View source for Uranium-235

{{short description|Isotope of uranium}}
{{redirect|U-235|the U-boat|German submarine U-235{{!}}German submarine ''U-235''}}
{{Infobox isotope
| image =  HEUraniumC.jpg
| image_caption = Uranium metal highly enriched in uranium-235
| alternate_names =
| mass_number = 235
| symbol = U
| num_neutrons = 143
| num_protons = 92
| abundance = 0.72%
| halflife = {{val|703,800,000|u=years}}
| decay_product = Thorium-231
| decay_mass = 231
| decay_symbol = Th
| parent = Protactinium-235
| parent_mass = 235
| parent_symbol = Pa
| parent_decay = 
| parent2 = Neptunium-235
| parent2_mass = 235
| parent2_symbol = Np
| parent2_decay =
| parent3 = Plutonium-239
| parent3_mass = 239
| parent3_symbol = Pu
| parent3_decay = 
| mass = 235.0439299
| spin = 7/2−
| excess_energy = {{val|40914.062|1.970}}
| binding_energy = {{val|1783870.285|1.996}}
| decay_mode1 = Alpha
| decay_energy1 = 4.679
| decay_mode2 =
| decay_energy2 =
| decay_mode3 =
| decay_energy3 =
| decay_mode4 =
| decay_energy4 =
}}
'''Uranium-235''' ('''{{SimpleNuclide|Uranium|235}}''' or '''U-235''') is an [[isotope of uranium]] making up about 0.72% of [[natural uranium]]. Unlike the predominant isotope [[uranium-238]], it is [[fissile]], i.e., it can sustain a [[nuclear chain reaction]]. It is the only fissile isotope that exists in nature as a [[primordial nuclide]].

Uranium-235 has a [[half-life]] of 703.8&nbsp;million years.  It was discovered in 1935 by [[Arthur Jeffrey Dempster]]. Its [[fission cross section]] for slow [[thermal neutron]]s is about {{val|584.3|1}} [[Barn (unit)|barns]].<ref name="StandardReactionIAEA">{{cite web |title=#Standard Reaction: 235U(n,f) |url=https://www-nds.iaea.org/standards/Data/standards-235U_xs-data.txt |website=www-nds.iaea.org |publisher=IAEA |access-date=4 May 2020}}</ref> For [[fast neutron]]s it is on the order of 1&nbsp;barn.<ref>{{cite web |url=http://www.uic.com.au/uicphys.htm |title="Some Physics of Uranium", ''UIC.com.au'' |access-date=2009-01-18 |url-status=usurped |archive-url=https://web.archive.org/web/20070717070625/http://www.uic.com.au/uicphys.htm |archive-date=July 17, 2007 }}</ref>
Most [[neutron absorption]]s induce fission, though a minority (about 15%) result in the formation of [[uranium-236]].<ref name=power-ratio>{{cite web |url=https://www.nuclear-power.com/nuclear-power/fission/capture-to-fission-ratio/ |title=Capture-to-fission Ratio |website=nuclear-power.com |access-date=June 26, 2024}}</ref><ref name=ratio-1962>{{cite journal |title=The ratio of neutron capture to fission for uranium-235 |first1=M. J. |last1=Cabell |first2=L. J. |last2=Slee |date=1962 |journal=Journal of Inorganic and Nuclear Chemistry |volume=24 |issue=12 |pages=1493–1500 |doi=10.1016/0022-1902(62)80002-5}}</ref>

== Fission properties ==
[[File:Nuclear fission reaction.svg|thumb|left|Nuclear fission seen with a uranium-235 nucleus]]
The fission of one atom of uranium-235 releases {{val|202.5|u=MeV}} ({{val|3.24|e=-11|u=J}}) inside the reactor. That corresponds to 19.54&nbsp;TJ/[[mole (unit)|mol]], or 83.14&nbsp;TJ/kg.<ref name="kayelaby">[https://web.archive.org/web/20190505175631/http://www.kayelaby.npl.co.uk/atomic_and_nuclear_physics/4_7/4_7_1.html Nuclear fission and fusion, and neutron interactions], National Physical Laboratory Archive.</ref> Another 8.8&nbsp;MeV escapes the reactor as anti-neutrinos. When {{chem|235|92|U}} nuclei are bombarded with neutrons, one of the many fission reactions that it can undergo is the following (shown in the adjacent image):

{{center|{{chem|1|0|}}[[neutron|n]] + {{chem|235|92|U}} → {{chem|141|56|[[Barium|Ba]]}} + {{chem|92|36|[[Krypton|Kr]]}} + 3 {{chem|1|0|}}[[neutron|n]] }}
[[Heavy water reactor]]s and some [[graphite moderated reactor]]s can use natural uranium, but [[light water reactor]]s must use [[low enriched uranium]] because of the higher [[neutron absorption]] of light water. [[Uranium enrichment]] removes some of the uranium-238 and increases the proportion of uranium-235. [[Highly enriched uranium]] (HEU), which contains an even greater proportion of uranium-235, is sometimes used in the reactors of [[Nuclear marine propulsion|nuclear submarines]], [[research reactors]] and [[nuclear weapon design|nuclear weapons]].

If at least one [[neutron]] from uranium-235 fission strikes another nucleus and causes it to fission, then the chain reaction will continue. If the reaction continues to sustain itself, it is said to be [[critical mass|critical]], and the mass of <sup>235</sup>U required to produce the critical condition is said to be a critical mass. A critical chain reaction can be achieved at low concentrations of <sup>235</sup>U if the neutrons from fission are [[neutron moderator|moderated]] to lower their speed, since the probability for fission with [[Thermal neutron|slow neutrons]] is greater. A fission chain reaction produces intermediate [[Fission product|mass fragments]] which are highly [[radioactive]] and produce further energy by their [[radioactive decay]]. Some of them produce neutrons, called [[delayed neutron]]s, which contribute to the fission chain reaction. The power output of [[nuclear reactor]]s is adjusted by the location of [[control rods]] containing elements that strongly absorb neutrons, e.g., [[boron]], [[cadmium]], or [[hafnium]], in the reactor core. In [[nuclear bomb]]s, the reaction is uncontrolled and the large amount of [[energy]] released creates a [[nuclear explosion]].

=== Nuclear weapons ===
The [[Little Boy]] [[Gun-type fission weapon|gun-type]] atomic bomb dropped on Hiroshima on August 6, 1945, was made of highly enriched uranium with a large [[Nuclear weapon design#Pure fission weapons|tamper]]. The nominal spherical critical mass for an untampered <sup>235</sup>U nuclear weapon is {{convert|56|kg}},<ref>{{cite web |url=https://fas.org/nuke/intro/nuke/design.htm |archive-url=https://web.archive.org/web/19990507100044/https://fas.org/nuke/intro/nuke/design.htm |title=FAS Nuclear Weapons Design FAQ |access-date=2010-09-02 |archive-date=1999-05-07 }}</ref> which would form a sphere {{convert|17.32|cm|in}} in diameter. The material must be 85% or more of <sup>235</sup>U and is known as [[weapons grade]] uranium, though for a crude and inefficient weapon 20% enrichment is sufficient (called ''weapon(s)-usable''). Even lower enrichment can be used, but this results in the required [[Critical mass (nuclear)|critical mass]] rapidly increasing. Use of a large tamper, [[Implosion-type nuclear weapon|implosion]] geometries, trigger tubes, [[polonium]] triggers, [[tritium]] enhancement, and [[neutron reflector]]s can enable a more compact, economical weapon using one-fourth or less of the nominal critical mass, though this would likely only be possible in a country that already had extensive experience in engineering nuclear weapons. Most modern nuclear weapon designs use [[plutonium-239]] as the fissile component of the primary stage;<ref name="FASdesign">{{cite book |url=https://fas.org/nuke/intro/nuke/design.htm |title=Nuclear Weapon Design |publisher=Federation of American Scientists |editor= |access-date=2016-06-04 |archive-url=https://web.archive.org/web/20081226091803/http://www.fas.org/nuke/intro/nuke/design.htm |archive-date=2008-12-26 |url-status=dead}}</ref><ref>{{cite book
|title = The Encyclopedia of the Chemical Elements
|chapter-url = https://archive.org/details/encyclopediaofch00hamp
|chapter-url-access = registration
|publisher = Reinhold Book Corporation
|location = New York (NY)
|year = 1968
|editor = Clifford A. Hampel
|last = Miner
|first = William N.
|author2=Schonfeld, Fred W.
|chapter = Plutonium
|page = [https://archive.org/details/encyclopediaofch00hamp/page/541 541]
|lccn = 68029938
}}</ref> however, HEU (highly enriched uranium, in this case uranium that is 20% or more <sup>235</sup>U) is frequently used in the secondary stage as an ignitor for the fusion fuel.

{| class="wikitable"
! Source
! Average energy<br />released [MeV]<ref name="kayelaby" />
|-
| style="background:#8989f9;" colspan=2          | Instantaneously released energy
|-
|                                                  Kinetic energy of fission fragments
| style="background:#e0ffe0; text-align:center;" | 169.1
|-
|                                                  Kinetic energy of prompt neutrons
| style="background:#e0ffe0; text-align:center;" | {{fsp}}{{fsp}}4.8
|-
|                                                  Energy carried by prompt γ-rays
| style="background:#e0ffe0; text-align:center;" | {{fsp}}{{fsp}}7.0
|-
| style="background:#8989f9;" colspan=2          | Energy from decaying fission products
|-
|                                                  Energy of β− particles
| style="background:#e0ffe0; text-align:center;" | {{fsp}}{{fsp}}6.5
|-
|                                                  Energy of delayed γ-rays
| style="background:#e0ffe0; text-align:center;" | {{fsp}}{{fsp}}6.3
|-
|                                                  Energy released when those prompt neutrons which do not (re)produce fission are captured
| style="background:#e0ffe0; text-align:center;" | {{fsp}}{{fsp}}8.8
|-
| style="background:#60c060;"                    | Total energy converted into heat in an operating thermal nuclear reactor
| style="background:#60c060; text-align:center;" | 202.5 MeV
|-
|                                                  Energy of anti-neutrinos
| style="text-align:center;"                     | {{fsp}}{{fsp}}8.8
|-
| style="background:#f96060;"                    | Sum
| style="background:#f96060; text-align:center;" | 211.3 MeV
|}

== Natural decay chain ==
<math chem="">\begin{array}{r}
\ce{^{235}_{92}U ->[\alpha][7.038 \times 10^8 \ \ce y] {^{231}_{90}Th} ->[\beta^-][25.52 \ \ce h] {^{231}_{91}Pa} ->[\alpha][3.276 \times 10^4 \ \ce y] {^{227}_{89}Ac}}
\begin{Bmatrix}
\ce{->[98.62\% \beta^-][21.773 \ \ce y] {^{227}_{90}Th} ->[\alpha][18.718 \ \ce d]} \\
\ce{->[1.38\% \alpha][21.773 \ \ce y] {^{223}_{87}Fr} ->[\beta^-][21.8 \ \ce{min}]}
\end{Bmatrix}
\ce{^{223}_{88}Ra ->[\alpha][11.434 \ \ce d] {^{219}_{86}Rn}} \\
\ce{^{219}_{86}Rn ->[\alpha][3.96 \ \ce s] {^{215}_{84}Po}}
\begin{Bmatrix}
\ce{->[99.99\% \alpha][1.778 \ \ce{ms}] {^{211}_{82}Pb} ->[\beta^-][36.1 \ \ce{min}]} \\
\ce{->[2.3\times 10^{-4}\% \beta^-][1.778 \ \ce{ms}] {^{215}_{85}At} ->[\alpha][0.10 \ \ce{ms}]}
\end{Bmatrix}
\ce{^{211}_{83}Bi}
\begin{Bmatrix}
\ce{->[99.73\% \alpha][2.13 \ \ce{min}] {^{207}_{81}Tl} ->[\beta^-][4.77 \ \ce{min}]} \\
\ce{->[0.27\% \beta^-][2.13 \ \ce{min}] {^{211}_{84}Po} ->[\alpha][0.516 \ \ce s]}
\end{Bmatrix}
\ce{^{207}_{82}Pb_{(stable)}}
\end{array}
</math>

== Astrophysical dating ==
Knowledge of current and theoretical production ratios of uranium-235 to uranium-238 allows [[radiometric dating]], the time since modern uranium nuclei were formed in [[stellar nucleosynthesis]].

The 1957 [[B2FH paper|B2FH]] landmark paper in astrophysics explained the [[r-process]] by which both nuclei form. The authors predicted their relative abundances, and those of their rapidly alpha-[[Decay chain|chain]] decaying [[Parent nuclide|parent nuclides]]. Thus they predicted 1.64 as the <sup>235</sup>U/<sup>238</sup>U ratio contributed to the [[interstellar medium]] by r-process events ([[supernovae]] and subsequently discovered [[Kilonova|kilonovae]]). This takes billions of years to diminish to their present value of 0.0072 (see [[natural uranium]]). They investigate scenarios for historical contribution to the [[solar nebula]], before contribution is cut off at the Sun's formation 4.5 billion years ago. The scenarios are: a single supernova, a finite continuous uniform series of supernovae representing the lifetime of the [[Milky Way]], and an infinite series representing the [[steady-state universe]]. From the second scenario, they estimated an age of the Milky Way at around 10 billion years, compared to a modern value of 13.61 billion years. Significantly, at this point the oldest known objects were [[stellar clusters]] at 6.5 billion years old.<ref name="l830">{{cite journal |last=Burbidge |first=E. Margaret |last2=Burbidge |first2=G. R. |last3=Fowler |first3=William A. |last4=Hoyle |first4=F. |date=1957-10-01 |title=Synthesis of the Elements in Stars |url=http://link.aps.org/pdf/10.1103/RevModPhys.29.547 |journal=Reviews of Modern Physics |volume=29 |issue=4 |pages=547–650 |doi=10.1103/RevModPhys.29.547 |issn=0034-6861 |access-date=2025-02-23 |doi-access=free}}</ref>

This technique is identical to [[uranium–thorium dating]] and [[Uranium-234-Uranium-238 Dating|uranium-234-uranium-238 dating]].

== References ==
{{reflist}}

== External links ==
* [http://atom.kaeri.re.kr/ Table of Nuclides].
* ''DOE Fundamentals Handbook: Nuclear Physics and Reactor Theory'' [https://energy.gov/sites/prod/files/2013/06/f2/h1019v1.pdf Vol. 1] ({{Webarchive|url=https://web.archive.org/web/20170731233821/https://www.energy.gov/sites/prod/files/2013/06/f2/h1019v1.pdf |date=2017-07-31 }}), [https://energy.gov/sites/prod/files/2013/06/f2/h1019v2.pdf Vol. 2] ({{Webarchive|url=https://web.archive.org/web/20161220052835/https://energy.gov/sites/prod/files/2013/06/f2/h1019v2.pdf |date=2016-12-20 }}).
* [http://www.epa.gov/radiation/radionuclide-basics-uranium Radionuclide Basics: Uranium]—[[US EPA]]
* [http://toxnet.nlm.nih.gov/cgi-bin/sis/search/r?dbs+hsdb:@term+@na+@rel+uranium,+radioactive NLM Hazardous Substances Databank – Uranium, Radioactive]
* [https://books.google.com/books?id=ftkDAAAAMBAJ&dq=Popular+Science+1930+plane+%22Popular+Mechanics%22&pg=PA1 "The Miracle of U-235"], ''[[Popular Mechanics]]'', January 1941—one of the earliest articles on U-235 for the general public

{{Isotope sequence
|element=uranium
|lighter=[[uranium-234]]
|heavier=[[uranium-236]]
|before=[[protactinium-235]]<br />[[neptunium-235]]<br />[[plutonium-239]]
|after=[[thorium-231]]
}}
{{Isotopes of uranium}}

[[Category:Actinides]]
[[Category:Fissile materials]]
[[Category:Isotopes of uranium]]
[[Category:Radionuclides used in radiometric dating]]
[[Category:Special nuclear materials]]