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

28constrained by rock properties at those depths, for example higher stresses andunfavourable chemical environment (e.g. salinity), which affect both safety andconstruction procedures (for discussion see e.g. Äikäs et al. 2000). The current plannedrepository depth is between 400–500 meters, the lowest point of the repository layoutbeing at -437 m (Anttila et al. 2009).4.3 Performance targets related to functionality of the engineeredbarriersSafety of the repository rests first and foremost on the long-term isolation andcontainment of radionuclides in the copper canisters, provided by the engineered barriersystem (EBS), and on favourable site properties. As interactions between the systemcomponents are also important for safety, requirements related to the performance of theEBS are briefly summarised in the following sections. The discussion is based oncurrent knowledge and it is expected that further studies, for example of bufferperformance, will bring new information that may lead to a revision of the presentedtargets. For a detailed account of EBS performance targets, see e.g. Posiva 2006, SKB2006, Smith et al. 2007c.4.3.1 CanisterContainment provided by the canister plays a key role in system safety. A basicrequirement is that the spent fuel inside the canister remains sub-critical. Theperformance target or design lifetime of the canister is to retain integrity over a periodof at least 100 000 years. Therefore canisters shouldhave low probability of initial penetrating defects,be resistant to corrosion, andhave high mechanical strength.The canister’s susceptibility to corrosion is dependent on the chemical environment inthe near field of the canister. The mechanical load on a canister results from hydrostaticpressure that depends on repository depth, ice sheet thickness during future glaciations,and swelling pressure of the bentonite. Shear stress on a canister can result from unevenswelling of the buffer and/or shear displacement caused by earthquakes (most likelyinduced by glaciation).The ability of the canister to withstand shear loads is strongly dependent on the bufferdensity. According to results by Börgesson et al. (2004) based on laboratory tests andmodelling (ABACUS software), where canister and buffer dimensions according tocurrent design were applied, the impact of shear displacements up to 10 cm on theplastic strain of the canister is minimal (1,6 %) if the buffer densities are in range of2000 kg/m 3 . With higher buffer densities or larger displacements, strain on the canisteris increased; at buffer density 2100 kg/m 3 and shear displacement 20 cm, the cast ironinsert is already strongly affected. Also velocity of the shear movement and location ofthe shear plane both affect the severity of consequences on the canister. Shear rates ofup to 1 m/s have been considered in the modelling. The rock shear criterion may have tobe reconsidered in case of new essential results from the ongoing studies on the canisterand buffer behaviour under shear load become available.