Recrystallization and Uptake of 226Ra into Ba-Rich (Ba,Sr)SO4 Solid Solutions

Abstract

226Ra is an important contributor to naturally occurring radioactive materials (NORM) and also considered in safety cases related to the disposal of spent nuclear fuel in a deep geological repository. Recrystallization and solid solution formation with sulfates is regarded as an important retention mechanism for 226Ra. In natural systems sulfates often occur as (Ba,Sr)SO4. Therefore, we have chosen this solid solution at the Ba-rich end for investigations of the 226Ra uptake. The resulting 226Ra-solubility in aqueous solution was assessed in comparison with a thermodynamic model of the solid solution-aqueous solution system (Ba,Sr,Ra)SO4 + H2O. The temperature and composition of the initial (Ba,Sr)SO4 solid solution were varied. Measurements of the solution composition were combined with microscopic observations of the solid and thermodynamic modeling. A complex recrystallization behavior of the solid was observed, including the dissolution of significant amounts of the solid and formation of metastable phases. The re-equilibration of Ba-rich (Ba,Sr)SO4 to (Ba,Sr,Ra)SO4 leads to a major reconstruction of the solid. Already trace amounts of Sr in the solid solution can have a significant impact on the 226Ra solubility, depending on the temperature. The experimental findings confirm the thermodynamic model, although not all solids reached equilibrium with respect to all cations.