Hydrothermal reactions of nuclear waste solids. A preliminary study
A simulated high-level waste glass, Supercalcine, and some common ceramic and metallic solids were exposed to hydrothermal conditions at 350 0 C and 250 0 C for time periods ranging from three days to three weeks. (a) Most of the experiments were done in salt brine, but the glass study did include deionized water tests so that the influence of salt could be better understood. Under the extreme hydrothermal conditions of these tests, all of the materials examined underwent measurable changes.
... surable changes. The glass is converted to a mixture of crystalline phases, depending upon conditions, giving NaFeSi 2 0 6 as the primary alteration product. The rate of alteration is higher in deionized water than in salt brine; however, under equivalent test conditions, 66% of the cesium originally in the glass is released to the salt brine, while only 6% is released to deionized water. Rubidium and molybdenum are the only other fission product elements significantly leached from the glass. Evidence is presented which shows that sintered Supercalcine undergoes chemical changes in salt brine that are qualitatively similar to those experienced by glass samples. High concentrations of cesium enter the aqueous phase, and strontium and molybdenum are mobilized. Scouting tests were made with a variety of materials including commercial glasses, granite, U0 2 , A1 2 0 3 , steel, and waste glasses. Weight losses under hydrothermal conditions are in a relatively narrow band, with glass and ceramic materials showing 3 to 20 times greater weight losses than 304L stainless steel in the 250°C test used. The conclusion from these studies is that virtually all solid materials show hydrothermal reactivity at temperatures between 250 0 C and 350 0 C, and that these extreme conditions are not desirable. Further work is needed to establish kinetic parameters for the hydrothermal reactions. (a) These conditions were selected to accelerate the hydrothermal reaction; actual temperature during waste storage, even for a high-activity waste, should be less than 200 0 C if water is present during the first 100-yr time period following closure of the repository. At 1000 yr the temperature will be less than 1000C for both wet and dry storage. iii SUMMARY INTRODUCTION .