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Detailed long term ground investigations are under way for a radioactive waste repository in Finland. Adrian Greeman reports.

There are not many site investigations which have a team of 40 engineers, permanent on site offi ces and canteen, their own largescale drilling jumbo rig, and a schedule lasting nearly a decade.

Or which use not just boreholes, but a full size tunnel excavation and a number of underground chambers.

But not many sites are being prepared for the burial of nuclear waste for a minimum of 1000 years.

In Olkiluoto in west Finland, just such a programme is in progress, and work is due to continue for some years at a cost which will approach 300M (£204M).

A 4km long tunnel is being bored, spiralling round to reach a depth of 420m where a series of chambers will be built. Storage buildings are being erected above ground. A fi nal additional tunnel may be added to 520m depth.

All this digging is for the disposal of nuclear waste. Like most countries Finland has chosen rock entombment to deal with the high level waste produced by its power stations. Sitting as it does on some of the oldest and most stable rock masses in Europe, the choice was perhaps easier to make over fanciful ideas of blasting the waste into space or plunging it into deep sea trenches.

The so-called Onkalo project will eventually be the central point for handling and embedding radioactive fuel waste coming from Finland's five nuclear power reactors.

Spent fuel rods will be sealed in for a minimum 1000 years - probably much more - while they cool and their radioactivity dies down. Once fi led up over decades, the entire site will be sealed, containing a maximum 6500t of used high level radioactive material.

Operational work to handle the radioactive canisters and to drill a continually growing network of disposal tunnels will be centred in the below ground chambers, a total of some total 330,000m 3 of space.

Above ground there will be sealing facilities for preparing the canisters before they go underground.

The work in progress has so far driven a 4.5m high, 5.5m wide tunnel about 1.5km into the rock in the past two years and there is about 2.5km to go. Disposal will not begin until 2020: all this effort is for further investigations on a huge scale. Most of the chambers will serve for the next decade as access points for further boreholing as well as laboratory and sample storage space.

These investigations are considered vital. Having the best quality hard rock at hand does not guarantee safe disposal. Waste has to remain in place and safe for at least 1000 years, and potentially much more, as its radioactivity decays.

It is relatively simple to keep people away above ground, but not as easy to guarantee there will be no leakage and contamination below ground. Hence the detailed rock investigation.

Preparing radioactive waste to minimise the possibility of leakage is a multi-level operation. The basic waste is processed to embed it in a low mobility ceramic medium.

Pellets of this are enclosed in iron frames which are then inserted into casings of high grade copper, proven to last tens of thousands of years in a metallic state.

If, despite these measures, some waste should escape from the sealed canisters, the next failsafe is to ensure that it does not move from where it is. Embedding it in solid rock, sealed in tunnels deep below ground, is a start. But it is not a total guarantee.

The problem is groundwater - even in the densest rock mass there will be cracks and the potential for chemical migration. Starting several hundred metres down helps, but it does not eliminate the problem.

'The next level of defence therefore is to encase the waste in bentonite, ' says Timo Niemitalo, site manager for the Posiva company which is carrying out the extended investigation works. Posiva is a joint venture of Finnish power companies TVO, Fortum Power and Heat which under Finnish law are responsible for the disposal of the waste they produce.

The exact method of storing the canisters, 4.5m long and each weighing between 15t and 20t has still to be decided. There are two alternatives. They can be deposited in vertical holes drilled into the oor of an eventual grid of tunnels which will comprise the repository. Each hole will be lined with bentonite pads below and around the canisters.

A nal bentonite plug will seal the hole and then once all the canisters are installed in each tunnel, that too will be lled with a bentonite and aggregate mix before being sealed with a concrete plug.

The other method involves horizontal storage of the canisters inside larger 'super-cannisters'.

These would be lled with bentonite with a perforated steel casing to allow in enough water to expand it, and would be slotted one after the other into the nal storage tunnel.

'Slow seepage of groundwater will soak into the bentonite and expand it to block the tunnel and all further groundwater movement, ' says Niemitalo. But the groundwater should be as minimal as possible.

'The main idea is to build a hydraulic and geological model of the rock to nd the places where the best rock quality is located, ' he adds. 'In particular the aim is to try and identify any continuous or long running cracks.'

First, deep cores are made from the surface. These are supplemented by cores created at deeper levels as the tunnelling proceeds, with the boreholes used for monitoring groundwater ws and chemical composition as well as the rock being analysed.

But the tunnelling is also being studied in detail both to gain further understanding of the rock and of the tunnelling process itself and the impact it has on the rock. Drill and blast methods are being used, but slowly and with extreme care.

Before the tunnel is drilled a 72mm pilot bore is made ahead for 50m to analyse the rock. A Tamrock jumbo machine then drills 5m long blasting rounds with as much care as possible; undercut is preferred to overbreak - unlike most normal tunnelling - with the intrusions carefully blasted out afterwards.

'We want to minimise the cracks that blasting might cause, ' explains Niemitalo.

The tunnel walls are cleaned and carefully inspected for rock quality with a detailed inspection and recording of cracks, their type, the mineral composition of the rock and hardness done by the Schmidt test.

The jumbo used for the drilling is a Tamrock Axera T11 Data - one of the latest computerised machines - which also provides more data from the digital logging equipment of the blast hole drilling. To make best use of that, Posiva recently took over tunnelling directly from the contractors it used for the rst 1km or so, and is now buying its own machines such as the Axera model.

'The work requires a continuity of recording information and you cannot achieve that with a contractor who may be on site for a year but then moves elsewhere, ' says Niemitalo '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.

The recording will help determine the best methods for building the main repository as well as give more information for the rock study.

Other work is also in hand to study the eventual disposal methods, including joint studies with Sweden's SKB, and various research programmes at Finnish universities, investigating the properties of the various materials being used.

Meanwhile the facility is just beginning to take shape. A shaft from the surface has recently been completed to 86m depth intersecting with the tunnel as it comes around its rst loop. A pilot was bored from above and then a 3.5m diameter raise boring machine used to create the vertical hole. For the moment it serves to short circuit the ventilation, cutting out the long exible tubing that would otherwise have been needed along the tunnel.

'It also gives us an escape route, ' adds Niemitalo. As the tunnel comes around on its further loops the shaft will be extended in phases.

There may also eventually be a bigger shaft of 6.5m diameter and in the permanent facility the two will provide lift shaft access for both people and the waste canisters.

Disposal will begin in 2020 and will continue for 100 years, after which the entire facility will be permanently sealed.

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