View source for Uranium-238

{{Short description|Isotope of uranium}}
{{Use mdy dates|date=September 2022}}
{{Infobox isotope|
 | image = Uranium03.jpg
 | image_caption = 10 gram sample
 | alternate_names =
 | mass_number = 238
 | symbol =U
 | num_neutrons = 146
 | num_protons = 92
 | abundance = 99.2745%
 | halflife = {{val|4.468|e=9|u=years}} 
 | decay_product = Thorium-234
 | decay_mass = 234
 | decay_symbol = Th
 | parent = Plutonium-242
 | parent_mass = 242
 | parent_symbol = Pu
 | parent_decay =a
 | parent2 =Protactinium-238
 | parent2_mass = 238
 | parent2_symbol = Pa
 | parent2_decay = b-
 | mass = 238.05078826
 | spin = 0
 | excess_energy =
 | binding_energy =
 | decay_mode1 =[[alpha decay]]
 | decay_energy1 = 4.267
 | decay_mode2 =
 | decay_energy2 =
 | decay_mode3 =
 | decay_energy3 =
 | decay_mode4 =
 | decay_energy4 =
 }}

'''Uranium-238''' ('''{{SimpleNuclide|Uranium|238}}''' or '''U-238''') is the most common [[isotope of uranium]] found in nature, with a relative abundance of 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|''fertile'']], meaning it can be [[Nuclear transmutation|transmuted]] to fissile [[plutonium-239]]. <sup>238</sup>U 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 <sup>238</sup>U's [[neutron absorption]] [[resonance]]s, increasing absorption as fuel temperature increases, is also an essential [[negative feedback]] mechanism for reactor control.

Around 99.284% of [[natural uranium]]'s mass is uranium-238, which has a half-life of 1.41{{e|17}} [[second]]s (4.468{{e|9}} years, or 4.468 billion years).<ref name="natortg">{{cite web | last1 = Mcclain | first1 = D. E. | last2 = Miller | first2 = A. C. | last3 = Kalinich | first3 = J. F. | title = Status of Health Concerns about Military Use of Depleted Uranium and Surrogate Metals in Armor-Penetrating Munitions | publisher = [[NATO]] | date = December 20, 2007 | url = http://www.afrri.usuhs.mil/www/outreach/pdf/mcclain_NATO_2005.pdf | access-date = November 14, 2010 | archive-url = https://web.archive.org/web/20110419080320/http://www.afrri.usuhs.mil/www/outreach/pdf/mcclain_NATO_2005.pdf | archive-date = April 19, 2011 | url-status = dead | df = mdy-all }}</ref>
Due to its natural abundance and half-life relative to other [[radioactive element]]s, <sup>238</sup>U produces ~40% of the radioactive heat produced within the Earth.<ref>{{Cite journal|last1=Arevalo|first1=Ricardo|last2=McDonough|first2=William F.|last3=Luong|first3=Mario|title=The K-U ratio of the silicate Earth: Insights into mantle composition, structure and thermal evolution|journal=Earth and Planetary Science Letters|volume=278|issue=3–4|pages=361–369|doi=10.1016/j.epsl.2008.12.023|bibcode=2009E&PSL.278..361A|year=2009}}</ref>  The <sup>238</sup>U [[decay chain]] contributes six [[Electron neutrino|electron anti-neutrinos]] per <sup>238</sup>U nucleus (one per [[beta decay]]), resulting in a large detectable [[geoneutrino]] signal when decays occur within the Earth.<ref>{{Cite journal|last1=Araki|first1=T.|last2=Enomoto|first2=S.|last3=Furuno|first3=K.|last4=Gando|first4=Y.|last5=Ichimura|first5=K.|last6=Ikeda|first6=H.|last7=Inoue|first7=K.|last8=Kishimoto|first8=Y.|last9=Koga|first9=M.|title=Experimental investigation of geologically produced antineutrinos with KamLAND|journal=Nature|volume=436|issue=7050|pages=499–503|doi=10.1038/nature03980|pmid=16049478|bibcode=2005Natur.436..499A|year=2005|s2cid=4367737}}</ref>  The decay of <sup>238</sup>U to daughter isotopes is extensively used in [[radiometric dating]], particularly for material older than approximately 1&nbsp;million years.

[[Depleted uranium]] has an even higher concentration of the <sup>238</sup>U 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 <sup>238</sup>U. [[Reprocessed uranium]] is also mainly <sup>238</sup>U, 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]].<ref>{{cite web
 |author       = Nuclear France: Materials and sites
 |title        = Uranium from reprocessing
 |url          = http://www.francenuc.org/en_mat/uranium4_e.htm
 |access-date  = 2013-03-27
 |archive-url  = https://web.archive.org/web/20071019023302/http://francenuc.org/en_mat/uranium4_e.htm
 |archive-date = 2007-10-19
 |url-status     = dead
}}</ref>

==Nuclear energy applications==
In a fission [[nuclear reactor]], uranium-238 can be used to generate [[plutonium-239]], which itself can be used in a [[nuclear weapon]] or as a nuclear-reactor fuel supply. In a typical nuclear reactor, up to one-third of the generated power comes from the fission of <sup>239</sup>Pu, which is not supplied as a fuel to the reactor, but rather, [[nuclear transmutation|produced]] from <sup>238</sup>U.<ref>{{cite web |url=https://world-nuclear.org/information-library/nuclear-fuel-cycle/fuel-recycling/plutonium.aspx|title=Plutonium - World Nuclear Association}}</ref> A certain amount of production of {{chem|239|Pu}} from {{chem|238|U}} is unavoidable wherever it is exposed to [[neutron radiation]]. Depending on [[burnup]] and [[neutron temperature]], different shares of the {{chem|239|Pu}} are converted to {{chem|240|Pu}}, which determines the "grade" of produced plutonium, ranging from [[weapons grade plutonium|weapons grade]], through [[reactor grade plutonium|reactor grade]], to plutonium so high in {{chem|240|Pu}} that it cannot be used in current reactors operating with a thermal neutron spectrum. The latter usually involves used "recycled" [[MOX fuel]] which entered the reactor containing significant amounts of plutonium{{citation needed|date=December 2022}}.

===Breeder reactors===
<sup>238</sup>U can produce energy via [[Fast fission|"fast" fission]]. In this process, a neutron that has a kinetic energy in excess of 1&nbsp;[[MeV]] can cause the nucleus of <sup>238</sup>U to split. Depending on design, this process can contribute some one to ten percent of all fission reactions in a reactor, but too few of the average 2.5 neutrons<ref>{{cite web |url=http://www.world-nuclear.org/information-library/nuclear-fuel-cycle/introduction/physics-of-nuclear-energy.aspx |title=Physics of Uranium and Nuclear Energy |website=World Nuclear Association |access-date=November 17, 2017}}</ref> produced in each fission have enough speed to continue a chain reaction.

<sup>238</sup>U can be used as a source material for creating plutonium-239, which can in turn be used as nuclear fuel. [[Breeder reactor]]s carry out such a process of [[Nuclear transmutation|transmutation]] to convert the [[fertile material|fertile]] isotope <sup>238</sup>U into fissile <sup>239</sup>Pu. It has been estimated that there is anywhere from 10,000 to five billion years worth of <sup>238</sup>U for use in these [[power station|power plants]].<ref>[http://www-formal.stanford.edu/jmc/progress/cohen.html Facts from Cohen] {{Webarchive|url=https://web.archive.org/web/20070410165316/http://www-formal.stanford.edu/jmc/progress/cohen.html |date=2007-04-10 }}. Formal.stanford.edu (2007-01-26). Retrieved on 2010-10-24.</ref> Breeder technology has been used in several experimental nuclear reactors.<ref>[http://www.world-nuclear.org/info/inf08.html Advanced Nuclear Power Reactors | Generation III+ Nuclear Reactors] {{Webarchive|url=https://web.archive.org/web/20100615004046/http://www.world-nuclear.org/info/inf08.html |date=June 15, 2010 }}. World-nuclear.org. Retrieved on 2010-10-24.</ref>

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 reactor|BN-800]], at the Beloyarsk Nuclear Power Station which became fully operational in November 2016. Also, Japan's [[Monju Nuclear Power Plant|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.{{Citation needed|date=January 2021}}

The breeder reactor as its name implies creates even larger quantities of <sup>239</sup>Pu or <sup>233</sup>U than the fission nuclear reactor.{{Citation needed|date=January 2021}}

The [[Clean And Environmentally Safe Advanced Reactor]] (CAESAR), a nuclear reactor concept that would use steam as a moderator to control [[delayed neutron]]s, will potentially be able to use <sup>238</sup>U as fuel once the reactor is started with [[Low-enriched uranium]] (LEU) fuel. This design is still in the early stages of development.{{Citation needed|date=January 2021}}

===CANDU reactors===
Natural uranium, with 0.711% {{SimpleNuclide|U|235}}, is usable as [[nuclear fuel]] in reactors designed specifically to make use of naturally occurring uranium, such as [[CANDU reactor]]s. By making use of non-enriched uranium, such reactor designs give a nation access to nuclear power for the purpose of electricity production without necessitating the development of fuel enrichment capabilities, which are often seen as a prelude to weapons production{{citation needed|date=December 2022}}.

===Radiation shielding===
<sup>238</sup>U is also used as a [[radiation shielding|radiation shield]] – its [[alpha radiation]] is easily stopped by the non-[[radioactive]] casing of the shielding and the uranium's high [[atomic weight]] and high number of [[electron]]s are highly effective in absorbing [[gamma ray]]s and [[X-ray]]s. It is not as effective as ordinary water for stopping [[fast neutron]]s. Both metallic [[depleted uranium]] and depleted [[uranium dioxide]] are used for radiation shielding. Uranium is about five times better as a gamma ray shield than [[lead]], so a shield with the same effectiveness can be packed into a thinner layer.{{Citation needed|date=January 2021}}

[[DUCRETE]], a concrete made with uranium dioxide [[Construction aggregate|aggregate]] instead of gravel, is being investigated as a material for [[dry cask storage]] systems to store [[radioactive waste]].{{Citation needed|date=January 2021}}

===Downblending===
The opposite of enriching is [[enriched uranium#Downblending|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 nuclear fuel.

<sup>238</sup>U from depleted uranium and natural uranium is also used with recycled <sup>239</sup>Pu 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.{{Citation needed|date=January 2021}}

==Nuclear weapons==
{{More citations needed section|date=December 2022}}
Most modern [[nuclear weapon]]s utilize <sup>238</sup>U as a "tamper" material (see [[nuclear weapon design]]). A tamper which surrounds a fissile core works to [[neutron reflector|reflect neutrons]] and to add [[inertia]] to the compression of the <sup>239</sup>Pu charge. As such, it increases the efficiency of the weapon and reduces the [[critical mass (nuclear)|critical mass]] required. In the case of a [[thermonuclear weapon]], <sup>238</sup>U
can be used to encase the fusion fuel, the high flux of very energetic [[neutron]]s from the resulting [[nuclear fusion|fusion]] reaction causes <sup>238</sup>U nuclei to split and adds more energy to the "yield" of the weapon. Such weapons are referred to as ''[[fission-fusion-fission]]'' weapons after the order in which each reaction takes place. An example of such a weapon is [[Castle Bravo]].

The larger portion of the total explosive yield in this design comes from the final fission stage fueled by <sup>238</sup>U, producing enormous amounts of radioactive [[fission product]]s. For example, an estimated 77% of the 10.4-[[Megatons|megaton]] yield of the [[Ivy Mike]] thermonuclear test in 1952 came from fast fission of the depleted uranium [[nuclear weapon design#tamper reflector|tamper]]. Because depleted 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 <sup>238</sup>U final stage had been replaced with lead. Had <sup>238</sup>U 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.

=={{Anchor|Radium_series}} Radium series (or uranium series)== 
The [[decay chain]] of <sup>238</sup>U is commonly called the "[[radium series]]" (sometimes "uranium series"). Beginning with naturally occurring uranium-238, this series includes the following elements: [[astatine]], [[bismuth]], [[lead]], [[polonium]], [[protactinium]], [[radium]], [[radon]], [[thallium]], and [[thorium]]. All of the [[decay product]]s are present, at least transiently, in any uranium-containing sample, whether metal, compound, or mineral. The decay proceeds as:

:<math chem="">\begin{array}{l}{}\\
\ce{^{238}_{92}U->[\alpha][4.468 \times 10^9 \ \ce y] {^{234}_{90}Th} ->[\beta^-][24.1 \ \ce d] {^{234\!m}_{91}Pa}}
\begin{Bmatrix} 
\ce{->[0.16\%][1.17 \ \ce{min}] {^{234}_{91}Pa} ->[\beta^-][6.7 \ \ce h]} \\ 
\ce{->[99.84\%\ \beta^-][1.17 \ \ce{min}]}
\end{Bmatrix}
\ce{^{234}_{92}U ->[\alpha][2.445 \times 10^5 \ \ce y] {^{230}_{90}Th} ->[\alpha][7.5 \times 10^4 \ \ce y] {^{226}_{88}Ra} ->[\alpha][1600 \ \ce y] {^{222}_{86}Rn}}
\\
\ce{^{222}_{86}Rn ->[\alpha][3.8235 \ \ce d] {^{218}_{84}Po} ->[\alpha][3.05 \ \ce{min}] {^{214}_{82}Pb} ->[\beta^-][26.8 \ \ce{min}] {^{214}_{83}Bi} ->[\beta^-][19.9 \ \ce{min}] {^{214}_{84}Po} ->[\alpha][164.3 \ \mu\ce{s}] {^{210}_{82}Pb} ->[\beta^-][22.26 \ \ce y] {^{210}_{83}Bi} ->[\beta^-][5.012 \ \ce d] {^{210}_{84}Po} ->[\alpha][138.38 \ \ce d] {^{206}_{82}Pb}}
\end{array}

</math>

{{Radium series/table}}

The [[mean lifetime]] of <sup>238</sup>U is 1.41{{e|17}} seconds divided by [[Natural logarithm of 2|ln(2)]]&nbsp;≈&nbsp;0.693 (or multiplied by 1/ln(2)&nbsp;≈&nbsp; 1.443), i.e. ca. 2{{e|17}} seconds, so 1 [[mole (unit)|mole]] of <sup>238</sup>U emits 3{{e|6}} alpha particles per second, producing the same number of thorium-234 [[atom]]s. In a closed system an equilibrium would be reached, with all amounts except for lead-206 and <sup>238</sup>U in fixed ratios, in slowly decreasing amounts. The amount of <sup>206</sup>Pb will increase accordingly while that of <sup>238</sup>U decreases; all steps in the decay chain have this same rate of 3{{e|6}} decayed particles per second per mole <sup>238</sup>U.

Thorium-234 has a mean lifetime of 3{{e|6}} seconds, so there is equilibrium if one mole of <sup>238</sup>U contains 9{{e|12}} atoms of thorium-234, which is 1.5{{e|-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.

While <sup>238</sup>U is minimally radioactive, its decay products, thorium-234 and protactinium-234, are [[beta particle]] emitters with [[half-life|half-lives]] 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 <sup>238</sup>U will emit three times the radiation due to <sup>238</sup>U itself, and most of this radiation is beta particles.

As already touched upon above, when starting with pure <sup>238</sup>U, within a human timescale the equilibrium applies for the first three steps in the decay chain only. Thus, for one mole of <sup>238</sup>U, 3{{e|6}} times per second one alpha and two beta particles and a gamma ray are produced, together 6.7 MeV, a rate of 3&nbsp;μW.<ref>{{Cite report |last=Enghauser |first=Michael |date=2018-04-01 |title=Uranium Gamma Spectroscopy Training Revision 00. |osti=1525592 |url=https://www.osti.gov/biblio/1525592 |language=English}}</ref><ref>{{Cite web |date=2017-07-26 |title=5.3: Types of Radiation |url=https://chem.libretexts.org/Courses/Furman_University/CHM101%3A_Chemistry_and_Global_Awareness_(Gordon)/05%3A_Basics_of_Nuclear_Science/5.03%3A_Types_of_Radiation |access-date=2023-05-16 |website=Chemistry LibreTexts |language=en}}</ref>

<sup>238</sup>U 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.<ref>{{Cite journal |last1=Huy |first1=N. Q. |last2=Luyen |first2=T. V. |date=2004-12-01 |title=A method to determine 238U activity in environmental soil samples by using 63.3-keV-photopeak-gamma HPGe spectrometer |url=https://www.sciencedirect.com/science/article/pii/S0969804304003513 |journal=Applied Radiation and Isotopes |language=en |volume=61 |issue=6 |pages=1419–1424 |doi=10.1016/j.apradiso.2004.04.016 |pmid=15388142 |issn=0969-8043}}</ref><ref>{{Cite web |last=Clark |first=DeLynn |date=December 1996 |title=U235: A Gamma Ray Analysis Code for Uranium Isotopic Determination |url=https://digital.library.unt.edu/ark:/67531/metadc688862/m2/1/high_res_d/310887.pdf |access-date=2023-05-21}}</ref>

==Radioactive dating==
<sup>238</sup>U abundance and its decay to daughter isotopes comprises multiple uranium dating techniques and is one of the most common radioactive isotopes used in [[radiometric dating]]. The most common dating method is [[Uranium–lead dating|uranium-lead dating]], which is used to date rocks older than 1 million years old and has provided ages for the oldest rocks on Earth at 4.4 billion years old.<ref>{{cite journal|first1=John W.|last1=Valley|first2=David A.|last2=Reinhard|first3=Aaron J.|last3=Cavosie|first4=Takayuki|last4=Ushikubo|title=Nano- and micro-geochronology in Hadean and Archean zircons by atom-probe tomography and SIMS: New tools for old minerals|url=http://ammin.geoscienceworld.org/content/gsammin/100/7/1355.full.pdf|journal=American Mineralogist|date=2015-07-01|issn=0003-004X|pages=1355–1377|volume=100|issue=7|doi=10.2138/am-2015-5134|first5=Daniel F.|last5=Lawrence|first6=David J.|last6=Larson|first7=Thomas F.|last7=Kelly|first8=David R.|last8=Snoeyenbos|first9=Ariel|last9=Strickland|bibcode=2015AmMin.100.1355V|doi-access=free}}</ref>

The relation between <sup>238</sup>U and <sup>234</sup>U gives an indication of the age of [[sediment]]s and seawater that are between 100,000 years and 1,200,000 years in age.<ref>{{Cite journal|title=Seawater (234U/238U) during the last 800 thousand years| doi=10.1016/S0012-821X(02)00556-3|bibcode=2002E&PSL.199...97H|volume=199|issue=1–2|journal=Earth and Planetary Science Letters|pages=97–110 | last1 = Henderson | first1 = Gideon M|year=2002}}</ref>

The <sup>238</sup>U daughter product, <sup>206</sup>Pb, is an integral part of [[lead–lead dating]], which is most famous for the determination of the [[age of the Earth]].<ref>{{Cite journal|last=Patterson|first=Claire|date=1956-10-01|title=Age of meteorites and the earth|journal=Geochimica et Cosmochimica Acta|volume=10|issue=4|pages=230–237|doi=10.1016/0016-7037(56)90036-9|bibcode=1956GeCoA..10..230P}}</ref>

The [[Voyager program]] spacecraft carry small amounts of initially pure <sup>238</sup>U on the covers of their [[voyager golden record|golden records]] to facilitate dating in the same manner.<ref>{{Cite web|url=https://voyager.jpl.nasa.gov/golden-record/making-of-the-golden-record/|title=Voyager - Making of the Golden Record|website=voyager.jpl.nasa.gov|language=en|access-date=2020-03-28}}</ref>

== Health concerns ==
Uranium emits [[alpha particle]]s through the process of [[alpha decay]]. External exposure has limited effect. Significant internal exposure to tiny particles of uranium or its decay products, such as thorium-230, [[radium-226]] and [[radon-222]], can cause severe health effects, such as cancer of the bone or liver.

Uranium is also a toxic chemical, 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.<ref>[https://www.cdc.gov/nceh/radiation/emergencies/isotopes/uranium.htm ''Radioisotope Brief''] CDC (accessed November 8, 2021)</ref><ref>[https://www.ncbi.nlm.nih.gov/books/NBK201047/ ''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.</ref>

==See also==
* [[Depleted uranium]]
* [[Uranium–lead dating|Uranium-lead dating]]

==References==
{{reflist}}

==External links==
* [http://toxnet.nlm.nih.gov/cgi-bin/sis/search/r?dbs+hsdb:@term+@na+@rel+uranium,+radioactive NLM Hazardous Substances Databank – Uranium, Radioactive]

{{Isotope sequence
|element=uranium
|lighter=uranium-237
|heavier=[[uranium-239]]
|before=[[plutonium-242]] ([[alpha decay|α]]) <br /> [[protactinium-238]] ([[beta decay|β<sup>−</sup>]])
|after=[[thorium-234]] (α)}}
{{Isotopes of uranium}}

[[Category:Actinides]]
[[Category:Fertile materials]]
[[Category:Isotopes of uranium]]
[[Category:Uranium]]
[[Category:Radionuclides used in radiometric dating]]