Application of the extended Pitzer equation to nuclear fuel reprocessing

A rigorous method for calculating the activity coefficients of ions in aqueous solutions during nuclear fuel reprocessing is presented along with predictions of the model during the criticality event that took place on October 17, 1978 at the Idaho Chemical Processing Plant. Determination and validation of the parameters for the extended Pitzer model were performed using OLI database simulations to supplement available experimental data. Values for the parameters for the neutral species uranyl nitrate (UO2(NO3)2) and nitric acid (HNO3) from Hlushak et al. were determined to accurately predict the activity coefficients of these species without the aluminum nitrate buffer solution present. The parameters for aluminum nitrate (Al(NO3)3) from Accornero and Marini needed slight modifications for the UO2(NO3)2/HNO3/Al(NO3)3/H2O system used in the extraction process during the event under consideration. The extended Pitzer model was used to calculate the equilibrium constant for extraction at steady?state and then used to predict distribution coefficients in the case of decreasing Al(NO3)3 buffer solution concentrations that led to the accident. Finally, the equilibrium constant was modified to determine the stage?wise distribution based on the uranium mass balance.