You need a particular kind of nuclear reactor to create the enriched uranium

*plutonium. Enriched uranium comes from taking natural uranium and enriching the content of a specific isotope (235U), typically with centrifuges, gaseous diffusion and/or magnetic separation in a synchrotron. The enriched uranium can be used in a weapon, or it can be used as fuel for a nuclear reactor to make 239Pu from 238U.