Characterization and Assessment of the Groundwater Pathway for the Low Level Waste Repository, UK

The Low Level Waste Repository (LLWR) is the UK’s principal facility for the disposal of solid low-level radioactive waste and is operated by LLW Repository Limited. Presently, LLWR Ltd is establishing the long-term environmental safety of disposals of solid radioactive waste at the LLWR, through the submission of the 2011 Environmental Safety Case for the LLWR. This Environmental Safety Case addresses the Environment Agency Guidance on Requirements for Authorisation. Aspects of the submission consider improved vault design, closure design, and quantitative assessments. Each of these issues requires an understanding of the movement of water through the facility and the surrounding geology during operations and following facility closure. Groundwater flow modelling has been used extensively in support of the interpretation of field investigations, the development of the engineering design, and an assessment of the groundwater pathway as one of the major pathways by which contaminants may reach the environment. This paper describes these important aspects of the Environmental Safety Case. The geological environment in the region of the LLWR consists of Quaternary age deposits overlying older bedrock. The facility involves shallow excavations into the Quaternary deposits, originally for trenches, with disposals to a vault system beginning in 1988. In the post-closure phase these disposals are covered by a cap and surrounded by a cut-off wall to minimise the water flow around or through the waste. An innovative modelling methodology has been developed to represent the range of scales that have to be considered from the regional groundwater flow patterns over several kilometres, the scale of tens of metres around the immediate site area, and down to about 1 metre for details of flows within the repository itself in three dimensions. Detailed finite-element models of the flow through geological media and the engineered features are used to interpret site data and assess a credible set of post-closure situations and model cases. In the radiological assessment, a more simplified compartment model is used to assess uncertainties in hydrogeological properties and the long-term evolution of the engineered barriers. Together the approach provides flexible tools for understanding and assessing a comprehensive range of aspects including details of flows within the repository, dilution and migration in the external geology, the long-term evolution of the hydrogeological system, the implications of spatial variability and alternative geological models, and effects of uncertainties.