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In Focus: Pile Fuel Cladding Silo

The PFCS is Sellafield’s oldest nuclear waste storage facility

The Pile Fuel Cladding Silo is Sellafield’s oldest nuclear waste storage facility and will be the last of the four ‘legacy’ ponds and silos to start the process of reducing the nuclear and environmental risk by getting the waste out of the old building into a safer place. Although there’s been a huge amount learned the hard way on how decommissioning needs to be considered right at the outset of planning and construction, in many ways the team’s ‘lead and learn’ early retrievals approach of today harks back to the ‘simplicity first’ ethos used when it was built.

It’s fair to say that anyone building an Intermediate Level Waste storage facility today wouldn’t look to a 1940s North American grain silo for engineering and architectural inspiration. But that’s what happened with the Pile Fuel Cladding Silo when it was designed and built in the aftermath of the Second World War. The rush to secure Britain’s place at the world’s atomic weapons table in the Cold War was paramount, meaning the solution for dealing with the waste and by-products of plutonium production was way down the pecking order. Put frankly, storage solutions that were ‘agricultural’ in every sense of the word had to be good enough at the time.

“The people who built the Pile Fuel Cladding Silo did extraordinary things in incredibly short timescales. Their mandate was to develop the first nuclear weapons, not to consider the long-term effects of radioactive waste storage,” said Gary Snow, head of the silo programme. “Anywhere in the world where there was a post-war weapons programme has the same issues: waste storage buildings which weren’t really built to last; poor and inconsistent record keeping on the contents; and no real thought given to eventual retrievals.”

Gary’s team is delivering the programme of emptying and decommissioning the building used as the UK’s first ever storage facility for intermediate level waste. He says if he could go back in time and say anything to these nuclear pioneers, it would be “make the building more robust and keep good records”. But, without the luxury of time travel, Sellafield has instead had to carefully manage and monitor the facility.

In the 1980s efforts were stepped up to upgrade and fortify the building which had been exposed to the Cumbrian weather for nearly 40 years. As well as repointing and rendering, the building has been strengthened by huge supporting bands wrapped around it, like a girdle around the waist of this pensioner of the plutonium age. Other crucial upgrades involved reducing the risk of fire by replacing the air in the silo with the inert gas argon. But a facility’s life can only be prolonged for so long and the focus is now firmly on getting the waste out. This means us, the Nuclear Decommissioning Authority and the nuclear regulators all having an understanding and acceptance that the retrievals process does involve an elevated state of risk, but that is more acceptable than the risk of leaving the waste in there any longer.

“The time for studies and assessments is over. It’s now time for safe action,” said Gary. That ‘action’ will involve getting the silo ready for retrievals to start and installing the retrievals machinery in the reinforced concrete ‘superstructure’ which has been custom-built right next to the silo. The business plan used in the current Sellafield Plan from 2014 predicts that retrievals will start in 2022 and the overall cost of the programme will be £904 million. However, a new plan currently being agreed hopes to bring forward the start of retrievals significantly and also reduce costs.

A key way of delivering more quickly and efficiently is to do things as simply as possible. “We’ve learned from other decommissioning programmes, particularly the Pile Fuel Storage Pond, how productive and effective the ‘decommissioning mindset’ can be. This means that rather than plan and mitigate for every single outlier or surprise you can think of, you’re better off getting on with the job as safely as practicable and dealing with challenges as they arise. We are also trying to make processes and equipment as simple as they can be right across the programme,” said Gary.

Pile Fuel Cladding Silo

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An early glimmer of this strive for simplicity was shown at the beginning of the millennium in the challenge of removing the waste which was blocking up the transfer tunnel at the top of the facility. Ideas were sought for how best to dislodge the tunnel’s excess waste into the compartments below. Diagrams were drawn, calculations made, engineering options considered. Yet through this haze of emerging complexity, one of the workers proffered up a low-tech, high-innovation option of simply using a large steel pole and ‘poking’ the waste so it dropped into the compartment below. It worked and was a lesson in the power of intuitive and simple thinking from our workforce – and a warning against over-engineering and unnecessary complication.

The team is using this learning today to drive the new ‘lead and learn’ approach through early retrievals from a single compartment rather than all six simultaneously. “We were originally going up there and building a ‘Rolls-Royce’ solution for all six compartments. That meant building a very complex system that took account for every single outlier that you didn’t yet know was actually going to be necessary. The sooner we start retrieving waste, the sooner we will have a view of how it actually behaves and also what the structural integrity is like. We’re going to go up there and hopefully prove that you can do this simply,” said Gary.

FACTFILE
  • The silo is 21m high and its six individual waste containment compartments hold over 3,400 cubic metres of intermediate level waste, the equivalent of 30 double decker buses.
  • It has been storing radioactive waste since 1952 – the same year that Queen Elizabeth II ascended to the throne.
  • Originally, the silo had no dedicated personnel assigned to it, and the facility sending the waste would arrange for its own personnel to tip the waste. This means there are big variances in the quality of information on what has been put there and Sellafield has conducted scores of interviews with now-retired staff to build intelligence on its contents.
  • It is situated in a highly-congested area of the Sellafield site, surrounded by a maze of active pipelines and similarly sensitive buildings. This makes it very challenging to carry out decommissioning work. Even the installation and use of a crane is a highly-detailed project requiring extensive safety assessments.
  • The superstructure built next to the silo to house the retrievals equipment is strong enough to withstand a seismic event.
£0904m

Overall cost of the programme

How are we getting the waste out?

In simple terms, getting the waste out of the Pile Fuel Cladding Silo involves cutting six holes in the side of the building, extending a telescopic boom and grabber into the silo, grabbing and lifting the waste so it can be loaded into a 3m3 metal box, putting the box into a shielded flask and then transporting it away. Sounds easy. But no humans would survive inside this environment, so everything has to be done remotely, and the silo’s needs to maintain the argon atmosphere inside, (even through hole cutting) to eliminate the risk of fire.

Highly-engineered containment doors will be attached to the side of the building and placed into the “open position” before penetrations are made to enable access to the waste inventory. As the holes are cut, the removed concrete monoliths will be placed into sealed bags to maintain contamination and atmospheric control, before the doors are closed. The process of grabbing the waste and loading it into the metal boxes also requires reliable and safe engineering solutions. The telescopic boom and grab will be deployed through the new doors in the side of the silo. This grab will pick up the waste from within the silo, retract and lower the grab full of waste into one of 2,200 3m3 metal storage boxes, which will be lidded, bolted, swabbed, placed in a flask, and exported prior to treatment and final storage.

The waste is being sent to the Box Encapsulation Plant Product Store – Direct Import Facility, a new above-ground facility which will import and store the nuclear waste until its planned final destination – a Geological Disposal Facility deep under the ground. The above ground store alone is a £267m programme, due for completion in 2019, illustrating the size and scope of just one of the enablers needed to deal with the legacy of the past.

We’ve learned from other decommissioning programmes, particularly the Pile Fuel Storage Pond, how productive and effective the ‘decommissioning mindset’ can be

Pile Fuel Cladding Silo Timeline

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