6.46. It may be necessary to define several different time windows within one safety case in order to deal with different scenarios. For example, some scenarios may include events that will disrupt and/or destroy certain types of disposal facility. At some sites, erosion or glaciation may destroy a near surface disposal facility. While the time frame over which glaciation is to be expected is not relevant for waste containing mainly short lived radionuclides, disposal facilities for uranium mill tailings may be affected by glaciation, which will limit the time frame for meaningful assessment of above surface impoundments for such waste. As another example, the need to prevent criticality will need to be considered for certain waste types, and the time frame over which criticality might conceivably occur should be defined largely by consideration of radionuclide decay and in-growth. It may also be desirable to define several different time windows within one safety assessment for presentational reasons. For example, definition of more than one assessment time window may enable assessment calculations to be undertaken and presented at different levels of detail and/or for different degrees of conservatism or realism.