The potential of natural analogues in assessing systems for deep disposal of high-level radioactive waste

Many of the processes which will lead to the breakdown of engineered barriers and the mobilisation of radionuclides in a deep waste repository have analogies in natural geological systems. These 'natural analogues' are seen as a particularly important means of validating predictive models, under the broad heading of radionuclide migration, which are used in long-term safety analyses. Their principal value is the opportunity they provide to examine processes occuring over geological timescales, hence allowing more confident extrapolation of short timescales experimental data. This report begins by reviewing the processes leading to breakdown of containment in a high-level radioactive waste repository in crystalline bedrock and the subsequent migration mechanisms for radionuclides back to the biosphere. Nine specific processes are identified as being of the most significance in migration models, based on available sensitivity analyses. While the report is aimed specifically at the Nagra (Switzerland) and KBS (Sweden) concepts for disposal of high-level wastes or spent fuel, the results are seen to be more widely applicable. A considerable amount of the information reviewed and presented could be used in the assessment of disposal of other waste types in other host rocks. (author)