Effects of Environmental Conditions on Estimating the Long-Term Performance of a Cementitious Waste Form for the Immobilization of Nuclear Waste
Portions of low-activity, high salt content radioactive tank waste and secondary liquid waste at the DOE Hanford Site are being considered for treatment with a mixture of fly ash, blast furnace slag, and ordinary Portland cement to produce Cast Stone as a waste form followed by near surface disposal. Performance assessment requires evaluation of retention of radionuclides by the waste form over at least 1000 years, so the long-term physical and chemical integrity of the waste form under aging conditions during disposal needs to be understood. The basis for detailed estimation of effects from the interaction between the waste form and its surrounding environment during its service life has not yet been established for use in the performance assessment for Cast Stone. This dissertation investigated several anticipated aging factors affecting the performance of Cast Stone including drying, carbonation, and leaching. Using a combination of aging experiments, extensive material characterization, and geochemical speciation and reactive transport modeling, the individual and coupled effects of drying and carbonation on water saturation, microstructure, chemical distribution, and leaching of constituents of potential concerns (COPCs) were evaluated. A moisture transport model was developed to predict drying in Cast Stone as a function of environmental relative humidity considering the evolution of capillary force and salt solution water activity. Geochemical speciation coupled with reactive transport models were used to simulate leaching and gain insights on the mechanism of drying and carbonation on the leaching of major constituents and COPCs. The reactive transport model provides the tool for predicting the long-term performance for the waste form. This study also informs the understanding of carbonation in alkali activated materials due to the similarity in the composition with Cast Stone.