Optimisation: an overview of concepts

Philippe Raimbault, consultant to the NEA, summarised and reviewed the concepts relevant to the 'optimisation' of geological disposal systems as they are outlined in national and international guidance as well as in the work of NEA groups. The main objective of optimisation is to find the 'optimal' or 'best' combination of characteristics in terms of balancing imperatives of current and future safety while respecting the interests of present and future generations. There are, however, several ways to tackle the issue as 'optimisation' may mean different concepts to different groups of people, as expounded in the written paper and in the NEA report supporting this workshop. Most of the international guidance on optimisation of geological disposal is provided by ICRP (ICRP-81 and ICRP-103), IAEA (WS-R-4) and the European Union IPPC (Integration Prevention Pollution and Control) Directive. Although international guidance on optimisation exists, it is varied and not always clear. In particular, there is no specific recommendation on methods to balance operational and post-closure safety. Countries have been preparing regulatory guidance on demonstrating long-term safety. The interpretation of international guidance and the level of detail of the implementation of the optimisation concept vary depending on the country but it remains on a very general level. Most countries observe that a stepwise approach should be followed, which means that design options may evolve with time. In this sense, one may view that protection becomes optimised as the result of the process of repository development. The principle of optimisation of the safety of the repository is closely linked to the decision making process. The latest ICRP recommendations state that the optimisation of protection is a forward-looking iterative process involving all parties and aimed at preventing and reducing future exposures. It would seem, however, ineffectual to base optimisation on just one criterion - that related to radiological exposure - when in fact system optimisation is, by its nature, a multi-criteria endeavour. There is currently a trend to consider optimisation more as a process than an outcome and to strive towards system optimisation. At the same time, people tend to expect hard criteria on which regulators can base their decisions, and it is difficult for regulation not to be normative