Nirex 97 an assessment of the post-closure performance of a deep waste repository at Sellafield. Volume 2; hydrogeological conceptual model development - effective parameters and calibration

United Kingdom Nirex Limited ('Nirex') is responsible for providing and managing facilities for the safe disposal of intermediate and certain low-level radioactive waste (ILW and LLW respectively). Government policy is that the preferred disposal route for such wastes is a deep geological repository. The repository concept aims to use a combination of natural and engineered barriers to achieve the necessary degree of long-term isolation and containment of the radioactive wastes. Since 1987, Nirex has carried out an extensive technical programme directed at the science of safe disposal. The work comprises a research programme into the long-term performance of waste forms and the engineered and natural barriers, including the characterisation of candidate geological settings to assess their suitability to host a deep waste repository ('DWR'). Between mid-1991 and March 1997 the geological characterisation programme was concentrated on establishing the suitability, or otherwise, of a candidate site at Sellafield, West Cumbria. In July 1994, as part of a detailed site investigation programme, Nirex applied for planning permission to develop an underground Rock Characterisation Facility (RCF) at Longlands Farm near Sellafield. This application was rejected by the planning authority and Nirex's appeal against that decision led to a local planning inquiry which ran from September 1995 until February 1996. In line with the Inspector's Report, in March 1997 the Nirex appeal was dismissed by the Secretary of State for the Environment. The Company's response to that decision, and its readiness to contribute to the new government's review of the way forward, are described in the Nirex Annual Report for 1996-97. This report - Nirex 97 - is founded on the understanding developed through the Nirex technical programme. It reports the outcome of an assessment of the post-closure safety performance, over hundreds of thousands of years, of a repository system located in a potential repository zone beneath Longlands Farm. In style, scope and presentation, Nirex 97 is primarily aimed at the scientific community, other radioactive waste disposal agencies and regulators. The report is published as part of Nirex's commitment to open publication of its scientific findings. The main value of the report currently is as a demonstration of the generic capability which has been developed to assess the radiological safety performance of candidate repository sites. The safety assessment reported as Nirex 97 was carried out between April 1996 and August 1997. It updates a preliminary assessment of the groundwater pathway for a repository at Sellafield, 'Nirex 95' published in July 1995. Nirex 97 takes account of further data obtained from the Nirex waste inventory, design, site characterisation and research programmes. In addition, Nirex 97 extends the Nirex 95 evaluation to include consideration of the potential radiological and flammability hazards in the biosphere arising from the effects of gas generation within the repository and the extent of pressurisation within the repository vaults. The assessment also takes account of the latest guidance from the Environment Agencies on requirements for authorisation of disposal facilities on land. Volume 2: Hydrogeological Model Development - Effective Parameters and Calibration, explains the steps followed to interpret and process field data in order to build a variety of two- and three-dimensional numerical models of groundwater flow. It describes the process of upscaling used to derive effective hydrogeological parameters for the hydrogeological units represented in these numerical models. The calibration of these numerical models is discussed