This project is investigating the chemical processes that govern actinide sequestration in grout materials with the goal of determining the long-term behavior of grouts used to stabilize actinides in source-terms such as high level waste tank heals. Two grouts contained portland cement, blast furnace slag and fly ash, with one formulation containing zeolite and the other fluorapatite. Earlier experimental work was conducted with funds from DOE/West Valley. CEMS funding allowed further

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... n of grout behavior, beyond the scope of the original work which consisted of both batch and flow-through column experiments. The primary focus was the late stage behavior of actinides in the grout system when it is expected to be open to the atmosphere and groundwater, resulting in decreases of pH and interactions of U (and other elements) with dissolved carbonate. Initially the solubility of contaminants will be constrained by chemistry dominated by the grout, primarily by the high pH, around 12. This is controlled and buffered by the portland cement and blast furnace slag components of the grout, which by themselves maintain a solution pH of about 12.5. Slowly the pH will diminish as Ca(OH) 2 and KOH dissolve, are carried away by water, and CaCO 3 forms. As these conditions develop, the behavior of U comes into question. The behavior of uranium in contact with two grouts formulated as potential backfill materials for emptied high-level radioactive waste facilities was investigated through a set of batch experiments, some extending up to 550 days. Solution U(VI) concentrations generally remained below the detection limit of 0.3 mg/L when access to atmospheric CO 2 was limited. As KHCO 3 was added to simulate weathering effects, U(VI) was released to solution but concentrations were limited to 3 mg/L or less for grouts and grout materials that maintained pH greater than 11.5. At pH values lower than 11.5, higher U(VI) concentrations were sometimes observed. Calcite was an important secondary mineral even when CO 2 was limited, and uranium partitioned into the calcite, resulting in concentrations as high as 9,600 mg/kg. Evidence from these batch experiments, supported by solid phase analysis, shows that partitioning of uranium to calcite is an important control on the long-term fate of U(VI), in a near-surface grout system. Uranium and the other elements that partition into calcite (e.g., As) may be effectively sequestered even in the end-state of the disposal system when it is open to