(HEU), "oralloy" meaning "Oak Ridge alloy", or "25" (a combination of the last digit of the atomic number of uranium-235, which is 92, and the last digit of its mass number, which is 235); and ²³⁹Pu, also known as plutonium-239, or "49" (from "94" and "239").

Uranium's most common isotope, ²³⁸U, is fissionable but not fissile, meaning that it cannot sustain a chain reaction because its daughter fission neutrons are not (on average) energetic enough to cause follow-on ²³⁸U