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

34infiltrating water. Upconing of the deep saline water due to disturbances caused by theconstruction or during glacial periods is possible. According to a description of siteevolution (Pastina & Hellä 2006), maximum salinities at the repository level areexpected to remain below 20–25 g/L, with occasional higher values possible. Also,higher salinities may be permitted depending on material selection, as some backfillmaterials studied by Johannesson & Nilsson (2006) could provide enough swellingpressure even in higher salinities (7%).In addition to performance of the backfill material, emplacement of the backfill maycreate further requirements on the rock. These requirements may consider the salinityand inflow to a certain tunnel section as well to the point inflow. Performance tests onthe backfill concept have been carried out using maximum salinity of 35 g/L. Thedevelopment of backfilling materials and concept needs to be considered in the futurerevisions of performance targets and criteria.As the backfill will contain swelling material (either bentonite or Friedland clay,Tanskanen 2007, see also Keto et al. 2009), the backfill material will pose similar hostrock requirements as the buffer:chemical environment that is favourable to backfill functionalitylow groundwater flow so that erosion will not take place or change the chemicalenvironmentsufficient repository depth in order to avoid freezing of the backfill or tominimise any adverse impact of the freezing of the backfill4.3.4 SummaryEBS functionality depends on the geochemical, hydrogeological and mechanicalconditions at the repository depth.With respect to the chemical environment, reducing conditions with no dissolvedoxygen (O 2 ) are required to avoid canister corrosion. Other corroding agents includesulphide (HS - ) and chloride (Cl - ) as well as nitrates and ammonia. Sulphide, present ingroundwater, the buffer and the backfill, is produced by sulphate-reducing bacteria inthe presence of organic matter, and by anaerobic methane oxidation involving bothmethane-oxidising bacteria and sulphate-reducing bacteria (sees e.g. Chapters 6 and 11in Pastina & Hellä 2006). To avoid chloride corrosion, the pH of groundwater must behigh enough, i.e. pH > 4 (SKB 2006).Chemical erosion of buffer can take place in the presence of groundwater with very lowionic strength. High salinity can impair buffer and backfill properties, especiallyswelling pressure and hydraulic conductivity. The presence of potassium can lead toillitisation of the buffer and, consequently, to a decrease in swelling pressure andincrease in hydraulic conductivity (Pastina & Hellä 2006, SKB 2006). Iron is alsoconsidered to be detrimental to the long-term behaviour of the buffer as it may lead tochloritisation and other mineral alterations and to a reduced swelling pressure.Erosion of the buffer can take place when there is high inflow in the deposition hole ortunnel section. Currently, maximum allowable inflow to the deposition hole is estimatedto be in the order of 0.1 L/min (SKB 2006, Smith et al. 2007c, see also Section 5.2).