Assessment of Disturbances Caused by Construction and Operation of ONKALO

Construction of ONKALO, the underground rock characterisation facility at Olkiluoto, is planned to be started in 2004. The facility consists of a system of exploratory tunnels accessed by a tunnel and a shaft. The main characterisation level will be located at the depth of 420 metres and the lower level at the depth of 520 metres. ONKALO is designed in such a manner that it can later function as access routes and auxiliary rooms of the repository for spent fuel. The ONKALO excavations may be in use during the whole operation period of the repository. If no sealing of rock were made, the inflow of groundwater into ONKALO could be as high as 600 000 m3/yr and drawdown of groundwater table and upconing of deep saline groundwater several hundred metres. The salinity of groundwater in the vicinity of the tunnels might locally rise up to 50 g/l. Intrusion of superficial water containing oxygen and carbon dioxide would consume the redox and pH buffering capacity of fracture fillings. A major part of the inflow comes from the subhorizontal fracture zones, which the access tunnel and shaft penetrate at the depth between 70 and 330 metres. The disturbances can be significantly reduced by applying methods commonly used in rock engineering, grouting of rock with cement and hydrostatic liners, to limit leakages. A plume of high pH originating from cement may affect transport processes in the geosphere and the performance of the EBS. The main concerns are related to the alteration of bentonite, stability of the fuel matrix, and validity of the performance assessment databases in a high pH environment. Development, transport and dilution of the plume and its intrusion into the EBS are limited by the same processes that govern release and transport of radionuclides. On the basis of mass transport analyses, it is estimated as unlikely that cement transported from ONKALO could significantly impair the key functions of the bentonite buffer and other components of the EBS in any of the deposition holes in the repository. Uncertainties in the assessment could be reduced by investigating pH-related disturbances around past groutings. To avoid potentially harmful effects, the amount of ordinary cement remaining in the host rock, especially in the deep parts of ONKALO below the subhorizontal fracture zone R20, should be kept as low as possible. This may be achieved by removing concrete structures before the sealing of the repository, by using grouts only at locations where they best reduce leakages, and by developing and using low-alkali cements or non-cementitious grouts. Effects of new materials introduced into the host rock should, of course, be first studied. A balance needs to be found between the disturbances caused by leakages and those caused by the methods used to limit them. In the early phase of the construction in the upper part of the bedrock, focus may be put on limiting the inflows as low as possible and learning effective sealing methods of rock in the actual in situ conditions. Based on the experiences and on the ongoing development of alternative grouts, the practices should be reviewed. Appropriate review points are after the penetrations of the subhorizontal fracture zones at the depths between 70 and 130 metres, and between 290 and 330 metres. It is crucial that the planned forthcoming use of ONKALO as access routes and auxiliary rooms of the repository will be taken into consideration in construction and operation. Some potentially harmful effects due to engineering and stray materials may be avoided by careful and clean construction, operation and backfilling practices.