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.
Highlights
The fate of 226 Ra is relevant to a number of environmental questions, mainly due to the fact that it is one of the main contributors to naturally occurring radioactive materials (NORM). 226 Ra containingNORM appears in many raw material production processes e.g., phosphate industry, unconventional gas production, geothermal energy production, and oil extraction [1,2,3,4,5,6]. 226 Ra is considered as a relevant radionuclide in safety cases that are prepared for the deep geological disposal of high-level nuclear waste [7,8,9]
Recent studies [11,12,13,14,15,16,17,18] have shown that the formation of a (Ba,Ra)SO4 solid solution significantly reduces the solubility of 226 Ra in aqueous systems
The chemical homogeneity and morphology of the initial solid solution particles is shown in the back-scatter electron (BSE) image of
Summary
The fate of 226 Ra is relevant to a number of environmental questions, mainly due to the fact that it is one of the main contributors to naturally occurring radioactive materials (NORM). 226 Ra containingNORM appears in many raw material production processes e.g., phosphate industry, unconventional gas production, geothermal energy production, and oil extraction [1,2,3,4,5,6]. 226 Ra is considered as a relevant radionuclide in safety cases that are prepared for the deep geological disposal of high-level nuclear waste [7,8,9]. 226 Ra is considered as a relevant radionuclide in safety cases that are prepared for the deep geological disposal of high-level nuclear waste [7,8,9]. There, it will occur as a fission product of the 238 U decay chain and may dominate the dose after about 100,000 years. Co-precipitation can lead to the formation of solid solutions in which the radionuclides are structurally incorporated in a host structure [6,8,10]. Rapid uptake via co-precipitation [11,12,13] as well as the slower recrystallization
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