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|244Cm||241Pu f||250Cf||243Cmf||10–30 y||137Cs||90Sr||85Kr|
|232U f||238Pu||f is for|
|69–90 y||151Sm nc➔|
|4n||249Cf f||242Amf||141–351 y||No fission product|
has half-life 102
to 2×105 years
|241Am||251Cf f||431–898 y|
|4n||245Cmf||250Cm||239Pu f||8–24 ky|
|233U f||230Th||231Pa||32–160 ky|
|248Cm||242Pu||340–373 ky||Long-lived fission products|
|237Np||4n+2||1–2 My||93Zr||135Cs nc➔|
|232Th||238U||235U f||0.7–12 Gy||fission product yield|
A weapons-grade substance is one that is pure enough to be used to make a weapon or has properties that make it suitable for weapons use. Weapons-grade plutonium and uranium are the most common examples, but it may also be used to refer to chemical and biological weapons. Weapons-grade nuclear material causes the most concern, but plutonium and uranium have other categorizations based on their purity.
Only fissile isotopes of certain elements have the potential for use in nuclear weapons. Additionally they must be produced in sufficient quantity and purity to be usable. Uranium-235 and plutonium-239 are well known examples for which this is the case. Experiments have been conducted with uranium-233. Neptunium-237 and a number of isotopes of americium might be usable, but it is not clear that this has ever been actually implemented.
Very 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.
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 downblended with U-238 to remove proliferation concerns.
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 like 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 65000 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.
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. 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. 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.
While it has been claimed that spent LWR fuel could be reprocessed to produce plutonium that, while not weapons grade, could be used to produce a nuclear explosion (even if only one of fizzle yield), this has never been demonstrated. In particular, a 1962 test at the US 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 US 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. 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 is estimated to have been at least 85%, much higher than typical spent fuel from currently operating reactors. Therefore, this test does not prove that constructing a bomb from plutonium sourced from modern spent fuel, which contains no more than 70% Pu-239, is possible.
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.
Less frequently, weapons-grade refers to a substance used in chemical warfare or an organism used in biological warfare. A chemical that is weapons-grade must be of a high enough purity and be relatively free of contaminants. When an organism, such as a bacterium or virus, is weapons-grade, it means that it is a strain of that species that is suitable for weapons use. This may mean that it has been made more infectious or deadly. It may also mean that person-to-person transmission has been made more difficult, which helps prevent a country's own troops and citizens from becoming infected.