Single-pass flow-through test elucidation of weathering behavior and evaluation of contaminant release models for Hanford tank residual radioactive waste

Contaminant release models are required to evaluate and predict long-term environmental impacts of residual amounts of high-level radioactive waste after cleanup and closure of radioactively contaminated sites such as the DOE’s Hanford Site. More realistic and representative models have been developed for release of U, Tc and Cr from Hanford Site tanks C-202, C-203, and C-103 residual wastes using data collected with a single-pass flow-through test (SPFT) method. These revised models indicate that contaminant release concentrations from these residual wastes will be considerably lower than previous estimates based on batch experiments. For U, a thermodynamic solubility model provides an effective description of U release, which can account for differences in pore fluid chemistry contacting the waste that could occur through time and as a result of different closure scenarios. Under certain circumstances in the SPFT experiments various Ca-rich precipitates (Ca phosphates and calcite) form on the surfaces of the waste particles, inhibiting dissolution of the underlying U phases in the waste. This behavior was not observed in previous batch experiments. For both Tc and Cr, empirical release models were developed. In the case of Tc, release from all three wastes was modeled using an equilibrium Kd model. For Cr release, a constant concentration model was applied for all three wastes.