Hydrological Evaluation of Placement Strategies for Disposal of Cementitious Waste Forms

Disposal facilities for cementitious waste forms containing radionuclides are being developed for long-term immobilization of radioactive and hazardous contaminants. Performance and risk assessments require an understanding of contaminant release rates from cementitious waste, which are influenced by evolving hydrological conditions and waste placement methodology. A variably saturated flow model was developed to evaluate how hydrologic conditions in a disposal facility are affected by placement strategies, to identify approaches that constrain contaminant release. Key factors examined included the hydraulic properties of backfill materials, the arrangement of cementitious waste blocks, and the degradation of the waste form (e.g., oxidation and carbonation). Their impact on water flow paths and degree of saturation in waste and backfill was evaluated for three operational periods: (1) preclosure without a cover (high meteoric infiltration), (2) early postclosure with an intact cover (low infiltration), and (3) late postclosure with a degraded cover (intermediate infiltration). Results indicate that vertical alignment of cementitious waste blocks and backfill promotes rapid flow, shorter water–waste contact times, and reduced contaminant release. More permeable backfill diverts water away from the waste, enabling faster drainage and reduced contact with the waste. Waste degradation slightly increases water exchange between waste and backfill but has a minimal effect on saturation. Compared to preclosure, backfill fluxes and saturation decrease substantially during early postclosure due to reduced infiltration by the intact cover. As the cover degrades, water fluxes and saturation in the backfills increase again. Cover placement also diverts water toward facility peripheries, reducing backfill saturation and extending residence times within the interior. These findings highlight the significance of backfill properties, waste arrangement, and cover performance in minimizing water–waste interactions and constraining contaminant release in cementitious waste disposal facilities.