NDE & inspection
Exploring the depths23 April 2012
At the UK’s largest and most complex nuclear site, Sellafield, in North West England, decommissioning of one of the oldest nuclear storage ponds has reached new depths in risk reduction thanks to the implementation of two remotely-operated vehicles.
More than 98% of the contents of the First Generation Magnox Storage Pond (FGMSP) have been successfully surveyed, with only 20 of the 1200-plus skips in the pond left to do.
The pond, which became operational in 1958, was used for 26 years to store and prepare natural uranium magnox fuel for reprocessing. In its lifetime the facility stored and prepared approximately 27,000 tonnes of fuel, which equates to nearly 2.5 million fuel rods. The facility comprises an inlet building for receiving fuel, an open-air concrete storage pond and a building containing wet bays for fuel cladding removal (decanning) and a sludge settling pond. The fuel pond is connected to a row of seven wet bays, a special-purpose bay and the magazine transfer bay. The facility was extended through the 1960s to expand pond storage capability and additional large caves were built to improve fuel cladding removal.
A lengthy and unforeseen reprocessing shutdown in the 1960s led to prolonged storage of fuel and cladding. Ultimately this led to increased radiation levels and extremely poor underwater visibility in the pond. These difficulties slowed the rate of decanning, increased residence times for spent fuel and so created a perpetual delay in throughputs. When the plant was taken off-line, considerable quantities of fuel had yet to be exported and reprocessed.
The storage pond in its current state now principally contains sludge, which covers the pond floor, contaminated pond water, spent magnox fuel, solid contamination and activated items, and large items of scrap plant equipment.
Two of the biggest challenges the project team face are the lack of detailed knowledge of the inventory in the facility and the difficulty of cross-referencing inventory to the existing databases. The priority is to address these issues to assist with future retrievals; all the existing inventory within the facility will have to be retrieved and placed in an interim store before final disposal. The retrieval programme was also started to satisfy stringent safeguarding requirements from the EU that have come into force.
Although surveys of the ponds had been carried out in the past, they have had very limited success. But in late 2005 a new design of ROV was introduced in the shape of the ‘Seeker’: a GBP 160,000, one-metre long remote-controlled submersible supplied by ROVTECH Systems, which could swim to most of the areas in the pond.
Remotely operated vehicles (ROV) are tethered underwater vehicles that are commonly used in deepwater industries such as offshore hydrocarbon extraction. ROVs are unoccupied, highly manoeuvrable and operated by a person on land or aboard a vessel. They are linked to their operator by a tether containing electrical power, video and data cables. Most ROVs are equipped with at least a video camera and lights. These may include sonars, magnetometers, a still camera, a manipulator or cutting arm, water samplers, and instruments that measure water clarity, light penetration and temperature.
This ROV was essentially an off-the-shelf item with only minor modifications to suit the pond conditions. After proof-of-concept, the decision was made to go straight into operational prototype. Before that could be sanctioned, a specially-designed, purpose-built training tank had to be built to test and develop the Seeker ROV in a clean, controlled environment before entering the pond. A 50-cubic-metre, open-top tank was developed at a nearby James Fisher Nuclear facility, complete with dummy skips to replicate the pond environment. Operations were witnessed by representatives from the nuclear regulator, who approved the ROV for nuclear use.
Once submerged in the real FGMSP, the ROV made a series of underwater surveys that involved surveying 768 upper level skips, leaving 467 second- and lower-level skips which at that time were inaccessible.
The Seeker has successfully inspected and mapped more than 60% of the skips in the pond, but its size meant that accessing many of the lower stacked skips was restricted. So in 2009 it was fitted with a boom camera to enable the team to gain access for the first time to the various lower-level skips. The boom camera can rotate 180° to survey skips on either side of an aisle. The boom is narrow enough to be able to fit into gaps smaller than 72 mm between the skips.
FGMSP head of operations Dave Skilbeck said that a ROV is much safer than some other alternatives. “After the initial launch into the pond it enables the operators to withdraw to a low-radiation dose area of the building and thus reduce human dose. There is no alternative that is as safe as this method, which is evidenced by the absence of high-quality information on the pond conditions prior to this project.
“It’s faster than the alternatives, as it can be deployed quickly, and operates in a very efficient manner, surveying large sections of the pond in a short space of time. The largest time savings come from the remote nature of the work, which means that risks to operators are correspondingly low.”
Phil Toomey, technical manager, Sellafield Ltd, said: “The decommissioning significance of this project has set the standard in surveying high-risk stores elsewhere. It is a long-term cost-effective solution to accelerating waste retrieval strategies, which otherwise would have taken many more years to achieve. The remote aspect of the technology is also a huge step forward in safeguarding the operators on site and minimising the environmental impact”.
Two units of a smaller, more compact model, the GBP 40,000 VideoRay ROV, were later procured from US vendor Atlantas Marine and used to survey in between lower-level skips. Their compact design (about a third as long as the Seeker) allowed them to fit into gaps of about 300 mm between rows of skips; they were also fitted with a boom camera.
The FGMSP floor has had to be cleared of sludge to enable skips to be laid down safely onto a clear area to accelerate the risk reduction programme. It is also necessary to identify the quantities of fuel and to determine its state, as well as ascertaining the actual condition of the facility itself. The ROVs have been utilised by the project team to carry out additional work, which also includes sludge volume estimation, sampling radiation surveys and bay inspections. After condition surveys, retrieval of the fuel skips can begin in order to eventually empty the pond of its inventory.
ROVs will also help with this latter project, picking up a loose fuel element off the pond floor and placing it in a skip, or transferring fuel elements from one skip to another. For this work, a bespoke skid with hydraulically-driven manipulator arm and power pack has been mounted underneath the Seeker. The arm extends vertically below the ROV. Before the newly-armed ROV can start work, it had to undergo demonstration trials.
Dave Skilbeck explains: “The future work of the ROV will be to prove its capability by continuing to trial and develop its manoeuvrability and longevity. This is now being carried out in a purpose-built facility based on the Sellafield site called the Underwater Test Facility. This will enable research and development to continue but in a clean, contamination free area”.
Supplier ROVTECH Systems were involved in other trials that saw a ROV pick up empty fuel rods and place them safely in empty skips in Barrow harbour.
ROVs are also used in two other Sellafield ponds, the pile fuel storage pond and THORP storage pond.
In charge of the three ROVs at Sellafield is a 17-member team consisting of nine staff, four contractor pilots, three contractor project design engineers and one contractor project manager. A team of two pilots work together to control the ROV during operations. As with any piece of machinery, training is a crucial part of the ROV operations. Sellafield Ltd worked closely with JFN Services to develop training and create a training environment for ROV operators to ensure that all operators are fully trained. Maritime Training and Competence Solutions Ltd of Windermere have also been involved with operator training.
This article was published in the April 2012 issue of Nuclear Engineering International.