Weathering Effect on 99Tc Leachability from Cementitious Waste Form

The mass transfer of contaminants from the solid phase to the waste form pore water, and subsequently out of the solid waste form, is directly related to the number and size distribution of pores as well as the microstructure of the waste form. Because permeability and porosity are controlled by pore aperture size, pore volume, and pore distribution, it is important to have some indication of how these characteristics change in the waste form during weathering. Knowledge of changes in these key parameters can be used to develop predictive models that estimate diffusivity or permeability of radioactive contaminants can be used to develop predictive models that estimate diffusivity or permeability of radioactive contaminants from waste forms for long-term performance assessment. It is known that dissolution or precipitation of amorphous/crystalline phases within waste forms alters their pore structure and controls the transport of contaminants our of waste forms. One very important precipitate is calcite, which is formed as a result of carbonation reactions in cement and other high-alkalinity waste forms. Enhanced oxidation can also increase Tc leachability from the waste form. To account for these changes, weathering experiments were conducted in advance to increase our understating of the long-term Tc leachability, especially out of the cementitious waste form. Pore structure analysis was characterized using both N2 absorption analysis and XMT techniques, and the results show that cementitious waste form is a relatively highly-porous material compared to other waste forms studied in this task, Detailed characterization of Cast Stone chunks and monolith specimens indicate that carbonation reactions can change the Cast Stone pore structure, which in turn may correlate with Tc leachability. Short carbonation reaction times for the Cast Stone causes pore volume and surface area increases, while the average pore diameter decreases. Based on the changes in pore volumes measured using gas absorption analysis, the total porosity was changed from 8.6 percent to 10.7 or 12.0 percent after carbonation. The accelerated weathering reactions produced with exposure to elevated CO2 or O2 condition were performed on cast stone monoliths and the resulting water leachates for the carbonated Cast Stone using the EPA 1315 method showed significantly decreased pHs, but increased Tc diffusivities for the entire 90-day period when compared to Cast Stone monoliths that were not weathered. The increased Tc diffusivity for Cast Stone after reacting with CO2 resulted from increased porosity as found in previous pore structure analyses. However, the increased Tc diffusivity in carbonated Cast Stone gradually decreased, and the decreased pH in effluents also gradually increased as leaching times increased, suggesting that calcite precipitate and pore structure changes might affect Tc diffusivity under long-term leaching conditions