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Uranium-238 ( or U-238) is the most common isotope of uranium found in nature, with a relative abundance above 99%. Unlike uranium-235, it is non-fissile, which means it cannot sustain a chain reaction in a thermal-neutron reactor. However, it is fissionable by fast neutrons, and is fertile material, meaning it can be transmuted to fissile plutonium-239. 238U cannot support a chain reaction because inelastic scattering reduces neutron energy below the range where fast fission of one or more next-generation nuclei is probable. Doppler broadening of 238U's neutron absorption , increasing absorption as fuel temperature increases, is also an essential negative feedback mechanism for reactor control.
The isotope has a half-life of 4.463 billion years (). Due to its abundance and half-life relative rate of decay to other radioactive elements, 238U is responsible for about 40% of the radioactive heat produced within the Earth. The 238U decay chain contributes six electron anti-neutrinos per 238U nucleus (one per beta decay), resulting in a large detectable geoneutrino signal when decays occur within the Earth. The decay of 238U to daughter isotopes is extensively used in radiometric dating, particularly for material older than approximately 1 million years.
Depleted uranium has an even higher concentration of the 238U isotope, and even low-enriched uranium (LEU), while having a higher proportion of the uranium-235 isotope (in comparison to depleted uranium), is still mostly 238U. Reprocessed uranium is also mainly 238U, with about as much uranium-235 as natural uranium, a comparable proportion of uranium-236, and much smaller amounts of other isotopes of uranium such as uranium-234, uranium-233, and uranium-232.
238U can be used as a source material for creating plutonium-239, which can in turn be used as nuclear fuel. carry out such a process of transmutation to convert the fertile material isotope 238U into fissile 239Pu. It has been estimated that there is anywhere from 10,000 to five billion years worth of 238U for use in these power station. Facts from Cohen . Formal.stanford.edu (2007-01-26). Retrieved on 2010-10-24. Breeder technology has been used in several experimental nuclear reactors. Advanced Nuclear Power Reactors | Generation III+ Nuclear Reactors . World-nuclear.org. Retrieved on 2010-10-24.
By December 2005, the only breeder reactor producing power was the 600-megawatt BN-600 reactor at the Beloyarsk Nuclear Power Station in Russia. Russia later built another unit, BN-800, at the Beloyarsk Nuclear Power Station which became fully operational in November 2016. Also, Japan's Monju breeder reactor, which has been inoperative for most of the time since it was originally built in 1986, was ordered for decommissioning in 2016, after safety and design hazards were uncovered, with a completion date set for 2047. Both China and India have announced plans to build nuclear breeder reactors.
The breeder reactor as its name implies creates larger quantities of 239Pu or 233U (the fissile isotopes) than it consumes.
The Clean And Environmentally Safe Advanced Reactor (CAESAR), a nuclear reactor concept that would use steam as a moderator to control , will potentially be able to use 238U as fuel once the reactor is started with Low-enriched uranium (LEU) fuel. This design is still in the early stages of development.
DUCRETE, a concrete made with uranium dioxide aggregate instead of gravel, is being investigated as a material for dry cask storage systems to store radioactive waste.
238U from depleted uranium and natural uranium is also used with recycled 239Pu from nuclear weapons stockpiles for making mixed oxide fuel (MOX), which is now being redirected to become fuel for nuclear reactors. This dilution, also called downblending, means that any nation or group that acquired the finished fuel would have to repeat the very expensive and complex chemical separation of uranium and plutonium process before assembling a weapon.
The larger portion of the total explosive yield in this design comes from the final fission stage fueled by 238U, producing enormous amounts of radioactive . For example, an estimated 77% of the 10.4-Megatons yield of the Ivy Mike thermonuclear test in 1952 came from fast fission of the depleted uranium tamper. Because depleted (or natural) uranium has no critical mass, it can be added to thermonuclear bombs in almost unlimited quantity. The Soviet Union's test of the Tsar Bomba in 1961 produced "only" 50 megatons of explosive power, over 90% of which came from fusion because the 238U final stage had been replaced with lead. Had 238U been used instead, the yield of the Tsar Bomba could have been well above 100 megatons, and it would have produced nuclear fallout equivalent to one third of the global total that had been produced up to that time.
Or in tabular form, including minor branches:
| 238U | α | 4.463×109 a | 4.270 | 234Th |
| 234Th | beta decay | 24.11 d | 0.195 | 234mPa |
| 234mPa | IT 0.16% β− 99.84% | 1.16 min | 0.079 2.273 | 234Pa 234U |
| 234Pa | β− | 6.70 h | 2.194 | 234U |
| 234U | α | 2.455×105 a | 4.858 | 230Th |
| 230Th | α | 7.54×104 a | 4.770 | 226Ra |
| 226Ra | alpha decay | 1600 a | 4.871 | 222Rn |
| 222Rn | α | 3.8215 d | 5.590 | 218Po |
| 218Po | α 99.98% β− 0.02% | 3.097 min | 6.115 0.257 | 214Pb 218At |
| 218At | α 100% β− | 1.28 s | 6.876 2.883 | 214Bi 218Rn |
| 218Rn | α | 33.75 ms | 7.262 | 214Po |
| 214Pb | β− | 27.06 min | 1.018 | 214Bi |
| 214Bi | β− 99.979% α 0.021% | 19.9 min | 3.269 5.621 | 214Po 210Tl |
| 214Po | α | 163.5 μs | 7.833 | 210Pb |
| 210Tl | β− β−n 0.009% | 1.30 min | 5.481 0.296 | 210Pb 209Pb (in neptunium series) |
| 210Pb | β− α 1.9×10−6% | 22.2 a | 0.0635 3.793 | 210Bi 206Hg |
| 210Bi | β− α 1.32×10−4% | 5.012 d | 1.161 5.035 | 210Po 206Tl |
| 210Po | α | 138.376 d | 5.407 | 206Pb |
| 206Hg | β− | 8.32 min | 1.307 | 206Tl |
| 206Tl | β− | 4.20 min | 1.532 | 206Pb |
| 206Pb | stable |
The mean lifetime of 238U (or any nuclide) is the half-life divided by ln(2) ≈ 0.693 (or multiplied by 1/ln(2) ≈ 1.443), which is about 2 seconds, so 1 mole of 238U emits 3 alpha particles per second, producing the same number of thorium-234 . In a closed system an equilibrium would be reached in which all members except the stable end-product have fixed ratios to one another, but in slowly decreasing amount. The amount of 206Pb will increase accordingly while that of 238U decreases; all steps in the decay chain have this same rate of 3 decayed particles per second per mole 238U.
While 238U is minimally radioactive, its decay products, thorium-234 and protactinium-234, are beta particle emitters with half-life of about 20 days and one minute respectively. Protactinium-234 decays to uranium-234, which has a half-life of hundreds of millennia, and this isotope does not reach an equilibrium concentration for a very long time. When the two first isotopes in the decay chain reach their relatively small equilibrium concentrations, a sample of initially pure 238U will emit three times the radiation due to 238U itself, and most of this radiation is beta particles.
As already touched upon above, when starting with pure 238U, within a human timescale the equilibrium applies for the first three steps in the decay chain only. Thus, for one mole of 238U, 3 times per second one alpha and two beta particles and a gamma ray are produced, together 6.7 MeV, for a rate of 3 μW.
The 238U atom is itself a gamma emitter at 49.55 keV with probability 0.084%, but that is a very weak gamma line, so activity is measured through its daughter nuclides in its decay series.
The relation between 238U and 234U gives an indication of the age of and seawater that are between 100,000 years and 1,200,000 years in age.
The 238U daughter product, 206Pb, is an integral part of lead–lead dating, which is most famous for the determination of the age of the Earth.
The Voyager program spacecraft carry small amounts of initially pure 238U on the covers of their golden records to facilitate dating in the same manner.
Uranium is also chemically toxic, meaning that ingestion of uranium can cause kidney damage from its chemical properties much sooner than its radioactive properties would cause cancers of the bone or liver. Radioisotope Brief CDC (accessed November 8, 2021) Uranium Mining in Virginia: Scientific, Technical, Environmental, Human Health and Safety, and Regulatory Aspects of Uranium Mining and Processing in Virginia, Ch. 5. Potential Human Health Effects of Uranium Mining, Processing, and Reclamation. National Academies Press (US); December 19, 2011.
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