Carrying out the broad tasks to develop the safety case Choosing the disposal system

The disposal system is chosen according to the disposal principles defined in Chapter 2 in order to provide passive safety and robustness, and to ensure that information critical for the demonstration of safety can be obtained. The choice of the disposal system is the result of an iterative repository development strategy (see Fig. 1.2-3). Refinements to the disposal system at any given stage are guided by earlier studies, including studies of long-term safety. The disposal principles also have elements that are derived from previous experience (arrow (1) in Fig. 3.7-2). Refinements to the disposal system, and also to relevant scientific understanding, are expected to continue until a safety case can be made that is adequate for the corresponding licence application. The stepwise approach adopted in the programme for the management of SF, HLW and ILW in Switzerland is described in Chapter 1.

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Fig. 3.7-1: The lines of argument contributing to the safety case and their relationship to the broad tasks for developing the safety case

Colours are used to indicate which of the broad tasks within this procedure contribute to particular lines of argument.

Lines of argument contributing to the

safety case Broad tasks for developing the safety case

The strength of geological disposal as a waste management option

The safety and robustness of the chosen disposal system

The good scientific understanding that is available and relevant to the

chosen disposal system and its

The lack of outstanding issues with the potential to compromise safety

Determine what is known about the system and its evolution,

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Fig. 3.7-2: The procedure for constructing the safety case

FEP management procedures indicated by shaded boxes are described in Section 3.7.5.

Deriving the system concept

Having chosen the disposal system, a safety assessment is carried out in accordance with the assessment principles. The starting point for the assessment is an evaluation of information that is relevant to the system, drawing on the expertise that is available to the project, and the results of scientific investigations and design studies (arrow (2) in Fig. 3.7-2). The resulting under-standing of the FEPs that characterise, and may influence, the disposal system and its evolution, together with a broad conceptualisation of the possible paths that its evolution might take and a comprehensive evaluation of uncertainties, is termed the system concept.

Deriving the safety concept

The derivation of the safety concept involves identifying the elements (i.e. physical or chemical features and their ensemble of associated processes) of the disposal system that are key contributors to the safety functions (as defined in Section 2.6.2). Scientific investigations and design studies are used to provide understanding and support for the reliability of these features (arrows (2) and (3) in Fig. 3.7-2), as well as evidence for their reliability and long-term effectiveness. Their importance and actual contribution to safety is investigated by means of mathematical modelling studies, including deterministic and probabilistic sensitivity analyses.

Those elements that are eventually confirmed as providing robust safety are termed "pillars of safety".

Safety case (including guidance for next stage) END POINT OF CURRENT STAGE

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Illustrating radiological consequences

A range of representative assessment cases is defined in order to illustrate the radiological consequences and their ranges of uncertainty. The results of preliminary calculations, including sensitivity analyses, are used to investigate the behaviour of the system with respect to perturba-tions and to identify key uncertainties, in order to restrict the assessment cases to a manageable number (arrow (4) in Fig. 3.7-2). A decision is made as to which cases are to be qualitatively discussed, and which need to be analysed quantitatively, and the data on which to base the discussions and analyses are compiled (choice of cases and data in Fig. 3.7-2). The calculations are then performed, and qualitative arguments made, using the assessment capability (i.e. the information and codes) that has been acquired. The majority of analyses are performed using a deterministic approach, but this is complemented by a number of probabilistic calculations.

Consequence analysis, including the use of deterministic and probabilistic calculations and the definition and analysis of assessment cases, is discussed in more detail in Section 3.7.4.

Compiling arguments and providing guidance for future stages

The results of the analyses of the assessment cases are combined with a range of supporting evidence and complemented with qualitative arguments in order to construct the safety case.

The arguments and analyses aim to provide a clear indication as to whether the disposal system has been well chosen, in that it has good prospects of providing the level of safety required by Swiss regulations, and thus deserves further consideration. Through the definition and analysis of a broad range of representative assessment cases, the expected levels of safety of the pro-posed site and design options are assessed, and the consequences of current uncertainties are evaluated. As discussed below, many of the sources of uncertainty are either avoidable or amenable to reduction. The analysis of the assessment cases allows the detrimental FEPs and uncertainties with the greatest impact to be identified, giving guidance for future project stages (arrow (5) in Fig. 3.7-2), which may include specific modifications to the repository design (arrow (1) in Fig. 3.7-2), until ultimately the safety case is adequate to support the correspond-ing licence applications.

3.7.3 Evaluation and treatment of uncertainties