Investigating colloid-mediated transport of repository safety-relevant radionuclides under realistic, in situ conditions (including very low radionuclide solubilities in the groundwater, radioprotection constraints, analytical limitations etc) is a very complex task and so it was decided to split the CRR project into two phases:

• Phase 1: feasibility phase including literature studies,scoping laboratory and field experiments and associated modelling efforts.
   
• Phase 2: the main field experiment accompanied by supporting laboratory experiments and an intensive modelling programme.

The results of Phase 1 showed that the in situ experiment would certainly be feasible. The main results of this Phase may be summarised as:

• Experiments simulating groundwater flow at the host-rock/bentonite interface indicate that bentonite colloid generation can occur.
   
• Relatively high bentonite colloid concentrations have been shown to remain stable in the alkaline groundwaters of the experimental zone.
   
• Conversely, bentonite colloids are not stable within bentonite porewater.

In the cases of Am, Pu, Cs and U, the radionuclides examined show a strong affinity for bentonite colloids, which may facilitate migration of these radionuclides through a repository host rock.

Colloid and Radionuclide Retardation Experiment - Experimental Approach