The type of Uranium used in nuclear weapons (fissile U235) isn't created in reactors; it's purified from natural Uranium, which is almost entirely non-fissile U

The type of Uranium used in nuclear weapons (fissile U235) isn't created in reactors; it's purified from natural Uranium, which is almost entirely non-fissile U238 but contains a small proportion of U235. Most commerical enrichment of Uranium uses gas centrifuges to separate the heavier isotopes from the lighter.