4.13. Within the step by step approach, the scientific understanding of the disposal system and the design of the disposal facility should be progressively advanced, and the safety case should become more focused on key areas of concern. It should not only be scientific understanding that is advanced, but also an understanding of the important contributors to risk. At each step (i.e. at each major decision point), safety assessment should be performed in a manner that will enable the current level of understanding of the disposal system to be evaluated and the associated uncertainties to be assessed before decisions are made to proceed to the next step. The safety case and supporting safety assessment should be reviewed and updated prior to each major decision point and periodically as necessary to reflect actual experience and increasing knowledge (e.g. knowledge gained from scientific research), with account taken of operational aspects that are relevant for long term safety. Following commencement of facility operation, revisions or updates to the safety case and supporting assessment should be conducted if significant changes are identified in operational practices, waste forms, design, etc.