Conceptos fundamentales del programa

soil and bedrock at the site location, and the interaction between them. To assess the impacts on groundwater, the location of the power plant unit with respect to groundwater areas and the possible risks imposed on groundwaters due to construction and operation have been examined. The available modelling data has been utilised in the assessment.

9.8.1 Geology and seismology in the Olkiluoto area

Soil, bedrock and groundwater

Extensive research of the bedrock such as quarrying, drilling and sounding has been and will be carried out at Olkiluoto particularly for the purpose of spent nuclear fuel disposal. The research investigates the properties of rock and the routes of groundwater flow and provides confirmation for the rock models in the Olkiluoto research area. The main rock type in Olkiluoto bedrock is migmatite, which is a compound of gneiss and granite. The bedrock in the area is approximately 1,800 to 1,900 million years old. The soil on Olkiluoto is mainly rocky moraine. There are also thin layers of clay and peat at low-lying spots. The power plant site also includes filled areas.

The Olkiluoto island is quite flat, with no major differences in altitude. The earth surface on the Olkiluoto island is approximately 5 metres above sea level. The highest point of the island (Liiklankallio) is approximately 18 metres above sea level.

The soil layers in the sea bed are moraine, clay and sand.

The level of groundwater loosely follows the topography of the earth surface; in areas covered by moraine, the groundwater is at a depth of 1 to 2 metres, and at the shoreline, the groundwater level joins the sea water level. There are no classified groundwater areas in Olkiluoto, and the area is not significant for the procurement of water for communities. The island has 11 bored wells belonging to private owners, five of which are in continuous or recreational use. The nearest classified groundwater area is located in Kuivalahti, approximately 6 km northeast of the power plant. Modelling

Posiva released a geological site model of Olkiluoto in early 2006. After the geological model was released, integration work to combine geological and hydrogeological data was initiated. As a result of this work, a hydrogeological structure model of Olkiluoto was completed in the autumn of 2006. In addition to said models, the hydrogeochemical and rock mechanical models were also updated in 2006.

The crucial objective of hydrogeological and hydrogeochemical site modelling is to combine hydrogeological material with groundwater chemical material and interpretations to achieve an unambiguous description of groundwater flow and geochemical development, as well as describe the most substantial characteristics of deep bedrock groundwater flow and chemistry in the Olkiluoto area before ONKALO is constructed. (Posiva 2007b.)

Seismology

Finnish bedrock belongs to the Precambrian Fennoscandian shield that is one of the seismically most stable areas in the world. However, there are tensions that may be discharged and cause weak earthquakes. These are often focused on weakness zones existing in the bedrock. 10 to 20 earthquakes occurring in Finland are registered each year. The earthquakes are relatively weak, having a magnitude of 1 to 4 (Richter). The most intense earthquake registered after 1965 occurred at Alajärvi on 17 February 1979. Its magnitude was determined at approximately 3.8. From 1977 to 2001, almost half of all earthquakes observed in Finland occurred in the Kuusamo region. There are known observations of earthquakes in Finland for almost 400 years. Occurrences of earthquakes in Finland from 1965 to 2006 are presented in Figure 9-44 (University of Helsinki 2007).

In Finland, earthquakes are usually caused by tension arising from the widening of the mid-oceanic ridge in the North Atlantic. The Eurasian and North American 1

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of the earthquake are moving in relation to each other. This movement usually occurs along existing faults in the crust. Other local reasons include uplift, which causes earthquakes mainly in the Gulf of Bothnia region.

(University of Helsinki 2007.)

The bedrock of Olkiluoto has been studied in particular detail during recent years. Geological surveys have already proven that the bedrock is stable and that earthquakes affecting plant operation are nonexistent. The risks of a seismic accident at the Olkiluoto nuclear power plant have been assessed in the probabilistic safety analysis. (EQE International Inc. 1997, ref. TVO 1997.)

The stability of the bedrock and the safety of the premises to be excavated will be ensured through structural means and continuous monitoring.

During the excavation work for the foundations of the power plant and the extension to the spent fuel interim storage facility (KPA Store), the extension of the VLJ Repository and the cooling water tunnels, bedrock groundwater will flow into the excavated premises. Groundwater will also percolate into the VLJ Repository during its operation. The quantity of water percolating into premises excavated in the rock will vary and depends on factors such as the size of the room, the tightness of the surrounding rock, the level and occurrence of groundwater, as well as any sealing actions carried out during excavation. This does not have any detrimental effect on the quality or quantity of groundwater at the power plant site or in the vicinity.

Discharges polluting the soil and groundwater at the power plant have been prevented using different types of structural solutions and sewage arrangements. The plant units are designed so that leak water and waste water from the process cannot come into contact with groundwater. Underground external structures are cast from waterproof concrete. Leak water, watering water and cleaning water are treated using separate leak collection and drainage systems. Sewage water from premises within the controlled area is collected using the controlled area floor drain system and treated mainly by evaporation. Floor water, watering and aeration water and sanitary water from other premises are collected using a separate sewage system and treated at a waste water treatment plant.

Leak water, watering water and cleaning water from the spent fuel storage facility are treated using separate leak collection and drainage systems. Contaminated and active filter rinsing water, leak water, watering and aeration water, as well as floor and cleaning water from the controlled area of the storage are pumped into the OL1 liquid waste treatment system. Any seawater leak and sprinkler water is conducted to the sea through the rain water drain system. The foundation water and leak water from the sea water pumping station is pumped directly into the sea. Water collected in the foundations of the storage building and its tunnels is conducted to the sea through the rain water drain system. Water can also be pumped into the controlled area floor drain system in case the foundation water is radioactive. The radioactivity of water is monitored by semi-annual sampling.

Diesel and heating oil tanks are surrounded by earthwork, and protective basins have been constructed. Rain water drainage from the earthwork goes through oil trap wells.

Figure 9-44 Earthquakes in Finland from 1965 to 2006 (University of Helsinki 2007).