U, Th and 226Ra vertical migration in sub-clay from an uranium tailngs impoundment

After 31 years operation of an uranium tailings impoundment, sub-clay samples beneath its bottom were taken for determining concentration distribution of U, Th and 226Ra migration from the impoundment. For study of nuclide migration, parameters were measured such as particle size of tailings, nuclide specific activities and leaching factors of tailings, and distribution coefficients of nuclides in sub-clay. One dimensional convection-dispersion model of nuclide migration was applied for fitting nuclide distribution in sub-clay. The result indicates that the sludge is the main component of tailings, and possess higher specific activities and lower leaching factor than coarse tailings. Sub-clay has a strong adsorption ability to nuclides, and the natural and man-made sub-clay layers beneath the impoundment bottom can effectively reduce nuclides seepage and vertical migration distance. Distribution coefficients of U, Th and 226Ra are 62, 1.3 x 103 and 9.8 x 102 mL/g, respectively. The vertical migration distances at which the concentrations of nuclides reduce to background level are 1.2 m for U and 0.2 m for Th and 226Ra. (authors)