The contaminated Windscale chimney tops at Sellafield are the testground for the UK's awesome nuclear plant decommissioning programme. David Hayward reports.
The End of the Nuclear Follies
Demolition of the first major section of any decommissioned UK nuclear power station is about to enter its most crucial stage. Next week a £250,000 remote controlled dismantling machine will be delivered to Sellafield nuclear fuel reprocessing plant to begin the painstakingly slow task of removing highly radioactive glass fibre lining from one of the old windscale reactor's twin chimneys.
'We are the first team to actually start dismantling a nuclear structure and we feel at the forefront of decommissioning technology says consultant WS Atkins & Partners project engineer Bob Mathews. 'We have to be lateral thinkers and adopt demolition techniques from a wide range of industries to ensure the best solutionFor decades the Cumbrian coast skyline has been dominated by two top heavy 125m high chimneys - chimneys that became notorious symbols following the UK's most serious nuclear accident, the Windscale reactor fire in October 1957.
Dismantling work of the past few weeks has already dramatically altered this familiar skyline. The uppermost box-like filter section of one stack has been slashed in half and the second chimney top is now scaffolded awaiting its robot dismantler. But from the ground the simplicity of the task. Removal of just the upper 27m of the two chimneys will cost at least £100M and take around 10 years to complete.
'The work cannot be equated with any conventional civils demolition contract as we can only plan a short way ahead' Mathews explains. 'We run it like a fast track project designing as we go and must award most of the work in short six month lump sum contracts'.
The identical 14m diameter reinforced concrete chimneys were erected at Windscale in the late 1940s to vent gases from the UK's first and only two air-cooled nuclear reactors. These small 180MW Windscale reactors - the only two items at Sellafield site retaining the original name - were built to produce plutonium for the Government's atomic bomb programme.
Construction of the 1.5m thick concrete walled chimneys was already underway when government scientist John Cockcroft ordered a major design change. Cockcroft was fearful of the remote possibility of a split occurring in the aluminium cladding around the reactor's uranium fuel rods. He insisted that the chimneys would need filters to reduce the consequences if contaminated cooling air were released up the stacks.
But it was too late to install them conventionally at the chimney bases. Large filter boxes were added to the tops of each stack and these 27m tall steel and concrete boxes became known at the plant as.
But Cockcroft's caution provedwell founded. Results of the fire in number one reactor in 1957 would have been even more severe but for the filters. The incident rates as one of the world's worst nuclear accidents.
Insufficient thermocouple monitoring of the reactor is thought to have initiated the fire. During the fission process within these reactors, stored Wigner energy was trapped within a 2000t cube of graphite surrounding the uranium rods. This stored energy had to be released occasionally by annealing the core with a controlled heating of the graphite.
It was during such an annealing operation in reactor number one that the core was accidentally overheated. One corner caught fire and over the following two days temperatures reached around 1300 C. Vast quantities of contaminated exhaust air vented up the chimney before the fire was quenched by some 4.5M litres of water. Both reactors were immediately shut down and have not been operated since. Apart from removal of the filters in reactor one's chimney neither stack has been touched since.
Inspection a few years ago revealed spalling concrete and rusting of both structural steel and reinforcement in the filter boxes, especially in the overhanging soffit of the inspection galleries. 'Concrete cover is low in places and we suspect corrosion was caused mainly by the aggressive marine environment at this coastal site' comments Mathews. 'But the structures remain sound and the chimneys are good for at least another 50 years'.
Instead of adopting increasingly frequent maintenance, the most economic solution was deemed by client British Nuclear Fuels to be total removal of the filter galleries, an operation that would have taken just a few months had they not been on top of two nuclear reactors. The initial problem was the prohibition of all craneage, for safety reasons. Windscale reactors were built on a virtual green field site but are now totally surrounded by Sellafield's reprocessing plant Ironically this enormous complex was initially developed mainly to store and treat fuel from the two military reactors themselves 'We had to consider not only a load falling from a crane and crashing onto a main reprocessing area but the possibility of a complete 150m tall tower crane itself toppling over' recalls Mathews. 'The danger area was so large that any of the site's numerous buildings posing a potential radiological hazard could have been hit' Because cranes are banned, all brick, concrete and steel demolition material must be cut up into sections small enough to travel in one of the twin Alimac hoists erected up both chimneys.
Demolition is undertaken in two distinct stages. The 7m tall upper brick shaft and concentrator unit beneath are removed in one go - leaving dismantling of the more intricate and potentially hazardous 15m high filter and diffuser sections as a separate operation.
First stage demolition on chimney two - the stack not affected by the fire - will be complete by Christmas, three months ahead of schedule.
Initially we had no idea what contamination levels to expect but they turned out to be little more than background radiation' says Mathews. 'Most of the brick, concrete and steel could be brought down by hand with little protective clothing needed.'
With such an exposed structure, the main fear was to avoid the spread of contaminated dust over the plant beneath and most demolition is carried out within small tented areas specially vented to filtered extraction units. The choice of cutting equipment is influenced by the need to keep operators as far back as practical from the work face.
Reinforcing steel is normally parted by disc cutters instead of being burnt. A large, specially modified, hydraulic circular diamond concrete saw, supplied by Marcrist International Ltd of Doncaster is used on the brickwork in preference to hammers and chisels. Waste cooling water from the saw, captured in a specially designed guard system, must be filtered and recycled to prevent any contaminated runoff. 'No one would think of using a large remote controlled diamond saw just for cutting brickwork, but we want to keep the operator away from the radioactive shine inside the chimney' Mathew stresses.
Even such stringent precautions pale in comparison with those needed for the same stage one demolition work about to start a few hundred metres away on the notorious chimney number one where most dismantling must be by remote control. Engineers expected high radiation levels lingering from the fire 32 years ago, but contamination was even worse than feared. Levels of up to 0.1 Mega Bequerals per gram translate into less than ten minutes working time for even a fully protected operator with built in respirator equipment.
But it is hoped that much of the contamination above the filter boxes is contained within the upper shaft's 75mm thick internal glass fibre insulation And it is to remove this lining that the first of several specially designed, remote handling machines will arrive next week.
The glass fibre is attached by chicken wire to aluminium sheeting, itself held in place by a steel space frame bolted to the inside brickwork of the upper stack. The support frame creates a convenient 700mm wide gap between insulation and brickwork. The telescopic cylindrical remote handling machine has to work within this space with its two arm-like grabs cutting the chicken wire to release the glass fibre mats.
The operator will be at the base of the chimney and work with the aid of twin stereo TV cameras. The stripped insulation will be dropped 7m down the gap to the base of the upper shaft where a second remote control machine will work through holes cut into the brickwork, pulling out, shredding and securely packaging the glass fibre.
Total cost of this year long operation to remove 37m³ of glass fibre lining is estimated at £3M compared with £150,000 spent several months ago stripping a similar quantity of relatively uncontaminated insulation in chimney two.
Overall, stage one demolition on the fire irradiated chimney will cost over £11M, nearly five times more than on the neighboring stack.
Engineers hope that once the dangerous insulation is out of the way, the upper stack brickwork can be removed 'semi remotely', possibly using a converted mini excavator arm braced to supporting scaffolding Crusher and cutter attachments on the arm could, it is expected, break up all brickwork, concrete and steel to pieces sufficiently small to be brought down by the hoists. On both chimneys initial stage two removal of filter galleries and lower diffuser sections will be totally remote. Controlled again by stereo cameras, another £2M remote cutting machine
will be hung from steel beams over the 20 banks of filters in each chimney. It will then cut up the Warren truss filter supports and lower them in pieces 110m down to the chimney base.
The same machine will then be progressively lowered down the chimney cutting up and removing the glass fibre and aluminium lining as it descends.
For its final operation the machine will be fitted with a Marcrist designed, and built remote controlled diamond shaving system which has been manufactured to enable an exact, predetermined amount of contaminated concrete to be shaved. from the curved walls.
Only the first few millimeter's of concrete is likely to be contaminated, which is why the machine has the capability of contour following the surface, to remove the exact amount of concrete to eliminate any radioactivity and to reduce the costs of waste storage. Only when this process has been completed will it be considered safe for the steel and concrete filter galleries themselves to be dismantled.
The project's enormous total cost is partly a result of the high charge levied for storage of demolition debris, all classed as intermediate level waste. Current costs of £290/m³ for disposal within steel containers at BNFL's nearby Drigg site have risen by 700% in the last few years and just completed improvements at Drigg are likely to soon push that bill up to £1000/m³ .
'One of our major problems is what to do with the waste. There are cost incentives not to dismantle anything' says Matthews 'It is more a political problem than an engineering one but it still lands us with high engineering costs'.
Decommissioining unit project manager Fred Shiel is closely monitering the chimney top demolition the first of an anticipated BNFL's 14 major structures at Sellafield that will need decommissioning over the next decade or so "From the nuclear point of view it is proving easier than we feared' he says. 'We have been very successful in containing the dust and preventing the spread of contamination around the site'.Sheil estimates that BNFL's total 'decommissioning liability' at Sellafield over the next 100 years is around £4600M. With the current project working out at around £2M per metre run to demolish, that estimate may well prove conservative.
NOTE FOR INFORMATION:The two 125m high Windscale chimneys are identical. Concrete walls of the 12.5m internal diameter shafts reduce in steps from 1.5m thickness at the base to 305mm at the top. Each 27m tall upper box is made up of four separate sections. The 7m high reinforced concrete diffuser unit above the main shaft sits on a cellular concrete ring beam slightly overhanging the chimney perimeter. Its internal cone shaped lining, channels and spreads air flow through the filters above. The floor and walls of the 21m square filter itself were formed with a series of steel warren trusses encased in 700mm thick reinforced concrete. Inside the 20 banks of glass fibre filter are supported on more Warren trusses. Above the filters, the 5m high concentrator section, designed to channel air flow up through the tap shaft, consists of a similar concrete clad space frame. Finally the 7m tall upper shaft was formed with 230mm brickwork walls lined internally with aluminium sheets backed by 75mm thick glass fibre insulation. This lining and insulation is tied to the lattice steel frame which is bolted to the inside of the brickwork.