Solubility Evaluation for Yucca Mountain TSPA-SR

Abstract

A systematic evaluation of radionuclide solubility has been conducted for the Yucca Mountain Project. The conventional thermodynamic approach was employed and geochemical model calculations were used to estimate radionuclide solubilities for the base case. The study uses the computer code EQ3/6 as the major geochemical modeling tool. The water composition and environmental conditions are based on the results of in-package chemistry and in-drift chemistry studies. Field observations, laboratory measurements, and thermodynamic and kinetic considerations are utilized to ensure the resulting solubilities are conservative. Fourteen radioelements have been studied (U, Np, Pu, Th, Am, Ac, Tc, I, C, Cs, Sr, Ra, Pa, and Pb) and their solubilities are presented as either functions of environmental conditions or statistical distributions. Alternative solubility models for Np and Pu were also developed. These alternative models are based on measurements of spent fuel dissolution experiments rather than conventional thermodynamic considerations. Comparison of the Np and Pu base case models with their alternative models suggests that they are needed in order to advance our understanding about the behaviors of Np and Pu during the process of spent fuel corrosion.