Final disposal

Aboveground Site Investigations

Site investigations related to final disposal of spent nuclear fuel have been carried out in Olkiluoto ever since the 1980s. These investigations have been principally conducted at the surface, but after the construction of the ONKALO research facility, new studies have also been started underground.
The original purpose of the site investigations was to produce information to underpin the selection of the final disposal site.  After Olkiluoto was chosen as the final disposal site, the site investigations have been continued, now with the aim of ensuring Olkiluoto’s suitability for final disposal.

Furthermore, information provided by the site investigations supports
  • the planning and designing of the repository
  • the assessment of the long-term safety of the final disposal
  • assists in the construction of ONKALO.
One of the major site investigation methods is to drill deep drillholes and to use them in various groundwater investigations. Other important methods are, among others, excavating investigation trenches and conducting variablegeophysical examinations. The surface environment, including the flora and fauna, is also actively studied.


The bedrock in Olkiluoto is investigated by drilling holes of up to one kilometre deep, from ground surface.  By 2014, a total of about 60 holes have been drilled.

This drilling has continued since the late 1980s. A few holes are still drilled every year.  During recent years, the drilling has concentrated on the previously scarcely-investigated eastern and north-eastern part of the island.

The drilling and investigations carried out in the drillholes will also continue in future years in order to allow the flexible planning of repository by utilising the entire area available.

Drilling produces a drill core sample of some five centimetres in diameter. It is carefully studied to establish the degree of fracturing and types of rock present in the bedrock. The in hole measurements and samples taken from the drillhole allow studying of the flow and qualities of groundwater. Geophysical measurements and imaging also take place in the holes.

The drilling investigations can be utilised for establishing the location and properties of particular bedrock structures. Geophysical methods also allow verifying the properties of modelled structures. The results of the investigations are, among others, utilised for planning the excavation and sealing of the ONKALO facility and for designing the repository.

In addition to holes drilled from surface, drilling also takes place underground, in ONKALO. They serve, among other things, to help predict the future excavation conditions and provide guidelines for planning and design. Long characterisation holes will also be drilled from ONKALO to the location of the repository.

Investigation Trenches

Investigation trenches help produce information of the surface of bedrock in Olkiluoto. A total of some 20 investigation trenches have been excavated.

The investigation trenches are done by removing all loose soil from on top of the rock and by cleaning the rock surface.

The rock surface is studied for the presence of different types of

  • rock
  • fractures
  • cleavage
  • folding
  • displacements.

After examination, the investigation trench is filled in and landscaped. The investigation trenches may be several hundred metres long and almost ten metres deep.

Often, investigation trenches are made in conjunction with drill holes so that the structures found by drilling can be seen on the rock surface. This information can be later utilised for preparing a geological model of the bedrock.

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