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

38in Research, Development and Technical Design (RTD) activities. For example, thestudies on bentonite buffer are likely to provide new information on buffer behaviourunder different flow conditions and chemical environments. Furthermore, developmentof the backfill concept and selection of the backfill material may cause changes toperformance targets for geosphere.An area that requires further evaluation is that of mechanical disturbances in tunnels anddeposition holes. Prediction of the probability of rock damage along the tunnel ispresented in the latest Site Descriptive Model report (Posiva 2009a), and estimates onthe spalling are reported by Hakala et al. (2008). In the current state, resolution of thesepredictions is not yet such that numerical tunnel or deposition hole scale criteria can beproposed. It might be possible to suppress the initiation of thermally induced spalling byengineering solutions and even if such spalling was to occur it may still be acceptablefrom a safety point of view. Furthermore, the uncertainties in spalling strength, reflectedby uncertainty in the estimated extent of spalling (both depth and area of the damages aswell as potential number of deposition holes affected), call for further study. Theongoing construction of ONKALO will provide data at relevant depths. Predictions onmechanical damage are also being developed. Currently, further development of thecriteria for buffer/rock and backfill/rock interfaces is taking place, however theserequirements are likely to concern mainly the buffer and are not expected to affect therequirements on the host rock to great extent. Properties at the buffer rock interfacewere a key issue in the KBS-3H safety assessment (Smith et al. 2007a, c).The performance targets defined here form a basis in developing rock suitability criteriafor the host rock (see Chapter 5). The rock suitability criteria translate these generalrequirements to observable and measurable (either directly or indirectly) properties ofthe host rock that can be verified by means of interpretation and modelling. Consideringthe set of performance targets listed in the Table 4-3, the following are taken asguidelines for criteria development:performance targets affecting the criteria for deposition holes are essentiallyones related to inflow, i.e. low flow rate around the deposition holes which isalso linked to transport resistance, and to limited rock shear in the depositionholes,criteria related to groundwater chemistry, retention and thermal propertiesshould be considered for rock volumes hosting the deposition tunnels, andcriteria related to limited mechanical disturbances need to be developed whenmore information is gained from ONKALO at relevant depths.