What are weapons-grade uranium and weapons-grade plutonium?
In principle, any HEU can fuel a nuclear bomb. However, the greater the concentration of U-235, the less HEU is needed for the bomb. Weapons-grade uranium consists of 90 percent or greater U-235. Despite this definition, however, nuclear bombs can be and have been made with less enriched material. The average enrichment of the HEU used in the Hiroshima bomb was roughly 80 percent. Weapon-grade plutonium typically contains 93 percent or more plutonium-239 (Pu-239), the isotope most useful in nuclear weapons. The plutonium in the spent fuel discharged from typical power reactors is “reactor-grade,” containing larger concentrations of Pu-240 and Pu-241, which are more troublesome for weapons designers. Nevertheless, government studies have concluded that any state or group that could make a bomb from weapon-grade plutonium could also make a bomb from reactor-grade plutonium.
