Alligator Rivers Analogue project. Geochemistry of 239Pu, 129I, 99Tc and 36Cl

One objective of this research programme has been to evaluate the applicability of uranium orebodies as natural analogues for testing radionuclide release-rate models used in performance assessment activities. The investigated nuclides included three of the most persistent radioactive constituents of high-level wastes from nuclear fission power reactors: plutonium-239, iodine-129, and technetium-99. The feasibility of uranium minerals as analogues for the behavior of these nuclear reaction products (NRP) in spent fuel relies upon a capability to characterise NRP concentrations in the source minerals. Measured abundances of natural 239Pu, 99Tc and 129I in uranium ores are compared to calculated abundances in order to evaluate the degree to retention of these radionuclides by the ore. This modelling study also shows the extent to which various NRP are correlated, such that one provides a constraint on the production rates of others. Under most conditions, 36Cl, another long-lived neutron-capture product found in uranium ores, is shown to be an ideal in-situ monitor of the 235U fission rate, which is the dominant source term for 129I and possibly a significant one for 99Tc. Similarly, 239Pu/U ratios can be used to establish limits on the 238U neutron-induced fission rate; the ratios measured in this study suggest that 238U induced fission comprises < 4% of the total fissions in most of the ore samples studied and hence can be neglected as a source of 129I and 99Tc. 79 refs., 21 tabs., 18 figs