News Extra - Demolition of Windscale chimneys a unique decommissioning challenge
23 December 2014
Fifty-seven years after the UK’s worst nuclear incident, the skyline in West Cumbria is set to change as Sellafield Ltd tackles the crucial task of bringing the second and final ventilation chimney of the Windscale Piles to the ground.
Windscale Pile chimney - Image: Sellafield Ltd
Standing at 110m tall, some 5,000 tonnes of concrete, steel and brick will be carefully dismantled, monitored to check for any remaining contamination and disposed of safely.
The Windscale Piles, with their two distinctive ventilation chimneys, were built in the early days of the Cold War to provide the Government of the day with plutonium for the production of a nuclear deterrent.
Sellafield was at that time at the forefront of pioneering nuclear technology, but the plants built back then had few of the safety features which would now be deemed as essential requirements in modern day nuclear facilities. In October 1957, a fire took hold in Pile 1 reactor and threatened much of the North of England with nuclear contamination.
Uranium fuel cells had ignited with the blaze reaching 1,300C, and workers fought to stop the whole facility exploding. Water failed to put out the blaze and the fire was only extinguished when operators closed off the air in the reactor room.
Thankfully, Nobel prize-winning physicist Sir John Cockcroft had insisted that the Windscale Pile chimneys be fitted with high performance filters. Since this was decided after the chimney barrels had been designed and partially built, they produced iconic bulges at the top of the structures.
Known locally as ‘Cockcroft’s Follies’, because of the additional expense of installing them so late in the process, it was the filters which prevented the Windscale fire from becoming a major catastrophe, with most of the radioactive contamination being captured by the filters.
The blaze burnt for three days and significant amounts of radioactive material, most notably iodine-131, were released and spread across the UK and Europe. It is estimated about 240 cases of thyroid cancer were caused by the radioactive leak and all milk produced within 310 square miles (800 square km) of the site was destroyed for a month after the fire.
The level of radioactive material which did escape is estimated to be 1,000 times less than at Chernobyl.
Both of the plutonium-producing piles were shut down following the 1957 fire. The contaminated filters themselves were removed shortly afterwards, and the chimneys sealed up to allow radiation levels to reduce before decommissioning work could start. The first chimney was reduced to the level of the adjacent reactor building in 2001. The second chimney posed more of a challenge due to radioactive contamination from the fire, with demolition beginning in 2013.
The decommissioning process has employed a combination of manual and semi-remote tools and techniques, and has involved stripping out thermal insulation lining, taking down the concentrator, dismantling filter in-fills and removing four 10 tonne winches that were used to raise and lower the head gear platform inside the chimney on which a remotely operated demolition vehicle was deployed to remove the chimney lining and steel work.
Steve Slater, Head of Decommissioning, said the plan was to remove the filter gallery by the end of 2015 and then the chimney diffuser by 2018. A tower crane will be built alongside the chimney and the chimney barrel itself will then be dismantled and lowered down in sections.
Work is now underway for the demolition of the filter gallery structure. Already 66 tonnes of brickwork has been removed from the filter gallery external walls, and taken down from the top of the chimney in a small goods hoist.
Chris Wilson, Pile Chimney Demolition Manager, said: “The decommissioning challenges posed by the Windscale Pile reactors and chimneys are unique. It’s a complex job in terms of both radiological and conventional safety, with no instruction manual to turn to, and so many more traditional demolition techniques are unavailable to us because Sellafield is a working nuclear site with other plants and stores in close proximity to the chimney.”
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