Enter the Tigers

ALL PREVIOUS RECORDS FOR TRANSFERRING nuclear waste have been broken at Sellafield, after it used adapted robotic vehicles, proven offshore, to handle waste in spent fuel ponds.

Reliability is key in the vehicle design as the need to limit maintenance intervention is essential in a highly corrosive and radioactive environment that is hazardous to humans.

The design came from a collaborative project between Sellafield and Saab Seaeye, the world’s largest manufacturer of electric underwater robotic systems. A Saab Seaeye Tiger vehicle was proven to work consistently for nine months and then stopped for routine maintenance.

The role of the Tiger is to clean and empty the legacy-waste storage ponds by gathering and sorting nuclear and contaminated material, including radioactive fuel rods. The Tiger was chosen for its long-established record of reliability. Introduced 20 years ago, its design has been proven in the most hostile environments on the planet and working in arduous conditions, particularly in the offshore energy industry. Indeed, one 19-year-old Tiger has been operating for a total of 60,000 hours underwater on various missions, and is still going strong.

Reliability vital

It is also the Tiger’s robust and reliable design, with its powerful thrusters and agility to manoeuvre in tight spaces, that appealed to Sellafield. Working in such a hazardous environment makes reliability, particularly thruster reliability, vitally important, says Phil Toomey, technical manager at Sellafield Ltd.

“Reliability is key for the health of our operators. They must wash down the Tigers during maintenance checks and recovery after operations. Exposure to radiation for operators is carefully limited.”

Sellafield expects the Tigers to work continuously for long periods of time between scheduled maintenance periods. “We wanted a proven robotic vehicle,” says Toomey, “and we liked that the Tiger’s skid technology makes it easy to swap skids for different tasks.”

An existing core system, rather than a custom-built one, could be easily replaced and supported; it need only be proved for nuclear tasks.

To do this engineering elements had to be transformed. This included swapping aluminium parts wherever possible for stainless steel alternatives, or protecting them with an epoxy ceramic coating. Aluminium thruster blades were replaced with plastic ones. No sharp edges were allowed, so soft cable ties were used. Water traps had to be avoided so cables were loomed separately rather than bundled.

The buoyancy sections were coated in a hot melt solvent resistant PU skin on all sides for easy decontamination. Stainless steel bumpers were fitted, rather than polypropylene, to avoid trapping contaminants in scratches.

Sellafield has so far acquired four Tigers. Their role is to swim in the ponds, using their underslung manipulator to grip metre-long, 15kg fuel bars, before each bar is measured and identified then transferred to a skip for removal from the pond for safer long-term storage in a more modern facility.

Tooling skid options

Nuclear waste is buried in about 30cm of sludge, which is also highly radioactive. To clear this and gain access to the waste, scoop and clamshell tool skids are used. The vehicle also has an under-slung manipulator skid and a four- function forward-facing manipulator skid, together with a water-jet cleaning skid and cutter skids.

The Tiger also has a colour zoom camera and downward and rear-view cameras.

Having a selection of task-specific skids readily available is transforming working arrangements in the ponds. The water-jet skid, for instance, can clean large pond items before export and also be used for final clean-down of the pond walls – a task at other facilities undertaken by people on pontoons.

Steve Wordworth, head of operations for the First Generation Magnox Storage Pond, on the Sellafield site, said: “It’s a shift into a ‘production line mentality’ for waste retrievals. These industrial machines are our new workhorses for hazard and risk reduction and will be working at the nuclear coalface for a long time to come.”

Sellafield plans to remove all bulk fuel from its legacy ponds by 2022.