Cleaning up Magnox ponds18 June 2020
Three of the UK’s Magnox sites have achieved major milestones in their ponds decommissioning programme. Work has been carried out in a challenging radioactive environment to remove empty skips, sludge and other debris before draining off the water.
Above: Workers and car on the drained pond floor at Sizewell
THE SPENT FUEL PONDS AT the UK’s Magnox sites are, like the sites themselves, completely different from each other. Some have two ponds, some one pond; and some are fairly clean while others are cluttered and highly contaminated.
The entire ponds cleanup programme across the ten UK Magnox reactor sites is expected to cost around £300 million. Because of their variety the cleanup process for Magnox fuel ponds is a far cry from ‘one size fits all’, but the common understanding and toolkits developed through the Magnox ponds programme have helped to build a bank of knowledge to share across the fleet. Simple innovative processes and shared learning mean the expected cost is £45 million less than the original estimate.
Most recently, Dungeness A, Oldbury and Sizewell A have all completed comprehensive programmes to drain and clean their ponds, deploying a variety of techniques. Some were conventional; some were original and innovative. Each three-year programme cost more than £20 million. Together, their achievements are a milestone in the Nuclear Decommissioning Authority group’s corporate targets.
Magnox chief executive Gwen Parry-Jones says achieving the milestone at three sites is the culmination of many years preparatory work and demonstrates the value of collaboration across the Magnox fleet, which has accumulated valuable knowledge and expertise through the Ponds Programme. “The teams involved have shown real commitment and innovation to overcome all the challenges.”
Teams of commercial diving specialists from the US were deployed at Dungeness and Sizewell to cut up and retrieve underwater items while Oldbury’s relatively clean ponds were emptied using more standard methods
Water now remains only in the Chapelcross ponds, and the site is set to reap the benefit from the learning and hands-on experience at other sites.
Diving in at Dungeness
Divers were first used in the UK nuclear industry at Winfrith during the 1990s, to cut up steelwork 8m below the surface of the Steam Generating Heavy Water Reactor’s pond. They first entered Magnox ponds at Dungeness in 2016, cutting up equipment, including skips, and retrieving manageable- sized items from the floor of both ponds.
As water is a radiation shield, the divers were able to work close to the empty skips and other equipment, and gain access to awkward areas.
When operational, Dungeness’s two ponds, each with five bays, held 2000 cubic metres of water, which has now been drained and filtered through the single treatment plant that handles all active site water. “It’s difficult to know exactly what’s in the ponds because the equipment agitates the water, limiting visibility,” says Tony Wells, programme delivery manager. “There was, in fact more sludge and debris than we anticipated.”
Above: Jet-washing pond walls at Dungeness
“We decided to use long-reach manual tools rather than remotely operated vehicles as the team already had experience in this technique from Hinkley A and Bradwell, so were familiar with the radiological conditions, and could build on previous learning.”
After the divers had completed their work, a mechanical long-reach grab retrieved items while residual sludge was removed by an industrial-sized wet vacuum cleaner (known as the ‘Big Brute’), and pumped into shielded tanks ahead of retrieval and treatment through the advanced vacuum drying system (AVDS). AVDS dries out the waste, leaving a smaller and more stable volume for storage.
Walls and floors were cleaned and stabilised using high-pressure water jets, and radiological surveys completed.
With the target milestone achieved, the next step is to decontaminate the large steel splitter flask used to transfer fuel lugs to lug vaults for storage, before being put through a dissolution process. All ancillary equipment, such as pipework and pumps, will be removed before the ponds are ready to hand over to the plant and structures team for eventual demolition.
Sizewell turns to submersible
Sizewell’s single pond, with five bays, contained 3300 cubic metres of water. When the divers left Dungeness, this is where they came, bringing their learning. Over 10 months, they cut up 35 skips and 70t of ponds equipment, collected miscellaneous contaminated and activated items and pumped most of the sludge (around 4 cubic metres), to a holding tank.
A conventional hoist removed size-reduced pond equipment, which was packaged and handed to the waste operations team.
A small remotely operated submersible vehicle, acquired from Bradwell, assisted with inspecting the pond structure and equipment, helping the team understand what was needed for decommissioning. In-house repairs to the vehicle focused the team on maximising its capabilities.
Above: The customised ROV underwater in the pond at Sizewell
The vehicle, a VideoRay Pro 4, was replaced with the more powerful the new-to-the-market Defender, which could lift up to 25kg. The team began experimenting with interchangeable sludge removal tools, which were designed and manufactured in-house before testing in a local dive pool.
“We were conscious that once in the water, it had to be just right. We bought a collapsible pool, filled it with sand and sludge to continuing testing. We linked it to a pumping system, and it worked like a dream,” says project engineer Nick Joy.
Ponds programme delivery manager Steve Franks adds: “The achievement is a tribute to the team’s innovative approach and determination to succeed.”
The remaining sludge was pumped into a storage tank and will be retrieved and dried out in ductile cast-iron containers for transfer to Bradwell’s Intermediate Storage Facility. Joy says, “These were new on the market and we’ve customised them to retrieve sludge and debris from spent fuel ponds. They’re now ready to use elsewhere, including Sellafield, Chapelcross or further afield ... anywhere really!”
With all pond bays drained, work is under way to decontaminate and stabilise walls and floors.
Oldbury benefits from lessons learned
The pond at Oldbury, one of the youngest Magnox sites, is smaller than Sizewell, with an Olympic swimming pool of water (2500 cubic metres) and has benefitted from the operational experience of the older stations, with pond conditions well controlled during its generating life.
A variety of wastes have been removed and disposed of since 2016. Along with 40t of redundant equipment, the project also processed highly contaminated IONSIV cartridges and filters from Dungeness and Sizewell that had been consolidated with those at Oldbury. The items were remotely packaged into self-shielded Mosaik containers. By working closely with the central waste team, the number of Mosaiks required was reduced from 24 to 10.
As divers proved to be of most value in large retrieval or size-reduction campaigns, involving equipment attached to pond floors, Oldbury opted to size-reduce skips in air using bespoke steel shielding designed by the in-house ponds team and manufactured locally. The shielding protected workers as they cut up skips, while a ventilation system extracted airborne contamination.
Above: Cutting up one of the pond skips at Oldbury using tailor-made steel cover
Slots in the shielding allowed a saw to fit through and cut the skip underneath, while a bracing mechanism prevented the sides from collapsing when the shielding was removed. “Our methods were simple and many tools we deployed had been developed and used on the Hinkley ponds, and were suitable for Oldbury,” says, ponds programme delivery manager Mark Liston. One of the successes was a miniature electric car, available for a few hundred pounds, that was fitted with a remote dose monitoring system to check conditions on the drained pond floor before people were allowed in.
The pond basin is now being decontaminated by pressure washing and being surveyed to ascertain residual radiological conditions.
About Magnox: Magnox Limited, owned by the Nuclear Decommissioning Authority, manages and operates 12 nuclear sites and one hydroelectric plant in the UK. It is responsible decommissioning of Berkeley, Bradwell, Chapelcross, Dungeness A, Harwell, Hinkley Point A, Hunterston A, Oldbury, Sizewell A, Trawsfynydd, Winfrith and Wylfa.
All photos courtesy Nuclear Decommissioning Authority