The readily fissionable isotope of uranium used to enrich natural uranium in nuclear fuels. It is present in uranium only to the extent of 0.7% and can be separated from it by any of several methods: the gaseous diffusion process using uranium hexafluoride, the gas centrifuge process, and the electromagnetic separation method. Its half-life is 7.13 × 10（8）years. It was the energy source used in the original atomic bomb. Its critical mass is about 33 lb. See fission; uranium.
