Weapons-grade nuclear material

Weapons-grade nuclear material is any fissionable nuclear material that is pure enough to make a nuclear weapon or 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.)

Actinides and fission products by half-life
Actinides[1] by decay chain				Half-life range (y)	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	†		^155Eu^þ
^244Cm^ƒ	^241Pu^ƒ	^250Cf	^227Ac^№	10–29		^90Sr	^85Kr	^113mCd^þ
^232U^ƒ		^238Pu^ƒ	^243Cm^ƒ	29–97		^137Cs	^151Sm^þ	^121mSn
^248Bk[3]	^249Cf^ƒ	^242mAm^ƒ		141–351	No fission products have a half-life in the range of 100–210 k years ...
	^241Am^ƒ		^251Cf^ƒ[4]	430–900
		^226Ra^№	^247Bk	1.3 k – 1.6 k
^240Pu	^229Th	^246Cm^ƒ	^243Am^ƒ	4.7 k – 7.4 k
	^245Cm^ƒ	^250Cm		8.3 k – 8.5 k
			^239Pu^ƒ	24.1 k
		^230Th^№	^231Pa^№	32 k – 76 k
^236Np^ƒ	^233U^ƒ	^234U^№		150 k – 250 k	‡	^99Tc^₡	^126Sn
^248Cm		^242Pu		327 k – 375 k			^79Se^₡
				1.53 M		^93Zr
	^237Np^ƒ			2.1 M – 6.5 M		^135Cs^₡	^107Pd
^236U			^247Cm^ƒ	15 M – 24 M			^129I^₡
^244Pu				80 M	... nor beyond 15.7 M years[5]
^232Th^№		^238U^№	^235U^ƒ№	0.7 G – 14.1 G
Legend for superscript symbols ₡  has thermal neutron capture cross section in the range of 8–50 barns ƒ  fissile m  metastable isomer №  primarily a naturally occurring radioactive material (NORM) þ  neutron poison (thermal neutron capture cross section greater than 3k barns) †  range 4–97 y: Medium-lived fission product ‡  over 200,000 y: Long-lived fission product

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. Neptunium-237 and some isotopes of americium might be usable, but it is not clear that this has ever been implemented.[6]

Critical mass

Main article: 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. This is, of course, infinite for any material that is not radioactive. 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]

Nuclide	Half-life (y)	Critical mass (kg)	Diameter (cm)	Ref
uranium-233	159,200	15	11	[8]
uranium-235	703,800,000	52	17	[8]
neptunium-236	154,000	7	8.7	[9]
neptunium-237	2,144,000	60	18	[10][11]
plutonium-238	87.7	9.04–10.07	9.5–9.9	[12]
plutonium-239	24,110	10	9.9	[8][12]
plutonium-240	6561	40	15	[8]
plutonium-241	14.3	12	10.5	[13]
plutonium-242	375,000	75–100	19–21	[13]
americium-241	432.2	55–77	20–23	[14]
americium-242m	141	9–14	11–13	[14]
americium-243	7370	180–280	30–35	[14]
curium-243	29.1	7.34–10	10–11	[15]
curium-244	18.1	13.5–30	12.4–16	[15]
curium-245	8500	9.41–12.3	11–12	[15]
curium-246	4760	39–70.1	18–21	[15]
curium-247	15,600,000	6.94–7.06	9.9	[15]
berkelium-247	1380	75.7	11.8-12.2	[16]
berkelium-249	0.9	192	16.1-16.6	[16]
californium-249	351	6	9	[9]
californium-251	900	5.46	8.5	[9]
californium-252	2.6	2.73	6.9	[17]
einsteinium-254	0.755	9.89	7.1	[16]

Countries that have produced weapons-grade nuclear material

Ten countries have produced weapons-grade nuclear material:[18]

• Five recognized "nuclear-weapon states" under the terms of the Nuclear Non-Proliferation Treaty (NPT): the United States (first nuclear weapon tested and two bombs used as weapons in 1945), Russia (first weapon tested in 1949), the United Kingdom (1952), France (1960), and China (1964)
• Three other declared nuclear states that are not signatories of the NPT: India (not a signatory, weapon tested in 1974), Pakistan (not a signatory, weapon tested in 1998), and North Korea (withdrew from the NPT in 2003, weapon tested in 2006)
• 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, which also had enrichment capabilities and developed nuclear weapons (possibly tested in 1979), but disassembled its arsenal and joined the NPT in 1991

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

See also: Reactor-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 U-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 y. No growth of Cf²⁴⁸ was detected, and a lower limit for the β⁻ half-life can be set at about 10⁴ y. No alpha activity attributable to the new isomer has been detected; the alpha half-life is probably greater than 300 y."
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. Nuclear Weapons Design & Materials, The Nuclear Threat Initiative website.
9. 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. Updated Critical Mass Estimates for Plutonium-238, U.S. Department of Energy: Office of Scientific & Technical Information
13. Amory B. Lovins, Nuclear weapons and power-reactor plutonium, Nature, Vol. 283, No. 5750, pp. 817–823, February 28, 1980
14. 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. II. Tokai, Ibaraki, Japan: Japan Atomic Energy Research Institute. pp. 618–623.
15. 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. 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. Holdren, John; Matthew Bunn (1997). "Managing military uranium and plutonium in the United States and the former Soviet Union" (PDF). Annual Review of Energy and the Environment. 22: 403–496. doi:10.1146/annurev.energy.22.1.403. Retrieved March 29, 2014.
23. J. Carson Mark (August 1990). "Reactor Grade Plutonium's Explosive Properties" (PDF). Nuclear Control Institute. 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. Retrieved February 28, 2010.

External links

• Reactor-Grade and Weapons-Grade Plutonium in Nuclear Explosives, Canadian Coalition for Nuclear Responsibility
• Nuclear weapons and power-reactor plutonium, Amory B. Lovins, February 28, 1980, Nature, Vol. 283, No. 5750, pp. 817–823
• Garwin, Richard L. (1999). "The Nuclear Fuel Cycle: Does Reprocessing Make Sense?". In B. van der Zwaan (ed.). Nuclear energy. World Scientific. p. 144. ISBN 978-981-02-4011-0. But there is no doubt that the reactor-grade plutonium obtained from 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.