Uranium-234

{{short description|Isotope of uranium}} {{Infobox isotope |image = U-234 oxide.png |image_caption = a sample of uranium-234 oxide |alternate_names = Uranium&nbsp;II ({{abbr|hist|historically}}) |mass_number = 234 |symbol = U |num_neutrons = 142 |num_protons = 92 |abundance = 0.0055% |halflife = {{val|245500|u=years}}<ref>{{NUBASE2020}}</ref> |decay_product = thorium-230 |decay_mass = 230 |decay_symbol = Th |parent = uranium-238 |parent_mass = 238 |parent_symbol = U |parent_decay = alpha, beta, beta |parent2 = protactinium-234 |parent2_mass = 234 |parent2_symbol = Pa |parent2_decay = b- |parent3 = plutonium-238 |parent3_mass = 238 |parent3_symbol = Pu |parent3_decay = a |decay_mode1 = Alpha emission |decay_energy1 = 4.858<ref>{{NNDC}}</ref> |decay_mode2 = Spontaneous fission |decay_energy2 = }} '''Uranium-234''' ('''{{SimpleNuclide|Uranium|234}}''' or '''U-234''') is an isotope of uranium. In natural uranium and in uranium ore, ²³⁴U occurs as an indirect decay product of uranium-238, but it makes up only 0.0055% (55 parts per million, or 1/18,000) of the raw uranium because its half-life of just 245,500 years is only about 1/18,000 as long as that of ²³⁸U. Thus the ratio of {{chem|234|U}} to {{chem|238|U}} in a natural sample is equivalent to the ratio of their half-lives. The primary path of production of ²³⁴U via nuclear decay is as follows: uranium-238 nuclei emit an alpha particle to become thorium-234. Next, with a short half-life, ²³⁴Th nuclei emit a beta particle to become protactinium-234 (²³⁴Pa or more usually the isomer ^234mPa). Finally, ²³⁴Pa or ^234mPa nuclei emit another beta particle to become ²³⁴U nuclei.

Uranium-234 nuclei decay by alpha emission to thorium-230, except for the tiny fraction (here less than 2 per trillion) of nuclei that undergo spontaneous fission.

Disequilibrium between the two uranium isotopes does occur in nature when the uranium is dissolved, and is restored again with the half-life of uranium-234; this is the basis of uranium–uranium dating and must be accounted for in the more common uranium–thorium dating.

Extraction of the rather small amount of ²³⁴U from natural uranium would be possible using isotope separation, similar to that used for regular uranium-enrichment. However, there is no real demand in chemistry, physics, or engineering for isolating ²³⁴U, and the small amounts that may be wanted for research can be separated chemically from plutonium-238 that have been aged enough to accumulate its alpha decay product, which is ²³⁴U.

Enriched uranium contains more ²³⁴U than natural uranium as a byproduct of the uranium enrichment process aimed at obtaining uranium-235, which concentrates lighter isotopes even more strongly than it does ²³⁵U. IAEA research paper TECDOC-1529 concludes the ²³⁴U content of enriched fuel is directly proportional to the degree of ²³⁵U—enrichment with 2% ²³⁵U resulting in 150&nbsp;g ²³⁴U/ton HM (heavy metals, special meaning, relating to nuclear fuels), and the most common 4.5% ²³⁵U enrichment resulting in 400&nbsp;g ²³⁴U/ton HM.<ref>{{cite web| url=https://www-pub.iaea.org/MTCD/publications/PDF/te_1529_web.pdf | title=Management of Reprocessed Uranium | access-date=2024-01-24}}</ref> The increased percentage of ²³⁴U in enriched natural uranium is not harmful to the operation of current nuclear reactors.

Uranium-234 has a neutron-capture cross section of about 100 barns for thermal neutrons, and about 700 barns for its resonance integral—the average of neutrons having a range of intermediate energies. In a nuclear reactor non-fissile isotopes ²³⁴U and ²³⁸U both capture a neutron, thereby breeding fissile isotopes ²³⁵U and ²³⁹Pu, respectively. ²³⁴U is converted to ²³⁵U more easily and therefore at a greater rate than ²³⁸U is to ²³⁹Pu (via neptunium-239) because ²³⁸U has a much smaller neutron-capture cross section of just 2.7 barns. In the reaction ²³⁴U + n → ²³⁵U reaction, the ²³⁴U content of 4.5% enriched fuel drops steadily over the irradiation period falling from 450g/ton HM to 205g/ton HM in fuel with an irradiation of 60GWd/ton HM.<ref>{{cite web | url=https://www.nuclear-power.net/wp-content/uploads/2015/06/uranium_234_irradiated_fuel.png | format=PNG | title=U234 Irradiation Graph | access-date=2020-03-24 | archive-date=2020-03-24 | archive-url=https://web.archive.org/web/20200324201555/https://www.nuclear-power.net/wp-content/uploads/2015/06/uranium_234_irradiated_fuel.png | url-status=dead }}</ref>

Additionally, (n,&nbsp;2n) reactions with fast neutrons also convert small amounts of ²³⁵U to ²³⁴U. This is countered by the rapid conversion of available ²³⁴U into ²³⁵U through thermal neutron capture. Uranium from spent nuclear fuel may contain as much as 0.010% ²³⁴U, or 100 parts per million, lower than the original fuel but still a higher fraction than natural uranium's 55 parts per million. Depleted uranium separated during the enrichment process contains much less ²³⁴U (around 0.001%<ref>[https://www.who.int/mediacentre/factsheets/fs257/en/ WHO | Depleted uranium] {{webarchive |url=https://web.archive.org/web/20120815092349/http://www.who.int/mediacentre/factsheets/fs257/en/ |date=August 15, 2012 }}</ref>), which reduces the alpha radioactivity almost half (the beta and gamma activity, which is from ²³⁴Th and ²³⁴Pa, is unchanged) compared to natural uranium having an equilibrium concentration of ²³⁴U in which an equal number of decays of ²³⁸U and ²³⁴U occur.

Uranium-234, as well as uranium-232, is a byproduct, through further neutron capture, in reactors breeding thorium-232 into uranium-233.

{{Isotope sequence |element=uranium |lighter=uranium-233 |heavier=uranium-235 |before=plutonium-238 '''(α)'''<br />protactinium-234 '''(β⁻)'''<br />neptunium-234 '''(β⁺)'''<nowiki/> |after=thorium-230 '''(α)'''<nowiki/> }}

==See also== *Uranium–uranium dating

==References== {{Reflist}} {{Isotopes of uranium}}

Category:Isotopes of uranium Category:Fertile materials