Modeling of Oxidized Cementitious Waste Forms: Oxidation Depths Estimation and Tc Leaching Assessment Under Field Scenarios - 22040

Cast Stone as a cementitious waste form has been developed to immobilize the secondary waste created during the vitrification treatment of low activity radioactive waste. Technetium (Tc-99) is a primary constituent of potential concern in Cast Stone because of its long half-life of 213,000 years and high mobility in oxic environments. The internal reducing environment of Cast Stone generated by blast furnace slag is beneficial for stabilizing Tc given the low solubility of Tc in reduced form (Tc(IV)). However, Cast Stone is subject to oxidation from oxygen diffused from the air or soil gas during its service life of over 1,000 years, which can accelerate the release of Tc into the environment. Knowledge of the ingress rate of oxidation front into Cast Stone under various levels of relative humidity (RH) is important for evaluating the long-term leaching behavior of Tc; however, it is difficult to directly measure oxidation depth. Herein, cylindrical Cast Stone samples doped with 0.2 wt.% Cr as a redox indicator were aged in air with one exposed surface under various RHs for different periods of time, and then the aged samples were subjected to a 1-dimensional monolithic diffusion leaching test (i.e., EPA Method 1315 test). With the good description of Cr leaching behavior by a monolithic diffusion model, the oxidation depth of each sample was estimated based on the modeled pe profile of the Cast Stone monolith. Compared to samples aged under higher RHs, samples aged under lower RHs were identified to have greater oxidation depths because the faster transport of moisture out of Cast Stone under lower RHs facilitated the diffusion of O₂ deeper into Cast Stone monoliths. Also, a functional relationship between oxidation depth and aging time was identified to estimate the oxidation depths of Cast Stone under long-term aging. Finally, a monolithic diffusion-based reactive mass transport model was developed to assess the long-term (i.e., 1000 years) leaching behavior of Tc from Cast Stone monoliths under field scenarios considering the aging and leaching conditions of Cast Stone at the Hanford Site. The total release of Tc from original Cast Stone (without oxidation) was in the range of 0.68 - 0.71 %, indicating a good retention ability of Cast Stone for Tc. However, oxidation reduced the retention ability of Cast Stone for Tc, and the accelerated release of Tc primarily happened during an early stage (e.g., pre-closure period). Therefore, collecting the leachate at the early stage with a functional collection system would be effective in preventing the transport of Tc from oxidized Cast Stone to the vadose zone. (authors)