Xenon-135

Last updated
Xenon-135, 135Xe
General
Symbol 135Xe
Names xenon-135, 135Xe, Xe-135
Protons (Z)54
Neutrons (N)81
Nuclide data
Natural abundance syn
Half-life (t1/2)9.14±0.02 h
Spin 3/2+
Excess energy −86413±4 keV
Binding energy 8398.476±0.028 keV
Decay products 135Cs
Decay modes
Decay mode Decay energy (MeV)
Beta decay 1.168
Isotopes of xenon
Complete table of nuclides

Xenon-135 (135Xe) is an unstable isotope of xenon with a half-life of about 9.2 hours. 135Xe is a fission product of uranium and it is the most powerful known neutron-absorbing nuclear poison (2 million barns; [1] up to 3 million barns [1] under reactor conditions [2] ), with a significant effect on nuclear reactor operation. The ultimate yield of xenon-135 from fission is 6.3%, though most of this is from fission-produced tellurium-135 and iodine-135.

Contents

135Xe effects on reactor restart

In a typical nuclear reactor fueled with uranium-235, the presence of 135Xe as a fission product presents designers and operators with problems due to its large neutron cross section 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 suspected the effect of Xe-135, and followed the advice of Emilio Segrè in contacting his student Chien-Shiung Wu. Wu's soon-to-be published paper on Xe-135 completely verified Fermi's guess that it absorbed neutrons and disrupted the B Reactor that was being used in their project. [3] [4]

During periods of steady state operation at a constant neutron flux 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, 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. [5]

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). [6] 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". [7] 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 must be carefully managed. As the control rods 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 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. [8]

Xenon poisoning was a contributing factor to the Chernobyl disaster; 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 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 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 demonstrated that spraying the liquid fuel as droplets through a gas space during recirculation can allow xenon and krypton to leave the fuel salts. Removing xenon-135 from neutron exposure improves neutron economy, but causes the reactor to produce more of the long-lived fission product caesium-135. The long lived (but 76000 times less radioactive) caesium-135 condenses in a separate tank after the decay of xenon-135, and is physically separate from the 30.05 year half life caesium-137 produced in the fuel, and it is practical to handle them separately (fission yield is appr. 6% for both).

Decay and capture products

A 135Xe atom that does not capture a neutron undergoes beta decay to 135Cs, one of the 7 long-lived fission products, 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%, [9] 39%–91% [10] and "essentially all". [11] For instance, in a (somewhat high) neutron flux of 1014 n·cm−2·s−1, the xenon cross section of σ = 2.65×10−18 cm2 (2.65×106 barn) would lead to a capture probability of 2.65×10−4 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 reactors, the ratio would be 50-50, and half the 135Xe would decay to 135Cs before neutron capture.

136Xe from neutron capture ends up as part of the eventual stable fission xenon which also includes 134Xe, 132Xe, and 131Xe produced by fission and beta decay rather than neutron capture.

Nuclei of 133Xe, 137Xe, and 135Xe that have not captured a neutron all beta decay to isotopes of caesium. 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) and highly radioactive 137Cs than 135Cs.

Spatial xenon oscillations

Large thermal reactors with low flux coupling between regions may experience spatial power oscillations [12] 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.

  1. 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.
  2. 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.
  3. 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.
  4. 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

Related Research Articles

<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">Nuclear fuel cycle</span> Process of manufacturing and consuming nuclear fuel

The nuclear fuel cycle, also called nuclear fuel chain, is the progression of nuclear fuel through a series of differing stages. It consists of steps in the front end, which are the preparation of the fuel, steps in the service period in which the fuel is used during reactor operation, and steps in the back end, which are necessary to safely manage, contain, and either reprocess or dispose of spent nuclear fuel. If spent fuel is not reprocessed, the fuel cycle is referred to as an open fuel cycle ; if the spent fuel is reprocessed, it is referred to as a closed fuel cycle.

<span class="mw-page-title-main">Natural nuclear fission reactor</span> Naturally occurring uranium self-sustaining nuclear chain reactions

A natural nuclear fission reactor is a uranium deposit where self-sustaining nuclear chain reactions occur. The conditions under which a natural nuclear reactor could exist were predicted in 1956 by Paul Kuroda. The remnants of an extinct or fossil nuclear fission reactor, where self-sustaining nuclear reactions have occurred in the past, are verified by analysis of isotope ratios of uranium and of the fission products. This was first discovered in 1972 in Oklo, Gabon by Francis Perrin under conditions very similar to Kuroda's predictions.

<span class="mw-page-title-main">Control rod</span> Device used to regulate the power of a nuclear reactor

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 many neutrons without themselves decaying. These elements have different neutron capture cross sections for neutrons of various energies. Boiling water reactors (BWR), pressurized water reactors (PWR), and heavy-water reactors (HWR) operate with thermal neutrons, while breeder reactors operate with fast neutrons. Each reactor design can use different control rod materials based on the energy spectrum of its neutrons. Control rods have been used in nuclear aircraft engines like Project Pluto as a method of control.

<span class="mw-page-title-main">Uranium-238</span> Isotope of uranium

Uranium-238 is the most common isotope of uranium found in nature, with a relative abundance of 99%. Unlike uranium-235, it is non-fissile, which means it cannot sustain a chain reaction in a thermal-neutron reactor. However, it is fissionable by fast neutrons, and is fertile, meaning it can be transmuted to fissile plutonium-239. 238U cannot support a chain reaction because inelastic scattering reduces neutron energy below the range where fast fission of one or more next-generation nuclei is probable. Doppler broadening of 238U's neutron absorption resonances, increasing absorption as fuel temperature increases, is also an essential negative feedback mechanism for reactor control.

<span class="mw-page-title-main">Nuclear fission product</span> Atoms or particles produced by nuclear fission

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 heat energy, and gamma rays. The two smaller nuclei are the fission products..

Naturally occurring samarium (62Sm) is composed of five stable isotopes, 144Sm, 149Sm, 150Sm, 152Sm and 154Sm, and two extremely long-lived radioisotopes, 147Sm (half life: 1.06×1011 y) and 148Sm (7×1015 y), with 152Sm being the most abundant (26.75% natural abundance). 146Sm is also fairly long-lived, but is not long-lived enough to have survived in significant quantities from the formation of the Solar System on Earth, although it remains useful in radiometric dating in the Solar System as an extinct radionuclide. A 2012 paper revising the estimated half-life of 146Sm from 10.3(5)×107 y to 6.8(7)×107 y was retracted in 2023. It is the longest-lived nuclide that has not yet been confirmed to be primordial.

Naturally occurring xenon (54Xe) consists of seven stable isotopes and two very long-lived isotopes. Double electron capture has been observed in 124Xe and double beta decay in 136Xe, which are among the longest measured half-lives of all nuclides. The isotopes 126Xe and 134Xe are also predicted to undergo double beta decay, but this has never been observed in these isotopes, so they are considered to be stable. Beyond these stable forms, 32 artificial unstable isotopes and various isomers have been studied, the longest-lived of which is 127Xe with a half-life of 36.345 days. All other isotopes have half-lives less than 12 days, most less than 20 hours. The shortest-lived isotope, 108Xe, has a half-life of 58 μs, and is the heaviest known nuclide with equal numbers of protons and neutrons. Of known isomers, the longest-lived is 131mXe with a half-life of 11.934 days. 129Xe is produced by beta decay of 129I ; 131mXe, 133Xe, 133mXe, and 135Xe are some of the fission products of both 235U and 239Pu, so are used as indicators of nuclear explosions.

<span class="mw-page-title-main">Isotopes of iodine</span> Nuclides with atomic number of 53 but with different mass numbers

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.

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 radioisotopes are 135Cs with a half-life of 2.3 million years, 137
Cs
 with a half-life of 30.1671 years and 134Cs with a half-life of 2.0652 years. All other isotopes have half-lives less than 2 weeks, most under an hour.

<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">Spent nuclear fuel</span> Nuclear fuel thats been irradiated in a nuclear reactor

Spent nuclear fuel, occasionally called used nuclear fuel, is nuclear fuel that has been irradiated in a nuclear reactor. It is no longer useful in sustaining a nuclear reaction in an ordinary thermal reactor and, depending on its point along the nuclear fuel cycle, it will have different isotopic constituents than when it started.

<span class="mw-page-title-main">Fission products (by element)</span> Breakdown of nuclear fission results

This page discusses each of the main elements in the mixture of fission products produced by nuclear fission of the common nuclear fuels uranium and plutonium. The isotopes are listed by element, in order by atomic number.

Uranium-236 (236U) is an isotope of uranium that is neither fissile with thermal neutrons, nor very good fertile material, but is generally considered a nuisance and long-lived radioactive waste. It is found in spent nuclear fuel and in the reprocessed uranium made from spent nuclear fuel.

Nuclear fission splits a heavy nucleus such as uranium or plutonium into two lighter nuclei, which are called fission products. Yield refers to the fraction of a fission product produced per fission.

Long-lived fission products (LLFPs) are radioactive materials with a long half-life produced by nuclear fission of uranium and plutonium. Because of their persistent radiotoxicity, it is necessary to isolate them from humans and the biosphere and to confine them in nuclear waste repositories for geological periods of time.

The iodine pit, also called the iodine hole or xenon pit, is a temporary disabling of a nuclear reactor due to buildup of short-lived nuclear poisons in the reactor core. The main isotope responsible is 135Xe, mainly produced by natural decay of 135I. 135I is a weak neutron absorber, while 135Xe is the strongest known neutron absorber. When 135Xe builds up in the fuel rods of a reactor, it significantly lowers their reactivity, by absorbing a significant amount of the neutrons that provide the nuclear reaction.

<span class="mw-page-title-main">Oklo Mine</span> Natural nuclear fission reactor discovered in 1972 in the Oklo region of Gabon

Oklo Mine, located in Oklo, Gabon on the west coast of Central Africa, is believed to have been the only natural nuclear fission reactor. Oklo consists of 16 sites at which self-sustaining nuclear fission reactions are thought to have taken place approximately 1.7 billion years ago, and ran for hundreds of thousands of years. It is estimated to have averaged under 100 kW of thermal power during that time.

References

  1. 1 2 "Livechart - Table of Nuclides - Nuclear structure and decay data".
  2. ↑ ""Xenon Poisoning" or Neutron Absorption in Reactors".
  3. ↑ Benczer-Koller, Noemie (January 2009). "Chien-shiungwu 1912—1997" (PDF).
  4. ↑ Lykknes, Annette (2019-01-02). Women In Their Element: Selected Women's Contributions To The Periodic System. ISBN   9789811206306.
  5. ↑ DOE Fundamentals Handbook: Nuclear Physics and Reactor Theory Volume 2 (PDF). U.S. Department of Energy. January 1993. Archived from the original (PDF) on 2013-02-14., pp. 35–42.
  6. ↑ DOE Fundamentals Handbook: Nuclear Physics and Reactor Theory Volume 2 (PDF). U.S. Department of Energy. January 1993. Archived from the original (PDF) on 2013-02-14., p. 35.
  7. ↑ Crist, J. E. "Xenon, A Fission Product Poison" (PDF). candu.org. Archived from the original (PDF) on February 3, 2007. Retrieved 2 November 2011.
  8. ↑ Xenon decay transient graph Archived June 24, 2018, at the Wayback Machine
  9. ↑ CANDU Fundamentals: 20 Xenon: A Fission Product Poison Archived July 23, 2011, at the Wayback Machine
  10. ↑ Utilization of the Isotopic Composition of Xe and Kr in Fission Gas Release Research Archived October 19, 2013, at the Wayback Machine
  11. ↑ Roggenkamp, Paul L. "The Influence of Xenon-135 on Reactor Operation" (PDF). Westinghouse Savannah River Company. Retrieved 18 October 2013.
  12. ↑ "Xenon-135". www.nuclear-power.net. Retrieved 2017-09-19. and "Xenon-135 Oscillations".

Further reading

This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.