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

304.3.2 BufferThe most important safety function of the buffer is to protect the canisters fromprocesses that could compromise the safety function of complete containment of spentfuel. The buffer will also retard the transport of radionuclides in case of canister failure.To protect the canister, the buffer should besufficiently plastic (or ductile) to protect canisters from small rock movements,including shear displacements at canister locations,sufficiently stiff to maintain canisters in a vertical position,sufficiently dense so that microbes are not active in the buffer and do not giverise to unfavourable chemical conditions at canister surface, andimpermeable enough that the movement of water is insignificant and diffusion isthe dominant transport mechanism for corrosive agents present in thegroundwater that may degrade the canisters.The buffer should have a swelling pressure and self-sealing capability, which mean thatany potential advective pathways for flow and transport that may arise, for example, asa result of piping and erosion, sudden rock movements or the release of gas formed in adamaged canister are rapidly closed. Its impermeability should limit and retard therelease of any dissolved radionuclides from the canisters, should any be damaged.Furthermore, it should have a fine pore structure such that microbes and colloids areimmobile (filtered) and microbe- or colloid-facilitated radionuclide transport will notoccur.Loss or redistribution of buffer mass or mineral alteration can lead to a reduction inswelling pressure, increase in hydraulic conductivity of the buffer, and to possiblechanges in buffer density (described in more detail by Smith et al. 2007c and Miller &Marcos 2007). These will in turn have a detrimental effect on the functionality of thebuffer. Buffer freezing would only occur at low temperatures (less than –5 C, see e.g. p.477 in SKB 2006) and the consequences may not be dramatic even if the buffer freezes.Furthermore, according to existing knowledge, freezing is not expected at repositorydepth (Hartikainen 2006), however, further studies on this issue are in process.Loss or redistribution of buffer mass can result, for example, from piping and erosion byflowing water. There are a number of factors affecting the loss of bentonite, for exampleinflow, erosion rate and duration of the erosion process and the properties of thebentonite like the self sealing. According to Börgesson & Hernelind (2006), the amountof eroded bentonite in one deposition hole can be in the order of 100 kg withoutdetrimental effects on the barrier safety function. This amount of erosion will result inreduced density at the buffer/rock interface but swelling pressure will remain above1 MPa, which is required for tightness and self-sealing. Based on results presented byBörgesson & Hernelind (2006), also summarised in SR-Can (SKB 2006), the buffer cantolerate inflow to the deposition hole in the order of 0.1 L/min eroding 10 g dry weightof bentonite per litre water during and after installation. The amount of eroded bentonitedepends on the duration of erosion, assumed by Börgesson & Hernelind (2006) to lastfor 12 weeks until the tunnel is plugged and sealed. For reference, at Olkiluoto theoperation of the tunnel will last for some 7 to 10 months (Section 4.1, Saanio et al.2006).