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19.06.2007 12:00

Ventilation shaft in ONKALO reaches down to a depth of 178 metres

The construction project of ONKALO, the underground facility for research related to final disposal of spent nuclear fuel, has reached another milestone, with the ventilation shaft completed at a depth of 178 metres. The shaft was excavated as a rise-boring operation, using a reamer bit with a 3.5 metre diameter. The Swedish Bergteamet Company was in charge of the rise-boring operation.

The ventilation shaft was bored in two stages, and in the first operation a year ago reached down to a depth of 90 metres. The second stage was started at the -90 m level by first drilling an "eyehole" ca. 30 cm in diameter to a depth of 178 metres. The 3.5 m reamer bit was then attached to the drill at the lower tunnel level and lifted up through the eyehole in a rotating movement.

The rise-boring operation continued 24 hours day, for about one week. Boring proceeded at a rate of half a metre to one metre in one hour. The rock round the shaft was pregrouted prior to the boring operation to minimise water leaks.

At the final disposal depth (-420 m), ventilation in ONKALO will be implemented by supplying air into the tunnel through the shaft. In this solution air is exhausted through the tunnel entrance.

The ONKALO project will later involve the excavation of another ventilation shaft as well as a personnel shaft. According to plans, the rise-boring of the personnel shaft will start this autumn. Once completed, the personnel shaft that has a diameter of 4.5 metres will also reach down to a depth of 178 metres.

At present, the total excavated length of the ONKALO tunnel is 2165 metres, and it now runs to a depth of ca. 200 metres. The excavation project proceeds as three-shift work, with the exception of weekends.

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