Coprecipitation of Ni with CaCO3: An experimental study

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 calcite coprecipitation only. This report gives a short introduction to the theory of coprecipitation. The report also presents some preliminary experimental work on the coprecipitation of trace concentrations of nickel with calcite under anoxic conditions. The experimental conditions were chosen such that no individual Ni solids would form; i.e. the only way for Ni to precipitate was via coprecipitation. Calculated mole fractions of NiCO3 in the coprecipitate formed, and estimates of the conditional solubility constants for the NiCO3 are also presented.