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Bringing down the curtain

The complex clean-up of Dounreay nuclear plant on the north east coast of Scotland is progressing as contractors near completion of the grout curtain which will isolate a contaminated waste shaft. Alexandra Wynne reports.

The distinctive white dome of the Dounreay Fast Reactor (DFR) rises like a giant golf ball as out of the Scottish landscape, signalling your arrival at the country's largest nuclear clean-up and demolition project.

But the dome is only one element of a vast site that for nearly four decades operated as the UK's centre of fast-reactor research and development. Dounreay Site Restoration (DSRL), a company owned by the United Kingdom Atomic Energy Authority (UKAEA), is now decommissioning the Nuclear Decommissioning Authority-owned site (NCE 1 February 2007).

In the 1950s, during construction of a sub-sea liquid effluent discharge tunnel (LEDT) for the site, a temporary shaft had been built to improve tunnelling access and remove spoil.

The 4.6m diameter, 60m deep shaft was later used as a dump for intermediate level nuclear waste. "In 1958 some bright spark physicists realised they had this shaft, so they applied for permission to dispose of intermediate level waste in there," says DSRL shaft isolation senior project manager Warren Jones.

The shaft, whose base is 60m below sea bed level, was unlined, allowing water to flow in freely through the Devonian Caithness Flags bedrock. Once work was complete, however, the shaft was sealed off from the tunnel system with a 1m concrete plug. The pumps used during tunnelling were then removed and it was allowed to flood.

The nuclear legacy facing site workers today comprises more than 11,000 items including lathes, laboratory equipment and drums of waste.

They were dumped there until 1977, when the shaft suffered a chemical explosion. Since then, environmental legislation has been tightened and DSRL must now remove all waste from the shaft.

Geotechnical firm Ritchies is carrying out the first part of this work for DSRL and is nearing completion of its £16M contract to seal the rock surrounding the shaft. This will allow the 60m of contaminated groundwater that has seeped inside the shaft to be pumped out, and stop new water flowing in through surrounding rock.

Ritchies' target is to seal the rock so tightly that water inflow can be handled by the existing water treatment plant at Dounreay.

When the waste retrieval begins, the water in the shaft will need to be controlled so waste can be removed in dry conditions.

DSRL ruled out the alternative to dry retrieval which would have meant pumping 300m3 of water from the hole each day.

This is because it is 25% salt water, and DSRL would have had to acquire evaporation treatment plant to handle the water.

Jones says this would have cost £220M to build at 2000 prices. Halcrow is designer for the shaft isolation and proposals for the main grout treatment involved a grout curtain similar to the shape of a cup that would surround the shaft and blocked-off tunnel section.

However, Jones says a redesign meant the eventual curtain resembles the shape of a boot. This surrounds the shaft more closely than the earlier design, reducing the internal surface area of the curtain. This is important because it also reduces the area through which surrounding groundwater can seep into the shaft.

The original plan meant that the inner part of the grout curtain could have been more than 20m from the shaft at its furthest point.

With the final design, this distance has been reduced to a maximum 10m from the shaft.

Ritchies is using remote-control wireline coring rigs for the boreholes on this project.

These have now completed drilling 250 holes – both vertical and inclined – with a 96mm diameter to depths of 10m to 100m. The main grouting for the shaft – completed by the beginning of April – has been done in stages with the help of a packer. This has rubber rings at the top and bottom of a 5m long grouting tube that are inflated to form a seal. This allowed the crew to change the grouting pressure in any section of the borehole according to the extent of the (mainly horizontal) fracturing in the rock.

The boot shape is formed in four stages. The first comprises an inner curtain of blocker grouting to protect the shaft from the grouting for the main outer curtain. This is done through boreholes at 3m spacings and the idea was to fill the fractures to create a protective barrier around the shaft.

"Physicists were worried that if the grout [for the main shaft isolation work] got into the shaft it could solidify the waste in there," says Jones.

Using higher pressures of up to 50 bar ensured the blocker grout reached beneath the shaft and blocked-tunnel section. Then moving away from the shaft, further out from the protective barrier, site workers completed primary grouting in three phases starting with the larger fissures, and gradually focusing on the smaller ones.

The final phase comprises a floor and curtain around the shaft and a floor and roof around the blocked section of the LEDT.

This included using 225m3 of ultra-fine cementitious grout capable of grouting fissures to a scale of thousandths of a millimetre. At Dounreay the tolerance is an exacting 25µm.

Ritchies must complete the sealing work to an efficiency of 95% or more, which is equivalent to reducing the water inflow from its original assessment at 350m3 down to 15m3 per day. The team was concerned that the water drill flush for drilling the boreholes could become contaminated by the small amounts of ground-up rock being washed out of the bores.

It came up with a way of using the same 100m3 of flush for the entire project. This was de-sanded and re-circulated to the rigs for the two-year drilling period to minimise contamination. Validation drilling and testing was expected to be complete earlier this month.

Over the next three months, the results of the validation testing will be assessed, groundwater inflow modelled and a report produced before the focus shifts to removing the nuclear waste.

click here for Dounreay Power Station plan and cross section

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