Chapter 10: Solid-solution formation and the long-term safety of nuclear waste disposal
The disposal of nuclear waste in geological formations poses major scientific and social challenges to be met in the coming decades. One of the key issues is the long-term safety of a waste-repository system over extended periods of time (up to 106 years). Demonstrating the safety over geological time scales requires a sound understanding of the migration and retention of radionuclides in the geosphere. Predicting the geochemical evolution of a waste-repository system over such time scales cannot be based on ‘simple’ extrapolations of empirical phenomena, but requires molecular-level process understanding. In recent years it has been demonstrated that understanding fundamental reactions can improve the reliability of geochemically based long-term predictions. A sound understanding of radionuclide binding to minerals by sorption reactions is an important issue when discussing the long-term safety of nuclear disposal. The need of the radiogeochemistry community for molecular-level system understanding of structural incorporation of key radionuclides and subsequent solid-solution formation will be discussed on the basis of two prominent examples: (1) the (Ra,Ba)SO4 solid solution series; and (2) the uptake of trivalent actinides and lanthanides by calcite. The current status of our knowledge of these systems will be discussed critically and knowledge gaps for the long-term safety assessment will be identified.
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