The colloid and radionuclide retardation experiment at the Grimsel Test Site: influence of bentonite colloids on radionuclide migration in a fractured rock

Abstract

Abstract The colloid and radionuclide retardation (CRR) experiment is dedicated to the study of the in situ migration behaviour of selected actinides and fission products in the absence and presence of bentonite colloids in a water-conducting feature (shear zone) in the Grimsel Test Site (GTS). The technical scenario considers the bentonite backfill/host rock interface as a potential source for colloids. The experiment investigates the migration behaviour of U, Th, Pu, Am, Np, Sr, Cs, I and Tc and the influence of smectitic bentonite colloids by two in situ tracer injections in a well-characterised dipole. The field experiments are supported by an extended laboratory and modelling programme. Colloid breakthrough is determined on-line by a mobile, laser-induced breakdown detection apparatus (LIBD), a mobile photon correlation spectrometer (PCS) and, afterwards in the laboratory, by a single particle counting method (SPC) using a laser light scattering technique. Bentonite colloids generated from bentonite backfill material were found to be stable in the experimental groundwater and the influence of pH and salinity on colloid stability was investigated. The in situ monitored breakthrough of the tri- and tetravalent actinides Am and Pu and of Cs followed the colloid breakthrough indicating some degree of colloid-mediated migration of these radionuclides in the experimental shear zone. But even when no colloids had been added to the tracer cocktail, part of Am(III) and Pu(IV) appears to migrate as colloids. The different colloid detection techniques revealed a colloid recovery between 80 and 90% of the injected bentonite colloids. The CRR experimental results are considered from the perspective of understanding the likely long-term behaviour of a deep geological repository for radioactive waste and as an indicator of the way forward to the next generation of in situ experiments.