RSC-Programme - Interim Report. Approach and Basis for - Posiva

21example, material selection (e.g. buffer and backfill material) can lead to morerequirements on the rock environment and thus needs to be considered when settingperformance targets. The performance targets for barriers are defined as conditionsunder which the safety functions are fulfilled, i.e. if target values are met then the safetyfunctions of barriers are maintained. If, during certain time frames of the assessmentperiod, deviations from target values occur and indicate that the corresponding safetyfunctions may not be provided in full, overall system safety assessment must be carriedout. A performance target is a property of the system component, including a preferablyquantitative limit or range for this property. Therefore, performance targets correspondto safety function indicators and criteria in SKB notation (see also Section 2.3). As anexample, Figure 3-2 shows the steps in defining rock suitability criteria from safetyfunctions and performance targets. A more detailed discussion on definition of safetyfunctions and performance targets is given in Chapter 4.Rock suitability criteria are site-specific. For example, they judge the most relevantparameters and confidence in models at the site which affect the performance targetsand engineering targets set. Interpretation and modelling are required to understand theimportance of these observations in the context of general site understanding. The linkbetween the site property used in criteria definition and the given performance orengineering target needs to be shown. In many cases, modelling of underlying processesand laboratory or in situ tests on barrier performance is needed. In defining the criteria,there is a need to assess whether the rock characterisation in current conditions willprovide sufficient information about future conditions. The challenge is to have thispredictive capacity in models, as well as the ability to observe and measure the mostrelevant site properties under disturbed conditions. Part of this work is the selection ofproper measurement techniques. Preliminary testing at the site will not only provideimportant information on the practical aspects of applying such criteria, but also help inestimating observational uncertainties. To estimate the effectiveness of the criteria, theinitial state defined by the criteria and potential for the perturbing processes and eventsduring the evolution of the repository and site needs to be considered. Further on, theconsequences on the safety needs to assessed.Definition of the rock suitability criteria is presented in more detail in Chapter 5. It isnoted, that the rock suitability criteria are often set conservatively. This means thatsome of the criteria can be overly strict for example to cope with the uncertainties in thedetermination of the parameter value and still giving confidence in the fulfilment of theperformance target. Thus the criteria could be referred to as indicators rather thancriteria. In any case, the criteria will be used in deciding whether to construct tunnels incertain volume during tunnel excavation and for acceptance of the deposition hole fordisposal use. Therefore, due to the fact that RSC is a tool in this decision makingprocess, it is considered justified to call them criteria rather than just indicators.