Estimation of Hanford SX tank waste compositions from historically derived inventories

Migration of radionuclides under the SX-tank farm at the Hanford nuclear waste complex involves interaction of sediments with concentrated NaOH-NaNO3-NaNO2 solutions that leaked from the tanks. This study uses a reaction path calculation to estimate tank supernatant compositions from historical tank inventory data. The Pitzer activity coefficient algorithm based on the computer code GMIN is combined with the reactive transport code FLOTRAN to carry out the simulations. An extended version of the GMIN database is used which includes Al and Si species. In order for the reaction path calculations to converge, a pseudo-kinetic approach employing a rate limiter for precipitation kinetics is introduced. The rate limiter enables calculations to be carried out with the reaction path approach which previously could only be accomplished using a Gibbs free energy minimization technique. Because the final equilibrium state is independent of the reaction path, the value used for the rate limiter does not affect the calculation for the tank supernatant composition. Three different tanks are considered: SX-108, SX-109 and SX-115, with supernatant compositions ranging from extremely to moderately concentrated. Results of the simulations indicate that sodium concentrations much higher than previously expected are possible for the SX-108 tank. This result has important implications for the migration of cesium released from the tank within the vadose zone. The mineral cancrinite was predicted to form in all three tanks consistent with recent experiments. The calculated supernatant pH ranged from 14 to 12.8 for the tanks considered and Eh was mildly reducing determined by the redox couple NO3-NO2. (C) 2003 Elsevier Science Ltd. All rights reserved.