1991:08 The Potential Significance of Permafrost tothe Behaviour of a Deep RadioactiveWaste Repository

Permafrost is one of the scenarios that is being considered as part of the groundwater flow and transport modelling for the Project-90 assessment. It is included as one of the primary Features, Events and Processes (FEPs) which are being kept outside the PROCESS SYSTEM in the SKBjSKI scenario development project (Andersson at aI, 1989). There is a large amount of evidence that Sweden has suffered several cycles of permafrost development over the Quaternary, approximately the last 2My, and climatic predictions for the next hundred thousand years suggest that similar climatic cycling is likely to occur. The presence of permafrost could have important effects on the hydrogeological regime and could therefore be important in modifying the release and dispersion of radionuclides from a repository. The climatic conditions of permafrost would also influence radionuclide migration and accumulation in the biosphere and the associated radiation exposure of man. These biosphere aspects are not considered here but the implications for discharge into the biosphere are examined, including the abstraction of groundwater by man in permafrost regions.

In order to structure the list of FEPs, it was found necessary to introduce the concepts of the PROCESS SYSTEM and external conditions. The PROCESS SYSTEM comprises the complete set of deterministic physical and chemical processes that might influence the release of radionuclides from the repository to the biosphere. The external conditions are events and processes that are not induced by the repository and can occur independently of the processes in the PROCESS SYSTEM. Using these definitions, most of the FEPs on the original list were assigned to the PROCESS SYSTEM. and only a small number, approximately 50, were left as FEPs representing external conditions. These remaining FEPs have been grouped (lumped) into a few (10) primary FEPs of external conditions, permafrost being one of these ten. This report reviews the evidence relating to permafrost development and discusses the possible implications for the long-term safety of a deep repository.