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

17A safety function is a role through which a repository component contributes tosafety.A safety function indicator is a measurable or calculable property of a repositorycomponent, which indicates the extent to which a safety function is fulfilled.A safety function indicator criterion is a quantitative limit, such that if therelevant safety function indicator fulfils the criterion, the corresponding safetyfunction is maintained.Fulfilment of the safety function indicator criteria implies that safety functions of thecomponent are maintained. On the other hand, if the criteria for one or more safetyfunction indicators are not fulfilled, then consequences of loss or degraded performanceof the corresponding safety function will be further analysed and potentially evaluatedin the safety assessment.As part of the preparation for its upcoming licence application, SKB is currentlydeveloping design premises of a KBS-3V repository for spent nuclear fuel from a longtermsafety point of view. The design premises is based on the experience from SKB’slatest published safety assessment, SR-Can, and some subsequent analyses. Thedevelopment work by SKB presents an elaboration of the design basis cases, and thefeedback to the canister and repository design in sections 13.4–13.6 of the SR-Can mainreport (SKB 2006). The purpose is to provide more detailed material in order toformulate requirements on the barriers for actual design decisions, includingrequirements on the host rock and underground excavation. Examples of requirementswith regard to the rock include maximum permitted inflow to deposition tunnels andholes, minimum distances from deposition holes to large deformation zones, a need toavoid large fractures in deposition holes to ensure mechanical stability (see e.g. Munier2006a, 2006b) and minimum distances between canisters to control buffer temperature.These design premises are key inputs to the SKB reference design, which will bepresented in the “production reports” for the final repository. The production reportsshould verify that the chosen design complies with given design premises, whereas thedesign premises report takes the burden of justifying why these design premises arerelevant.2.4 KBS-3H workSKB and Posiva have carried out a joint project for developing the KBS-3H repositoryalternative. The approach based on safety functions and safety function indicatorspresented in the SR-Can and applied for the Swedish candidate sites (SKB 2006, seealso Section 2.3), was applied for Olkiluoto in compiling the safety case for KBS-3H(Smith et al. 2007a, b, c). It is noted that work carried out by Smith et al. (2007a) wasnot a full safety case, as the focus was mainly in differences between the features ofKBS-3V and KBS-3H. Accordingly, the emphasis in the KBS-3H work was on safetyfunctions, indicators and criteria for the engineered barrier system, rather than thegeosphere. Some modifications were done by Smith et al. (2007c) to the safetyfunctions, safety function indicators and safety function indicator criteria given in SR-Can (SKB 2006) to make them applicable for KBS-3H.Smith et al. (2007c) also distinguished between design requirements on the componentsand the safety function indicators, although the two are often closely related. Smith etal. (2007c) defines the design requirements to be “attributes that the repository is