uranium-234

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 to 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 likely a nuclear isomer denoted ^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 (parts per billion) of nuclei that undergo spontaneous fission.

Extraction of rather small amounts of ²³⁴U from natural uranium would be feasible 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. Very small pure samples of ²³⁴U can be extracted via the chemical ion-exchange process from samples of plutonium-238 that have been aged somewhat to allow some decay to ²³⁴U via alpha emission.

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 g ²³⁴U/ton HM, and the most common 4.5% ²³⁵U enrichment resulting in 400 g ²³⁴U/tonHM. The increased percentage of ²³⁴U in enriched natural uranium is acceptable in current nuclear reactors. Recycled (re-enriched) reprocessed uranium contains even higher fractions of ²³⁴U. This is advantageous because while ²³⁴U is not fissile, it tends to absorb slow neutrons in a nuclear reactor breeding ²³⁵U. This is much more efficient than the series of steps ²³⁸U + n → ²³⁹Np → ²³⁹Pu in replacing fissile isotope consumption.

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.

Additionally, (n, 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. Spent fuel may contain as much as 0.010% ²³⁴U, or 100 parts per million, a higher fraction than in natural uranium's 55 parts per million. Depleted uranium separated during the enrichment process contains much less ²³⁴U (around 0.001%WHO , Depleted uranium
), which makes the radioactivity of depleted uranium about half of that of natural uranium. Natural uranium has an "equilibrium" concentration of ²³⁴U—the point at which an equal number of decays of ²³⁸U and ²³⁴U will occur.

Uranium-234 as well as uranium-232 are common byproducts in reactors breeding thorium-232 into uranium-233.

See also

* Uranium–uranium dating

References

{{Isotopes of uranium

Actinides
Isotopes of uranium
Fertile materials