What is weapons-grade fissile material?
“Weapons-grade” refers to purified fissile material that is most suitable for use in a nuclear weapon. A concentration of more than 90 percent is optimal for both uranium-235 and plutonium-239. Nuclear material with much lower concentrations can be used in nuclear weapons, however — and is sometimes referred to as “weapons-usable.” At uranium concentrations less than 20 percent, producing an explosive chain reaction is almost impossible.
