Tritium
Tritium or hydrogen-3 is a rare and radioactive isotope of hydrogen with a half-life of 12.32 years. The tritium nucleus contains one proton and two neutrons, whereas the nucleus of the common isotope hydrogen-1 contains one proton and no neutrons, and that of non-radioactive hydrogen-2 contains one proton and one neutron. Tritium is the heaviest particle-bound isotope of hydrogen. It is one of the few nuclides with a distinct name. The use of the name hydrogen-3, though more systematic, is much less common.
Naturally occurring tritium is extremely rare on Earth. The atmosphere has only trace amounts, formed by the interaction of its gases with cosmic rays. It can be produced artificially by irradiation of lithium or lithium-bearing ceramic pebbles in a nuclear reactor and is a low-abundance byproduct in normal operations of nuclear reactors.
Tritium is used as the energy source in radioluminescent lights for watches, night sights for firearms, numerous instruments and tools, and novelty items such as self-illuminating key chains. It is used in a medical and scientific setting as a radioactive tracer. Tritium is also used as a nuclear fusion fuel, along with more abundant deuterium, in tokamak reactors and in hydrogen bombs. Tritium has also been used commercially in betavoltaic devices such as NanoTritium batteries.
History
Tritium was first detected in 1934 by Ernest Rutherford, Mark Oliphant and Paul Harteck after bombarding deuterium with deuterons. Deuterium is another isotope of hydrogen, which occurs naturally with an abundance of 0.015%. Their experiment could not isolate tritium, which was first accomplished in 1939 by Luis Alvarez and Robert Cornog, who also realized tritium's radioactivity. Willard Libby recognized in 1954 that tritium could be used for radiometric dating of water and wine.Decay
Tritium decays into helium-3 by beta-minus decay as shown in this nuclear equation:releasing 18.6 keV of energy in the process. The electron's kinetic energy varies, with an average of 5.7 keV, while the remaining energy is carried off by the nearly undetectable electron antineutrino. Beta particles from tritium can penetrate only about of air, and they are incapable of passing through the dead outermost layer of human skin. Because of their low energy compared to other beta particles, the amount of bremsstrahlung generated is also lower. The unusually low energy released in the tritium beta decay makes the decay useful for attempts at absolute neutrino mass measurement, none of which has yet succeeded.
The low energy of tritium's radiation makes it difficult to detect tritium-labeled compounds except by using liquid scintillation counting.
Production
Lithium
Tritium is most often produced in nuclear reactors by neutron activation of lithium-6. The release and diffusion of tritium and helium produced by the fission of lithium can take place within ceramics known as breeder ceramics. Production of tritium from lithium-6 in such breeder ceramics is possible with neutrons of any energy, though the cross section is higher when the incident neutrons have lower energy, reaching more than 900 barns for thermal neutrons. This is an exothermic reaction, yielding 4.8 MeV. In comparison, fusion of deuterium with tritium releases about 17.6 MeV. For applications in proposed fusion energy reactors, such as ITER, pebbles consisting of lithium bearing ceramics including LiTiO and LiSiO, are being developed for tritium breeding within a helium-cooled pebble bed, also known as a breeder blanket.High-energy neutrons can also produce tritium from lithium-7 in an endothermic reaction, consuming 2.466 MeV. This was discovered when the 1954 Castle Bravo nuclear test produced an unexpectedly high yield. Prior to this test, it was incorrectly assumed that would absorb a neutron to become, which would beta-decay to, which in turn would decay to two nuclei on a total timeframe much longer than the duration of the explosion.
The slowed neutrons from this reaction can still react with in the first, exothermic reaction; thus lithium can generate more tritium atoms than neutrons absorbed.
Boron
High-energy neutrons irradiating boron-10, also occasionally produce tritium:A more common result of boron-10 neutron capture is Li and a single alpha particle.
Especially in pressurized water reactors which only partially thermalize neutrons, the interaction between relatively fast neutrons and the boric acid added as a chemical shim produces small but non-negligible quantities of tritium.
Deuterium
Tritium is also produced in heavy water-moderated reactors whenever a deuterium nucleus captures a neutron. This reaction has a small absorption cross section, making heavy water a good neutron moderator, and relatively little tritium is produced. Even so, cleaning tritium from the moderator may be desirable after several years to reduce the risk of its escaping to the environment. Ontario Power Generation's "Tritium Removal Facility" is capable of processing up to of heavy water a year, and it separates out about of tritium, making it available for other uses.CANDU reactors typically produce of tritium per year, which is recovered at the Darlington Tritium Recovery Facility attached to the 3,512 MW Darlington Nuclear Generating Station in Ontario. The total production at DTRF between 1989 and 2011 was – with an activity of : an average of about per year.
Deuterium's absorption cross section for thermal neutrons is about 0.52 millibarn, whereas that of oxygen-16 is about 0.19 millibarn and that of oxygen-17 is about 240 millibarns. While O is by far the most common isotope of oxygen in both natural oxygen and heavy water; depending on the method of isotope separation, heavy water may be slightly richer in O and O. Due to both neutron capture and reactions they are net "neutron consumers" and are thus undesirable in a moderator of a natural uranium reactor which needs to keep neutron absorption outside the fuel as low as feasible. Some facilities that remove tritium also remove O and O, which can – at least in principle – be used for isotope labeling.
India, which also has a large fleet of pressurized heavy water reactors, also removes at least some of the tritium produced in the moderator/coolant of its reactors but due to the dual use nature of tritium and the Indian nuclear bomb program, less information about this is publicly available than for Canada.
Fission
Tritium is an uncommon product of the nuclear fission of uranium-235, plutonium-239, and uranium-233,, with a production of about one atom per 10 fissions, meaning it is in fact produced at all reactors. The release or recovery of tritium needs to be considered in the operation of nuclear reactors, especially in the reprocessing of nuclear fuel and storage of spent nuclear fuel. The production of tritium is not a goal, but a side-effect. It is discharged to the atmosphere in small quantities by some nuclear power plants. Voloxidation is an optional additional step in nuclear reprocessing that removes volatile fission products before an aqueous process begins. This would in principle enable economic recovery of the produced tritium, but even if the tritium is only disposed of and not used, it has the potential to reduce tritium contamination in the water used, reducing radioactivity released when the water is discharged since tritiated water cannot be removed from "ordinary" water except by isotope separation.Given the specific activity of tritium at, one TBq is equivalent to roughly 2.8 mg.
Fukushima Daiichi
In June 2016 the Tritiated Water Task Force released a report on the status of tritium in tritiated water at Fukushima Daiichi nuclear plant, as part of considering options for final disposal of the stored contaminated cooling water. This identified that the March 2016 holding of tritium on-site was 760 TBq in a total of 860,000 m of stored water. This report also identified the reducing concentration of tritium in the water extracted from the buildings etc. for storage, seeing a factor of ten decrease over the five years considered, 3.3 MBq/L to 0.3 MBq/L.According to a report by an expert panel considering the best approach to dealing with this issue, "Tritium could be separated theoretically, but there is no practical separation technology on an industrial scale. Accordingly, a controlled environmental release is said to be the best way to treat low-tritium-concentration water." After a public information campaign sponsored by the Japanese government, the gradual release into the sea of the tritiated water began on 24 August 2023 and is the first of four releases through March 2024. The entire process will take "decades" to complete. China reacted with protest.
The IAEA has endorsed the plan. The water released is diluted to reduce the tritium concentration to less than 1500 Bq/L, far below the limit recommended in drinking water by the WHO.
Helium-3
Tritium's decay product helium-3 has a very large cross section for reacting with thermal neutrons, expelling a proton; hence, it is rapidly converted back to tritium in nuclear reactors.This could allow tritium to be recycled as it decays, maintaining the desired inventory.
Cosmic rays
Tritium occurs naturally due to cosmic rays interacting with atmospheric gases. In the most important reaction for natural production, a fast neutron interacts with atmospheric nitrogen:Worldwide, the production of tritium from natural sources is about 4 megacuries per year. The global equilibrium inventory of tritium created by natural sources remains approximately constant at 70 megacuries, as a balance between the fixed production rate and nuclear decay. These may be taken as 415 g and 7,250 g respectively.
Production history
USA
Tritium for American nuclear weapons was produced in special heavy water reactors at the Savannah River Site until their closures in 1988. With the Strategic Arms Reduction Treaty after the end of the Cold War, the existing supplies were sufficient for the new, smaller number of nuclear weapons for some time.of tritium was produced in the United States from 1955 to 1996. Since it continually decays into helium-3, the total amount remaining was about at the time of the report,
and about as of 2023.
Tritium production was resumed with irradiation of rods containing lithium, at the reactors of the commercial Watts Bar Nuclear Plant from 2003 to 2005 followed by extraction of tritium from the rods at the Tritium Extraction Facility at the Savannah River Site beginning in November 2006. Tritium leakage from the rods during reactor operations limits the number that can be used in any reactor without exceeding the maximum allowed tritium levels in the coolant.