Main contractor Nuvia is using a 25-tonne self-climbing platform held up by mere friction to demolish a chimney that rises 122 m above the ground.
Project: Primary separation plant chimney
Contract value: £45m
Main contractor: Nuvia
Access subcontractor: Delta International
Start date: November 2016
Completion date: September 2020
The Sellafield nuclear site in Cumbria has had an eventful history.
Made up of the Windscale and adjacent Calder Hall sites, the whole 6 sq km site was renamed Sellafield in 1981.
It became the world’s first atomic energy power station when it was commissioned in 1956. However, in October the following year Windscale’s first reactor – known as Pile 1– overheated, caught fire and released radioactive waste in what remains Britain’s worst nuclear accident.
Despite this difficult start, Sellafield remains a unique site that was at the forefront of the nation’s atomic ambitions during the cold war. As well as providing domestic energy to the grid, the site also produced weapons-grade plutonium for the UK’s nuclear arsenal during the height of the atomic arms race with the Soviet Union.
During this time of military posturing, many of the facilities were built rapidly with little thought as to how they would be decommissioned and demolished. As a result, Sellafield is now home to the largest inventory of untreated nuclear waste in the world – the Nuclear Decommissioning Authority spends around £2bn of its annual budget slowly decommissioning one facility at a time.
With no blueprint as to how the facilities on the site should be decommissioned, today Sellafield is still awash with cutting-edge innovation – albeit for very different purposes.
No ordinary chimney
The latest part of the site to be decommissioned is a 61 m-high chimney, which sits on top of a 10-storey radioactive building that is also 61 m in height.
The chimney is made up of 350 tonnes of concrete and has a stainless steel interior liner. Its diameter measures 5.9m at the base that gradually tapers to 4.6 m at the top, with a thickness of 150-250 mm.
Sellafield Ltd, which operates and manages the decommissioning of the site on behalf of the Nuclear Decommissioning Authority, identified the chimney as having a high risk profile. The structure failed seismic assessments due to its age and height, leaving it susceptible to minor earthquakes.
“Delta had a rudimentary climbing platform. We took that original concept and adapted it into a one-off bespoke access platform”
John Ball, Nuvia
When Sellafield put the contract for its demolition out to tender nearly a decade ago, it had already consulted on the best way to complete the job, reaching the conclusion that a self-climbing platform would have to be used.
The client eventually chose Nuvia as its main contractor for the project. A specialist in the nuclear, defence, oil and gas industries, Nuvia came equipped with prior experience of nuclear decommissioning projects.
Part of the reason Sellafield handed the work to Nuvia was that the contractor had conducted a global search of chimney demolition companies. Nuvia proposed teaming up with work-at-height specialist Delta International, which had developed a self-climbing platform to scale the large chimneys at Battersea Power Station.
“Delta had a rudimentary climbing platform, which was scaffolded together,” explains Nuvia operations director John Ball. “We took that original concept and adapted it into a one-off bespoke access platform.”
Working at Height: Special Report
The team got to work in 2011 redesigning the original two-storey platform that had been used at Battersea. The redesign, manufacture and testing of the new three-storey self-climbing platform took 13 months.
Following a delay in the project after this first stage, work eventually recommenced in 2014.
Nuvia built a replica 18 m dummy chimney on which to test the platform, which has three storeys that wrap around the circumference of the chimney. It also has a load-bearing roof that offers weather protection and supports a centrally positioned suspended platform.
The whole platform weighs 25 tonnes, is 8.5 m in height and 12 m in diameter, with the platform’s working areas measuring 1.4 m wide.
Sellafield chimney Nuvia Delta 006
“The material that is actually held between the chimney and the rig is rubber,” Mr Ball says. “I know it sounds bonkers but it’s true – it’s held by friction. Clearly the tension of the actual band is that tight that it works. We did trials on what is the best material to use in all weather conditions and rubber was best.”
The platform has 84 rubber pads, each measuring 361 x 114 mm. All 84 pads press up against the chimney when the platform is stationary; only half of them press against the chimney when it is in motion.
The pads are held in place by a series of bands, which are split into upper and lower sets on each level of the platform.
When the platform is moving up, the upper set of bands are loosened, raised and tightened. The two lower sets of bands are then loosened, raised and tightened.
“We decided to drill out cores through the concrete structure. The reason for drilling the cores was to prevent any crack propagation”
John Ball, Nuvia
This allows Nuvia to climb up the chimney metre by metre.
Delta pioneered the self-climbing solution, which uses a radial array of linear actuators to clamp the steel bands to the chimney, and moves the arms attached to the bands up the platform.
“It is a working-at-height access solution which takes away the need for something like scaffolding,” says Delta director Eleanor Hill. “This is a bespoke solution. It is a platform from which they can demolish.”
Construction of the self-climbing platform was completed in May 2016.
As well as designing and manufacturing this one-off access solution, Nuvia also had to consider the best method for demolishing the concrete and steel chimney while 122 m above the ground.
Sellafield chimney Nuvia Delta with diagram
Compressed air and water-related demolition methods were ruled out due to the difficulty of getting them up to the top of the chimney. Concerns around radioactive dust and contaminated water escaping from the platform meant these methods also posed further risks.
“It was determined that a dry cut system would be used,” Mr Ball says. “We decided to drill out cores through the concrete structure. The reason for drilling the cores was to prevent any crack propagation.”
The stainless steel liner in the chimney is being dismantled in segments with a plasma cutter.
Getting the job done
The self-climbing platform was assembled at the base of the chimney and ready to start its 61 m ascent in November 2016 – five years after the original design work began.
Climbing one metre at a time, the platform reached the top of the chimney nine months later in August 2017, having had to accommodate weather delays and maintenance of the platform as it climbed the structure.
Nuvia started the demolition programme in January this year, and is targeting a demolition rate of 1 m every seven days. However, this can only be achieved if the demolition team has a clear seven-day run at the work.
Sellafield chimney Nuvia Delta DSC09071
Unfortunately for the operatives, challenges presented by Cumbria’s weather and the platform’s maintenance requirements have meant this hasn’t always been the case. Now six months into demolition, Nuvia is only two cycles away from reaching the first major milestone on the job.
“[There are] different safety-critical points,” Delta’s Ms Hill explains. “The 47 m mark is the first of these points, where it comes down from category A to category B. They are two cycles off from moving from category A to category B in terms of criticality. So that means it’s not so high profile. This will be a big moment on site because it’s the first demolition milestone.”
“The platform itself is three levels, so there will be a stub left because when the platform is grounded, there will be a point where you can’t work on the platform anymore”
John Ball, Nuvia
Work is due to complete in September 2020, but how the final stages of the project will be completed is yet to be decided. “The platform is going to go right down to the base of the chimney,” Mr Ball says. “But bear in mind the platform itself is three levels, so there will be a stub left because when the platform is grounded, there will be a point where you can’t work on the platform anymore.”
Nuvia’s contracted terms are to demolish the chimney down to the final 9 m. Mr Ball says the final leg of the job could be completed using conventional scaffolding to provide access to the operatives.
Whatever shape the chimney takes at the end of 2020, it will mark the end of a decade-long journey to demolish just one part of Sellafield, highlighting the sheer complexity of decommissioning a radioactive structure – especially one that is 122 m in the air.
This article has been amended to specify that the platform’s bands are made of steel