# Uranium-234

Uranium-234 ($^{234}$ U or U-234) is an isotope of uranium. In natural uranium and in uranium ore, $^{234}$U 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 $^{238}$U. Thus the ratio of $^{234}$ U to $^{238}$ U in a natural sample is equivalent to the ratio of their half-lives. The primary path of production of $^{234}$U via nuclear decay is as follows: uranium-238 nuclei emit an alpha particle to become thorium-234. Next, with a short half-life, $^{234}$Th nuclei emit a beta particle to become protactinium-234 ($^{234}$Pa), or more likely a nuclear isomer denoted $^{234m}$Pa. Finally, $^{234}$Pa or $^{234m}$Pa nuclei emit another beta particle to become $^{234}$U 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 $^{234}$U 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 $^{234}$U. Very small pure samples of $^{234}$U can be extracted via the chemical ion-exchange process from samples of plutonium-238 that have been aged somewhat to allow some decay to $^{234}$U via alpha emission.
Enriched uranium contains more $^{234}$U 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 $^{235}$U. IAEA research paper TECDOC-1529 concludes the $^{234}$U content of enriched fuel is directly proportional to the degree of $^{235}$U—enrichment with 2% $^{235}$U resulting in 150 g $^{234}$U/ton HM, and the most common 4.5% $^{235}$U enrichment resulting in 400 g $^{234}$U/tonHM. The increased percentage of $^{234}$U in enriched natural uranium is acceptable in current nuclear reactors. Recycled (re-enriched) reprocessed uranium contains even higher fractions of $^{234}$U. This is advantageous because while $^{234}$U is not fissile, it tends to absorb slow neutrons in a nuclear reactor breeding $^{235}$U. This is much more efficient than the series of steps $^{238}$U + n → $^{239}$Np → $^{239}$Pu 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 $^{234}$U and $^{238}$U both capture a neutron, thereby breeding fissile isotopes $^{235}$U and $^{239}$Pu, respectively. $^{234}$U is converted to $^{235}$U more easily and therefore at a greater rate than $^{238}$U is to $^{239}$Pu (via neptunium-239) because $^{238}$U has a much smaller neutron-capture cross section of just 2.7 barns. In the reaction $^{234}$U + n → $^{235}$U reaction, the $^{234}$U 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 $^{235}$U to $^{234}$U. This is countered by the rapid conversion of available $^{234}$U into $^{235}$U through thermal neutron capture. Spent fuel may contain as much as 0.010% $^{234}$U, 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 $^{234}$U (around 0.001%), which makes the radioactivity of depleted uranium about half of that of natural uranium. Natural uranium has an "equilibrium" concentration of $^{234}$U—the point at which an equal number of decays of $^{238}$U and $^{234}$U will occur.
Uranium-234 as well as uranium-232 are common byproducts in reactors breeding thorium-232 into uranium-233.
| Lighter: uranium-233 | Uranium-234 is an isotope of uranium | Heavier: uranium-235 |
| Decay product of: plutonium-238 (α) protactinium-234 (β$^−$) neptunium-234 (β$^+$) | Decay chain of uranium-234 | Decays to: thorium-230 (α) |