One of the trickiest nuclear waste clear up operations in the UK is scheduled to start around 2009.
But before work can begin, a complex grouting project must be completed. Adrian Greeman reports.
It seems incredible now that an old rock shaft, linked to the sea, unlined and simply plugged with concrete at the bottom, should ever have been used to store deadly radioactive waste. But this is what happened at the Dounreay nuclear facility from 1959 onwards.
The 4.5m diameter shaft had been excavated in 1953 as a mucking adit to facilitate the construction of the low level liquid waste sea outfall tunnel (NCE 25 June 1998). And whoever decided to start pitching bits of lab apparatus, reactor coolant pipes, flasks of uranium solvents and plutonium swarf into the shaft's 65m depth left a difficult legacy for today's clean-up teams - 700m 3 of assorted nasties, in fact.
Just how to clear it out has been teasing the minds of engineers for some time at the experimental reactor plant, which until it was shut down from active operation last decade, was at the cutting edge of Britain's nuclear research. It hosted the country's only fast-breeder type reactors, the secondary waste from which is now jumbled together in the rock shaft, along with redundant irradiated equipment from a number of nuclear science laboratories.
The wisdom of dumping nuclear waste in the shaft was brought into sharp focus by the now infamous 1977 explosion of hydrogen produced by chemical reactions between the waste and the water. Its force blew off the lid covering the shaft and scattered waste around the surface. It also jumbled the mix even more. But at least dumping stopped.
From the start of dumping the shaft was allowed to fill with a mixture of groundwater and seawater. But engineers believe the final clean out will have to be done by some kind of remote handling operation in the dry, which means dewatering.
That immediately creates problems, says Warren Jones, senior project manager for UK Atomic Energy Authority's (UKAEA) major projects and engineering division. 'In old pictures during construction of the outfall you can see the workers wearing waterproofing. That's because the rock lets through a lot of water - an estimated 400m 3/day.' That much water would create havoc for the remote operations needed to lift out the waste and would create another disposal issue. Everything that touches the radioactive waste itself also needs to be disposed of, so a sealed pumping circuit would have to be set up.
Critically 'we would have to store and process the wastewater', says Jones, which would require the building of a new evaporative liquid treatment plant, at a estimated cost of £220M.
The alternative, therefore, is to reduce the inflow of water 3 a day, within the capacity of existing reprocessing facilities at the site. The chosen solution is to use a state of the art grout curtain to seal the rock tighter than Camp X-Ray.
Grout tests have been in progress for about a year and groundworks contractor Ritchies, part of Edmund Nuttall, began site drilling tests recently in a clean area about 100m from the shaft. These will continue over the rest of the year to trial various drilling patterns and grout types.
The detailed shape of Ritchies' £16M design and build project will depend on these results, but the overall scheme is more or less clear, Jones says.
Access is a problem, as the top of the shaft sits close to the sloping rocky foreshore. 'We will create a solid work platform around the shaft about 40m across using a lean mix rolled concrete similar to that used in dam building and with a rock armour on the seaward side, ' Jones explains. '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 fissures in the hard Caithness Flagstone around the shaft. Before that happens, however, work has been done to isolate the shaft from the outfall tunnel.
Shaft and outfall were linked by a stub tunnel, Jones explains.
'There is a concrete plug in the stub tunnel, but it's far from 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 flood. 'There is now a small differential head of about 4m in the other direction - inwards, ' says Jones. 'We keep the shaft just below sea level [to prevent radioactivity from the shaft migrating into the surrounding rock under positive water pressure].' If something large was dropped into the shaft, which might well happen during its clean out, the resultant pressure surges could damage the plug, Jones fears. An added complication is that there are no 'as built' drawings of the plug and a sonic survey through probe drilling has already shown it sits 1m further inwards than designed.
To reinforce the plug on the seaward side a grout bag will be inserted and inflated further up the tunnel through a hole drilled down from the surface (see diagram). Once the tunnel is sealed, grout can be pumped down a second drill hole into the space between the grout bag and the original plug while water is extracted through a third.
'The new plug serves a second purpose as a medium for the grout curtain, which cuts through the tunnel line here, ' says Jones.
Similar ling is needed for the landward section of outfall tunnel, running from the stub tunnel up to the surface. Where the tunnel line passes through the grout curtain it will be infilled with a low strength cement bentonite or concrete 'weak enough to excavate again in the future as the need might arise', explains Jones.
The grout curtain itself is also complex - in horizontal cross-section, an oval envelope enclosing the shaft and the tunnel junction. Ritchies' project manager Iain Robertson explains that the first part will be an inner ring of drillings around the shaft, which will be sealed with a fast gelling cement grout.
'What they don't want is for the grout to start entering the shaft itself, which would make disposal [of the contents] much more difficult. So we use a blocker grout which does not travel very far to seal the biggest inflows.' A much wider curtain will then be built up around the inner ring using ultra-microfine grouts that can penetrate even 50 micron fissures for up to 15m.
The first part of this curtain is to be a base seal reaching across the bottom of the shaft. Holes will be drilled to 15m below the shaft before special twin packers are inserted. These are steel cylinders with inflatable rubber ends made by Michelin, which are dropped down the hole to a set point with ends expanded hydraulically to seal the bore.
Grout will be pumped into the packers and forced out between the top and bottom seals into the surrounding rock. This technique allows the base seal to be positioned very precisely to within a 5m horizon beneath the foot of the shaft. Very fluid grouts will be used to ensure penetration right across the sub-shaft area.
To aid the efficiency and efficacy of the grouting, drilling of the grout holes is being done with wireline core drills. Coring rather than grinding the rock away will greatly reducing the amount of crushed spoil, so reduce the risk of fissures being clogged. All spoil will be screened for radioactivity and removing cylindrical rock cores rather than crushed material will make separation of clean from contaminated material easier, Robertson adds.