Since World War II, more than 1,200 metric tons of plutonium have been produced in nuclear reactors. Of those 1,200 metric tons, approximately 260 are weapon-grade plutonium, defined as containing less than 7 percent (typically 5-6 percent) plutonium 240, an isotope with a high rate of spontaneous fission. Commercial-grade inventories presumably are composed of a combination of fuel-grade plutonium (7 to 18 percent 240) and reactor-grade (more than 19 percent 240). Despite its high plutonium 240 content, nuclear weapons can be made with reactor-grade plutonium.
Throughout the 1950s and 1960s, weapon designers worked to reduce the amount of plutonium needed to produce a yield in the 10- to 20-kiloton range, roughly in the range of the Hiroshima and Nagasaki bombs. Modern U.S. warheads use two to four kilograms of weapon-grade plutonium per warhead. At an average of three kilograms per warhead, the world's approximately 260 tons of weapon-grade plutonium produced since 1945 would be enough for more than 85,000 warheads.