View source for Weapons-grade nuclear material

{{Short description|Nuclear material pure enough to be used for nuclear weapons}}
{{Use mdy dates|date=September 2011}}
{{Actinidesvsfissionproducts}}
{{Nuclear weapons}}
'''Weapons-grade nuclear material'''  is any fissionable [[nuclear material]] that is pure enough to make a [[nuclear weapon]] and has properties that make it particularly suitable for nuclear weapons use. [[Plutonium]] and [[uranium]] in grades normally used in nuclear weapons are the most common examples. (These nuclear materials have [[special nuclear material|other categorizations]] based on their purity.)
Only [[fissile]] [[isotope]]s of certain elements have the [[Critical mass#Critical mass of a bare sphere|potential]] for use in nuclear weapons. For such use, the concentration of fissile isotopes [[uranium-235]] and [[plutonium-239]] in the element used must be sufficiently high. Uranium from natural sources is enriched by [[isotope separation]], and plutonium is produced in a suitable [[nuclear reactor]].
Experiments have been conducted with [[uranium-233]] (the fissile material at the heart of the [[thorium fuel cycle]]). [[Neptunium-237]] and some isotopes of [[americium]] might be usable, but it is not clear that this has ever been implemented. The latter substances are part of the [[minor actinides]] in [[spent nuclear fuel]].<ref>{{cite web
| author = [[David Albright]] and Kimberly Kramer
| date = August 22, 2005
| title = Neptunium 237 and Americium: World Inventories and Proliferation Concerns
| url = http://isis-online.org/uploads/isis-reports/documents/np_237_and_americium.pdf
| publisher = [[Institute for Science and International Security]]
| access-date = October 13, 2011
}}</ref>
==Critical mass ==
{{Main|Critical mass}}
Any weapons-grade nuclear material must have a [[critical mass]] that is small enough to justify its use in a weapon.  The critical mass for any material is the smallest amount needed for a sustained nuclear chain reaction.  Moreover, different [[isotope]]s have different critical masses, and the critical mass for many radioactive isotopes is infinite, because the mode of decay of one atom cannot induce similar decay of more than one neighboring atom.  For example, the critical mass of [[uranium-238]] is infinite, while the critical masses of [[uranium-233]] and [[uranium-235]] are finite.
The critical mass for any isotope is influenced by any impurities and the physical shape of the material.  The shape with minimal critical mass and the smallest physical dimensions is a sphere. Bare-sphere critical masses at normal density of some [[actinide]]s are listed in the accompanying table. Most information on bare sphere masses is classified, but some documents have been declassified.<ref>[https://www.osti.gov/servlets/purl/4765098 Reevaluated Critical Specifications of Some Los Alamos Fast-Neutron Systems]</ref>
{{Critical mass}}
==Countries that have produced weapons-grade nuclear material==
At least ten countries have produced weapons-grade nuclear material:<ref name=IEER-2004>{{dubious|source refers only to uranium, not plutonium|date=May 2017}}{{Cite web |url=http://www.ieer.org/reports/uranium/enrichment.pdf |title=Uranium Enrichment: Just Plain Facts to Fuel an Informed Debate on Nuclear Proliferation and Nuclear Power |first1=Arjun |last1=Makhijani |first2=Lois |last2=Chalmers |first3=Brice |last3=Smith |date=October 15, 2004 |publisher=[[Institute for Energy and Environmental Research]] |access-date=May 17, 2017}}</ref>
* Five recognized "[[nuclear-weapon states]]" under the terms of the [[Nuclear Non-Proliferation Treaty]] (NPT): the [[United States and weapons of mass destruction|United States]] ([[Trinity (nuclear test)|first nuclear weapon tested]] and two bombs [[Atomic bombings of Hiroshima and Nagasaki|used as weapons in 1945]]), [[Russia and weapons of mass destruction|Russia]] ([[RDS-1|first weapon tested in 1949]]), the [[United Kingdom and weapons of mass destruction|United Kingdom]] ([[Operation Hurricane|1952]]), [[France and weapons of mass destruction|France]] ([[Reggane series, French nuclear tests|1960]]), and [[China and weapons of mass destruction|China]] ([[596 (nuclear test)|1964]])
* Three other declared nuclear states that are not signatories of the NPT: [[India and weapons of mass destruction|India]] (not a signatory, [[Smiling Buddha|weapon tested in 1974]]), [[Pakistan and weapons of mass destruction|Pakistan]] (not a signatory, [[Chagai-I|weapon tested in 1998]]), and [[North Korea and weapons of mass destruction|North Korea]] (withdrew from the NPT in 2003, [[2006 North Korean nuclear test|weapon tested in 2006]])
* [[Nuclear weapons and Israel|Israel]], which is widely known to have developed nuclear weapons (likely first tested in the 1960s or 1970s) but has not openly declared its capability
* [[South Africa and weapons of mass destruction|South Africa]], which also had enrichment capabilities and developed nuclear weapons (possibly [[Vela incident|tested in 1979]]), but disassembled its arsenal and joined the NPT in 1991
==Weapons-grade uranium==
{{anchor|Weapons-grade#Weapons-grade uranium}}
[[Natural uranium]] is made weapons-grade through [[isotopic enrichment]]. Initially only about 0.7% of it is [[fissile]] U-235, with the rest being almost entirely [[uranium-238]] (U-238). They are separated by their differing [[mass]]es. [[Highly enriched uranium]] is considered weapons-grade when it has been enriched to about 90% U-235.{{Citation needed|date=November 2013}}
U-233 is produced from [[thorium-232]] by [[neutron capture]].<ref>{{Cite web |title=Thorium - World Nuclear Association |url=https://world-nuclear.org/information-library/current-and-future-generation/thorium |url-status=live |archive-url=https://archive.today/20241018182956/https://world-nuclear.org/information-library/current-and-future-generation/thorium |archive-date=2024-10-18 |access-date=2024-10-18 |website=world-nuclear.org}}</ref> The U-233 produced thus does not require enrichment and can be relatively easily chemically separated from residual Th-232. It is therefore regulated as a [[special nuclear material]] only by the total amount present. U-233 may be intentionally down-blended with U-238 to remove proliferation concerns.<ref>[http://www.ornl.gov/info/reports/1998/3445606041609.pdf Definition of Weapons-Usable Uranium-233] [[ORNL]]/TM-13517</ref>
While U-233 would thus seem ideal for weaponization, a significant obstacle to that goal is the co-production of trace amounts of [[uranium-232]] due to side-reactions. U-232 hazards, a result of its highly radioactive decay products such as [[thallium-208]], are significant even at 5 [[parts per million]]. [[Implosion nuclear weapon]]s require U-232 levels below 50 PPM (above which the U-233 is considered "low grade"; [[cf.]] "Standard weapon grade plutonium requires a [[Pu-240]] content of no more than 6.5%." which is 65,000 PPM, and the analogous [[Pu-238]] was produced in levels of 0.5% (5000 PPM) or less). [[Gun-type fission weapon]]s would require low U-232 levels and low levels of light impurities on the order of 1 PPM.<ref>[http://nuclearweaponarchive.org/Nwfaq/Nfaq6.html Nuclear Materials] FAQ</ref>
==Weapons-grade plutonium==
{{anchor|Weapons-grade#Weapons-grade plutonium}}
{{See also|Reactor-grade plutonium}}
Pu-239 is produced artificially in [[nuclear reactor]]s when a neutron is absorbed by U-238, forming U-239, which then [[radioactive decay|decays]] in a rapid two-step process into Pu-239.<ref>{{Cite web |title=All about plutonium {{!}} Orano |url=https://www.orano.group/en/unpacking-nuclear/all-about-plutonium |url-status=live |archive-url=https://archive.today/20241018184443/https://www.orano.group/en/unpacking-nuclear/all-about-plutonium |archive-date=2024-10-18 |access-date=2024-10-18 |website=orano.group |language=en}}</ref> It can then be separated from the uranium in a [[nuclear reprocessing]] plant.<ref>{{Cite web |date=2024-10-24 |title=Development of Advanced Reprocessing Technologies |url=https://www.iaea.org/sites/default/files/gc/gc52inf-3-att4_en.pdf |website=[[International Atomic Energy Agency]]}}</ref>
Weapons-grade plutonium is defined as being predominantly [[Pu-239]], typically about 93% Pu-239.<ref>{{cite web|title=Reactor-Grade and Weapons-Grade Plutonium in Nuclear Explosives|url=http://www.ccnr.org/plute.html|work=Nonproliferation and Arms Control Assessment of Weapons-Usable Fissile Material Storage and Excess Plutonium Disposition Alternatives (excerpted)|publisher=U.S. Department of Energy |access-date=September 5, 2011 |date=January 1997}}</ref> Pu-240 is produced when Pu-239 absorbs an additional neutron and fails to fission. Pu-240 and Pu-239 are not separated by reprocessing. Pu-240 has a high rate of [[spontaneous fission]], which can cause a nuclear weapon to pre-detonate. This makes plutonium unsuitable for use in [[Gun-type fission weapon|gun-type nuclear weapons]]. To reduce the concentration of Pu-240 in the plutonium produced, weapons program plutonium production reactors (e.g. [[B Reactor]]) irradiate the uranium for a far shorter time than is normal for a [[nuclear power reactor]]. More precisely, weapons-grade plutonium is obtained from uranium irradiated to a low [[burnup]].
This represents a fundamental difference between these two types of reactor. In a nuclear power station, high burnup is desirable. Power stations such as the obsolete British [[Magnox]] and French [[UNGG]] reactors, which were designed to produce either electricity or weapons material, were operated at low power levels with frequent fuel changes using [[online refuelling]] to produce weapons-grade plutonium. Such operation is not possible with the [[light water reactor]]s most commonly used to produce electric power. In these the reactor must be shut down and the pressure vessel disassembled to gain access to the irradiated fuel.
Plutonium recovered from LWR spent fuel, while not weapons grade, can be used to produce nuclear weapons at all levels of sophistication,<ref>{{cite Q|Q56853752}}<!-- Managing military uranium and plutonium in the United States and the former Soviet Union -->.</ref> though in simple designs it may produce only a [[Fizzle (nuclear test)|fizzle]] yield.<ref>{{cite web |url=http://www.nci.org/NEW/NT/rgpu-mark-90.pdf |title=Reactor Grade Plutonium's Explosive Properties |author=J. Carson Mark |date=August 1990 |publisher=Nuclear Control Institute |access-date=May 10, 2010 |author-link=J. Carson Mark |archive-date=May 8, 2010 |archive-url=https://web.archive.org/web/20100508105622/http://www.nci.org/NEW/NT/rgpu-mark-90.pdf |url-status=dead }}</ref> Weapons made with reactor-grade plutonium would require special cooling to keep them in storage and ready for use.<ref>{{cite web|last=Rossin|first=David|title=U.S. Policy on Spent Fuel Reprocessing: The Issues|url=https://www.pbs.org/wgbh/pages/frontline/shows/reaction/readings/rossin.html|work=PBS|access-date=29 March 2014}}</ref>  A 1962 test at the U.S. [[Nevada National Security Site]] (then known as the Nevada Proving Grounds) used non-weapons-grade plutonium produced in a Magnox reactor in the United Kingdom. The plutonium used was provided to the United States under the [[1958 US–UK Mutual Defence Agreement]]. Its isotopic composition has not been disclosed, other than the description ''reactor grade'', and it has not been disclosed which definition was used in describing the material this way.<ref name=DOE-199406>{{cite web|url=http://permanent.access.gpo.gov/websites/osti.gov/www.osti.gov/html/osti/opennet/document/press/pc29.html|title=Additional Information Concerning Underground Nuclear Weapon Test of Reactor-Grade Plutonium|publisher=[[US Department of Energy]]|date=June 1994|access-date=March 15, 2007}}</ref> The plutonium was apparently sourced from the Magnox reactors at Calder Hall or Chapelcross. The content of Pu-239 in material used for the 1962 test was not disclosed, but has been inferred to have been at least 85%, much higher than typical spent fuel from currently operating reactors.<ref>{{cite web|url=http://www.world-nuclear.org/info/inf15.html|title=Plutonium|publisher=World Nuclear Association|date=March 2009|access-date=February 28, 2010|archive-date=March 30, 2010|archive-url=https://web.archive.org/web/20100330221426/http://www.world-nuclear.org/info/inf15.html|url-status=dead}}</ref>
Occasionally, low-burnup spent fuel has been produced by a commercial LWR when an incident such as a fuel cladding failure has required early refuelling. If the period of irradiation has been sufficiently short, this spent fuel could be reprocessed to produce weapons grade plutonium.
==References==
<references/>
==External links==
*[http://www.ccnr.org/plute.html Reactor-Grade and Weapons-Grade Plutonium in Nuclear Explosives], Canadian Coalition for Nuclear Responsibility
*[http://www.rmi.org/images/other/Security/S80-01_NucWeaponsAndPluto.pdf Nuclear weapons and power-reactor plutonium] {{Webarchive|url=https://web.archive.org/web/20070316181800/http://www.rmi.org/images/other/Security/S80-01_NucWeaponsAndPluto.pdf |date=March 16, 2007 }}, [[Amory B. Lovins]], February 28, 1980, [[Nature (journal)|Nature]], Vol. 283, No. 5750, pp.&nbsp;817–823
*{{cite book|chapter-url=https://books.google.com/books?id=cjhGcg_98kQC&pg=PA144|title=Nuclear energy|chapter=The Nuclear Fuel Cycle: Does Reprocessing Make Sense?|editor=B. van der Zwaan|last=Garwin|first=Richard L.|page=144|quote=But there is no doubt that the [[reactor-grade plutonium]] obtained from [[nuclear reprocessing|reprocessing]] [[LWR]] [[spent fuel]] can readily be used to make high-performance, high-reliability nuclear weaponry, as explained in the 1994 [[Committee on International Security and Arms Control]] (CISAC) publication. | isbn=978-981-02-4011-0 | year=1999 | publisher=World Scientific}}
[[Category:Nuclear weapons]]
[[Category:Nuclear materials]]
[[Category:Plutonium]]
[[Category:Uranium]]