Radium solubility control in the $BaSO_4$ – $RaSO_4$ solid-solution aqueous solution system

Radium is a decay product of 238U and 232Th and plays an important role regarding the dose rate in long term safety assessments for the direct disposal of nuclear fuel elements. The solubility control of Ra by the formation of a RaxBa1-xSO4 solid solution has been demonstrated in many cases [1]. Coprecipitation leads to the formation of structurally incorporated Radium within the barite structure. Such solid solutions are ubiquitous in natural systems – most minerals in nature are atomistic mixtures of elements rather than pure compounds. In many cases the formation of solid solutions leads to a thermodynamically more stable situation compared to the formation of pure compounds, due to a negative excess Gibbs energy of mixing. However, radionuclide solubility controlled by solid solutions is currently not considered in long term safety assessments for a nuclear waste repository system. One reason is related to the fact that only a limited number of rather simple solid solution systems have been studied to a sufficient level. Nevertheless, the thermodynamic concepts for solid solution formation under repository relevant conditions are very well developed. Here we have combined microscopic and spectroscopic experimental approaches to study in detail how a Ra containing solution will equilibrate with solid BaSO4 under repository relevant conditions. In combination with atomistic modeling a molecular level mixing model has been derived. In general, Radium solubility is significantly reduced if it is controlled by a binary (Ra,Ba)SO4 solid solution. Atomistic modeling indicates that a regular solid solution model may be applied. The thermodynamic parameters for the solid solution have been derived and can now be applied in the safety assessment of the direct disposal of nuclear fuel elements. Literature: [1] H. A. Doerner, & W. M. Hoskins, (1925) Journal of the American Chemical Society, 47, 662-67