The Pile Fuel Storage Pond at Sellafield holds several accolades. Not only was it the very first nuclear fuel storage pond built on the site in the 1940s, it is also the largest single, open-air nuclear fuel storage pond in the world today.
Paul Nichol, Head of the Pile Fuel Storage Pond, reflects on what makes one of Sellafield Ltd’s priority clean-up projects so special. He said: “There’s a real sense of history about the place. We’ve all seen the lovely old photos of the pond being built and it really engenders a sense of pride. The nuclear pioneers who designed and built the pond didn’t have any blueprints to copy and worked to unbelievable short timescales, so that they could produce nuclear materials for defence purposes.”
Paul added: “It was really an amazing feat of science and engineering to build the Windscale suite of buildings in less than five years. I am proud to see that our workforce today has recaptured some of this old pioneering spirit, and are coming up with all sorts of innovative solutions for the incredibly difficult job we’re faced with cleaning-up Sellafield.
“Decommissioning the Pile Fuel Storage Pond involves emptying it of all of the radioactive materials and redundant plant and equipment so that we can drain the pond water and finally demolish the structure.
“It might sound simple but it is a complex job that will see us remove around 20,000 individual items from the pond. Record keeping during the pond’s early operations was poor but It’s reckoned that there are 1000s of different types of wastes in the pond and these all have to be removed and a home found for each one. The home for most of the wastes will be one of the modern, nuclear stores specially built to store the waste safely above ground. However, some of the waste is what is rather quaintly called orphan wastes, which means it doesn’t have a home to go to. New waste treatment plants and stores are being designed and built for these wastes.
This waste is being turned into solids when they are currently liquids or sludges, and are being placed into large, robust containers and immobilised – which means that the waste can’t spill, leak or run away. It has to be solid, safe and stable for long term storage above ground and suitable for final disposal.
What are the main wastes in the pond?
The majority of the canned fuel in the pond hailed from the Windscale Advanced Gas-cooled Reactor – the iconic golf ball structure in the Sellafield skyline – and was put into the pond in the 1960s. There is also a range of exotic fuels from test and research reactors in both the UK and abroad which had accumulated in the pond over its 65 year lifetime.
The last of the 191 cans of fuel was taken out of the pond in October 2015, a key milestone that means the radioactive inventory of the pond has now been halved. Paul added: “Getting this 2.5 tonnes of canned fuel out has also effectively eliminated the criticality risk associated with the pond – which means there is no chance of an uncontrolled nuclear chain reaction in.”
The National Nuclear Laboratory provided technical support, remotely cutting the fuel out of old cans in special cell within the facility where physical access is not allowed because of radiation levels. The fuel was then identified and assessed before being placed into new containers to keep the fuel safe in a more modern storage pond.
Nick Hanigan, director for Waste Management and Decommissioning at the National Nuclear Laboratory, said: “This project is a fantastic example of how we and Sellafield Ltd are working together to accelerate decommissioning and reduce risk on the Sellafield site. The past few years have been a showcase of how integrated team working can drive innovative solutions and flexible working. I’m so proud for NNL to have been a key part of this success story.”
Andy Lindley, director of the Office for Nuclear Regulation’s Sellafield Programme said: “We are encouraged at the progress made by Sellafield Ltd and National Nuclear Laboratory to remove and process canned fuel from the pond. The remediation of fuel and sludges from the legacy ponds has been gathering pace during 2015 and the success in removing canned fuel is a key milestone in the long term goal of achieving risk and hazard reduction at the site.”
In the solids category there was originally 750 tonnes of contaminated metal wastes, much of what has been lurking at the bottom of the pond and its associated bays for over 60 years. This included various pieces of installed equipment, machinery, pond furniture and fuel skips that are red with rust; so far 180 tonnes have been removed and cleaned up wherever possible.
This contaminated and redundant kit is being progressively removed from the pond and the first of two large fuel decanners, weighing in at 6.5 tonnes each, has recently been successfully lifted out of the pond. This underwater giant was used to strip off the fuel cladding and had to be cut into two using an underwater diamond saw before this hulk of rusty, contaminated metal could be lifted clear of the pond.
This group of material also includes 10 tonnes of non-fuel nuclear wastes including Calder fuel end cones and cobalt cartridges, and a right old cocktail of isotopes, radioactive cartridges, nuclear ‘pennies’ and so-called nuclear ‘ammunition boxes’ containing various radioactive materials. This is one of the most difficult groups of historical materials to find homes for within existing facilities at Sellafield.
One of the big successes has been with the 800 cobalt isotopes, which are being re-packaged using mini submarines that can literally grab the end of the one metre long cartridges and lift them into new skips to prepare them for shipping out. This delicate operation is done using surgical precision by the submarine pilots who work remotely, minimising their exposure to radiation.
Paul said: “One of our targets this year was to remove 50 tonnes and so far 10 tonnes of solids have been consolidated into containers. In just one shift we recently consolidated one quarter of a tonne, which is quite some feat considering the mini submarines can only lift 10kg at a time.
“In the past this sort of task would have been carried out manually with the workforce dressed in PVC and using tools to move the material from the side of the pond. The use of mini-submarines has lifted this burden. We have calculated that the mini submarine has taken a 20 sievert dose in the last 12 months, which is five times more than the combined Sellafield workforce dose of 4.3 sieverts – so they really are a vital part of our team.”
It is estimated that there is 350m3 of radioactive sludge at the bottom of the seven metre-deep pond. Getting this radioactive sludge – made up of fuel corrosion products and algae that have built up – out of the pond at the same time as leaving the water in place to shield the nuclear fuel is just one of the challenges that the team is facing and overcoming.
Sludge has been progressively pumped from the bottom of the pond into an under-water corral. This temporary solution was not 100% effective as it proved difficult to stop the sludge floating out of the corral and back onto the floor of the pond. The construction of a new building alongside the pond will allow the team to pump the sludge out of the pond and into engineered drums so that it can be treated in other buildings on the Sellafield site.
Paul said: “The new building has an important job to do and the team has made best use of fit-for-purpose technology already in use by other industries, proving that Sellafield’s unique challenges do not always need a unique solution.
“Using a simplified petrol-pump style design, the drum filling plant will safely export the sludge from the pond while also saving the UK taxpayer more than £50m compared to original bespoke designs, and will accelerate sludge retrieval by more than three years.”
Remotely operated mini submarines have provided a valuable underwater help towards consolidating the remaining 12 tonnes of metal fuel in the pond into skips to prepare it for export.
Paul is confident that the team has almost conquered the metal fuel challenge. He said: “By the end of March 2016 we aim to have removed all of the remaining metal fuel, meaning that over 70% of the pond’s radioactive inventory will have left the building.”
This category includes the residual solids and sludges that will be left after the bulk retrievals have been completed. Some of it is orphan wastes and it won’t be easy to access as it is likely to be what is left at the bottom of various process bays within the pond. Current plans indicate that remote-controlled submarines will be used to access the darkest corners of the pond to remove the final wastes before the water is drained out of the pond.
Other nuclear materials produced by Windscale included:
• Cobalt cartridges – produced for external beam radiotherapy, sterilisation of medical supplies and medical waste, radiation of foods for sterilisation and industrial radiotherapy of welds.
• Caesium sources – kilocurie radioactive sources were manufactured for medical uses to treat cancers using radiotherapy.
• Plutonium isotopes – radioactive material was produced for the Department of Health to power batteries used in early pacemakers.