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The 110m structure the scene of Britains worst nuclear accident will begin to disappear later this year.
A giant crane has been constructed to bring it down.
The 152m crane is the tallest structure ever built at Sellafield, just six metres shorter than the Blackpool Tower.
It will begin work this autumn, removing and lowering chunks of the chimney cut out using diamond wire saws.
Duncan Thompson from the Nuclear Decommissioning Authority said:
The complex task of decommissioning and demolishing the Windscale Pile One stack has reached an important stage.
It is another example of the ingenuity that goes into solving the UKs decommissioning problems.
Once demolition begins it will be a very visible demonstration of the work being done to make Sellafield a safer place.
The Windscale Pile One chimney, with its distinctive top-heavy appearance, has dominated the Sellafield skyline for nearly 70 years.
Famously, its filtration system was a last-minute addition, placed unusually at its summit.
Despite being mockingly referred to as Cockrofts Folly, after its designer Sir John Cockroft, it turned out to be a masterstroke.
In 1957, fire broke out in the Windscale Pile One reactor. The sky-high filters captured an estimated 95 per cent of the radioactive dust created.
Now, the chimney is a decommissioning project.
Teams from Sellafield Ltd and its supply chain are working together to safely pull it down.
The first piece to go will be the square-shaped diffuser at the top.
Because buildings containing nuclear material surround the stack, traditional demolition techniques like explosives cannot be used.
George Frost from Sellafield Ltd, said:
Were making visible progress on this demolition, and it wont be long now until we start to see the diffuser removed.
The chimney is one of the iconic legacies of Sellafields past, so the skyline change as the chimney is removed will be significant.
This has been a challenging piece of work, so everyone is pleased to see work progressing. This is thanks not only to the Sellafield and supply chain