THE DEVELOPMENT AND APPLICATION OF INTEGRATED PROCEDURES FOR POST-CLOSURE ASSESSMENT, BASED UPON MONTE-CARLO SIMULATION - THE PROBABILISTIC SYSTEMS ASSESSMENT (PSA) APPROACH

PSA seeks to define a comprehensive, integrated model of the disposal system and to explain the implications of the uncertainties and associated subjective judgements, under which forecasting about the long-term post-closure period must inevitably be conducted, by the use of Monte Carlo simulation. A general procedure for integrated assessment is described together with an overall representation of the evolving environmental system into which the repository and its wastes are emplaced. Following a brief history of the development of PSA within the OECD countries, during the past 15 years, some recent applications in the UK and the USA are discussed. Conclusions are drawn as to the likely developments that will occur in the next few years. In particular, the formal but imaginative treatment of uncertainty traced from after acquisitions, through the interpreter of geological site data, interpretation of interacting processes and the spatial and temporal variations of the system into justifiable estimates of radiological performance suited to particular regulatory interests. As a secondary issue, the article provides a basis for resolving current debates ove the use of 'simulation' and 'scenario' methods. Given the current level of perceived knowledge and articulation of the implications of uncertainties through PSA it is not yet clear how logically to decide on further site geological studies (for instance) and whether or not these can reliably improve the long-term performances estimates that result from further analyses.