This paper describes a generic methodology for building the safety case for a geological repository, which is currently being applied to a possible facility for spent fuel, vitrified highlevel waste and long-lived intermediate-level waste in the Opalinus Clay of Northern Switzerland. The methodology involves:
1. the identification of certain basic disposal principles,
2. the choice of a disposal system, via a flexible repository development strategy,
3. the derivation of the system concept, based on current understanding of the phenomena that characterise, and may influence, the disposal system and its evolution,
4. the derivation of a safety concept, based on reliable, well understood and effective pillars of safety,
5. the illustration of the radiological consequences of the disposal system through the definition and analysis of a wide range of assessment cases, and
6. the compilation of the arguments and analyses that constitute the safety case, as well as guidance for future stages of the repository programme. A range of measures, including audits, are used to promote completeness of the phenomena considered in the safety case, and to avoid inadvertent bias.