Evolution of the Groundwater Chemistry Around a Nuclear Waste Repository.

Perturbations to the natural chemistry of the geology surrounding a nuclear waste repository may affect the overall nuclide release rates through properties such as their solubilities and sorption characteristics. In this paper, we develop some of the necessary techniques to construct a detailed research model of the evolution of the groundwater under the influence of the backfill material. These techniques involve various extensions to the coupled ionic migration and chemical equilibria code, CHEQMATE and are used in the first stages of such a model. Although the design and location of a deep low- and intermediate-level nuclear waste repository have not yet been finalised in the U.K., we have considered a concrete backfill material embedded in a clay geology. Preliminary results suggest that over timescales of about a thousand years the natural buffering action of the clay against changes in pH has a significant effect on the scale of perturbation by the ingress of highly alkaline porewater. It seems likely therefore that this type of modelling will have considerable relevance to the safety assessment models.