Overview of SR-Can; Äspö Task Force meeting Jan 17 th 2007 Overview of the safety assessment SR-Can Allan Hedin, SKB

Overview of SR-Can; Äspö Task Force meeting Jan 17 th 2007 The SR-Can team Kastriot Spahiu, fuel Lars Werme, canister Patrik Sellin, buffer and backfill Ignasi Puigdomenech, geochemistry Raymond Munier, geology Jan-Olof Selroos, flow and transport geosphere Ulrik Kautsky, biosphere Lena Morén, FHA issues Jens-Ove Näslund, future climate Fredrik Vahlund, radionuclide transport and dose calculations, input data Karin Pers (Kemakta), Initial state engineered barriers Kristina Skagius (Kemakta), FEP database, methodology, link to SDM Forsmark Johan Andersson (JA Streamflow AB), methodology, coordination with site investigations and design, coordination of mechanics issues, input data Lisa Wedin, project administration Allan Hedin, project leader, editor main report, methodology

Overview of SR-Can; Äspö Task Force meeting Jan 17 th 2007 Context Application for Encapsulation plant filed November 8:th Site investigations at Forsmark and Oskarshamn progressing according to plan –SR-Can is based on data from the initial site investigation stage SR-Can is a preparatory step for the SR-Site assessment. SR-Site will support SKB’s application for a final repository, planned for SR-Can is not part of any application Interim SR-Can report 2004, reviewed 2005 –Appendix C: SKB’s handling of main review findings

Overview of SR-Can; Äspö Task Force meeting Jan 17 th 2007 Purposes of SR-Can 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 to build the encapsulation plant; 2.To provide feedback to design development, to SKB’s R&D 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.

Overview of SR-Can; Äspö Task Force meeting Jan 17 th 2007 The KBS-3 concept Primary safety function: Complete isolation Secondary safety function: Retardation

Overview of SR-Can; Äspö Task Force meeting Jan 17 th 2007 Relation to other projects

Overview of SR-Can; Äspö Task Force meeting Jan 17 th 2007 Applicable regulations The Swedish Nuclear Power Inspectorate’s regulations concerning safety in final disposal of nuclear waste, SKIFS 2002:1 –… and General guidance to SKIFS 2002:1 Regulations for final disposal of spent nuclear fuel, SSI FS 1998:1 –… and General advice to SSI FS 1998:1 Regulations reproduced in Appendix A in the main report, with references to “implementation” in main text. Risk criterion: –The annual risk of harmful effects after closure must not exceed 10 −6 for a representative individual in the group exposed to the greatest risk. –“Harmful effects” refer to cancer and hereditary effects. –Limit corresponds to effective dose limit ≈1.4∙10 −5 Sv/yr ≈ 1% of natural background radiation in Sweden. –Applicable 100,000 years after closure Time scale for the assessment: One million years

Overview of SR-Can; Äspö Task Force meeting Jan 17 th 2007 Report hierarchy All ; Main report TR Site descriptive model Forsmark v layout D1 Site descriptive model Laxemar v layout D1

Overview of SR-Can; Äspö Task Force meeting Jan 17 th 2007 Methodology in ten steps

Overview of SR-Can; Äspö Task Force meeting Jan 17 th 2007 Safety functions A safety function is a role through which a repository component contributes to safety –Example: The canister should withstand isostatic load A safety function indicator is a measurable or calculable property of a repository component that indicates the extent to which a safety function is fulfilled –Example: Isostatic stress in canister A safety function indicator criterion is a quantitative limit such that if the safety function indicator to which it relates fulfils the criterion, the corresponding safety function is maintained –Example: Isostatic stress < isostatic collapse load

Overview of SR-Can; Äspö Task Force meeting Jan 17 th 2007 Safety functions, canister and buffer

Overview of SR-Can; Äspö Task Force meeting Jan 17 th 2007 Safety functions, deposition tunnel backfill and geosphere

Overview of SR-Can; Äspö Task Force meeting Jan 17 th 2007 Reference evolution Definition –Reference initial state conditions (IS report + SDMs + layouts D1) –Processes handled according to Process report “recipes”; data from Data report –External: Assumed repetitions of the last 120,000 year glacial cycle –Variant of external: Increased greenhouse effect Repository evolution analysed in four time frames; focusing on isolation –Excavation/operation period –Initial temperate period –Initial glacial cycle –Remaining seven glacial cycles Modelling of T, H, M, C, R, B processes in each time frame Evaluation of all safety functions after each time frame –Detailed evaluation after initial temperate period and initial glacial cycle Analysis of greenhouse variant essentially as comparison to base variant Quantification of radiological consequences as subsequent task

Overview of SR-Can; Äspö Task Force meeting Jan 17 th 2007 Definition of the main scenario Essentially the reference evolution… …with more precise definitions of handling of uncertainties identified in reference evolution

Overview of SR-Can; Äspö Task Force meeting Jan 17 th 2007 Selection of additional scenarios based on safety functions Consider every safety function: How can it be lost? Example: The canister should withstand isostatic load Define scenario “Canister failure due to isostatic load” Look for all possible ways this could occur –Evaluate uncertainties not considered in the reference evolution/main scenario –Higher than reference buffer density leading to high buffer swelling pressures on canister? –Severe design flaws in canister insert, weakening the structure? –More massive ice sheets in future glaciations than assumed in reference evolution? –I.e. evaluate uncertainties related to all FEPs of relevance for this safety function

Overview of SR-Can; Äspö Task Force meeting Jan 17 th 2007 Scenarios, cont. Bottom line: Could it happen? If yes: –estimate probability or assume pessimistically p=1 –calculate consequences and include in risk summation If no: –consider as residual scenario not to be included in risk summation –In some cases: calculate consequences for illustrational purposes Not all safety functions used to generate a scenario –Lumping necessary since functions are connected and not fully independent

Overview of SR-Can; Äspö Task Force meeting Jan 17 th 2007 Selected scenarios Main scenario –Base variant –Greenhouse variant Buffer advection Buffer freezing Buffer transformation Results of buffer scenarios propagated to analyses of Canister failure due to corrosion Canister failure due to isostatic load Canister failure due to shear load In addition: Scenarios related to future human actions

Overview of SR-Can; Äspö Task Force meeting Jan 17 th 2007 Conclusions; Compliance with regulatory risk criterion No canisters are assessed to fail during the initial temperate period, expected to last several thousand years A repository at Forsmark is assessed to comply with the regulatory risk criterion A repository at Laxemar is preliminarily assessed to comply with the regulatory risk criterion – but more representative data is required

Overview of SR-Can; Äspö Task Force meeting Jan 17 th 2007 Conclusions; Issues related to glacial conditions Freezing of an intact buffer is assessed as ruled out – even for very pessimistically chosen climate conditions Canister failure due to isostatic load is assessed as ruled out – even for very pessimistically chosen climate conditions Oxygen penetration is preliminarily assessed as ruled out – even for very pessimistically chosen conditions The risk contribution from earthquakes is assessed to be small Loss of buffer may occur from exposure to glacial melt waters but the extent is uncertain – further studies are required Substantial loss of buffer may lead to canister failures in very long time perspectives An prolonged period of warm climate (increased greenhouse effect) before the next glacial period is assessed to be primarily beneficial for repository safety

Overview of SR-Can; Äspö Task Force meeting Jan 17 th 2007 Conclusions; Other issues related to barrier performance and design Crucial to avoid deposition positions intersected by large or highly water conductive fractures – further studies are required The heat from the canister may fracture the rock in the deposition hole wall, which may enhance the in- and outward transport of dissolved substances – further studies are required The importance of the backfilled deposition tunnels as a transport path for radionuclides is limited The importance of the excavation damaged zone in the rock around the deposition tunnels as a transport path for radionuclides is limited

Overview of SR-Can; Äspö Task Force meeting Jan 17 th 2007 Risk summation Several pessimistic assumptions; e.g. –Hydraulic interpretation of the Forsmark site –Occurrence of buffer erosion –Geochemical conditions (sulphide concentrations) Data for Laxemar are not representative –Recently obtained data of intended repository volume indicate more favourable conditions Risk criterion, applicable for ≈100,000 years, fulfilled Higher calculated risk for one million year period

Overview of SR-Can; Äspö Task Force meeting Jan 17 th 2007 Risk summation