Diffuser removed from Windscale Pile Chimney

15 November 2021

After three years of careful dismantling, the square-shaped diffuser has now been removed from the top of the Windscale Pile Chimney on the Sellafield site in Cumbria, UK. The achievement has removed the seismic risk associated with the chimney, the remainder of which will now be dismantled.

Removal of the diffuser has reduced the height of the stack by over 15 metres (Image: Sellafield Ltd)

Two 125-metre-tall chimneys providing ventilation for the Windscale piles - early reactors built to produce plutonium for the UK's nuclear weapons programme - were sealed after Pile One was damaged beyond repair in a 1957 fire. The Windscale chimneys, with the characteristic bulge of the filters at their top, remained a feature of the Sellafield skyline until 2001 when the chimney servicing Pile Two was reduced to the height of adjacent buildings.

The chimneys were built in the late 1940s and early 1950s, and were retrofitted with high-performance filters at the insistence of Nobel prize-winning physicist John Cockcroft. The filters quickly became known as 'Cockcroft's Follies', but proved their worth when the filter at the top of Pile One's chimney trapped most of the radionuclides released when the reactor caught fire in what was, at the time, the world's worst nuclear accident.

The contaminated filters themselves were removed not long after the fire, and the chimney sealed up to allow radiation to decay. It was reopened in 2013 to allow demolition work to commence, starting with the removal of the filter gallery, a 530-tonne structure of steel, brick and concrete. Conventional demolition techniques were used, breaking up the concrete and rubble by core drilling. The rubble was then transferred to the ground, one tonne at a time, in a small goods hoist.

Demolition of the diffuser started in December 2018, with the first block being removed in January the following year. The diffuser had to be cut up in-situ, more than 100 metres above ground. A total of 226 blocks - each weighing between 2 and 9 tonnes - have now been removed from the top of the chimney stack.

The sections of the diffuser structure have been cut by machine using diamond wire saws. These cut blocks were removed from the top of the stack and lowered to the ground using a tower crane. The blocks were then remediated and transferred to the bagging station on the Sellafield site where a Goliath crane was used to manoeuvre the blocks ready for transport to interim storage prior to disposal. This was either to the Calder Landfill Extension Segregated Area on the Sellafield site or the Low Level Waste Repository at nearby Drigg, dependent on contamination levels and conditions for acceptance at each location.

Sellafield Ltd, which is carrying out the work on behalf of the Nuclear Decommissioning Authority (NDA), said work is now taking place to remove a metal frame platform and reduce the access shaft. Once that work is complete, a key delivery milestone will be met. Following this, work will start on demolishing the chimney barrel.

"Removing the seismic risk is a huge achievement," said Geoff Carver, Sellafield Ltd project manager. "This is a clear demonstration of progress towards our purpose of delivering a clean and safe environment for future generations.

"Many people have contributed to this success. Sellafield Ltd employees have worked closely with their colleagues from our partners ADAPT, Doosan, Mammoet, KDC, Kaefer, the Design Services Alliance, and Nuvia."

"This is an excellent example of the pioneering work taking place on our sites to deal with the legacy of the earliest days of the nuclear industry," said NDA Chief Executive David Peattie. "In reaching this significant milestone of removing the Windscale Pile Chimney diffuser, we take another step forward in our clean-up mission."

Researched and written by World Nuclear News