Activities of Water and Nitric Acid in Simulated Reprocessing Waste Solutions

Simulations were carried out for thermodynamic activities of water and nitric acid in simulated reprocessing solutions under a high level liquid waste (HLLW) evaporation condition. A prediction method based on a “Hydration Model” was developed to evaluate the activity coefficients of nitric acid and water in nitric acid solutions containing fission products. Suitability of the “Hydration Model” was confirmed; the changes in activity coefficient were well explained. Models predictions agreed well with the reported experimental results for changes of nitric acid concentration in the HLLW solution during the evaporation concentration process. The calculated activities of water and nitric acid were constant during the concentration process, except for the initial transient stage. Therefore nitric acid concentration changes should not have significant effects on chemical reactions, such as precipitation or volatilization, in the solution during HLLW concentration.