Design premises for a KBS-3V repository based on results from the safety assessment SR-Can and some subsequent analyses

The objective with this report is to: - provide design premises from a long term safety aspect of a KBS-3V repository for spent nuclear fuel, to form the basis for the development of the reference design of the repository. The design premises are used as input to the documents, called production reports, that present the reference design to be analysed in the long term safety assessment SR-Site. It is the aim that the production reports should verify that the chosen design complies with the design premises given in this report, whereas this report takes the burden of justifying why these design premises are relevant. The more specific aims and objectives with the production reports are provided in these reports. The following approach is used: - The reference design analysed in SR-Can is a starting point for setting safety related design premises for the next design step. - A few design basis cases, in accordance with the definition used in the regulation SSMFS 2008:211 and mainly related to the canister, can be derived from the results of the SR-Can assessment. From these it is possible to formulate some specific design premises for the canister. - The design basis cases involve several assumptions on the state of other barriers. These implied conditions are thus set as design premises for these barriers. - Even if there are few load cases on individual barriers that can be directly derived from the analyses, SR-Can provides substantial feedback on most aspects of the analysed reference design. This feedback is also formulated as design premises. - An important part of SR-Can Main report is the formulation and assessment of safety function indicator criteria. These criteria are a basis for formulating design premises, but they are not the same as the design premises discussed in the present report. Whereas the former should be upheld throughout the assessment period, the latter refer to the initial state and must be defined such that they give a margin for deterioration over the assessment period. The basic approach for prescribing such margins is to consider whether the design assessed in SR-Can Main report was sufficient to result in safety. In case this design would imply too strict requirements, and in cases the SR-Can design was judged inadequate or not sufficiently analysed in the SR-Can report, some additional analyses have been undertaken to provide a better basis for setting the design premises. The resulting design premises constitute design constraints, which, if all fulfilled, form a good basis for demonstrating repository safety, according to the analyses in SR-Can and subsequent analyses. Some of the design premises may be modified in future stages of SKB's programme, as a result of analyses based on more detailed site data and a more developed understanding of processes of importance for long-term safety. Furthermore, a different balance between design requirements may result in the same level of safety. This report presents one technically reasonable balance, whereas future development and evaluations may result in other balances being deemed as more optimal. It should also be noted that in developing the reference design, the production reports should give credible evidence that the final product after construction and quality control fulfils the specifications of the reference design. To cover uncertainties in production and quality control that may be difficult to quantify in detail at the present design stage, the developer of the reference design need usually consider a margin to the conditions that would verify the design premises, but whether there is a need for such margins lies outside the scope of the current document. The term 'withstand' is used in this document in descriptions of load cases on repository components. The statement that a component withstands a particular load means that it upholds its related safety function when exposed to the load in question. For example, if the canister is said to withstand a particular corrosion load, this means that it will uphold its containing function when exposed to this load. The time period during which the safety functions are assessed is one million years after repository closure in accordance with applicable regulations.