239PU-238U DISEQUILIBRIUM STUDIES AT THE NOPAL I URANIUM ORE DEPOSIT, PEñA BLANCA, MEXICO

Understanding environmental Pu mobility is necessary in order to evaluate the potential impacts on human health from a radioactive waste repository. Plutonium-239 (t_1/2 = 24,100 a) is produced naturally in ore deposits by neutron capture from ²³⁸U. Deviations from the expected Pu/U ratio (nuclear equilibrium) suggest either Pu or U mobility in the past 5 ²³⁹Pu half-lives, most likely through mineral-water interaction. In this study we present U and Pu concentrations and isotopic ratios for samples from the fractures surrounding the Nopal I natural uranium ore deposit at Peña Blanca, Mexico. These results were obtained via a highly-sensitive isotope dilution thermal ionization mass spectrometry technique.

Our results show much lower plutonium concentrations (~0.2-1.2 fg/g) compared to previously reported data from other locations. Previous U-series work on most of these Peña Blanca samples yielded ²³⁸U-²³⁴U-²³⁰Th closed system ages >200 ka (Denton et al., 2016), and so the ²³⁸U-²³⁹Pu system should be in nuclear equilibrium. Hence, the measured ²³⁹Pu/²³⁸U ratios (1.9 ± 1.3 x10^-13 (2σ)) suggest a nuclear equilibrium value for ²³⁹Pu/²³⁸U that is a factor of 40 lower than previously reported at other uranium ore deposits. One possible explanation for this is a lower neutron flux at this site resulting in less plutonium being produced. Our Pu/U results support previous work at Peña Blanca demonstrating uranium loss at one particular location in the unsaturated zone due to mineral-water interaction. These new results for the nuclear equilibrium value together with those of previous groundwater studies at Peña Blanca allow us to estimate that the retardation factor in the saturated zone for Pu is >10 times that of U, or >~13,000 (Goldstein et al., 2010). Consequently we conclude that there is very little plutonium mobility due to mineral-water interaction in the fractured, rhyolitic tuff at Peña Blanca.