The Onkalo depository project has the kind of leisurely pace that most tunnellers can only dream about as they battle production targets, tight construction schedules and delay penalty clauses. For just over 4.2km of tunnel through mainly good-quality competent rock, there is nearly six years allowed for completion.

And that is for a medium size tunnel only, 4.5m high and 5.5m wide, with occasional broadening out to 8m at right angle corners, where it changes direction to form a downward square-spiral shape. There are some slighter larger sections at the deepest level, 420m down, where a fully connected loop of the tunnel will be formed, along with some chambers (figure 1).

Between June 2004 and the end of 2005 the first 990m of drill and blast was completed and by November 2006 another 570m had been driven to a depth of just over 120m. “We have a progress rate of 20m-40m a week,” says Timo Niemitalo site manager for Posiva, who is building the facility for two of Finland’s electrical power suppliers. The two, TVO and Fortum Power & Heat are also the joint shareholders (see ‘nuclear waste management policy’ box).

Niemitalo says a maximum technical excavation rate of 50m a week could possibly be managed, but allowances for extensive grouting and rock investigation reduces the average to around 25m. He is relatively satisfied with that.

But then this project is like no ordinary tunnel. In fact the entire length, and a possible 1km additional loop of the spiral, to a depth of 520m, can be seen as an enormous ground investigation programme. If, and only if, that should prove successful, demonstrating the rock to be good enough, the tunnel will take on a practical function as access for the main scheme. All being well, in 2015 the construction of a huge comb-like network of tunnels (figure 2) will begin at the deep rock level where nuclear waste canisters will be stored from 2020 onwards (see ‘final repository for spent fuel’ box).

That depository will remain open for 100 years as the network is extended and the nuclear waste rods from Finland’s five nuclear reactors are buried there within the tunnels. It will then be sealed permanently to allow the radioactivity to decay, far from any contact with the environment above, with a time span counted in tens of thousands of years.

The idea of deep disposal of nuclear waste is now fifty years old, with a number of variations in the exact method proposed. Some countries have investigated burying waste in clays or sedimentary rock and the USA’s Yucca Mountain scheme in Nevada uses relatively shallow disposal in igneous rock. Controversy and political and environmental issues have surrounded all of them, as arguments have raged on how safe they are over the huge time spans involved (see ‘securing support and interest’ box).

Scandinavian countries have looked meanwhile at disposal in crystalline rock, most notably Sweden first and then Finland. Both, sitting on the huge Baltic Shield, have plenty of it, ancient hard granites and gneisses dating back as much as 2000 million years or more. Sweden initially developed a rock entombment system specifically for such rock.

But crystalline rock, while making a substantial barrier, has a drawback in that it is subject to cracking and therefore to groundwater penetration and movement. However slow that might be, it would be enough to move radioactive nucleotides over the course of some thousands, or ten of thousands of years perhaps. Sweden’s KBS-3 system is designed to counter that using sealed copper canisters, further sealed within bentonite (figure 3).

All the elements of the method are subject to intensive research at test facilities like Sweden’s Äspö Rock Laboratory and universities in Sweden and also Finland – but Finland has gone a step further in beginning construction of a depository at Olkiluoto on the west Baltic coast by the small town of Eurajoki. Very detailed assessment of the rock can be achieved thereby.

Investigations began with a national country survey 30 years ago and initial selection of five possible sites, later narrowed to a shortlist of two and finally this one. Since 2000 the site has been further probed in detail with surface trenches and borehole core sampling, including seven deep cores in 2005 and a total of over 50 to date.

While this continues, tunnelling allows even closer inspection of the rock, both during construction itself and from further core sampling done from inside the bore up to 1200m either side. The latter cores allow inspection of the rock and collection of groundwater to monitor chemical composition. An aspect of the final disposal plan is that it puts the waste at a deep enough level for the water table to be well away from the atmosphere. That means it is chemically reducing rather than oxidising and therefore less able to corrode the waste canisters. But this needs confirming and other chemical and physical properties need investigating that might affect solubility and transportability of radioactive particles.

Tunnel walls are also subject to detailed inspection and mapping of cracks and rock type, primarily by close visual inspection and Schmidt hammer tests as well as ultrasound and radar. A complete rock mass map is being developed.

The tunnelling itself is also being studied says Niemitalo to discover the best methods to minimise the impact on the rock and further cracking it might cause. Drill and blast is carefully and slowly done. Before the tunnel is drilled a 72mm pilot bore is made ahead for 50m to analyse the rock. A three boom jumbo then drills 5m long blasting rounds with as much care as possible; undercut is preferred to overbreak, unlike most normal tunnelling, with intrusions into the tunnel envelope carefully drilled and blasted out afterwards. After three rounds another pilot hole is made.

Posiva recently bought a Normet liquid explosive charging machine believing it can get better control of the charging that way.

After mucking out, the tunnel walls are cleaned for follow on inspection and the jumbo itself, an Axera T11 machine – one of the latest computerised types – also provides more data from the digital logging equipment of the hammers. The chosen machine also comes with a basket to allow further inspection.

To make best use of drilling data, Posiva recently took over tunnelling directly from the contractor it used for the first 1000m of tunnel, buying its own machine for the work. Kalliorakennus had used an Axera T10.

“The work requires a continuity of recording information over some years,” says Niemitalo “and you cannot achieve that with a contractor who may be on site for a year but then could move elsewhere.” Direct experience is more useful he says rather than mediate the work via a third party.

The chance of contractors moving on has increased recently, he adds, because of a number of major projects due to start in Finland for rail and road. Kalliorakennus requested that it be released from a long-term commitment at the project.

His team also wants to acquire experience in the high pressure grouting being used to seal cracks that are found. Again the client has bought its own equipment including two Swiss Häny grouting machines.

The first stretch of tunnel proved more cracked than expected and required substantial grouting work though this should ease off as the project descends.

Meanwhile there is also some shaft construction getting underway, including a first 101m deep shaft to intercept the first loop of the tunnel. The 3.5m diameter shaft has been raise bored from the tunnel following a pilot drilling from the tunnel entrance area. A second short length of 4.5m diameter shaft, currently just 11m deep has been created.

“That allows us a more direct route for ventilation in particular, saving on running it along the tunnel” explains Niemitalo. “The 3.5m diameter shaft is also the main escape route in the event of fire or accident with an emergency elevator.”

He adds that safety pods are installed along the tunnel with air supply and fire resistance where workers can shelter for several hours if there is a problem.

All this work is helping build up a very detailed model of the rock, which will be further expanded when the final sections of the investigation tunnel are done. The downward spiral will level out at 420m into a loop tunnel that will be the main “characterisation” level with a series of chambers also excavated there. These will provide space for support and laboratory facilities for geotechnical investigators carrying out further core drilling and monitoring of the rock, which will be used eventually for the main depository.

The two shafts will be extended from level to level of the spiral as it proceeds. The larger will be fitted with the main elevator system for personnel and inward ventilation, while the other is used for exhaust air.

An application to government for final go ahead for the main project will be made in 2012, assuming the rock is not found to have major problems. Final deposition tunnels are required to have no significant cracking within 30m.

After this the base loop will acquire a new function as the operations area for drilling the smaller 3.5m diameter deposition tunnels, preparing them with drilled holes for the canisters, and then refilling them with bentonite and sealing them. Some of the stockpiled spoil from the tunnels being kept near the project may well be reused at this point; the concept envisages that while pure bentonite surrounds the canisters themselves, a bentonite/rock mixture is used for filling the tunnel network.

Some 240,000m3 of spoil is produced by the investigation project, much of it stockpiled although some is being used for aggregate and fill material for local roads and the nearby construction of Finland’s new Olkiluoto 3 pressurised water reactor.

Specialised handling machinery for the canisters will be kept in the operations level and there will be living and safety facilities for operating and monitoring staff.

Finally when the tunnel network is complete, involving another 1.1Mm3 of excavation, the entire complex will be sealed and left for slow decay of the radioactivity in the containers.