Coprecipitation: Basic theory and some preliminary experimental results

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 (Read 1994). 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. The report gives a short introduction to coprecipitation phenomena based on literature studies. The report also presents some preliminary experimental work on coprecipitation of trace concentrations of strontium in calcite under anoxic conditions. The applicability of ICP-MS to these types of studies was investigated and found to be promising for future work.