BerkeleySmith CollegePrinceton UniversityColumbia UniversityZhejiang Unive ...
. Wu's soon-to-be published paper on Xe-135 completely verified Fermi's guess that it absorbed neutrons and disrupted the
B Reactor
The B Reactor at the Hanford Site, near Richland, Washington, was the first large-scale nuclear reactor ever built. The project was a key part of the Manhattan Project, the United States nuclear weapons development program during World War II. I ...
that was being used in their project.
During periods of steady state operation at a constant
neutron flux
The neutron flux, φ, is a scalar quantity used in nuclear physics and nuclear reactor physics. It is the total length travelled by all free neutrons per unit time and volume. Equivalently, it can be defined as the number of neutrons travelling ...
level, the
135Xe concentration builds up to its
equilibrium value for that reactor power in about 40 to 50 hours. When the reactor power is increased,
135Xe concentration initially decreases because the burn up is increased at the new higher power level. Because 95% of the
135Xe production is from decay of
iodine-135
There are 37 known isotopes of iodine (53I) from 108I to 144I; all undergo radioactive decay except 127I, which is stable. Iodine is thus a monoisotopic element.
Its longest-lived radioactive isotope, 129I, has a half-life of 15.7 million year ...
, which has a 6.57 hour half-life, the production of
135Xe remains constant; at this point, the
135Xe concentration reaches a minimum. The concentration then increases to the new equilibrium level (more accurately steady state level) for the new power level in roughly 40 to 50 hours. During the initial 4 to 6 hours following the power change, the magnitude and the rate of change of concentration is dependent upon the initial power level and on the amount of change in power level; the
135Xe concentration change is greater for a larger change in power level. When reactor power is decreased, the process is reversed.
Iodine-135 is a fission product of uranium with a yield of about 6% (counting also the iodine-135 produced almost immediately from decay of fission-produced tellurium-135). This
135I decays with a 6.57 hour half-life to
135Xe. Thus, in an operating nuclear reactor,
135Xe is being continuously produced.
135Xe has a very large neutron absorption cross-section, so in the high-neutron-flux environment of a nuclear reactor core, the
135Xe soon absorbs a neutron and becomes almost-stable
136Xe. Thus, in about 50 hours, the
135Xe concentration reaches equilibrium where its creation by
135I decay is balanced with its destruction by neutron absorption.
When reactor power is decreased or
shut down by inserting neutron-absorbing control rods, the reactor neutron flux is reduced and the equilibrium shifts initially towards higher
135Xe concentration. The
135Xe concentration peaks about 11.1 hours after reactor power is decreased. Since
135Xe has a 9.2 hour half-life, the
135Xe concentration gradually decays back to low levels over 72 hours.
The temporarily high level of
135Xe with its high neutron absorption cross-section makes it difficult to restart the reactor for several hours. The neutron-absorbing
135Xe acts like a control rod, reducing reactivity. The inability of a reactor to be started due to the effects of
135Xe is sometimes referred to as xenon-precluded start-up, and the reactor is said to be "poisoned out". The period of time that the reactor is unable to overcome the effects of
135Xe is called the "xenon dead time".
If sufficient
reactivity control authority is available, the reactor can be restarted, but the xenon burn-out
transient
ECHELON, originally a secret government code name, is a surveillance program ( signals intelligence/SIGINT collection and analysis network) operated by the five signatory states to the UKUSA Security Agreement:Given the 5 dialects that ...
must be carefully managed. As the
control rod
Control rods are used in nuclear reactors to control the rate of fission of the nuclear fuel – uranium or plutonium. Their compositions include chemical elements such as boron, cadmium, silver, hafnium, or indium, that are capable of absorbing ...
s are extracted and
criticality is reached,
neutron flux
The neutron flux, φ, is a scalar quantity used in nuclear physics and nuclear reactor physics. It is the total length travelled by all free neutrons per unit time and volume. Equivalently, it can be defined as the number of neutrons travelling ...
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 and neutron flux increases, and the control rods must be gradually reinserted to counter the loss of neutron absorption by the
135Xe. Otherwise, the reactor neutron flux will continue to increase, burning off even more xenon poison, on a path to
runaway criticality. The time constant for this burn-off transient depends on the reactor design, power level history of the reactor for the past several days, and the new power setting. For a typical step up from 50% power to 100% power,
135Xe concentration falls for about 3 hours.
Xenon poisoning was a contributing factor to the
Chernobyl disaster
The Chernobyl disaster was a nuclear accident that occurred on 26 April 1986 at the No. 4 reactor in the Chernobyl Nuclear Power Plant, near the city of Pripyat in the north of the Ukrainian SSR in the Soviet Union. It is one of only two nuc ...
; during a run-down to a lower power, a combination of operator error and xenon poisoning caused the reactor thermal power to fall to near-shutdown levels. The crew's resulting efforts to restore power placed the reactor in a highly unsafe configuration. A flaw in the
SCRAM
A scram or SCRAM is an emergency shutdown of a nuclear reactor effected by immediately terminating the fission reaction. It is also the name that is given to the manually operated kill switch that initiates the shutdown. In commercial reactor ...
system inserted positive reactivity, causing a thermal transient and a steam explosion that tore the reactor apart.
Reactors using continuous reprocessing like many
molten salt reactor
A molten salt reactor (MSR) is a class of nuclear fission reactor in which the primary nuclear reactor coolant and/or the fuel is a molten salt mixture. Only two MSRs have ever operated, both research reactors in the United States. The 1950's Ai ...
designs might be able to extract
135Xe from the fuel and avoid these effects. Fluid fuel reactors cannot develop xenon inhomogeneity because the fuel is free to mix. Also, the
Molten Salt Reactor Experiment
The Molten-Salt Reactor Experiment (MSRE) was an experimental molten salt reactor research reactor at the Oak Ridge National Laboratory (ORNL). This technology was researched through the 1960s, the reactor was constructed by 1964, it went critica ...
demonstrated that spraying the liquid fuel as droplets through a gas space during recirculation can allow xenon and
krypton
Krypton (from grc, κρυπτός, translit=kryptos 'the hidden one') is a chemical element with the symbol Kr and atomic number 36. It is a colorless, odorless, tasteless noble gas that occurs in trace amounts in the atmosphere and is often ...
to leave the fuel salts. However, removing xenon-135 from neutron exposure also causes the reactor to produce more of the
long-lived fission product
Long-lived fission products (LLFPs) are radioactive materials with a long half-life (more than 200,000 years) produced by nuclear fission of uranium and plutonium. Because of their persistent radiotoxicity it is necessary to isolate them from man ...
caesium-135
Caesium (55Cs) has 40 known isotopes, making it, along with barium and mercury, one of the elements with the most isotopes. The atomic masses of these isotopes range from 112 to 151. Only one isotope, 133Cs, is stable. The longest-lived radioisoto ...
.
Decay and capture products
A
135Xe atom that does not
capture a neutron undergoes
beta decay
In nuclear physics, beta decay (β-decay) is a type of radioactive decay in which a beta particle (fast energetic electron or positron) is emitted from an atomic nucleus, transforming the original nuclide to an isobar of that nuclide. For ...
to
135Cs, one of the 7
long-lived fission product
Long-lived fission products (LLFPs) are radioactive materials with a long half-life (more than 200,000 years) produced by nuclear fission of uranium and plutonium. Because of their persistent radiotoxicity it is necessary to isolate them from man ...
s, while a
135Xe that does capture a neutron becomes almost-stable
136Xe.
The probability of capturing a neutron before decay varies with the neutron flux, which itself depends on the kind of reactor, fuel enrichment and power level; and the
135Cs /
136Xe ratio switches its predominant branch very near usual reactor conditions.
Estimates of the proportion of
135Xe during steady-state reactor operation that captures a neutron include 90%, 39%–91% and "essentially all".
For instance, in a (somewhat high) neutron flux of 10
14 n·cm
−2·s
−1, the xenon cross section of σ = cm
2 ( barn) would lead to a capture probability of s
−1, which corresponds to a half-life of about one hour. Compared to the 9.17 hour half-life of
135Xe, this nearly ten-to-one ratio means that under such conditions, essentially all
135Xe would capture a neutron before decay. But if the neutron flux is lowered to one-tenth of this value, like in
CANDU
The CANDU (Canada Deuterium Uranium) is a Canadian pressurized heavy-water reactor design used to generate electric power. The acronym refers to its deuterium oxide ( heavy water) moderator and its use of (originally, natural) uranium fuel. C ...
reactors, the ratio would be 50-50, and half the
135Xe would be converted into
135Cs before neutron capture.
136Xe from neutron capture ends up as part of the eventual stable fission
xenon
Xenon is a chemical element with the symbol Xe and atomic number 54. It is a dense, colorless, odorless noble gas found in Earth's atmosphere in trace amounts. Although generally unreactive, it can undergo a few chemical reactions such as the ...
which also includes
136Xe,
134Xe,
132Xe, and
131Xe produced by fission and
beta decay
In nuclear physics, beta decay (β-decay) is a type of radioactive decay in which a beta particle (fast energetic electron or positron) is emitted from an atomic nucleus, transforming the original nuclide to an isobar of that nuclide. For ...
rather than neutron capture.
Nuclei of
133Xe,
137Xe, and
135Xe that have not captured a neutron all
beta decay
In nuclear physics, beta decay (β-decay) is a type of radioactive decay in which a beta particle (fast energetic electron or positron) is emitted from an atomic nucleus, transforming the original nuclide to an isobar of that nuclide. For ...
to
isotopes of caesium
Caesium (55Cs) has 40 known isotopes, making it, along with barium and mercury, one of the elements with the most isotopes. The atomic masses of these isotopes range from 112 to 151. Only one isotope, 133Cs, is stable. The longest-lived radioisoto ...
. Fission produces
133Xe,
137Xe, and
135Xe in roughly equal amounts but, after neutron capture, fission caesium contains more stable
133Cs (which however can become
134Cs on further
neutron activation
Neutron activation is the process in which neutron radiation induces radioactivity in materials, and occurs when atomic nuclei capture free neutrons, becoming heavier and entering excited states. The excited nucleus decays immediately by emittin ...
) and highly radioactive
137Cs than
135Cs.
Spatial xenon oscillations
Large thermal reactors with low flux coupling between regions may experience spatial power oscillations
[ and ] because of the non-uniform presence of xenon-135. Xenon-induced spatial power oscillations occur as a result of rapid perturbations to power distribution that cause the xenon and iodine distribution to be out of phase with the perturbed power distribution. This results in a shift in xenon and iodine distributions that causes the power distribution to change in an opposite direction from the initial perturbation.
The instantaneous production rate of xenon-135 is dependent on the iodine-135 concentration and therefore on the local neutron flux history. On the other hand, the destruction rate of xenon-135 is dependent on the instantaneous local neutron flux.
The combination of delayed generation and high neutron-capture cross section produces a diversity of impacts on nuclear reactor operation. The mechanism is described in the following four steps.
# An initial lack of symmetry (for example, axial symmetry, in the case of axial oscillations) in the core power distribution (for example as a result of significant control rods movement) causes an imbalance in fission rates within the reactor core, and therefore, in the iodine-135 buildup and the xenon-135 absorption.
# In the high-flux region, xenon-135 burnout allows the flux to increase further, while in the low-flux region, the increase in xenon-135 causes a further reduction in flux. The iodine concentration increases where the flux is high and decreases where the flux is low. This shift in the xenon distribution is such as to increase (decrease) the multiplication properties of the region in which the flux has increased (decreased), thus enhancing the flux tilt.
# As soon as the iodine-135 levels build up sufficiently, decay to xenon reverses the initial situation. Flux decreases in this area, and the former low-flux region increases in power.
# Repetition of these patterns can lead to xenon oscillations moving about the core with periods on the order of about 24 hours.
With little change in overall power level, these oscillations can significantly change the local power levels. This oscillation may go unnoticed and reach dangerous local flux levels if only the total power of the core is monitored. Therefore, most PWRs use tandem power range excore neutron detectors to monitor upper and lower halves of the core separately.
See also
*
Isotopes of xenon
Naturally occurring xenon (54Xe) consists of seven stable isotopes and two very long-lived isotopes. Double electron capture has been observed in 124Xe (half-life ) and double beta decay in 136Xe (half-life ), which are among the longest measured ...
*
Shutdown (nuclear reactor)
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 no ...
References
{{Reflist
Further reading
"Xenon Poisoning" or Neutron Absorption in Reactors
Fission products
Isotopes of xenon
Neutron poisons