LIMITED DIFFUSION OF U-SERIES RADIONUCLIDES AT DEPTH IN DEEP-SEA SEDIMENTS

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. © 1990 Nature Publishing Group.