Long-term safety for KBS-3 repositories at Forsmark and Laxemar - a first evaluation. Main Report of the SR-Can project

This document is the main report from the safety assessment project SR-Can. The SR-Can project is a preparatory stage for the SR-Site assessment, the report that will be used in support of SKB's application for a final repository. The purposes of the safety assessment SR-Can are the following: 1. To make a first assessment of the safety of potential KBS-3 repositories at Forsmark and Laxemar to dispose of canisters as specified in the application for the encapsulation plant. 2. To provide feedback to design development, to SKB's RandD programme, to further site investigations and to future safety assessment projects. 3. To foster a dialogue with the authorities that oversee SKB's activities, i.e. the Swedish Nuclear Power Inspectorate, SKI, and the Swedish Radiation Protection Authority, SSI, regarding interpretation of applicable regulations, as a preparation for the SR-Site project. The assessment relates to the KBS-3 disposal concept in which copper canisters with a cast iron insert containing spent nuclear fuel are surrounded by bentonite clay and deposited at approximately 500 m depth in saturated, granitic rock. Preliminary data from the Forsmark and Laxemar sites, presently being investigated by SKB as candidates for a KBS-3 repository are used in the assessment. An important aim of this report is to demonstrate the proper handling of requirements placed on the safety assessment in applicable regulations. Therefore, regulations issued by the Swedish Nuclear Power Inspectorate and the Swedish Radiation Protection Institute are reproduced in an Appendix where references are given to sections in the main text where the handling of the different requirements is discussed. The principal acceptance criterion requires that 'the annual risk of harmful effects after closure does not exceed 10-6 for a representative individual in the group exposed to the greatest risk'. 'Harmful effects' refer to cancer and hereditary effects. The risk limit corresponds to an effective dose limit of about 1.4x10-5 Sv/yr. This, in turn, corresponds to around one percent of the effective dose due to natural background radiation in Sweden. The time frame for the assessment is one million years after repository closure, in accordance with regulatory requirements. The above risk limit is applicable as a quantitative regulatory limit during approximately the first one hundred thousand years, and thereafter as a basis for discussing the protective capability of the repository, according to SSI. Following the introductory Chapter 1, this report outlines the methodology for the SR-Can assessment in Chapter 2, and presents in Chapter 3 the handling of features, events and processes, FEPs, of importance for long-term safety. Chapters 4, and 6 present the initial state of the system for both of the sites, and the plans and methods for handling external influences and internal processes, respectively. Safety functions and safety function indicators are discussed in Chapter 7. The collection of input data for the assessment is described in Chapter 8. The Material presented in the first eight chapters is utilised in the analysis of the reference evolution in Chapter 9, focussing on isolation, and Chapter 10, addressing radionuclide transport and dose assessments. Scenarios for the further evaluation of safety are selected in Chapter 11 and the selected scenarios are analysed in Chapter 12. Finally, conclusions and feedback are provided in Chapter 13. References are provided in Chapter 14. Appendix A is an account of how applicable regulations are addressed in the assessment. Miscellaneous calculation models are presented in Appendix B and Material relating to reviews of previous assessments in Appendix C. A glossary of abbreviations and specialised terms used in SR-Can is found in Appendix D