The use of uranium-series disequilibrium for site characterization and as an analogue for actinide migration

Uranium-series isotopic disequilibrium in any rock or mineral phase is a powerful indicator for determining 1) how recently alteration has occurred, 2) the type of rock-water interaction that has occurred and, hence, the chemical conditions prevailing at that location, and 3) how actinides migrate under natural subsurface conditions. Since all geological formations contain trace amounts of U and Th, the disequilibrium technique is, therefore, an important tool in any site selection and characterization studies of prospective sites for geological disposal of nuclear wastes. To illustrate this application, measurements of uranium-series disequilibrium in three plutons in the Canadian Shield have been obtained together with supporting data from groundwater analyses. Disequilibrium between 238U, 234U, 230Th and 226Ra is generally not observed in visibly unaltered rock nor in fracture-filling minerals and adjacent rock where the minerals are of a high-temperature origin. Disequilibrium is observed, however, in rock immediately adjacent to, or rubble within, water-bearing fractures, and in some fractures containing low-temperature secondary minerals. Uranium-series data for a buried hydrothermal granite in Montana, USA, indicate negligible actinide mobility over the last 106 years. This result suggests that radionuclide migration from a vault in a hydrothermal regime will be minimal. A procedure is described for applying decay-series methods to characterization studies involved in site selection for nuclear waste disposal. (author)