Technical Report NTB 97-08

Coprecipitation of radionuclides: basic concepts, literature review and first applications

Coprecipitation of radionuclides with solid alteration products is currently not analysed quantitatively in safety assessments for nuclear waste repositories, although this process is thought to be an important mechanism for limiting nuclide concentrations in solution. This is due to the fact that neither the solid phases controlling coprecipitation nor the parameter values necessary to describe this process are known sufficiently.

This introductory report provides basic knowledge on this subject and a review of experimental data from the literature. Emphasis is placed on experiments of trace metal coprecipitation with calcite, because this mineral is a dominating alteration product of cement in the Swiss L/ILW repository. This resulted in a database of partition coefficients, which allow to describe empirically the distribution of trace elements between calcite and solution and thus to quantify coprecipitation processes.

Since laboratory data on coprecipitation with calcite are lacking for many safety-relevant radioelements, their partition coefficients were inferred with the help of estimation techniques. Such techniques rely on empirical correlations, which relate the uptake of trace metals in calcite (measured in laboratory tests) with selected chemical properties of the coprecipitated metals (e.g. ionic radius, sorption properties, solubility products of the pure trace metal carbonates). The combination of these correlations with independent geochemical evidence allows the extrapolation of radioelement-­specific partition coefficients, which are then used for the quantitative modelling.

In a first step the potential role of radionuclide coprecipitation during cement degradation in the L/ILW repository planned at Wellenberg is assessed. The results of model calculations indicate for many radionuclides that a large fraction of the total inventory would remain trapped in the secondary calcite formed during cement degradation, considerably reducing their solution concentration. Such calculations confirm the large potential of coprecipitation as a solubility-limiting mechanism for many radionuclides and suggest that including this process in future safety analyses could lead to a significant decrease in doses.