1.24. The disposal system (i.e. the disposal facility and the environment in which it is sited) is developed in a series of steps in which the scientific understanding of the disposal system and of the design of the disposal facility is progressively advanced. Safety assessment is an important tool for guiding site selection and evaluation and for assisting with the design of the facility. It is also used for evaluating the prevailing level of understanding of the disposal system and for assessing the associated uncertainties through the various steps in the development of the facility. The extent and complexity of such an assessment will vary with the type of facility and will be related to the hazard potential of the waste.