Re: What is Weapons Grade

["Robert Jolley" <rjolley@mail.state.tn.us>]

Zack,

A couple of definitions offered for considerations:

Uranium 235. Uranium (U) has 92 electrons and 92 protons (the atomic number). Of the 14 isotopes in the sequence uranium 227 to uranium 240 (the mass numbers), uranium 235 and uranium 238 are the most important. With half-lives of 700 million and 4,500 million years respectively, uranium 235 and uranium 238 are relatively stable isotopes. They are not strongly radioactive and can be handled without the need for substantial protection.

Naturally occurring uranium consists of 99.283 percent (by weight) of uranium 238, 0.711 percent of uranium 235, and 0.0055 percent of uranium 234. Uranium 235 is a fissile isotope. Uranium 238 is not fissile, and no amount of it can sustain a chain reaction. It is fertile, which means it can be readily transformed into a fissile isotope by neutron irradiation.

For nuclear weapons, and for fuel burned in many types of nuclear reactors, it is necessary to increase concentrations of uranium 235. This is the process known as "enrichment" (see section II).

The following five grades of uranium are commonly recognized:

1. Depleted uranium, containing less than 0.71 percent uranium 235.
2. Natural uranium, containing 0.71 percent uranium 235.
3. Low-enriched uranium (LEU), containing more than 0.71 percent and less than 20 percent uranium 235. 
4. Highly enriched uranium (HEU), containing more than 20 percent uranium 235.
5. Weapon-grade uranium, HEU containing more than 90 percent uranium 235.

Plutonium isotopes. Unlike uranium, all but trace quantities of plutonium (Pu) are manufactured material. The most common plutonium isotopes are highly radioactive, complicating their handling.

Plutonium 239 is produced in a nuclear reactor when uranium 238 is irradiated with neutrons. Its half-life is 24,000 years, and it is a fissile material. When it absorbs neutrons in a reactor, plutonium 240 is formed. Subsequent neutron captures lead to accumulations of plutonium 241 and plutonium 242. Plutonium 241 is fissile, but plutonium 240 and plutonium 242 are not. However, all of these plutonium isotopes are fissionable by fast neutrons, and thus can be used either in combination or alone in nuclear explosives. Although the weapon designer's preference is always for material with high concentrations of plutonium 239 and low fractions of other plutonium isotopes, militarily useful weapons can be made out of plutonium with low concentrations of plutonium 239 and high concentrations of plutonium 240, plutonium 241, or plutonium 242.

The plutonium used in nuclear weapons typically contains mostly plutonium 239 and relatively small fractions of other plutonium isotopes. Plutonium discharged in power reactor fuel typically contains significantly less plutonium 239 and more of other plutonium isotopes.

The following grades of plutonium are widely used:

1. Weapon-grade plutonium, containing less then 7 percent plutonium 240.
2. Fuel-grade plutonium, containing from 7 to 18 percent plutonium 240.
3. Reactor-grade plutonium, containing over 18 percent plutonium 240.

The term "super-grade plutonium" is sometimes used to describe plutonium containing less than 3 percent plutonium 240. The term "weapon-usable plutonium" is often used to describe plutonium that is in separated form and, thus able to be quickly turned into weapons components (see key terms).

Source: Institute for Science and International Security (ISIS)
www.isis-online.org/publications/fmct/primer/tableofcontents.html

Robert Jolley
rjolley@mail.state.tn.us




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