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DEWATERING DOUNREAY

At the Dounreay nuclear plant in Scotland work to clear a shaft full of radioactive waste begins in four years. By then it must be watertight. Adrian Greeman reports on an unusual grouting job.

It seems incredible now that an old rock tunnel mining shaft, linked to the sea, unlined and simply plugged with concrete at the bottom, should ever have been used to store deadly radioactive waste.

Whoever decided to start pitching laboratory apparatus, reactor coolant pipes, fl asks of uranium solvents and the like into its 65m depth left a difficult clean-up task for today's teams.

The problem has been teasing the minds of engineers at the experimental reactor plant for some time.

Until its active operations were shut down in the mid-1990s, Dounreay was at the cutting edge of Britain's nuclear research and housed the country's only fast-breeder reactors. Secondary waste from those (though not the warhead-suitable plutonium they generated) now lies gradually decaying in the rock shaft, jumbled together with equipment from nuclear science laboratories.

This situation was not helped by the infamous explosion in the shaft in 1977, thought to have been caused by a build-up of hydrogen, which blew the lid off and scattered waste around on the surface. It certainly mixed the jumble even more.

Engineers think the clean-out will need to be done in the dry by remote handling, which means first dewatering the shaft; from the start it was allowed to fi ll with a mixture of groundwater and seawater which now surrounds the waste.

That creates problems says Warren Jones, senior project manager for the UK Atomic Energy Authority's (UKAEA) major projects and engineering division. 'Old pictures of the construction show the workers wearing waterproofing. That is because the rock lets through a lot of water, perhaps 400m 3 a day.' That amount of water would create havoc for the difficult remote picking operations needed to lift out the waste and, more importantly, would create another disposal issue.

Everything that touches the radioactive waste also needs to be disposed of, so a sealed pumping circuit would have to be set up - 'and then we would have to store and process the waste water, ' Jones says.

This would require the building of an evaporative liquid treatment plant, at an estimated cost of £220M. With budgets limited and nuclear waste disposal costs becoming a hot political issue again, that is not likely to happen.

The alternative is to cut the infl ows and that is what a current project aims to do, using a state-ofthe-art grout curtain to seal the rock (GE November 2004). It is hoped daily infl ows can be cut to about 15m 3, which could be handled by existing reprocessing facilities.

Grout tests have been in progress for about a year. Recently groundworks contractor Ritchies, part of Edmund Nuttall, began site drilling tests in a clean area about 100m from the shaft. Trials of drilling patterns and grout types will continue over the rest of this year.

The detailed shape of Ritchies' £16M design and build project will depend on these results but Jones says the overall scheme is more or less clear. The shaft sits close to the sloping rocky foreshore by the sea and is not easily accessible.

'We will create a solid work platform around the shaft about 40m across using a lean mix rolled concrete with rock armour on the seaward side, ' he says. 'From there we can drill a sequence of boreholes around the shaft to build up a grout curtain.' The aim is to seal all but the tiniest fi ssures in the hard rock around the shaft, which is Caithness Flagstone, a mixture of siltstones, limestone and sandstone.

Before that, work is needed on the tunnel below. The shaft was used to excavate the long sea outfall used for disposal of low level liquid waste by offshore seabed diffusers, and is still connected by a stub tunnel.

Jones says: 'There is a concrete plug in the stub tunnel, but it is not clear how strong it is.' The plug was designed for a 60m head of water from the shaft to the dry tunnel, but eventually the main tunnel was also allowed to fl ood.

The plug could be vulnerable to pressure surges if something large was dropped into the shaft, which might well happen during its cleanout. An added complication is that there are no 'as built' drawings of the plug and a sonic survey through a probe drilling has shown it sits 1m further inwards than designed.

The plug will be reinforced with an additional grout plug behind it on the seaward side. To create this a grout bag will be inserted through a drilling further up the tunnel and then infl ated. Once it seals the tunnel, the space between can be fi lled carefully, balancing pressures by extracting water through one drillhole as the grout goes in another.

'The new plug serves a second purpose as a medium for the grout curtain which cuts through the tunnel line here, ' says Jones.

Similar filling is needed for a second tunnel. This is the upper section of the effl uent tunnel which joins the sea tunnel at one end and runs up to the surface for the sea waste pipes.

Where the line passes through the grout curtain it will be infi lled with a low strength cement bentonite or concrete -'weak enough to excavate again in the future as the need might arise, ' Jones explains.

The grout curtain itself is quite complex, an oval shape enclosing the shaft and the tunnel junction.

Ritchies' onsite project manager, Iain Robertson, says the first part will be an inner ring of boreholes around the shaft which will be sealed with a fast gelling cement grout.

'What they [UKAEA] don't want is for the grout to start entering the shaft itself, which would make disposal much more diffi cult, ' he says.

'So we use a blocker grout which does not travel very far to seal the biggest inflows [close to the shaft].' A much wider curtain will be built up around the inner ring using ultramicrofi ne grouts that can travel long distances in the rock, penetrating even 50 micron fissures for up to 15m.

The initial part of the main fine grout curtain will be a base seal reaching across the bottom of the shaft. To do this, boreholes will be made to 15m below the shaft and then inflatable packers will be used within the holes. These are steel cylinders with inflatable rubber ends.

These are expanded hydraulically as the high pressure grout being used means pneumatic expansion would not be fi m enough. Once expanded, they lock off a section of the bore which means grout can be injected at a specific point, in this case the bottom 5m of the bore.

The very thin grouts will be used to ensure complete lateral penetration underneath the shaft area.

'After that we may go to a chemical grout, either colloidal silica or an acrylic resin to seal down to about 20 microns. But that depends on how the rest goes, ' Robertson says.

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