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.