For nuclear weapons you need a certain level of isotope purity of either plutonium-239 or uranium-235.
And the easiest way by far to achieve this purity is to produce plutonium-239 in a breeder reactor, which allows you to get it out with much cheaper chemical processes rather than costly isotope separation methods.
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Weapons-grade nuclear material refers to fissile material that is suitable for use in nuclear weapons. The most commonly discussed materials in this context are uranium and plutonium. 1. **Uranium**: For uranium to be considered weapons-grade, it typically needs to contain a high percentage of the fissile isotope uranium-235 (U-235). Generally, uranium enriched to about 90% or more U-235 is classified as weapons-grade. Natural uranium contains about 0.