 a sample of uranium-234 oxide | |
| General | |
|---|---|
| Symbol | 234U |
| Names | uranium-234, Uranium II (hist) |
| Protons(Z) | 92 |
| Neutrons(N) | 142 |
| Nuclide data | |
| Natural abundance | 0.0055% |
| Half-life(t1/2) | 245500 years[1] |
| Parent isotopes | 238U (alpha, beta, beta) 234Pa (β−) 238Pu (α) |
| Decay products | 230Th |
| Decay modes | |
| Decay mode | Decay energy (MeV) |
| Alpha emission | 4.858[2] |
| Isotopes of uranium Complete table of nuclides | |
Uranium-234 (234
U orU-234) is anisotope of uranium. Innatural uranium and in uranium ore,234U occurs as an indirectdecay product ofuranium-238, but it makes up only 0.0055% (55parts per million, or 1/18,000) of the raw uranium because itshalf-life of just 245,500 years is only about 1/18,000 as long as that of238U. Thus the ratio of234
U to238
U in a natural sample is equivalent to the ratio of their half-lives. The primary path of production of234U vianuclear decay is as follows: uranium-238 nuclei emit analpha particle to becomethorium-234. Next, with a short half-life,234Th nuclei emit abeta particle to becomeprotactinium-234 (234Pa or more usually theisomer234mPa). Finally,234Pa or234mPa nuclei emit another beta particle to become234U nuclei.
Uranium-234 nuclei decay byalpha emission to thorium-230, except for the tiny fraction (here less than 2 per trillion) of nuclei that undergospontaneous 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 ofuranium-uranium dating and must be accounted for in the more commonuranium-thorium dating.
Extraction of the rather small amount of234U from natural uranium would be possible usingisotope separation, similar to that used forregular uranium-enrichment. However, there is no real demand inchemistry,physics, orengineering for isolating234U, and the small amounts that may be wanted for research can be separated chemically fromplutonium-238 that have been aged enough to accumulate itsalpha decay product, which is234U.
Enriched uranium contains more234U than natural uranium as abyproduct of the uranium enrichment process aimed at obtaininguranium-235, which concentrates lighter isotopes even more strongly than it does235U. IAEA research paper TECDOC-1529 concludes the234U content of enriched fuel is directly proportional to the degree of235U—enrichment with 2%235U resulting in 150 g234U/ton HM, and the most common 4.5%235U enrichment resulting in 400 g234U/tonHM.[3] The increased percentage of234U in enriched natural uranium is not harmful to the operation of currentnuclear reactors.
Uranium-234 has aneutron-capture cross section of about 100barns forthermal neutrons, and about 700 barns for itsresonance integral—the average of neutrons having a range of intermediate energies. In anuclear reactor non-fissile isotopes234U and238U both capture a neutron, thereby breeding fissile isotopes235U and239Pu, respectively.234U is converted to235U more easily and therefore at a greater rate than238U is to239Pu (vianeptunium-239) because238U has a much smaller neutron-capture cross section of just 2.7 barns. In the reaction234U + n →235U reaction, the234U 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.[4]
Additionally, (n, 2n) reactions with fast neutrons also convert small amounts of235U to234U. This is countered by the rapid conversion of available234U into235U throughthermal neutron capture. Uranium fromspent nuclear fuel may contain as much as 0.010%234U, or 100parts 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 less234U (around 0.001%[5]), which reduces the alpha radioactivity almost half (the beta and gamma activity, which is from234Th and234Pa, is unchanged) compared to natural uranium having an equilibrium concentration of234U in which an equal number of decays of238U and234U occur.
Uranium-234, as well asuranium-232, is a byproduct, through further neutron capture, in reactors breedingthorium-232 intouranium-233.
| Lighter: uranium-233 | Uranium-234 is an isotope ofuranium | 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) |
