5.63. Scenario uncertainties are usually dealt with by performing the assessment for a range of scenarios, usually comprising a base case scenario and several alternate evolution scenarios. These scenarios should be derived using appropriate, well defined procedures in which choices and decisions are structured, guided and documented. This comparison of assessments for various scenarios will provide an indication of the significance of the uncertainty in respect of the evolution of the site and disposal system. It may be determined that the scenario uncertainty is acceptable in the overall context of the safety case, or the comparison may indicate that changes to the design should be considered.