5.16. Where appropriate, the need to consider complex processes in the postclosure evolution of the disposal system should be reduced to the extent possible. Whereas passive features are not necessarily free from complexity, in the choice of site and design features complex processes should be avoided to the extent possible. Avoidance of complex processes may reduce the need to couple processes in the models developed for safety assessment; furthermore, it may be possible to restrict consideration of other factors that could affect the evolution of the disposal system in more complex settings. The spatial and temporal variability of features, events and processes [16] that need to be taken into account could also be more clearly presented for a relatively simple environment if complex processes are avoided. In this way, the number of key parameters to be included in the safety assessments may be reduced and simpler models could be used to assess safety. Although simplicity is a desirable feature, the containment and isolation capabilities of the site afforded by its natural characteristics are of primary importance and these should be the deciding criteria employed in the choice of a site.