Uranium-235, with a half-life of 700 million years, is the isotope used in most nuclear reactors but is much rarer than Uranium-238.

Ores containing uranium (mostly U-238) must be mined from rock deposits.

Milling separates uranium from other minerals in an acidic or alkaline bath, yielding uranium oxide (yellowcake).

Yellowcake is enriched; reactor grade uranium is 3-5% U-235 formed into pea-sized pellets.