Production of carbonate minerals will be a dominant alteration reaction during cement degradation in the near-field of the L/ILW repository planned at Wellenberg (Switzerland). Very large amounts of calcite are predicted to precipitate as a result of (a) the interaction between bicarbonate-rich ground water and cement phases and (b) the in-situ degradation of organic waste components in the repository.

A preliminary model was developed to estimate to which extent coprecipitation with calcite could reduce radionuclide concentrations in solution. Since the main objective was only to determine the order of magnitude of this effect, a very simple, closed-system model was considered to be sufficient. The model assumes instantaneous dissolution of the entire radionuclide inventory in the water-saturated pore space of the repository caverns and simultaneous reversible sorption on the cement. Coprecipitation of radionuclides with calcite and residual solution concentrations are calculated assuming in-situ precipitation of a given amount of calcite and using nuclide-specific partition coefficients.

The model calculations show that the solution concentration of several radionuclides (tetravalent and trivalent actinides, Sr and Ni) is likely to decrease by orders of magnitude as a consequence of coprecipitation with large amounts of secondary calcite. Further, radionuclide coprecipitation is found to depend on the extent to which the same radionuclide is sorbed on cement phases. The results confirm that coprecipitation with calcite will be an important process in determining concentration limits for dissolved radionuclides in the Wellenberg repository.