image_caption = 10 gram sample
abundance = 99.284%
halflife = 4.46 billion years
decay_product = Thorium-234
decay_mass = 234
decay_symbol = Th
parent = Plutonium-242
mass = 238.0507826
decay_energy1 = 4.267
Uranium-238 (U-238), is the most common isotope of
uraniumfound in nature. When hit by a neutron, it becomes uranium-239(U-239), an unstable isotope which decays into neptunium-239 (Np-239), which then itself decays, with a half-lifeof 2.355 days, into plutonium-239(Pu-239).
Around 99.284% of
last = Military / NATO
title = See Page three
date = December 20.
year = 2007
url = http://www.afrri.usuhs.mil/www/outreach/pdf/mcclain_NATO_2005.pdf
accessdate = 14 Nov.] is uranium-238, which has a half-life of 1.41 × 1017
seconds (4.46 × 109 years, or 4.46 billion years). Depleted uraniumconsists mainly of the 238 isotope, and enriched uraniumhas a higher-than-natural quantity of the uranium-235isotope. Reprocessed uraniumis also mainly U-238, but contains significant quantities of uranium-236, and in fact all the isotopes of uranium between uranium-232and uranium-238 except uranium-237. [Citation
author-link = Nuclear France: Materials and sites
title = Uranium from reprocessing
url = http://www.francenuc.org/en_mat/uranium4_e.htm
accessdate = 14 Nov.]
Nuclear energy applications
In a fission
nuclear reactor, uranium-238 can be used to breed plutonium-239, which itself can be used in a nuclear weapon or as a reactor fuel source. In fact, in a typical nuclear reactor, up to a third of the generated power does come from the fission of plutonium-239, which is not supplied as a fuel to the reactor, but transmuted from uranium-238.
Breeder reactors use the waste Plutonium-239 from fissile reactors as a fuel source.Uranium-238 is not usable directly as
nuclear fuel; however, it can be used as a source material for creating the element plutonium. Breeder reactors carry out such a process of transmutation to convert fertile isotopes such as uranium-238 into fissile plutonium. It has been estimated that there is anywhere from 10,000 to five billion years worth of uranium-238 for use in these power plants [http://www-formal.stanford.edu/jmc/progress/cohen.html] . Breeder technology has been used in several reactors [http://www.world-nuclear.org/info/inf08.html] .
As of December 2005, the only breeder reactor producing power is the 600-megawatt
BN-600 reactorat the Beloyarsk Nuclear Power Stationin Russia. Russia has planned to build another unit, BN-800, at the Beloyarsk nuclear power plant. Also, Japan's Monjubreeder reactor is planned for a re-start, having been shut down since 1995, and both Chinaand Indiahave announced intentions to build breeder reactors.
The breeder reactor as its name implies creates even larger quantities of plutonium-239 than the fission nuclear reactor.
Clean And Environmentally Safe Advanced Reactor(CAESAR), a nuclear reactor concept that would use steam as a moderator to control delayed neutrons, will potentially be able to burn uranium-238 as fuel once the reactor is started with LEU fuel. This design is still in the early stages of development.
Uranium-238 is also used as a radiation shield — its
alpha radiationis easily stopped by the non- radioactivecasing of the shielding and the uranium's high atomic weightand high number of electrons is highly effective in absorbing gamma rays and x-rays. However, it is not as effective as ordinary water for stopping fast neutrons. Both metallic depleted uranium and depleted uranium dioxideare being used as materials for radiation shielding. Uranium is about five times better as a gamma ray shield than lead, so a shield with the same effectivity can be packed into a thinner layer. DUCRETE, a concrete made with uranium dioxide aggregate instead of gravel, is being investigated as a material for dry cask storagesystems to store radioactive waste.
The opposite of enriching is downblending. Surplus highly-enriched uranium can be downblended with depleted uranium or natural uranium to turn it into low enriched uranium suitable for use in commercial
Uranium-238 from depleted uranium and natural uranium is also used with recycled plutonium from weapons stockpiles for making
mixed oxide fuel(MOX) which is now being redirected to become reactor fuel. 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 enrichment and separation processes before assembling a weapon.
nuclear weapons utilize uranium-238 as a "tamper" material (see nuclear weapon design). A tamper which surrounds a fissile core works to reflect neutrons and add inertiato the compression of the plutoniumcharge. As such, it increases the efficiency of the weapon and reduces the amount of critical mass required. In the case of a thermonuclear weapon uranium-238can be used to encase the fusion fuel, the high flux of very energetic neutrons from the resulting fusion reaction causes the uranium-238 to fission and adds energyto the yield of the weapon. Such weapons are referred to as " fission-fusion-fission" weapons after the three consecutive stages of the explosion.
The larger portion of the total explosive yield in this design comes from the final fission stage fueled by uranium-238, producing enormous amounts of radioactive
fission products. For example, 77% of the 10.4 megaton yield of the Ivy Mikethermonuclear test in 1952 came from fast fissionof the depleted uranium tamper. Because depleted uranium has no critical mass, it can be added to thermonuclear bombs in almost unlimited quantity. The 1961 Soviet test of Tsar Bombaproduced "only" 50 megatons, over 90% from fusion, because the uranium-238 final stage was replaced with lead. Had uranium-238 been used, the yield could have been as much as 100 megatons, and would have produced fallout equivalent to one third of the global total at that time.
Radioactivity and decay
Uranium-238's decay product
uranium-234has a half-lifeof 246,000 years and so is useful for determining the age of sediments that are between 100,000 years and 1,200,000 years in age. [Citation
last = Encyclopædia Britannica
author-link = Encyclopædia Britannica
title = uranium-234–uranium-238 dating
date = 14 Nov.
year = 2007
url = http://school.eb.com/eb/article-9074426
accessdate = 14 Nov.]
mean lifetimeof uranium-238 is 1.41 × 1017 seconds divided by 0.693 (or multiplied by 1.443), i.e. ca. 2 × 1017 seconds, so 1 mole of uranium-238 emits 3 × 106 alpha particles per second, producing the same number of thorium-234 (Th-234) atoms. In a closed system an equilibrium would be reached, with all amounts except lead-206 and uranium-238 in fixed ratios, in slowly decreasing amounts. The amount of Pb-206 will increase accordingly while U-238 decreases; all steps in the decay chain have this same rate of 3 × 106 decayed particles per second per mole uranium-238.
Thorium-234 has a mean lifetime of 3 × 106 seconds, so there is equilibrium if 1 mole of uranium-238 contains 9 × 1012 atoms of thorium-234, which is 1.5 × 10-11 mole (the ratio of the two half-lives). Similarly, in an equilibrium in a closed system the amount of each decay product, except the end product lead, is proportional to its half-life.
As already touched upon above, when starting with pure uranium-238, within a human timescale the equilibrium applies for the first three steps in the decay chain only. Thus, per mole of uranium-238, 3 × 106 times per second one alpha and two beta particles and gamma ray are produced, together 6.7 MeV, a rate of 3 µW. Extrapolated over 2 × 1017 seconds this is 600 GJ, the total energy released in the first three steps in the decay chain
* [http://toxnet.nlm.nih.gov/cgi-bin/sis/search/r?dbs+hsdb:@term+@na+@rel+uranium,+radioactive NLM Hazardous Substances Databank – Uranium, Radioactive]
* [http://www.nucleonica.net:81/wiki/index.php/Help:MCRD#U238 Simulation of U238 using the Monte Carlo method]
Wikimedia Foundation. 2010.
Look at other dictionaries:
Uranium-238 — Уран 238 природный изотоп урана с атомной массой 238. Содержание урана 238 в природном уране 99,28%, период полураспада 4,5·109 лет. Уран 238 подвергается ядерному делению под действием быстрых нейтронов и может использоваться в качестве… … Термины атомной энергетики
Uranium 238 — L uranium 238, noté 238U, est l isotope de l uranium dont le nombre de masse est égal à 238 : son noyau atomique compte 92 protons et 146 neutrons avec un spin 0+ pour une masse atomique de 238,0507826 g/mol. Il est caractérisé par un excès… … Wikipédia en Français
uranium 238 — noun the commonest isotope of uranium; it is not fissionable but when irradiated with neutrons it produces fissionable plutonium 239 • Hypernyms: ↑uranium, ↑U, ↑atomic number 92 * * * noun : an isotope of uranium of mass number 238 that is the… … Useful english dictionary
uranium 238 — Chem. the radioactive uranium isotope having a mass number 238, comprising 99.28 percent of natural uranium: used chiefly in nuclear reactors as a source of the fissionable isotope plutonium 239. Also called U 238, U 238. [1940 45] * * * … Universalium
uranium 238 — noun Date: 1942 an isotope of uranium of mass number 238 that is the most stable uranium isotope, that constitutes over 99 percent of natural uranium, that is not fissile but can be used to produce a fissile isotope of plutonium, and that has a… … New Collegiate Dictionary
uranium 238 — n. chem. the radioactive uranium isotope having a mass number of 238, used chiefly in nuclear reactors as a source of the fissionable isotope plutonium 239 • Etymology: 1940–45 … From formal English to slang
uranium-234-uranium–238 dating — Method of age determination that makes use of the radioactive decay of uranium 238 to uranium 234; the method can be used for dating sediments from either a marine or a playa lake environment. Because this method is useful for the period с… … Universalium
uranium-234–uranium-238 dating — method of age determination that makes use of the radioactive decay of uranium 238 to uranium 234; the method can be used for dating of sediments from either a marine or a playa lake environment. Because this method is useful for the period … Universalium
Uranium — (pronEng|jʊˈreɪniəm) is a silvery gray metallic chemical element in the actinide series of the periodic table that has the symbol U and atomic number 92. It has 92 protons and 92 electrons, 6 of them valence electrons. It can have between 141 and … Wikipedia
Uranium — Protactinium ← Uranium → Neptunium Nd … Wikipédia en Français