The issue of colloid-facilitated radionuclide transport (CFRT) was last addressed by the Swedish nuclear regulators (SKI at that time, now integrated into SSM) in 2001 – 2002. SKI had commissioned the Collage code with subsequent development as Collage 2. This code was employed to investigate the potential role for colloids to have been involved in the transport of radionuclides at the Nevada Test Site and to examine the implications for CFRT in the Swedish disposal programme. It was concluded that colloids could not be ruled out as a mechanism for rapid transport and early release from the geosphere.
Recently the “bentonite erosion scenario” has become of concern. In it the generation of large quantities of bentonite colloids in fractures as a result of fresh water ingress at repository depth is possible. Potentially, these could carry radiologically significant quantities of radionuclides to an early release to the surface system.
Recent developments in CFRT (reviewed here) indicate that additional parameters needed to be added to the existing Collage 2 plus code in order to adequately represent colloid transport in Swedish fractures.
This report looks at modifications to the model and discusses the implications of the implementation of the new processes. Authors conclude that the process of colloid filtration is an important mitigating mechanism. A new code – Collage 3 – is demonstrated and suggestions for further work are given.