6.44. The assessment time frame should be defined by taking account of national regulations and regulatory guidance, as well as the characteristics of the particular disposal facility, the site and the waste to be disposed of. Other factors that should be considered when deciding on the time frame and time windows for the assessment include the following: @@- Safety assessment calculations should cover a time period that is long enough to determine the maximum, or peak, dose or risk. However, this may not always be possible. For example, in the case of disposal of long lived waste (e.g. from uranium mining) on or near the surface where there is uncertainty in the durability of engineered barriers (e.g. dams and covers), doses and risks may remain constant or may even increase long into the future, through time frames in which uncertainties in the assessment increase significantly and limit the meaningfulness of the assessment. This may limit the timescale for the assessment in general, or at least the timescale for quantitative assessments.@@- Several factors that can significantly affect safety assessment results may change with time. As an example, the landscape and hydrological regime at and around a facility may change in response to climate change, and with these changes receptors and their habits may change. Assessments for long lived waste should consider such possible changes. As a means to assess the possible evolution of a disposal system, some assessments consider one or more climate scenarios involving future glacial periods or cycles. Assessment time frames should be defined as appropriate for the possible changes at the site.@@- The decision about the time frame for the assessment has implications for the type and severity of disturbing events that are considered in the safety assessment. For example, a flood or earthquake that is expected to occur once in a thousand years can be more disruptive than a flood or earthquake that is expected to occur once in a hundred years.