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

32geosphere (Miller & Marcos 2007). This is supported by observations from theONKALO control holes for grouting, which show that the pH decreases rather quickly(Posiva 2009a, Pitkänen et al. 2008). The effect of high pH water on the buffer can beexpected to be limited considering the amount bentonite in the deposition holecompared to the potential amount of high pH water. It is however recommended thatsome distance be kept between the deposition holes and the tunnel sections containingeven lower-pH cementitious material, for example in the form of grouts.A summary of the performance targets and target values (safety function indicators andassociated criteria according to SKB) for the buffer according to SKB (2006) and KBS-3H work (Smith et al. 2007c) is given in Table 4-2. Future development of therequirements will likely address whether, for example, density and swelling pressurerequirements apply to the average properties of the buffer (bulk of buffer), allowingsome local deviations, such as at the buffer-rock interface. Given the buffer material, therequirements related to hydraulic conductivity, swelling pressure, density given in Table4-2 can be also given as a (saturated) density range. The performance of the buffer isbased on the following assumptions of the host rock and these assumptions need to beconsidered in setting up the performance targets for the host rock:chemical environment that is favourable to buffer functionalitylow groundwater inflows and low groundwater flow rates in long-term so thaterosion will not take place or change the chemical environmentlimited shear displacementssufficient repository depth in order to avoid freezing of the buffer orminimise any adverse impact of freezing on the bufferto4.3.3 BackfillThe function of the backfill together with plugs is to support the safety functions of thecanister, buffer and rock. The backfill prevents formation of transport paths, contributesto mechanical stability of the tunnels, and prevents inadvertent human intrusion to therepository. Furthermore, the backfill should provide enough counter-pressure againstthe buffer swelling pressure so as to prevent the buffer from swelling out of thedeposition hole.Currently, the refence backfill option is block backfill with Friedland clay blocks(Tanskanen 2007). In this concept, most of the tunnel is filled with pre-compactedblocks of clayish material and the remaining space between the blocks and rock withbentonite pellets (Gunnarsson et al. 2007, Keto & Rönnqvist 2006). Alternative conceptof in-situ backfilling is being studied as well as alternative backfill materials includingbentonite and a mixture of bentonite and crushed rock are still under study (Gunnarssonet al. 2007, Keto 2006, Keto et al. 2009).