Uranium-234 (234U 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 234U to 238U 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 (heavy metals, special meaning, relating to nuclear fuels), and the most common 4.5% 235U enrichment resulting in 400 g 234U/ton HM. 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
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