The sludge retrieval project has taken nine years of work requiring significant engineering on the plant, as well as the design of bespoke equipment to operate in an underwater, radioactive environment.
The machine known as the Sludge Retrieval Hood has been designed and built in order to suck up radioactive sludge from the floor of the open air Pile Fuel Storage Pond (PFSP). The PFSP is the largest open air nuclear fuel storage pond in the world containing over 15,000 cu m of radioactive water, more than 300 cu m of sludge, various nuclear wastes and legacy spent nuclear fuel. It poses one of the most challenging decommissioning projects on the Sellafield site and an accelerated programme of decommissioning work is underway.
Radiation levels mean that the Sludge Retrieval Hood has to be deployed and operated remotely at the bottom of the 6 metres deep pond, where if sucks up some 25 cu m of material per hour. The sludge is then transferred into the Sludge Corale, where it will be stored pending treatment through the Local Sludge Treatment Plant, currently undergoing inactive commissioning.
The sludge retrieval project is also cleaning sludge from open topped metal skips currently stored within the pond. There were around 180 skips in the pond, 24 of these have now been emptied, cleaned and exported by the project team to create space within the pond.
The remaining skips contain miscellaneous nuclear wastes and spent nuclear fuel; these skips will be washed to remove the sludge then moved round the pond following a co-ordinated plan which will allow the team to work their way down the full length of the pond floor removing sludge – a project which is expected to take five years to complete.
Steve Cottam, Head of PFSP Programme Acceleration, said: “Decommissioning the PFSP has been hampered by the layer of sludge on the pond floor and within the metal skips. We’ve had to come up with an innovative solution to remove this radioactive sludge to allow the pond to be emptied, as part of our commitment to decommission this high priority project.
“It is quite exciting to see the plant make the transition into full blown retrieval operations. Getting the plant to this stage has however been a series of challenges. Equipment was initially designed for the legacy pond based on existing drawings and plant records, but we quite often discovered that the conditions were not as expected, and sometimes had to make last minute modifications to equipment. Even now we still find unexpected things!”
The PFSP was the very first nuclear fuel storage pond at Sellafield built in 1948 and sludge in the form of algae, corrosion products and wind blown material has been accumulating in this open air pond ever since. The pond was built to store nuclear fuel and isotopes from the Windscale Reactors that produced nuclear materials for the defence industry. Sludge retrieval operations commenced in 2008, when water lances were deployed in the pond bays to flush the sludge into the main pond. However it is only now that a machine has been developed to begin moving sludge from the main pond itself.
Kevin Newland, PFSP Technical Manager, added: “Decommissioning the PFSP is a unique piece of work. Not only do we have to deal with the radiological challenges posed by the pond and its contents, we also have to manage what is effectively a small reservoir. Waste water treatment compounds the nuclear problems because we have to consider biomass treatment solutions as well.
“In addition because the pond is open air, we have to proceed with the utmost of caution as there is no building to contain any radioactive contamination. In order to safely decommission the building we need to prevent any radioactive materials being released into the environment and so we’re looking for new and innovative techniques to successfully complete the project.”
Did you know?
• Built and commissioned between 1948 and 1952, the pond and adjoining decanning building provided the storage and cooling facility for irradiated fuel and isotopes from the two Windscale reactors. The pond processed 2100 tonnes of pile fuel and 300 tonnes of Magnox fuel.
• The pond is a sub-divided outdoor storage pond containing skips of irradiated fuel. The building contains a series of sub-ponds, otherwise known as bays, connected underwater to the main pond.
• All operations in the pond ceased in the 1970s. The plant was left in a care and maintenance regime, whilst operations were then transferred to more modern facilities. Extensive refurbishment and re-equipping took place in the 1980s and decommissioning started in the 1990s.
• The pond is 100 metres long, 25 metres wide and 7 metres deep, containing over 15 million litres of water. It contains metal skips each holding up to 6m3 of fuel and waste. 24 redundant skips have already been removed from the pond and disposed of.
• The pond also contains over 700 tonnes of activated and/or contaminated metal material. Five tonnes of metal from the storage pond wet bays and a number of empty waste skips was successfully exported from the pond in April 2011.