APPLICATION OF SINGLE-ION ACTIVITY-COEFFICIENTS TO DETERMINE THE SOLVENT-EXTRACTION MECHANISM FOR COMPONENTS OF HIGH-LEVEL NUCLEAR WASTE

The TRUEX solvent extraction process is being developed to remove and concentrate transuranic (TRU) elements from high-level and TRU radioac tive wastes that are currently stored at U.S. Department of Energy sit es. Phosphoric acid is one of the chemical species of concern in the a pplication of solvent extraction processes for removal of actinides, f or instance at the Hanford site, where bismuth phosphate was used to r ecover plutonium. The mechanism of phosphoric acid extraction with TRU EX-NPH solvent at 25 degrees C was determined from phosphoric acid dis tribution ratios, which were measured by using a phosphoric acid radio tracer and a variety of aqueous phases containing different concentrat ions of nitric acid and nitrate. A model was developed for predicting phosphoric acid distribution ratios as a function of the thermodynamic activities of nitrate ion, and hydrogen ion. The Generic TRUEX Model (GTM) was used to calculate these activities based on the Bromley meth od. The derived model supports extraction by CMPO and TBP in TRUEX-NPH solvent of a phosphoric acid-nitric acid complex and a CMPO-phosphori c acid complex.