Sellafield’s First-Generation Magnox Storage Pond – which is the size of two Olympic swimming pools Sellafield Ltd

Sellafield has been called the most dangerous place in the UK, the most hazardous place in Europe and the world’s riskiest nuclear waste site. At its heart is a giant pond full of radioactive sludge, strewn with broken metal, dead animals and deadly nuclear rods. The solution to clearing up Sellafield’s nuclear waste and retrieving the missing nuclear fuel? Robots, of course. And to tackle this mammoth task, the robots are being forced to evolve.

Sellafield’s First-Generation Magnox Storage Pond is a giant outdoor body of water that’s the same size as two Olympic swimming pools. It was built in the 1960s to store used fuel rods from the early Magnox reactors – which had magnesium alloy cladding on the fuel rods – as part of Britain’s booming nuclear program. In 1974, there was a delay in reprocessing; fuel rods started corroding and the pond became murky. The pool was active for 26 years until 1992 and is now finally being decommissioned as part of the £1.9 billion spent each year on Sellafield’s mammoth cleanup operation.


The pond contains about six metres of radioactive water and half a metre of sludge, composed of wind-blown dirt, bird droppings and algae – the usual debris that builds up in any open body of water. Unlike other mud, it conceals everything from dropped tools and bird carcasses to corroded Magnox cladding and the remains of uranium fuel rods.

A number of robotic creations have bee used to get to the bottom of the pool’s sludge but struggle to break through the hostile environment. Tethered swimming robots do not have the sensors to find objects in the fine mud, and lack the leverage to lift chunks of metal. Experience at Fukushima has shown robots that are not well adapted to the environment are a waste of time.

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Enter Cthulhu, a tracked robot that can drive along the pond bed, feeling its way with tactile sensors and sonar. The robot, which is currently in development, is approaching Sellafield’s problem differently. The robot will be able to identify nuclear rods and then pick them up. "Rather than trying to mimic a human, we’re building a robot that can do things humans can’t do with senses that humans don’t have," says Bob Hicks of QinetiQ, which is leading the project.

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The name stands for ‘Collaborative Technology Hardened for Underwater and Littoral Hazardous Environment,’ but it’s also a nod to Cthulhu, the godlike alien created by HP Lovecraft: both are amphibious, dwell in strange surroundings, and have sensory feelers. "Much like a walrus detecting molluscs, we hope to be able to detect and identify objects in the sludge with the whiskers,” says Plamen Angelov of Lancaster University’s School of Computing and Communications.

QinetiQ is supplying the tracked body, originally from a bomb disposal robot, and Bristol Maritime Robotics is developing the tactile sensors, while Angelov’s team is providing the neural network AI. It is planned the robot will use deep learning to fuse tactile and sonar data into a single picture of the world. Existing neural networks can handle video data, and ‘image classifiers’ to distinguish objects are well-established. But nobody has tried to fuse data from different types of sensor before.

Cthulhu's classifier will learn to divide objects into ‘fuel rods’ and ‘everything else’. The classifier will further be able to identify what type a fuel rod is, and whether it is still whole or partial. It will also be able to avoid the many obstacles on the pond floor. The fused data will be too sparse for a human to understand, but will provide accurate enough information for Cthulhu to pick up a fuel rod with a manipulator and place it in a collecting bin.

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The team is keen to learn from the Japanese experience at the destroyed Fukushima reactor site. Fukushima is full of dead robots which were unable to carry out their tasks. Some were killed by the intense radiation, but others were simply turned off when they failed in their mission. A broken caterpillar track, or getting stuck in a gap, is game over. The high level of radioactivity means they cannot be retrieved for repair.

Cthulhu is therefore designed for high reliability. Technically, it probably could be recovered and re-used but this will remain to be seen. Its body was designed for military use, which includes full chemical, biological and radiological decontamination, so it has no crevices and can be jet-washed easily, but the plan is for it to remain on site.

There’s a long way to go for the robotics project, which won’t be completed for another two years. In the meantime, other robots are attempting to clear-up Sellafield, in what is being considered as a proving ground for new robotic technologies. Another robot designed to pick-up radioactive fuel bars is the 15kg Tiger from Saab Seaeye. The Tigers are planned to be used at Sellafield for continuous six-month spells where they will use grippers to pick up nuclear waste.


Cumbrian robotics firm Forth Engineering had created a bot called Avexis, which has the appearance of a tadpole and has been designed to go inside the nuclear plant’s silos, record footage of what it sees and dislodge blockages. Videos have shown it diving in nuclear pools in Japan. Forth has also been working on creating six-legged robots that can move along the bottom of Sellafield’s pools. Elsewhere drones that can map nuclear waste are being developed.

The work at Sellafield is due to take several decades to complete fully. Nuclear waste is spread through several buildings in a variety of silos and pools. Each has its own challenges for cleaning-up. For the First Generation Magnox Pond, documents from the government show all the bulk fuel should be removed by the early 2030s.

Hicks suggests a later version of Cthulhu could crawl along the seabed to carry out pipeline inspections. Unlike swimming robots, it could remain perfectly stationary to carry out high-resolution sonar scans, recognising issues with advanced image classifiers and inspecting things like possible cracks minutely with tactile sensors. Such robots would have many uses in the offshore industry and other environments. If Cthulhu can survive and work effectively in the radioactive sludge pond of Sellafield, it can probably cope with anything.