Underwater Construction Corporation (UCC) UK Ltd (a subsidiary of US-based UCC) says it has achieved a significant decommissioning milestone at the UK Sellafield site. In December 2022, UCC diver Josh Everett became the first person in over 60 years to descend the ladder into Sellafield’s Pile Fuel Storage Pond (PFSP) to help complete clean-up work in the site’s oldest legacy pond.
The PFSP, one of six such ponds onsite, was constructed between the late 1940s and early 1950s. It was used for the cooling, storage, and de-canning of irradiated fuel and isotopes from the two Windscale reactors as part of the UK’s post-war atomic weapons programme. it was in full operation until 1962. Until the mid-1970s, the open-aid 100-metre-long outdoor pond was then used for storage of intermediate-level waste and fuel from the UK nuclear programme. During those years, sludges formed from the decaying nuclear fuel, algae and other debris. Sellafield now faces the huge task of cleaning up the legacy of the site’s early operations. The PFSP is one of Europe’s most hazardous nuclear facilities.
UCC UK Ltd, in partnership with Sellafield Ltd, spent more than four years developing customised procedures, processes and equipment to optimise safety and working conditions for its nuclear divers while minimising exposure to radioactive materials. Dive trials were conducted in an offsite facility that closely mimicked the PSFP, utilising specially developed dive platforms, CCTV monitoring, diver manipulated long reach tooling, cutting equipment, and radiation monitoring equipment.
According to Sellafield Ltd and the Nuclear Decommissioning Authority (NDA), “the last time a human entered PSFP was in 1958 when records show a maintenance operator and health physics monitor carried out a dive into the newly constructed pond to repair a broken winch.”
Sellafield says that, in the last decade significant progress has been made in retrieving and safely storing fuel, bulk sludges and redundant equipment from the pond. However, it remains one of the most complex decommissioning challenges in the world and a priority project for Sellafield Ltd, NDA and the Office for Nuclear Regulation (ONR). While significant progress has been made in removing historic waste from the facility with more than 75% of the waste safely taken out so far, divers were needed to help extract the remaining inventory.
Divers entered the pond via a specially installed access platform and position themselves on a metal dive stand with a shielded floor. From here they were able to work in shifts of up to 3.5 hours at a time to retrieve sludges and debris from the pond floor, corners and other hard-to-reach areas. Josh and his team have years of experience in safely diving in radioactive environments around the world, including at Dungeness A and Sizewell A in the UK. Lessons learned at these Magnox sites have been applied to the team’s work at the PSFP.
Carl Carruthers, Sellafield Ltd head of programme delivery for legacy ponds, said the PFSP diver project was five years in the planning “and has been a huge success”. He added: “Safety has been our priority throughout, and the divers are monitored and communicating with the dive supervisor at all times. Their work has helped us make real progress in cleaning up the pond and our site mission to deal with the nuclear legacy and create a clean and safe environment for future generations.”
Once all of the waste materials have been removed from the pond the water will be removed, ready for final demolition of the building. Current plans estimate the decommissioning work will cost around £212m ($14.9m) and will be completed by 2039.
“Since the 1970s, our nuclear diving experience has shown that the safe use of divers lowers overall programme radiation exposure, reduces overall schedule and cost, and is regulator endorsed, proven, and repeatable,” said Philip McDermott, Chief Operating Officer, UCC UK Ltd. Underwater Construction Corporation launched its wholly-owned UK subsidiary in 2015 to deliver safe and economical underwater solutions to NDA site licence companies, such as Magnox, Dounreay, and Sellafield.
ONR says it granted permission to Sellafield Ltd for underwater divers to access the PFSP following a rigorous series of assessments and appraisals of the work in Cumbria. “The use of divers replicates a well-established technique previously carried out internationally and at other UK nuclear sites during their decommissioning phase,” it noted.
After a detailed assessment of the safety case by ONR inspectors from a range of different specialisms including fault studies, radiation protection, human factors and conventional health and safety, ONR issued a licence instrument granting Sellafield Ltd permission to pilot the use of divers in bays 11 and 12 of the pond.
According to ONR, during the last 10 years, Sellafield Ltd had undertaken significant clearance work inside these two bays using a range of techniques including remotely-operated vehicles and workers deploying long reach tooling from the edge of the bays. “Good progress has been made, but these established clearing techniques have gradually delivered diminishing returns. This is because of the many integral structures attached to the floor of the pond complicate the recovery of the remaining sludge, debris and fuel and hinder easy access to the radioactive material,” ONR explained
Alan Wylie, ONR’s Superintending Inspector, said: “Due to the work that has been completed in bays 11 and 12 during the last decade to remove significant amounts of the waste, and because of the isolation of these two bays from the rest of the pond, this area presented the lowest risk in which to pilot the use of divers.” ONR’s specialist inspectors assessed Sellafield Ltd’s safety submission and were satisfied that the risks to divers entering the pond had been demonstrated to be as low as reasonably practicable and that Sellafield Ltd had taken suitable and sufficient measures to reduce risk. “As such we were satisfied that it was safe for divers to enter and work in the pond,” said Wylie. “The use of divers has allowed Sellafield Ltd to make progress with retrieving the remaining material from bays 11 and 12, which existing retrieval techniques are no longer able to do. “Our inspectors will continue to monitor the retrievals work from this facility as part of our inspection activities on the site.”
The PSFP was prioritised for clean-up by NDA in the early 2000s, along with three other Sellafield facilities collectively known as the legacy ponds and silos. In 2016, the last of its fuel inventory was retrieved, removing 70% of its radioactive inventory. Removing solid waste such as old equipment out of the pond began in 2010. There was an estimated 750 tonnes in the facility originally. The work involved removing equipment previously used for underwater fuel reprocessing operations, which was surrounded by radioactive sludge.
Each item had to be assessed for the most suitable and cost-effective route for disposal or storage. Some were sent to more modern plants at Sellafield for storage, pending final disposal. Items with low levels of radioactivity were disposed of at the UK’s Low Level Waste Repository.
Image: UCC diver Josh Everett descend the ladder into Sellafield’s Pile Fuel Storage Pond (PFSP) to help complete clean-up work in the site’s oldest legacy pond (courtesy of UCC)
