Your browser is no longer supported

For the best possible experience using our website we recommend you upgrade to a newer version or another browser.

Your browser appears to have cookies disabled. For the best experience of this website, please enable cookies in your browser

We'll assume we have your consent to use cookies, for example so you won't need to log in each time you visit our site.
Learn more

Onkalo below

Finland waste storage - With the UK tentatively edging towards a nuclear renaissance, eyes are on Finland, the first country to tackle underground disposal. Words and pictures by Adrian Greeman.

The usual rueful head-shaking over 'insufficient site investigation' will hardly apply at Onkalo. The nuclear waste storage project in western Finland is, in a sense, nothing but investigation on a grand scale.

An enormous ground-probing tunnel is being built, over 4km long and spiralling on a 10% gradient to 420m depth in hard granite and gneiss. Depending on the conditions discovered another 1km length of the 4.5m high tunnel may be added. From the basement loop of the tunnel and various rock chambers further research will be done over several years, using long core drill sampling and ground water monitoring far into the rock.

'The project is for characterisation of the ground, producing a map of the entire rock mass in great detail, ' says project manager Timo Niemitalo from client Posiva.

The company is a joint venture formed by two of Finland's power companies, TVO and Fortum Power & Heat, which under Finnish law must dispose of the waste they produce.

If the results of this study ? plus 25 years of previous work, including major multiple deep core drilling from the surface ? prove convincing and satisfy the government and its Radiation & Nuclear Safety Authority (STUK), the tunnel complex will be converted in 2015 into an entry ramp and operations centre for the main disposal project, an estimated 3bn ($4bn) worth.

From its base loop, kilometres of disposal tunnels will be driven in a comb-like pattern over the next 100 years (see diagram).

Small shafts will then be drilled to hold the canisters that contain the radioactive waste (see box).

Eventually the complex will be sealed and the slow process of radioactive decay will tick away for the next 100,000 years without any contact with the outside environment.

At least that's the theory.

The problem of turning theory into reality is groundwater. Sitting as it does on some of the oldest and most stable rock on the planet, the millions of year old Baltic Shield, it makes sense for the Finns to put their waste deep inside it. But the rock is crystalline and therefore has cracks, which in turn means some groundwater movement.

'Other ground disposal schemes in Europe have tended to look at sedimentary rock or clay layers, ' says Charles McCombie, an independent consultant in disposal based in Switzerland. 'These are much more impermeable.' The huge timescales involved could mean leakage if radioactivity escapes in hard rock. The groundwater, and any chemicals in it, could also contribute to the corrosion of the containers, although it is hoped the great depth will mean it has no dissolved oxygen ? essential for corrosion, as any GCSE student will tell you. 'There must be no significant cracking within 30m of any of the disposal tunnels, ' says Niemitalo.

Serious construction on the $390M investigation project began in 2004 after a long process of approvals and final site selection passed through the Finnish parliament. Moving Posiva's headquarters to Onkalo ? and the presence of a nuclear power station nearby already providing jobs ? helped win over the local Eurajoki municipality.

For the first year, tunnelling was carried out by Finnish contractor Kalliorakennus drilling and blasting the 4.5m high and 5.5m wide tunnel to 990m length. Since then, Posiva has taken on the work, buying its own Tamrock three-boom jumbo rig to complete about 600m more this year. Work will continue until 2010.

Kalliorakennus stopped work on the project, says Niemitalo, because there is a lot of major work coming on road and rail in the country and it did not want to be tied to the scheme long term.

Posiva also needs long-term experience from the tunnelling and decided it wanted direct contact with the process.

The tunnelling itself is part of the research, he explains, with careful cleaning, inspection and recording of the tunnel walls carried out throughout its length, as well as other follow on rock-probing work. Posiva wants to understand the drilling and blasting process in great detail to ensure that additional tunnel wall cracking is minimised.

Blasting errs on the undercut side rather than overbreaking and any intrusions into the tunnel envelope are subsequently drilled and blasted out 'gently'.

Progress is therefore slow, about 25m to 30m a week. More cracking than expected in the first kilometre also meant a lot of grouting that slowed work.

But work is on schedule. A first section of a 3.5m diameter shaft has also been raise bored just over 100m from the first loop and will be extended as the second loop comes round. This is used for ventilation and an emergency escape elevator.

The permanent facility will have a second larger shaft with elevators and inward ventilation, while the smaller one takes exhaust.

Nuclear new build Waste for disposal will come from five reactors, four currently operating plus the large Olkiluoto 3 reactor under construction nearby on the Baltic coast. Finland, highly dependent on Russian gas, is one of the first countries in Europe to revive nuclear power.

The $4bn turnkey project is under construction by a FrancoGerman joint venture of Areva NP (formerly Framatome) and Siemens and uses the latest variant of the pressurised water reactor, the European PWR. Bouygues is the main civils contractor.

The reactor will have a capacity of 1,650MW, almost equalling the two 840MW units which TVO already operates at the site.

Two reactors of 488MW capacity each are operated by Fortum at Loviisa in the Gulf of Finland.

Work on Olkiluoto 3, the first reactor to be built in Europe for some time, is over six months behind schedule. Concrete quality problems earlier this year, and a long learning curve for various suppliers for the components, are cited by the joint venture.

Disposal solutions Finland has adopted disposal techniques developed by Sweden, with Finnish input. They rely on a series of barriers to prevent escape of radionuclides. Spent fuel is thought to be reasonably resistant to the escape of radioactivity since the fuel rods are made up from uranium embedded in insoluble ceramic pellets.

After they have cooled, the rods will be stored for 40 years in interim storage water tanks before they are transported to the repository where a surface ecapsulation building will prepare them for long-term disposal.

The rods are then bundled inside copper cylinders which are filled with neutral argon gas and then welded shut. The fuel inside the 50mm thick copper casing is held in an iron framework that supports and separates them.

The cylinders sit in 7.5m deep vertical holes drilled at intervals in the base of the tunnels. They are packed in with blocks of bentonite that will gradually absorb groundwater and expand blocking off groundwater flow around the cylinders. More bentonite mixed with rock chip spoil will fill each tunnel once full.

The rock itself is the final barrier of course, as long as it is sufficiently sound, restraining groundwater movement to a very slow pace.

Have your say

You must sign in to make a comment

Please remember that the submission of any material is governed by our Terms and Conditions and by submitting material you confirm your agreement to these Terms and Conditions. Please note comments made online may also be published in the print edition of New Civil Engineer. Links may be included in your comments but HTML is not permitted.