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Cooling off period

Technical feature Nuclear

All signs of the world's first commercial nuclear power station are being erased from the muddy shores of the Severn Estuary.

Andrew Mylius reports on the demolition of Berkeley's redundant cooling water structures.

Just upstream from the glistening Severn Estuary mudflats that are home to the Slimbridge wildfowl and wetland trust, the hulking relic of a bygone nuclear age is being dismantled.

Berkeley nuclear power station was the first in the world to generate electricity commercially when it went into operation in 1962, producing 276MW from two Magnox reactors. But it was hurriedly shut down in 1989 due to fears over the safety of the reactors' refuelling cranes.

Berkeley has now been earmarked for 'safe store'.

The de-fuelled reactor cores will be left intact, contained within their pressure vessels, to radiologically 'cool off' for the best part of the next century.

It is calculated that natural radioactive decay will make the reactor structures substantially safer to work on by around 2100 than they are at present.

While contaminated structures will become less hazardous over time, there is little point in delaying demolition of other, uncontaminated buildings on the Berkeley site, however. This is why the rust-streaked steel and concrete of the station's cooling water structures have this month been erased from the Severn's muddy shoreline.

Berkeley's cooling system drew cold water from, and discharged hot water into, the Severn. To prevent hot water being drawn straight back into the cooling circuit, a 500m long baffle wall was erected in the estuary 150m off the high water line, separating fresh inlet water from heated outlet flows.

Cooling water was drawn in from outside the baffle through a 2.9m diameter pipeline, tunnelled under the sea bed, 30m beneath the mean high water mark. In order to regulate flows, an inlet gate house was constructed at the centre of the baffle. And to provide easy access to the gate structure, for maintenance of the inlet valve and control equipment, a 2m diameter access tunnel was dug parallel to the inlet pipe. The gate house was equipped with a lift.

Built like a bunker using 1,200m 3of reinforced concrete, the inlet gate house stood 14m tall from the estuary bed. The arms of the baffle wall were composed of interlocking boxsection steel piles. As part of its £3.17M package of works for client BNFL, contractor Nuttall was briefed to remove the baffle wall down to ground level, leaving nothing but chocolatebrown silts behind.

Tidal range in the Severn is up to 13m, leaving Nuttall with an almost impossibly tight two hour window of working time at low tide, says project manager Dan Hockey. To increase the amount of time it could spend attacking the offshore structure, the contractor started work by laying a causeway and working platform of local limestone rock. The low tide operating window was stretched to four and a half hours, but even so, with such restricted access night working has been essential to making progress on the project.

A pair of hydraulic peckers hacked down the central, concrete structure. Rebar was hauled off for recycling, and the concrete rubble used to extend the platform. Nuttall set about removing the steel piled wall using powerful 'lobster claw' hydraulic shears. However, here it hit a problem.

At high tides the open-ended box-section wall was largely immersed. Sediments borne on the Severn's racing currents had fallen out of suspension, filling the wall with dense, plastic mud. Instead of being able to shear easily through hollow box piles, Nuttall found the mud had created a composite structure, calling for an altogether different approach.

Holes were flame cut in alternate boxes, front and back to see if, having filled the box piles with silt, the tides could wash it out again. This was semi-successful, reports Hockey, but still did not make shearing a viable option.

So instead, in groups of four, the piles were cut across their front and back faces, then to be pushed over by an excavator - about 3t of force was needed to tear the lateral webs and bring the sections down, Hockey says.

They were then hitched to a bulldozer and hauled onto dry land.

All of the concrete and limestone used to build the causeway and working platform is being reclaimed from the estuary. The concrete is being crushed and stockpiled. This will be used to fill some of the huge voids associated with the cooling structure demolition works: The inlet tunnel extends 90m inland from the shore to the pump house, which drove water round the cooling system. From here, two smaller diameter cut and cover tunnels carried cold water the 220m to the old turbine halls. Two more cut and cover tunnels then fed hot water back to the shoreline where it was discharged from pipes run along a short, piled jetty, now removed.

On-shore, all of the tunnels and pumping facilities are being backfilled to prevent future collapse and subsidence - it is likely that the site will be built on at some future date.

There is little point in filling the sub-sea section of inlet pipe or the access tunnel, though.

Extending beyond the high tide mark into the estuary, if these collapse sedimentary deposits will fill any resulting depression.

It was decided to block off both tunnels with sandbags and groutfilled fabric formwork bags.

Since Berkeley closed 13 years ago the access tunnel had flooded, and to block off it and the inlet pipeline it was necessary to use divers. Diving in 30m of water meant relatively high pressures and restricted working time - far from ideal when the divers were being asked to handle bulky objects in cramped, heavily silted spaces, with zero visibility. To get around this challenge, diving time was extended by reducing the head of water. The inlet valve was shut tight, reducing but not eliminating ingress of water. Both tunnels were then partially pumped out, reducing the head of water to 13m.

To backfill the inlet and discharge tunnels, and also the vertical shaft for the access tunnel, 20,000m 3of pulverised fuel ash (PFA) grout has been used. Low grade PFA from nearby Didcot power station was cheap but adequate to provide the long-term strength required.

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