“nuclear material” means plutonium except that with isotopic concentration exceeding 80% in plutonium238; uranium233; uranium enriched in the isotopes 235 or 233; uranium containing the mixture of isotopes as occurring in nature other than in the form of ore or oreresidue; any material containing one or more of the foregoing;

“uranium enriched in the isotope 235 or 233” means uranium containing the isotopes 235 or 233 or both in an amount such that the abundance ratio of the sum of these isotopes to the isotope 238 is greater than the ratio of the isotope 235 to the isotope 238 occurring in nature.