Solubility constraint: an important consideration in safety assessment of nuclear waste disposal

Solubilities of key solid compounds that are either present in the waste form or can readily precipitate from waste package leachates under repository conditions can be used to set maximum limits on radionuclide concentrations expected in ground water. This is because the solubility limited concentrations are independent of the release scenarios, hydrologic transport characteristics, and absorption-desorption reactions. Some of the important factors that control solubilities are pH, pe, type of solid phase, and nature of complexing ligands in the ground waters. Most of the above factors are affected by radiolysis due to the inherent radiation field of the waste form. Experimental results pertaining to the solubilities of selected Am, U, Np, and Pu compounds and the effects of radiolysis are discussed. These results show that: (1) at expected repository pH and reducing conditions, solubility controlled concentrations of several actinides are low and near acceptable limits, (2) the redox conditions at the waste form-water interface may be very oxidizing due to radiolytic effects, despite the fact that normal repository conditions are assumed to be reducing, (3) additional data on solubility limits and key thermodynamic parameters are needed. 26 references, 4 figures