Limited diffusion of U-series radionuclides at depth in deep-sea sediments

Abstract

LONG-TERM radionuclide mobility is a primary concern for evaluating options for the disposal of radioactive waste. Information relevant to the long timescales necessary may be obtained from natural radionuclides. Here we examine a characteristic irregular profile of uranium concentration with depth, in deep-sea turbidite units more than 500,000 years old. This is enough time either for secular equilibrium to have been established, or for daughter radionuclides to have achieved steady-state distributions in response to local advection and diffusion. We find that advection is absent and that the ²³⁸U-series radionuclides ²³⁴U and ²³⁰Th are remarkably immobile, with very small effective diffusion coefficients, D < 5 x 10⁻¹³ cm² s⁻¹. But a fraction of the total ²²⁶Ra (another daughter radionuclide in the ²³⁸U decay series) is mobile, with D = 10⁻⁹–10⁻¹⁰ cm² s⁻¹. This work suggests that the reducing nature of the sediments at depth is important in maintaining such low D values. The in situ diffusion behaviour observed must be correctly predicted by any model used for evaluating waste disposal options.