Reevaluation of Actinide(IV)-Humic Complexation in the WIPP Performance Assessment - 16547

The legacy humic colloid model for tetravalent actinides (Th(IV), U(IV), Np(IV), and Pu(IV)) in the performance assessment (PA) of the Waste Isolation Pilot Plant (WIPP) is highly conservative. The model structure is feasible, but substantial reductions are needed for two constants, PHUMSIM and PHUMCIM. These constants are proportionality constants representing the equilibrium aqueous concentration ratio of humic-bound An(IV) to non-colloidal An(IV) for the Salado and Castile formations. In the WIPP PA model, both constants are set at 6.3 based on observed colloidal partitioning of Th(IV) in seawater. This value greatly enhances the mobile An(IV) concentration in the WIPP PA. Actual humic partitioning of An(IV) in WIPP brines is expected to be significantly lower than in seawater because the pH of WIPP brines (∼9) is higher, concentrations of competing cations (e.g., Mg²⁺) are higher, and concentrations of aqueous humic substances may be lower. In this work, the results of recent studies of Th(IV)-humic and Ca²⁺-humic complexation are used to simulate competitive humic complexation under WIPP conditions. The PHUMSIM and PHUMCIM values calculated for these simulations are in the range of 0.0013-0.0016 and 0.0047-0.0056, respectively. These values reduce humic-bound An(IV) to less than 1% of aqueous non-colloidal concentrations and would reduce mobile An(IV) concentrations in the WIPP PA by as much as 85%. (authors)