Vertical migration of nuclides seeped from an uranium tailings impoundment

After 31 year operation of an uranium tailings impoundment, sub-clayey samples beneath its bottom were taken for determining concentration distribution of U, Th and Ra seeped from the impoundment. For fitting nuclide migration, one dimensional convection-dispersion model of nuclide migration in groundwater was applied, and parameters were measured such as particle size of tailings, leaching factor of nuclides from tailings and distribution coefficients of nuclides in sub-clay. Results indicate that fine tailings are the main portion of tailings, and possess higher specific activities and lower leaching factor than coarse tailings. The sub-clay has a strong adsorption ability to nuclides, and distribution coefficients of U, Th and Ra are 62, 1.3 x 103 and 9.8 x 102 mL/g, respectively. The natural and man-made sub-clay layers beneath the impoundment can reduce effectively nuclides seepage and migration. (authors)