Development of Waste Acceptance Criteria at 221-U Building: Initial Flow and Transport Scoping Calculations

Abstract

SummaryThis report documents numerical flow and transport simulations that were performed under a contract from Fluor Hanford, Inc. to establish initial waste acceptance criteria for the potential waste streams that may be safely sequestered in the 221-U Building and similar canyon structures. Specifically, simulations were executed to identify the maximum loading of contaminant activity (without considering volume limitations) that can be emplaced within the 221-U Building with no more than 1 pCi/m 2 of contaminant exiting the bottom of the structure within a 1,000 year time period. Contaminants considered in the analysis were 3 H, 90 Sr, 99 Tc, 113 Cd, 129 I, 137 Cs, and 238 U. The initial scoping simulations were executed in one dimension to assess important processes, and then two dimensions to establish waste acceptance criteria. Two monolithic conditions were assessed: (1) a grouted canyon monolith and (2) a canyon monolith filled with sand. A three-stage approach was taken to account for different processes that may affect the amount of contaminant that can be sequestered safely in the canyon structure. In the first stage, flow and transport simulations established waste acceptance criteria based on a linear (K d ) isotherm approach. In the second stage, effects of thermal loading were examined, and the differences in waste acceptance criteria were quantified. In the third stage of modeling, precipitation/dissolution reactions were considered using reactive transport simulations, and the subsequent effect on the maximum contaminant loading was evaluated. The reactive transport modeling demonstrates a reactive transport capability that can be used for future performance predictions once site-specific data have been obtained.Results of the simulations demonstrated that contaminants that were transported with water flow and did not undergo retardation (e.g., 99 Tc and 3 H), cannot be safely sequestered in the 221-U Building using the waste stabilization approach assumed in this study. By contrast, the amount of moderately to strongly retarded (e.g., K d ≥ 1.0) contaminants such as 238 U and 113 Cd that can be safely sequestered was immense, far exceeding known inventories at the Hanford Site. For contaminants that were only slightly retarded (e.g., 0 < K d < 1.0), specific limits on waste acceptance were established, but uncertainties in the K d and half-life may introduce significant uncertainties in the estimated maximum loading capacities.The simulations demonstrated that the use of grout or sand as backfill material had little effect on establishment of waste acceptance criteria. A sensitivity analysis of the preclosure barrier recharge rate demonstrated only a small effect on the flux through the contaminant source and hence a small effect on the waste acceptance criteria. This was due to a surface gravel layer that permitted runoff to flow around the concrete structure of the 221-U Building.Heat loading had a more significant effect on the maximum loading due to increases in diffusive transport and...