Estimated Duration of the Subsurface Reducing Environment Produced by the Z-Area Saltstone Disposal Facility

The formula for saltstone includes approximately 25 percent wt slag to create a reducing environment for mitigating the subsurface transport of a number of radionuclides, including technetium-99. Based on laboratory measurements and mass balance calculations, it was estimated that the Z-Area saltstone waste form will maintain a reducing environment for more than a thousand years and likely for more than 10,000 years. The calculations were very sensitive to infiltration flow rate, underscoring the importance of the proposed moisture barrier for mitigating contaminant transport. Laboratory measurements indicated that the slag used in the formulation of the saltstone has an exceptionally high reduction capacity. Furthermore, measurements of a subsurface SRS sediment indicated that it also had a significant reduction capacity, albeit almost an order of magnitude less than that of the slag. Approximately 78 percent of the reduction capacity in the disposal system came from the saltstone, 14 percent from the geological materials in the overlying moisture barrier, and 9 percent from the vault made from reducing grout