ONKALO prepares for test boring of final disposal holes
The excavation of the underground characterisation facility ONKALO, commenced by Posiva in 2004, will soon be finished. The excavation will continue at a depth of 437 metres on the technical level where ONKALO's ventilation and personnel shafts will be completed during 2012. A second demonstration tunnel is being excavated above the technical level at a depth of 420 metres. The tunnel will be completed at the beginning of 2012. Full-scale test boring of final disposal holes will be launched in the demonstration tunnels in the next few days. For the boring, Posiva has created a project called KapRe, which involves acquisition of the required boring equipment, for example. The equipment has been designed and manufactured by TRB Raise Borers Oy, which is based in Tampere, Finland, and supplied by Sandvik Mining and Construction Finland Oy.
"When we started to design the equipment, the goal was to build a machine that would be optimally small but efficient enough to bore large holes. The demonstration tunnels are almost the same size as the final disposal tunnels and are essentially lower and narrower than the ONKALO access tunnel, which has had to be taken into consideration in the machine dimensions. On the other hand, the machine must have enough power and torsion because the canister hole diameter is almost 1.8 m and the depth is about eight metres," says Sanna Mustonen, Design Coordinator at Posiva.
During the excavation of the demonstration tunnels, Posiva has tested the rock suitability classification that acts as the basis for placing the final disposal tunnels and the canister holes on rock that provides optimal long-term safety. "The rock suitability criteria guide the facility design so that the final disposal tunnels and holes are implemented at a sufficient distance from deformation zones and water-conducting structures," says Mustonen.
A total of eight canister holes will be bored in the demonstration tunnels to test the suitability classification of the rock. From now on, the bentonite barrier and canister installation in disposal holes will be tested in the tunnels, and the tunnels will be filled with hard-compressed clay blocks and pellets.
"The full-scale testing in ONKALO is an important part of the permit procedure for the final disposal plant for spent nuclear fuel. Through testing, we will show that the installation of the technical release barriers – the canister, bentonite buffer and tunnel – has been performed in compliance with the requirements," says Reijo Sundell, President of Posiva.
The final disposal facility for spent nuclear fuel comprises an above ground encapsulation plant and its auxiliary facilities, as well as underground final disposal facilities. The goal of Posiva is to submit a construction license application for the final disposal facility to the Finnish Government by the end of 2012. Posiva intends to commence final disposal according to the Government decision in about 2020.