Windscale chimney decommissioning nears final phase 16 April 2014

The Windscale chimneys – a major part of the Sellafield skyline since the 1940s – are now close to final demolition, with the last remaining filter galley now half way through decommissioning.

Sellafield is demolishing the 530 tonne filter gallery before the chimney itself, which worked as a ventilation shaft for Windscale Pile One, can be dismantled.

Windscale Pile One caught fire in 1957 and the filters fitted at the top of the chimney prevented much of the radioactive contamination escaping to the local area.

Nobel prize-winning physicist John Cockcroft famously insisted that the Windscale Pile chimneys be fitted, at great expense, with high-performance filters.

It is these iconic bulges at the top of the structures, known as the Cockcroft's Follies – which prevented the Windscale Fire disaster turning into a catastrophe – that are now on their way out.

"The contaminated filters themselves were removed not long after the fire and the chimney was then sealed to allow the radiation to decay," explains Chris Wilson, pile chimney demolition project manager.

"We're now working to dismantle the filter gallery itself, which provided access to change and clean the filters," he continues.

"The filter gallery is a large, robust structure built of steel, brick and concrete at the top of the chimney... We're using conventional demolition techniques but in an unconventional environment at the top of a nuclear chimney."

Wilson explains that core drilling is being used to break up the concrete, while the rubble is then manually transferred to the ground in the small goods hoist that runs up and down the chimney.

"We can only take down one tonne of rubble at a time, due to the lift capacity. So, doing the maths, this means hundreds of trips up and down the chimney."

It is estimated that 172 tonnes of steelwork, 66 tonnes of brick and 150 tonnes of concrete have been brought down so far. The waste is monitored to check for contamination and most of it so far has been found to be suitable for disposal to the Sellafield land fill facility.

Last year the chimney was opened up for the first time in over 17 years and the filter dismantling access gantry was pulled apart to open the chimney to the elements in preparation for dismantling.

The plan is that the filter gallery will be fully dismantled by October this year.

The next stage is to remove the diffuser, which is the box-like structure sitting below the filter gallery. Preparations will be made next year to erect a tower crane next to the chimney and the 1,400 tonne concrete diffuser box will be cut up, using diamond wire, and lowered down in 7-tonne blocks.

This new approach is estimated to improve the efficiency of demolition by more than 700%. It is also safer and more cost effective.

Brian Tinham

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