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The objective of the work described here is to determine to what extent a variety of contaminants, including fission products, actinides, and RCRA elements are sequestered by the two grout formulations. The conceptual model for this study is as follows: a large mass of grout having been poured into a high-level waste tank is in the process of aging and weathering for thousands of years. The waste remaining in the tank will contain radionuclides and other contaminants, much of which will adhere to tank walls and internal structures. The grout will encapsulate the contaminants. Initially the grout will be well sequestered, but over time rainwater and groundwater will gain access to it. Ultimately, the grout/waste environment will be an open system. In this condition water will move through the grout, exposing it to O{sub 2} and CO{sub 2} from the air and HCO{sub 3}{sup -} from the groundwater. Thus we are considering an oxic environment containing HCO{sub 3}{sup -}. Initially the solubility of many contaminants, but not all, will be constrained by chemistry dominated by the grout, primarily by the high pH, around 11.8. 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){sub 2} and KOH dissolve, are carried away by water, and CaCO{sub 3} forms. As these conditions develop, the behavior of these elements comes into question. In our conceptual model, although the grout is formulated to provide some reducing capacity, in order to be conservative this mechanism is not considered. In addition to solubility constraints imposed by pH, the various contaminants may be incorporated into a variety of solid phases. Some may be incorporated into newly forming compounds as the grout sets and cures. Others (like soddyite, (UO{sub 2}){sub 2}SiO{sub 4}(H{sub 2}O){sub 2}) are the result of slower reactions but may become important over time as contaminants are exposed to evolving chemistry in the grout. Still other solid phases may form from reactions between the waste and grout components, not only the cementitious materials, but also the additives used in the grout. Another process that may exert some control on contaminant concentrations is adsorption onto solids within the grout. These may be additives such as the fluorapatite or zeolite that are substantial percentages of the grouts or they may be minerals, typically Ca-Al-Si materials, that form in the grout system as cement sets. In addition, as the grout weathers over time, CaCO{sub 3} minerals, such as calcite and aragonite, will form as a rind on the grout and as a fracture filling mineral. Some contaminants are likely to be incorporated into these minerals, to a greater or lesser extent, as they precipitate. For some elements, such as U, there is a significant literature exploring the incorporation into CaCO{sub 3}, but for others there is essentially no information. This is also the case for much of the chemical regime of the grouts. Initial conditions are at pH values around 12 and information is often sparse

Fate of Contaminants in Contact with West Valley Grouts