ABSTRACT
There exist three common nuclear fuels: 233U, 235U, and 239Pu. These nuclides have a high probability of undergoing –ssion when irradiated with neutrons of any energy. Of the three fuels, only 235U occurs in nature, where it constitutes about 0.72% of naturally occurring U. Most of the balance of natural Uconsists of 238U, which –ssions ef–ciently only through interactions with high-energy (>1.2-MeV) neutrons. There is also asmall admixture of 234U (t1/2 = 2.46 × 105 years) in natural U, in secular equilibrium with the longer-lived 238U isotope. Although 233U and 236U are long-lived (t1/2 = 1.59 × 105 years and 2.34 × 107 years, respectively), neither exists in nature in readily detectable concentrations. The presence of signi–cant admixtures of 233U or 236U in a uranium sample can only arise through anthropogenic nuclear processes.
