Actinides and fission products by half-life
Actinides[1] by decay chain Half-life
range (a)		 Fission products of 235U by yield[2]
4n 4n + 1 4n + 2 4n + 3 4.5–7% 0.04–1.25% <0.001%
228Ra№ 4–6 a 155Euþ
244Cmƒ 241Puƒ 250Cf 227Ac№ 10–29 a 90Sr 85Kr 113mCdþ
232Uƒ 238Puƒ 243Cmƒ 29–97 a 137Cs 151Smþ 121mSn
248Bk[3] 249Cfƒ 242mAmƒ 141–351 a

No fission products have a half-life in the range of 100 a–210 ka ...

241Amƒ 251Cfƒ[4] 430–900 a
226Ra№ 247Bk 1.3–1.6 ka
240Pu 229Th 246Cmƒ 243Amƒ 4.7–7.4 ka
245Cmƒ 250Cm 8.3–8.5 ka
239Puƒ 24.1 ka
230Th№ 231Pa№ 32–76 ka
236Npƒ 233Uƒ 234U№ 150–250 ka 99Tc₡ 126Sn
248Cm 242Pu 327–375 ka 79Se₡
1.53 Ma 93Zr
237Npƒ 2.1–6.5 Ma 135Cs₡ 107Pd
236U 247Cmƒ 15–24 Ma 129I₡
244Pu 80 Ma

... nor beyond 15.7 Ma[5]

232Th№ 238U№ 235Uƒ№ 0.7–14.1 Ga

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 other categorizations based on their purity.)

Only fissile isotopes of certain elements have the 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.[6]

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 isotopes 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 actinides are listed in the accompanying table. Most information on bare sphere masses is classified, but some documents have been declassified.[7]

NuclideHalf-life
(y)		Critical mass
(kg)		Diameter
(cm)		Ref
uranium-233159,2001511[8]
uranium-235703,800,0005217[8]
neptunium-236154,00078.7[9]
neptunium-2372,144,0006018[10][11]
plutonium-23887.79.04–10.079.5–9.9[12]
plutonium-23924,110109.9[8][12]
plutonium-24065614015[8]
plutonium-24114.31210.5[13]
plutonium-242375,00075–10019–21[13]
americium-241432.255–7720–23[14]
americium-242m1419–1411–13[14]
americium-2437370180–28030–35[14]
curium-24329.17.34–1010–11[15]
curium-24418.113.5–3012.4–16[15]
curium-24585009.41–12.311–12[15]
curium-246476039–70.118–21[15]
curium-24715,600,0006.94–7.069.9[15]
berkelium-247138075.711.8-12.2[16]
berkelium-2490.919216.1-16.6[16]
californium-24935169[9]
californium-2519005.468.5[9]
californium-2522.62.736.9[17]
einsteinium-2540.7559.897.1[16]

Countries that have produced weapons-grade nuclear material

At least ten countries have produced weapons-grade nuclear material:[18]

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 masses. Highly enriched uranium is considered weapons-grade when it has been enriched to about 90% U-235.

U-233 is produced from thorium-232 by neutron capture. 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.[19]

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 weapons 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 weapons would require low U-232 levels and low levels of light impurities on the order of 1 PPM.[20]

Weapons-grade plutonium

Pu-239 is produced artificially in nuclear reactors when a neutron is absorbed by U-238, forming U-239, which then decays in a rapid two-step process into Pu-239. It can then be separated from the uranium in a nuclear reprocessing plant.

Weapons-grade plutonium is defined as being predominantly Pu-239, typically about 93% Pu-239.[21] 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 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 reactors 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,[22] though in simple designs it may produce only a fizzle yield.[23] Weapons made with reactor-grade plutonium would require special cooling to keep them in storage and ready for use.[24] 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.[25] The plutonium was apparently sourced from the military 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.[26]

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

  1. ↑ Plus radium (element 88). While actually a sub-actinide, it immediately precedes actinium (89) and follows a three-element gap of instability after polonium (84) where no nuclides have half-lives of at least four years (the longest-lived nuclide in the gap is radon-222 with a half life of less than four days). Radium's longest lived isotope, at 1,600 years, thus merits the element's inclusion here.
  2. ↑ Specifically from thermal neutron fission of uranium-235, e.g. in a typical nuclear reactor.
  3. ↑ Milsted, J.; Friedman, A. M.; Stevens, C. M. (1965). "The alpha half-life of berkelium-247; a new long-lived isomer of berkelium-248". Nuclear Physics. 71 (2): 299. Bibcode:1965NucPh..71..299M. doi:10.1016/0029-5582(65)90719-4.
    "The isotopic analyses disclosed a species of mass 248 in constant abundance in three samples analysed over a period of about 10 months. This was ascribed to an isomer of Bk248 with a half-life greater than 9 [years]. No growth of Cf248 was detected, and a lower limit for the β− half-life can be set at about 104 [years]. No alpha activity attributable to the new isomer has been detected; the alpha half-life is probably greater than 300 [years]."
  4. ↑ This is the heaviest nuclide with a half-life of at least four years before the "sea of instability".
  5. ↑ Excluding those "classically stable" nuclides with half-lives significantly in excess of 232Th; e.g., while 113mCd has a half-life of only fourteen years, that of 113Cd is nearly eight quadrillion years.
  6. ↑ David Albright and Kimberly Kramer (August 22, 2005). "Neptunium 237 and Americium: World Inventories and Proliferation Concerns" (PDF). Institute for Science and International Security. Retrieved October 13, 2011.
  7. ↑ Reevaluated Critical Specifications of Some Los Alamos Fast-Neutron Systems
  8. 1 2 3 4 Nuclear Weapons Design & Materials, The Nuclear Threat Initiative website.
  9. 1 2 3 Final Report, Evaluation of nuclear criticality safety data and limits for actinides in transport, Republic of France, Institut de Radioprotection et de Sûreté Nucléaire, Département de Prévention et d'étude des Accidents.
  10. ↑ Chapter 5, Troubles tomorrow? Separated Neptunium 237 and Americium, Challenges of Fissile Material Control (1999), isis-online.org
  11. ↑ P. Weiss (October 26, 2002). "Neptunium Nukes? Little-studied metal goes critical". Science News. 162 (17): 259. doi:10.2307/4014034. Archived from the original on December 15, 2012. Retrieved November 7, 2013.
  12. 1 2 Updated Critical Mass Estimates for Plutonium-238, U.S. Department of Energy: Office of Scientific & Technical Information
  13. 1 2 Amory B. Lovins, Nuclear weapons and power-reactor plutonium, Nature, Vol. 283, No. 5750, pp. 817–823, February 28, 1980
  14. 1 2 3 Dias, Hemanth; Tancock, Nigel; Clayton, Angela (2003). "Critical Mass Calculations for 241Am, 242mAm and 243Am" (PDF). Challenges in the Pursuit of Global Nuclear Criticality Safety. Proceedings of the Seventh International Conference on Nuclear Criticality Safety. Vol. II. Tokai, Ibaraki, Japan: Japan Atomic Energy Research Institute. pp. 618–623.
  15. 1 2 3 4 5 Okuno, Hiroshi; Kawasaki, Hiromitsu (2002). "Critical and Subcritical Mass Calculations of Curium-243 to -247 Based on JENDL-3.2 for Revision of ANSI/ANS-8.15". Journal of Nuclear Science and Technology. 39 (10): 1072–1085. doi:10.1080/18811248.2002.9715296.
  16. 1 2 3 Institut de Radioprotection et de Sûreté Nucléaire: "Evaluation of nuclear criticality safety. data and limits for actinides in transport", p. 16
  17. ↑ Carey Sublette, Nuclear Weapons Frequently Asked Questions: Section 6.0 Nuclear Materials February 20, 1999
  18. ↑ Makhijani, Arjun; Chalmers, Lois; Smith, Brice (October 15, 2004). "Uranium Enrichment: Just Plain Facts to Fuel an Informed Debate on Nuclear Proliferation and Nuclear Power" (PDF). Institute for Energy and Environmental Research. Retrieved May 17, 2017.
  19. ↑ Definition of Weapons-Usable Uranium-233 ORNL/TM-13517
  20. ↑ Nuclear Materials FAQ
  21. ↑ "Reactor-Grade and Weapons-Grade Plutonium in Nuclear Explosives". Nonproliferation and Arms Control Assessment of Weapons-Usable Fissile Material Storage and Excess Plutonium Disposition Alternatives (excerpted). U.S. Department of Energy. January 1997. Retrieved September 5, 2011.
  22. ↑ Matthew Bunn and; John P. Holdren (November 1997). "MANAGING MILITARY URANIUM AND PLUTONIUM IN THE UNITED STATES AND THE FORMER SOVIET UNION". Annual Review of Energy and the Environment. 22 (1): 403–486. doi:10.1146/ANNUREV.ENERGY.22.1.403. ISSN 1056-3466. Wikidata Q56853752..
  23. ↑ J. Carson Mark (August 1990). "Reactor Grade Plutonium's Explosive Properties" (PDF). Nuclear Control Institute. Archived from the original (PDF) on May 8, 2010. Retrieved May 10, 2010.
  24. ↑ Rossin, David. "U.S. Policy on Spent Fuel Reprocessing: The Issues". PBS. Retrieved March 29, 2014.
  25. ↑ "Additional Information Concerning Underground Nuclear Weapon Test of Reactor-Grade Plutonium". US Department of Energy. June 1994. Retrieved March 15, 2007.
  26. ↑ "Plutonium". World Nuclear Association. March 2009. Archived from the original on March 30, 2010. Retrieved February 28, 2010.
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