| 词条 | Uranium-234 |
| 释义 |
| image = | background = #fc6 | text_color = | isotope_name = uranium-234 | isotope_filename = | alternate_names = | mass_number = 234 | symbol = U | num_neutrons = 142 | num_protons = 92 | abundance = 0.0054% | halflife = 246,000 years | error_halflife = | decay_product = thorium-230 | decay_mass = 230 | decay_symbol = Th | parent = uranium-238 | parent_mass = 238 | parent_symbol = U | parent_decay = alpha, beta, beta | parent2 = protactinium-234 | parent2_mass = 234 | parent2_symbol = Pa | parent2_decay = b- | parent3 = plutonium-238 | parent3_mass = 238 | parent3_symbol = Pu | parent3_decay = a | mass = | spin = | excess_energy = | error1 = | binding_energy = | error2 = | decay_mode1 = alpha emission | decay_energy1 = | decay_mode2 = spontaneous fission | decay_energy2 = | decay_mode3 = | decay_energy3 = | decay_mode4 = | decay_energy4 = Uranium-234 is an isotope of uranium. In natural uranium and in uranium ore, U-234 occurs as an indirect decay product of uranium-238, but it makes up only 0.0055% (55 parts per million) of the raw uranium because its half-life of just 245,500 years is only about 1/18,000 as long as that of U-238. The primary path of production of U-234 via nuclear decay is as follows: U-238 nuclei emit an alpha particle to become thorium-234 (Th-234). Next, with a short half-life, Th-234 nuclei emit a beta particle to become protactinium-234 (Pa-234), or more likely a nuclear isomer denoted Pa-234m. Finally, Pa-234 or Pa-234m nuclei emit another beta particle to become U-234 nuclei. U-234 nuclei decay by alpha emission to thorium-230, except for the tiny fraction (parts per billion) of nuclei which undergo spontaneous fission. Extraction of rather small amounts of U-234 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 U-234. Very small pure samples of U-234 can be extracted via the chemical ion-exchange process - from samples of plutonium-238 that have been aged somewhat to allow some decay to U-234 via alpha emission. Enriched uranium contains more U-234 than natural uranium as a byproduct of the uranium enrichment process aimed at obtaining U-235, which concentrates lighter isotopes even more strongly than it does U-235. The increased percentage of U-234 in enriched natural uranium is acceptable in current nuclear reactors, but (re-enriched) reprocessed uranium might contain even higher fractions of U-234, which is undesirable. This is because U-234 is not fissile, and tends to absorb slow neutrons in a nuclear reactor - becoming U-235. U-234 has a neutron-capture cross section of about 100 barns for thermal neutrons, and about 700 barns for its resonance integral - the average over neutrons having various intermediate energies. In a nuclear reactor non-fissile isotopes capture a neutron breeding fissile isotopes. U-234 is converted to U-235 more easily and therefore at a greater rate than U-238 is to Pu-239 (via neptunium-239) because U-238 has a much smaller neutron-capture cross-section of just 2.7 barns. However, (n, 2n) reactions with fast neutrons also convert small amounts of U-235 to U-234, so that spent nuclear fuel may contain about 0.010% U-234, a much higher fraction than in non-irradiated uranium.[1] Depleted uranium contains much less U-234 (around 0.001%[2]) which makes the radioactivity of depleted uranium about one-half of that of natural uranium. Natural uranium has an "equilibrium" concentration of U-234 at the point where an equal number of decays of U-238 and U-234 will occur. {{Isotope|element=uranium|lighter=uranium-233 |heavier=uranium-235 |before=plutonium-238 (α) protactinium-234 (β-) neptunium-234 (β+) |after=thorium-230 (α) }} See also
References1. ^{{Cite web |url=http://www.francenuc.org/en_mat/uranium4_e.htm |title=Archived copy |access-date=2013-03-29 |archive-url=https://web.archive.org/web/20071019023302/http://francenuc.org/en_mat/uranium4_e.htm |archive-date=2007-10-19 |dead-url=yes |df= }} {{Isotopes of uranium}}2. ^WHO | Depleted uranium {{webarchive |url=https://web.archive.org/web/20120815092349/http://www.who.int/mediacentre/factsheets/fs257/en/ |date=August 15, 2012 }} 3 : Actinides|Isotopes of uranium|Fertile materials |
| 随便看 |
|
开放百科全书收录14589846条英语、德语、日语等多语种百科知识，基本涵盖了大多数领域的百科知识，是一部内容自由、开放的电子版国际百科全书。