Raise drilling of ventilation shaft in ONKALO completed
The ventilation shaft of ONKALO, the underground facility for research on final disposal, has been completed down to a depth of ca. 90 metres. The shaft was realised using the raise drilling technique, in which a raise borer at ground level pulls a 3.5-metre diameter reamer bit upward from the bottom of the pilot hole. The raise drilling operation was implemented by the Swedish company Bergteamet.A pilot hole with a diameter of ca. 30 cm was fist bored down to a depth of 90 metres. After that, drill strings were lowered in the pilot hole, and a reamer bit was attached to the strings.
The raise drilling was completed in one week as a 24-hour operation. The drilling rate was 50 cm to 100 cm an hour. Before the operation was started, the rock surrounding the shaft was pre-injected to minimise water leaks.
Ventilation down to the final disposal depth (-420 m) will be realised by supplying air to the tunnel through the shaft. A shaft chamber for fans will be built on the ground level during the summer, and down at the shaft base acceleration fans are installed in the drift. In this solution, air is exhausted through the tunnel mouth. After ONKALO has been completed at the beginning of the 2010s, the ventilation of the tunnel system will be based on the existing ventilation shaft and a second ventilation/lift shaft that will be excavated later.
The total length of ONKALO excavated so far is 1089 metres. Excavation proceeds as a three-shift operation, excluding weekends.