A single-site reactive transport model of Cs+ for the in situ diffusion and retention (DR) experiment

Abstract

In situ diffusion experiments are performed in underground research laboratories for understanding and quantifying radionuclide diffusion from underground radioactive waste repositories. The in situ diffusion and retention, DR, experiment was performed at the Mont Terri underground research laboratory, Switzerland, to characterize the diffusion and retention parameters of the Opalinus clay. Several tracers were injected instantaneously in the circulating artificial water and were then allowed to diffuse into the clay rock through two porous packed-off sections of a borehole drilled normal to the bedding of the clay formation. This paper presents a single-site multicomponent reactive transport model of Cs+, a tracer used in the DR experiment which sorbs onto Opalinus clay via cation exchange. The reactive transport model accounts for the diffusive-reactive transport of 11 primary species and 22 aqueous complexes, and the water–rock interactions for 5 cation exchange and 3 mineral dissolution/precipitation reactions. Most of the solutes except for Cs+ diffuse from the Opalinus clay formation into the injection interval because the concentrations in the initial Opalinus clay pore water are larger than those of the initial water in the circulation system. Calcite dissolves near the borehole while dolomite precipitates. Dissolved Cs+ sorbs by exchanging with Ca2+ in the exchange complex. The computed dilution curve of Cs+ in the circulating fluid is most sensitive to the effective diffusion, D e, of the filter, the selectivity coefficient of Na+ to Cs+, K Na–Cs and D e of the borehole disturbed zone. The apparent distribution coefficient of Cs+, $$K_{\text{d}}^{\text{a}}$$ , in the formation varies in space and time from 100 to 165 L/kg due to the temporal changes in the water chemistry in the formation. The results of a sensitivity run in which the initial chemical composition of the Opalinus pore water is the same as the initial chemical composition of the water in the circulation system show that the changes in $$K_{\text{d}}^{\text{a}}$$ are negligible. The dilution curve of Cs+ computed with the reactive transport model coincides with that obtained with the K d model. The tracer concentrations along the overcoring profiles computed with the K d model, however, differ significantly from those computed with the reactive transport model. Therefore, a reactive transport model is needed for the appropriate interpretation of the Cs+ overcoring data from the DR diffusion experiment.