Long-term results from unsaturated durability testing of actinide-doped DWPF and WVDP waste glasses

Results from durability drip tests designed to simulate the unsaturated conditions in the proposed Yucca Mountain Repository are reported for two actinide-doped glasses used as model waste forms. These tests are being conducted with reference glass compositions doped with neptunium, plutonium, and americium from the Defense Waste Processing Facility (DWPF) and the West Valley Demonstration Project (WVDP), and have been ongoing for over 8 years. Solution compositions, including transuranics, have been periodically determined, and selected analyses of colloid formation and composition, glass corrosion layers, and solid alteration phases have been obtained by scanning and analytical transmission electron microscopies. The importance of integrated testing has been demonstrated, as complex interactions among the glass, the groundwater, and the sensitized stainless steel have been observed. The cumulative releases of both glass-forming and dopant elements are presented along with identification of reaction phases and their partitioning between solution and solid phases. Alteration phases, including smectite clay, iron silicates, uranium silicates, and calcium thorium phosphate, have been observed forming on the glass and stainless steel and have occasionally been found suspended in solution as colloids. Actinides, except neptunium, concentrate into alteration phases or sorb onto the stainless steel. The subsequent transport of the actinides is then controlled by these phases.