12.08.2011 13:36

Construction of the repository is assessed in the demonstration tunnels of ONKALO

In the ONKALO research facility in Olkiluoto, some 420 metres below ground level, excavation work has been going on since April 2011 for the demonstration tunnels, or the “DEMO” tunnels as they are known. In connection with the DEMO tunnels, the method for assessing the suitability of bedrock as well as methods associated with the construction of disposal tunnels and disposal holes are being assessed. Excavation of the first DEMO tunnel was completed in June 2011. Work for the second DEMO tunnel began in July 2011, and the aim is to excavate it to an approximate length of 120 metres by the end of 2011.

Geologist Paula Kosunen, why and how is the suitability of demonstration tunnels assessed?

“From the point of view of future repository facilities, it is important to demonstrate that suitable locations for disposal tunnels and disposal holes can be identified in the bedrock. In other words, we are investigating the suitability of bedrock when constructing the demonstration tunnels; for example, the disposal hole must not be in a brittle fracture zone or in a water-conducting zone; this affects the selection of locations for disposal holes.”

“After completion, the demonstration tunnels and disposal holes bored in them will be used for tests that are important for disposal operations. These are associated, amongst others things, with the installation of disposal canisters and buffer bentonite as well as with the backfilling of disposal tunnels. The facilities will remain in use for research.”

“Investigations indicate that two disposal holes can be located in the recently completed DEMO tunnel no. 1. Initially, pilot holes will be bored in the planned locations of holes for investigating in more detail whether they are suitable locations for disposal holes. The actual boring of disposal holes in DEMO tunnel no. 1 can commence when the suitability of hole locations has been verified. We currently expect that to happen in early October."

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Geologist Paula Kosunen (on the left) with collegues Topias Siren and Mari Lahti in ONKALO

How will the disposal holes be made in the DEMO tunnels?

“The small dimensions of the disposal tunnels — and hence also those of the DEMO tunnel — pose certain challenges: the tunnel is only 3.5 metres wide and less than five metres high. The disposal holes will be 7.8 metres deep and have a diameter or 1.75 metres, and they will be bored vertically downwards from the tunnel floor. That is why a raise boring machine of a new type will arrive in September to bore the disposal holes – the Rhino 500HSP, developed in cooperation with Tunnel Raise Borers, a subsidiary of Sandvik. The Rhino is a prototype, and it will be further developed as required."

Rhino 500HSP

What results are expected from constructing DEMO tunnels?

“During the construction phase, the goal is to demonstrate that it is technically possible to construct facilities that meet the requirements for disposal tunnels and disposal holes. The requirements for the quarried surfaces of tunnels, for example, are strict because the tunnels will have to be backfilled tightly after spent nuclear fuel has been finally disposed of. It is also important to seamlessly integrate the bedrock suitability assessment work and its associated investigations with the facilities’ construction process. And, naturally, to produce suitable facilities for future tests," says Paula Kosunen, a geologist at Posiva.

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The first DEMO tunnel is about 50 metres long, five metres high and 3.5 metres wide.

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