Calculations of risk or dose from the groundwater flow pathway are a central part of the assessment of post closure performance for a deep repository system for radioactive waste. These calculations need to be supported by a knowledge of the various components of the disposal system. Site characterisation provides a site specific geological database which can be used to build, test and refine numerical models of the geosphere and, to some extent, the biosphere components of the disposal system in order to evaluate its future evolution.

Site characterisation programmes, and groundwater flow modelling within the performance assessment need to progress in an interactive manner to ensure that both remain focused on the resolution of key uncertainties which impact the overall performance assessment. The data derived from site investigations can be classified in terms of their application within the performance assessment: establishing the framework for numerical models; enabling the parameterisation of numerical models; testing model output; refining models; and demonstrating their credibility. Site characterisation programmes need to be established to provide the appropriate level of information for each of these stages. It is therefore not particularly useful to classify information into the traditional disciplines of geology, hydrogeology and geochemistry.

A key issue for developing a geological and hydrogeological database for performance assessment is determining when sufficient information has been obtained. Primarily, this is dictated by the needs and sensitivities of the performance assessment and any regulatory requirements. However, radioactive waste disposal agencies work in a political environment that is strongly influenced by public perception. In order to progress, any programme must attain a broad measure of credibility and acceptability to other key “stakeholders”.

The paper draws on examples and experiences from the extensive programme of work which has been performed by Nirex to understand the characteristics of a site near Sellafield in Cumbria, and to develop groundwater flow models to support the assessment of the post closure performance of a potential repository located at that site. Uncertainties important in influencing calculated risk within an interim assessment, published as the “Nirex 95 assessment” were used to develop further site-specific investigations. The resulting site characterisation activities generated an enhanced and refined site-specific geological and hydrogeological database for a subsequent assessment - the “Nirex 97 assessment”.