The long-term safety of any disposal system for hazardous waste must be demonstrated prior to its implementation. A safety assessment cannot rely on direct observation, because timescales are far beyond the normal horizon of technical planning. Therefore, conceptual models of the engineered and natural barriers of the disposal system and computer simulations serve to illustrate the potential system performance.
Key activities in development of a repository performance assessment are the comprehensive identification of the relevant factors that should be included in quantitative analyses. Identification and selection of alternative futures (e.g ice-ages) relevant to assessment of repository performance is termed scenario development.
The objective of post-closure performance assessment (in conjunction with e.g. observations on natural systems) is to develop confidence that the disposal system will perform within certain bounds which correspond to acceptably low impacts or risks.