The Windscale chimneys have been a major part of the UK's Sellafield skyline since the 1940s and work on decommissioning the last remaining chimney is now progressing in earnest.
The chimney has been opened up for the very first time in over 17 years, which is a tangible sign that real decommissioning progress is being made. The filter dismantling access gantry, which is made up of 52 tonnes of structural steel work, was pulled apart to open the chimney to the elements in preparation for dismantling.
Jeremy Hunt, Head of Decommissioning Projects said: “The decommissioning challenges posed by the pile chimney are unique and no other structure in the world provides the same complexity in terms of both radiological and conventional decommissioning constraints. Bringing the chimney down will be a real visual demonstration of our commitment to cleaning up Sellafield.”
The original two pile chimneys were built at Windscale between 1947 and 1950. They were actually ventilation shafts, rather than chimneys, and discharged cooling air from the Windscale Pile Reactors. The Piles were conceived in the aftermath of the war, as part of the drive to develop a nuclear deterrent. Their primary purpose was to provide nuclear materials and the heat generated was released to atmosphere rather than harnessed to generate electricity.
The remaining pile chimney worked as a ventilation shaft for Windscale Pile One which caught fire in 1957 and the filters fitted at the top of the chimney prevented much of the contamination escaping to the local area. The decision was then taken to shut down both Windscale Piles.
Nobel prize-winning physicist John Cockcroft famously insisted that the Windscale Pile chimneys be fitted, at great expense, with high performance filters. Since this was decided after the stacks had been designed and partially built, they produced iconic bulges at the top of the structures - known as the Cockcroft’s Follies – which prevented the Windscale Fire disaster from becoming a catastrophe.
Steve Slater, Head of Decommissioning said: “Over 50 year after the Windscale Pile reactors ceased operation, the familiar landmarks of the West Cumbrian skyline are disappearing. The chimneys were a real technical achievement in terms of construction, which minimised the effect of the fire in 1957 and are testament to the nuclear pioneers who built them.
“Today, we’re using the considerable nuclear expertise built up at Sellafield to safely bring the final chimney down. The plan is to remove the filter gallery by the end of next year and then the chimney diffuser by 2018 to meet the requirements of our customer the Nuclear Decommissioning Authority. 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 major demolition of the filter gallery structure. Some 66 tonnes of brickwork have been removed from the filter gallery external walls and all of this has been transferred from the top of the chimney in a small goods hoist which runs up and down the outside of the chimney.
Chris Wilson, Pile Chimney Demolition Manager: “It’s taken many years of real effort and energy to develop a robust, safe and effective plan for the chimney demolition. For the first time in decades, we are able to confidently progress the safe dismantling and demolition of this historic and iconic UK nuclear legacy.
“There have been many significant challenges to overcome in preparing for the physical demolition of the chimney, not least coming up with an agreed plan with the neighbouring nuclear plants on the congested Sellafield site. We are also working hard to put in place a sizable workforce with the right skills to carry out the job.”
The chimney is 110 metres tall and there is approximately 500 tonnes of structural materials including steel work, bricks and masonry in the filter gallery section of the chimney, with over 5000 tonnes of materials in total to be removed during full demolition of the chimney to ground level.
The two chimneys were identical in structure comprising a 14 metre diameter reinforced concrete structure.
They consisted of seven major sections: foundations; main shaft; diffuser; filter section with plant rooms; concentrator section; upper section and access shaft.
Following the fire, the chimneys were sealed at the top, the contaminated filters removed and air inlet ducts isolated. Radiation levels had to reduce significantly before decommissioning work could start. The first chimney was reduced to the level of the adjacent reactor building in 2001, however the second chimney posed more of a challenge due to radioactive contamination from the fire.
Decommissioning progress on the remaining chimney has also involved: stripping out the thermal insulation lining; taking down the concentrator; dismantling the filter in-fills; removing the 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.
A combination of manual and semi remote tools and techniques will be used to provide the safest demolition approach. This will include concrete and steel cutting techniques, lifting capability through cranes and standard lifting apparatus, climbing platforms commonly used to access chimney stacks.
For more information, visit www.sellafieldsites.com