DOE OSTI2018
In salt repositories like the Waste Isolation Pilot Plant (WIPP), reducing conditions are expected, supporting two aqueous species of plutonium: Pu(III) and Pu(IV). Of these states, Pu(III) is more soluble than Pu(IV). Because Pu behavior is uncertain in WIPP brines under reducing conditions, performance assessment calculations assume a 50/50 partitioning between the two, justified through calculations using dilute solutions. Recent experimental evidence suggests an increasing importance for Pu(III). To reexamine this assumption, we modeled Pu speciation in NaCl and dilute WIPP brine solutions equilibrated with a Pu solid, including PuO{sub 2} and Pu(OH){sub 3}. To maintain reducing conditions, sulfate and sulfide were added. Models were performed using PHREEQC and the Specific Ion Theory (SIT) database on dilutions ranging from 0.1 M to 3 M. Models were compared to calculations using the Pitzer database with PuO{sub 2} and with analogs Am(III) and Th(IV), all up to 11 M. Results indicate that under reducing conditions, Pu(III) concentrations increase with ionic strength and can be up to 80 fold higher than Pu(IV). While concentrations varied due to differences in solubility for each solid, Pu(IV) concentrations remained nearly constant in all solutions. Analogs showed similar trends, though Am(III) is 3 times more soluble than Pu(III) while Th(IV) is 6 times more soluble than Pu(IV). Pu(IV) concentrations using Pitzer calculations were similar to those with SIT. Our study indicates that assuming equivalent partitioning of Pu species may under-represent concentrations during a release. They also show that in PA calculations there is a need to consider redox state at high ionic strength rather than adopting an assumption of equal partitioning between the two charges. (authors)
12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗