Sellafield Ltd is demolishing the 530 tonne filter gallery before the chimney itself can be dismantled. This 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 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. 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 as the Cockcroft’s Follies – which prevented the Windscale Fire disaster from becoming a catastrophe.
Chris Wilson, Pile Chimney Demolition Project Manager: “The contaminated filters themselves were removed not long after the fire and the chimney was then sealed up to allow the radiation to decay. We’re now working to dismantle the filter gallery itself, which provided access to change and clean the filters.
“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 common in other industries but in an unconventional environment at the top of a nuclear chimney.
“Core drilling is breaking up the concrete and the rubble is then manually transferred to the ground in the small goods hoist which 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.”
Over half of the filter gallery has been demolished and it is estimated that 172 tonnes or steelwork, 66 tonnes of brick and 150 tonnes of concrete have been brought down so far. The waste is monitored to check for any contamination and most of it has been found to be suitable for disposal to the Sellafield land fill facility
Last year the chimney was opened up for the very 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 1400 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 the demolition by over 700% – which is both safer and more cost effective. This will ensure another skyline change at Sellafield by 2017/18 and the demolition of the chimney barrel a few years later.
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. There’s no instruction manual for the job and we have to prove the decommissioning techniques chosen can be used 100% safely on the congested Sellafield site.”
The chimney is 110 metres tall and there is over 5000 tonnes of materials in total to be removed during full demolition of the chimney to ground level. It was built in the late 1940s as a ventilation shaft, rather than chimney, and discharged cooling air from one of 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 then harnessed to generate electricity.
Steve Slater, Head of Decommissioning said: “Almost 60 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, that early pioneering spirit has been captured again, as our engineers and scientists are leading the field in developing and applying innovative technology and techniques to decommission Sellafield. We’re using the considerable nuclear expertise built up at Sellafield to safely bring the final chimney down.”