5.26. Identifying and addressing uncertainties is a major part of post-closure assessments. A range of techniques should be used to evaluate uncertainties in post-closure performance of the facility. Detailed models of particular parts of the disposal system and of particular events and processes should be used to investigate behaviour and to decide how to handle system components and features, events and processes in the overall safety assessment. Sensitivity analysis, uncertainty analysis and bounding calculations can be used at the detailed level and at the system level. Probabilistic and deterministic calculations can be made for both time varying and steady state situations. The aim is to reduce uncertainties concerning safety where possible, and where this is not possible, to characterize uncertainties quantitatively or qualitatively. Care should be exercised in applying criteria for periods of time for which the uncertainties are such that the criteria may no longer be appropriate as a basis for decision making.