Weapons-grade

From Wikipedia, the free encyclopedia
  (Redirected from Weapons-grade plutonium)
Jump to: navigation, search
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 155Euþ
244Cm 241Puƒ 250Cf 227Ac 10–29 90Sr 85Kr 113mCdþ
232Uƒ 238Pu 243Cmƒ 29–97 137Cs 151Smþ 121mSn
248Bk[3] 249Cfƒ 242mAmƒ 141–351

No fission products
have a half-life
in the range of
100–210k years…

241Am 251Cfƒ[4] 430–900
226Ra 247Bk 1.3k–1.6k
240Pu 229Th 246Cm 243Am 4.7k–7.4k
245Cmƒ 250Cm 8.3k–8.5k
239Puƒ 24.1k
230Th 231Pa 32k–76k
236Npƒ 233Uƒ 234U 150k–250k 99Tc 126Sn
248Cm 242Pu 327k–375k 79Se
1.53M 93Zr
237Np 2.1M–6.5M 135Cs 107Pd
236U 247Cmƒ 15M–24M 129I
244Pu 80M

...nor beyond 15.7M[5]

232Th 238U 235Uƒ№ 0.7G–14.1G

Legend for superscript symbols
₡  has thermal neutron capture cross section in the range of 8–50 barns
ƒ  fissile
metastable isomer
№  naturally occurring radioactive material (NORM)
þ  neutron poison (thermal neutron capture cross section greater than 3k barns)
†  range 4a–97a: Medium-lived fission product
‡  over 200ka: Long-lived fission product

A weapons-grade substance is one that is pure enough to be used to make a weapon or has properties that make it particularly suitable for 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. Neptunium-237 and some isotopes of americium might be usable, but it is not clear that this has ever been implemented.[6]

Substances for chemical and biological warfare may also be described as weapons-grade.

Countries that have produced weapons-grade nuclear material[edit]

Relatively few countries have produced weapons-grade nuclear material. The only countries known to have done so are China, France, India, Israel, North Korea, Pakistan, Russia, South Africa, the United Kingdom, and the United States.

Weapons-grade uranium[edit]

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[citation needed].

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.[7]

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.[8]

Weapons-grade plutonium[edit]

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.[9] 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 predetonate. 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,[10] though in simple designs it may produce only a fizzle yield.[11] Weapons made with reactor-grade plutonium would require special cooling to keep them in storage and ready for use.[12] A 1962 test at the U.S. Nevada National Security Site (then known as the Nevada Proving Grounds) using non-weapons-grade plutonium used 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 for this test as reactor grade.[13] The plutonium was apparently sourced from the military Magnox reactors at Calder Hall or Chapelcross. The content of plutonium-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.[14]

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[edit]

  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 isotopes have half-lives of at least four years (the longest-lived isotope in the gap is radon-222 with a half life of less than four days). Radium's longest lived isotope, at a notable 1600 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. doi:10.1016/0029-5582(65)90719-4.  edit
    "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 isotope with a half-life of at least four years before the "Sea of Instability".
  5. ^ Excluding those "classically stable" isotopes 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.
  6. ^ David Albright and Kimberly Kramer (2005-08-22). "Neptunium 237 and Americium: World Inventories and Proliferation Concerns". Institute for Science and International Security. Retrieved 2011-10-13. 
  7. ^ Definition of Weapons-Usable Uranium-233 ORNL/TM-13517
  8. ^ Nuclear Materials FAQ
  9. ^ "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 5 September 2011. 
  10. ^ Holdren, John; Matthew Bunn (1997). "MANAGING MILITARY URANIUM AND PLUTONIUM IN THE UNITED STATES AND THE FORMER SOVIET UNION". Annual Review of Energy and the Environment 22: 403–496. doi:10.1146/annurev.energy.22.1.403. Retrieved 29 March 2014. 
  11. ^ J. Carson Mark (August 1990). "Reactor Grade Plutonium's Explosive Properties". Nuclear Control Institute. Retrieved May 10, 2010. 
  12. ^ Rossin, David. "U.S. Policy on Spent Fuel Reprocessing: The Issues". PBS. Retrieved 29 March 2014. 
  13. ^ "Additional Information Concerning Underground Nuclear Weapon Test of Reactor-Grade Plutonium". US Department of Energy. June 1994. Retrieved March 15, 2007. 
  14. ^ "Plutonium". World Nuclear Association. March 2009. Retrieved February 28, 2010. 

External links[edit]