Estimated water inflow into the repository

ONKALO is underground bedrock research facility for the repository of spent nuclear fuel. Posiva is the builder and ONKALO is being constructed to Olkiluoto in Eurajoki. Construction of ONKALO began at 2004 and it is planned to be completed in the year 2014. After this starts construction of the repository. This work is based on groundwater research, which is done in the drillholes at the area of ONKALO and repository. The purpose of this work is to predict the amount of groundwater flow into ONKALO's lowest part and deep repository with different suppositions. At the same time this work helps to estimate the need of grouting. In this water inflow estimation ONKALO has been examined from the depth -300 meters downwards, which includes the repository. The most important starting points of the inflow estimation are the hydrogeological model (Ahokas et al. 2007) and the layouts of ONKALO and repository (Chapter 2 and Appendix 1). The hydrogeological model (Chapter 4) describes essential water conductive structures of the Olkiluoto area and their properties. The hydrozones HZ20A, HZ20B, HZ20Balt, HZ20AE, HZ21, HZ21B, BFZ099, HZ001 and HZ004 intersects the layouts of ONKALO and repository based on the model. These structures are significant for the inflow estimation. Vaittinen et al. (2007) have estimated the expected occurrence frequency of the water conductive fractures at the depth range of -300 - -600 m. This statistic has been used in advantage in this inflow estimation. Groundwater inflow to the tunnels caused by hydrogeological zones and separate fractures outside of the zones is studied in this inflow estimation with analytical methods. Connecting these, the estimation can be presented to the different layouts and suppositions of the properties of the zones and fractures, and for example different amounts of open spaces in the case of repository. For every zone, in the case of water conductive zones, was calculated minimum, maximum and mean flow using minimum and maximum values and geometric mean of the trasmissivity. In the case of intact rock the flows of fractures were calculated with the mean and median values of the transmissivity. Many different cases were examined in this water inflow estimation. The amount of water inflow was defined to the case, where all tunnels were open at the same time. Correspondingly the amounts of water inflows were defined for the cases, where only part of the central tunnels (varied between 30-70%) and repository tunnels (varied between 5-25%) were open, while ONKALO was completely open. Water inflows from the hydraulically conductive fracture zones are different depending on how many times the zones intersect the facility and which parts are open. Besides, all cases were studied with the supposition that grouting reduces the water inflow. Because ONKALO is partly excavated, the water inflow estimations were connected with the already measured water inflows to obtain the total water inflow estimation for the whole ONKALO and repository. (orig.)