Weapons-grade means that a substance 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 is 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 certain isotopes of plutonium and uranium can be used in nuclear weapons. For plutonium, it is plutonium-239 (Pu-239), while uranium has uranium-233 (U-233) and uranium-235 (U-235). U-235 is made weapons-grade through isotopic enrichment. It only makes up 0.7% of natural uranium, with the rest being almost entirely uranium-238 (U-238). They are separated by their differing masses. U-235 is considered weapons-grade when it has been enriched to about 90% U-235. U-233 is produced artificially by bombarding thorium-232 (Th-232) with neutrons. It can be made highly pure because it can be chemically separated from Th-232 rather than by mass, which is far easier. Therefore, there is no weapons-grade concentration for U-233. Since it can relatively easily be made pure, it is regulated as a special nuclear material only by the total amount present rather than by concentration or concentration combined with the amount.

Pu-239 is produced artificially in nuclear reactors when a neutron is absorbed by U-238. Pu-240 has a high rate of spontaneous fission, which can cause a nuclear weapon to predetonate, and its concentration must be less than 7% for the plutonium to be weapons-grade. It is produced when Pu-239 absorbs a neutron. To avoid this the uranium fuel in a reactor must typically be replaced four to six times per year. This is necessary because the concentration of Pu-240 rises over time and its mass and chemical properties are too similar for it to be separated from Pu-239. With any reactor, plutonium is separated from the nuclear fuel, U-235 and U-238, chemically in a nuclear reprocessing plant.

It is difficult to produce weapons-grade plutonium with a light water reactor because the reactor must be shut down frequently to replace the nuclear fuel rods, so weapons-grade plutonium is generally produced in small, specialized military reactors. However, a test of a nuclear weapon that used reactor-grade plutonium was successfully detonated, although the yield was relatively low.

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.

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] , Amory B. Lovins, February 28 1980, Nature, Vol. 283, No. 5750, pp. 817-823

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