Interactions of Ni and Ca at the calcite-solution interface

The performance assessment of repositories for spent nuclear fuel need, among other things, data describing the solubilities of radionuclides in the near field and far field. The solubility limits are often used in order to estimate the maximum concentrations of radionuclides during their possible transport to the biosphere. The solubilities used are mostly the individual solubilities for pure solids of the actual radionuclides. This way of using solubility limits represents a conservative performance assessment where the estimated nuclide concentrations are unrealistically high. This is acceptable from a performance assessment point of view but very unsatisfactory for an optimal design of the repository. In order to make the assessment more realistic, coprecipitation and solid solution formation should be taken into account. Only solids which are, in geological terms, formed in fast reactions need to be considered, which in practice restricts the number of radionuclide scavengers to calcite and iron(III)oxihydroxide. This work focuses on the Ni coprecipitation with calcite. The systems were studied under anoxic conditions and consisted of calcite-saturated 0.05 M NaCl solutions in equilibrium with synthetic calcite. The solutions were initially spiked with 63Ni and 45Ca and the concentrations of these elements were determined using liquid scintillation counting. The Ni coprecipitation was mainly governed by the calcite recrystallization. The results show the extent to which these processes occurred during a 42-day period. Our results indicate that the value of the conditional solubility product for the coprecipitated Ni is at least 4 - 5 orders of magnitude lower than the individual solubility product for NiCO3, and thus a corresponding decrease in the solubility limit for Ni.