The aim of long-term performance assessment (PA) is generally to develop confidence that the disposal system will perform within certain bounds and give rise to acceptably low impacts or risks. The PA procedure must be able to deal with:
- uncertainty due to incomplete understanding of the relevant processes;
- uncertainty due to limited sampling of the structures and characteristics of the system and especially the geological media;
- uncertainty about future conditions and processes leading to changes in the engineered barriers and natural environment that may affect radionuclide release, transport and exposure pathways.
A number of approaches have been taken to solving the problem of how to deal with such uncertainties. The differences in approach may be due to the stage of project development, differences in national regulatory guidance and the different r6les of the implementor and the regulator. This paper examines the rôles of the implementor and regulator and discusses possible approaches appropriate to both points of view.