Devising Scenarios for Future Repository Evolution: A Rigorous Methodology
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DEVISING SCENARIOS FOR FUTURE REPOSITORY EVOLUTION: A RIGOROUS METHODOLOGY.
Neil A. Chapman', Johan Andersson', Peter Robinson', Kristina Skagius 3 , Clas-Otto Wene4 , Marie Wiborgh 3 & Stig Wingefors 2 . 'Intera Information Technologies, Melton Mowbray, UK; 2 Swedish Nuclear Power Inspectorate, Stockholm, Sweden; 3Kemakta, Stockholm, Sweden; 4Chalmers Institute of Technology, G6teborg, Sweden.
ABSTRACT The Swedish Nuclear Power Inspectorate is developing a new methodology for the construction of scenarios for radiological consequence analysis as part of its SITE94 performance assessment project. SITE-94 involves the incorporation of site specific data from the Asp6 site into a performance assessment (PA) of a hypothetical high-level waste repository. This paper describes a systems analysis approach that has been developed based on the concept of organising all the events and processes which need to be taken account of in PA into a 'process system' and a much smaller residual group which are used to generate scenarios. The methodology used for developing scenarios, producing calculation cases and addressing the various types of uncertainty involved in PA consequence analysis is described.
INTRODUCTION Scenario analysis in radioactive waste performance assessment has often been accused of being an unstructured and rather arbitrary process which incorporates untraceable decisions and does not allow other interested groups to use the same data to test their own ideas about repository behaviour. In some cases it is seen as providing a posteriorijustification for a particular group of performance assessment calculations. The objective of the present work is to make scenario construction and the production of calculation cases for consequence analysis a scientifically rigorous and traceable procedure. To do this it was decided to adopt a systems analysis approach. At the outset, it is useful to consider the philosophical basis on which scenarios are constructed. First, a working definition of scenarios was developed: A scenario is a hypothetical sequence of processes and events, and is one of a set devisedfor the purpose of illustratingthe range offuture behaviours and states of a repository system, for the purposes of making or evaluating a safety case. It is not necessary, or indeed possible, to describe individually all possible scenarios. However, consideration of the complete set should provide an adequately robust test of repository safety by addressingthe most likely futures together with less likely futures which exhibitfeatures of widely acknowledged concern. Experience in SITE-94, and an extensive review of the scenario literature, strongly influenced by the early work of Kahn', leads us to challenge the view that scenario analysis can ever provide a comprehensive description of the full range of future behaviour of a repository system or, consequently, that it can provide meaningful probabilistic safety assessment results. It is not possible to analyse all mathematically possible combinations of future poss
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