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 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 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 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.

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. 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%), 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.

See also

• Uranium–uranium dating

References