epa06104726 A handout photo made available by the International Research Institute for Nuclear Decommissioning (IRID) via Tokyo Electric Power Company (TEPCO) on 22 July 2017, shows the rear view of an underwater robot camera used to investigate inside the pedestal of number 3 reactor at TEPCO's Fukushima Daiichi Nuclear Power Plant in Okuma, Fukushima Prefecture, northern Japan. Media outlets report that the images, taken by the underwater robot, show possible melted nuclear fuel deposits inside the reactor. The facility was damaged when an earthquake on 11 March 2011, triggered a tsunami and sent three reactors into meltdown, killing more than 18,500 people. EPA/International Research Institute for Nuclear Decommissioning HAN MANDATORY CREDIT; BEST QUALITY AVAILABLE HANDOUT EDITORIAL USE ONLY/NO SALES HANDOUT EDITORIAL USE ONLY/NO SALES
An underwater robot camera used to probe number 3 reactor at Tepco's Fukushima nuclear plant © EPA

A miniature submarine operating inside the stricken Fukushima nuclear reactor has sighted what experts said was likely to be melted fuel debris, marking a potential milestone in the disaster’s clean-up effort.

After three days of robotic exploration inside Fukushima’s number three reactor, operator Tokyo Electric said it was “likely” that fused, lava-like deposits were a mixture of fuel debris and metal parts from the reactor.

The images represent a breakthrough in the exploration of the damaged reactors but also highlight the challenges ahead in decommissioning them. Tepco said the data would help it to prepare a road map to dismantle the reactors.

“It is hard to judge definitively that something is fuel debris from an image,” said Takahiro Kimoto, a general manager in Tepco’s nuclear division. Further data may potentially be gathered by bringing back samples from inside the reactor.

Three reactors at the Fukushima site suffered a catastrophic meltdown in 2011 after a tsunami knocked out their cooling systems, causing one of the worst nuclear disasters in history.

The robot was operating inside the primary containment for reactor number three, the centre of the structure. It entered an area called the “pedestal”, just below the reactor vessel. The images suggest that fuel melted through the bottom of the reactor vessel and settled into the containment below.

The robot was able to take pictures of the control rod drives, which regulate a working reactor. Mr Kimoto said the images showed the remains of various gratings and pipes but he was reluctant to speculate on the nature of seemingly corroded orange patches in the images.

Earlier this year, Tepco deployed a “scorpion robot” to explore Fukushima’s reactor number two. But high radiation damaged its cameras and the robot’s path was blocked by debris.

Reactor number three was more seriously damaged than number two but the high water level inside its containment vessel, which forced engineers to develop a submersible to work within it, has helped the exploration.

The water acted as a radiation shield, protecting the cameras, and has also allowed the submarine to move in three dimensions, enabling more extensive pictures of the devastation.

Developed by Toshiba, the submersible robot is 30cm long and has a diameter of just 13cm so it can fit down a narrow pipe into the reactor. The robot is controlled by a wire leading back to technicians.

Six years on from the disaster the situation at the Fukushima site has stabilised but decommissioning is expected to last decades. Tepco is still working to remove spent nuclear fuel from pools at the ruined reactors and to manage water flowing through the site. Decommissioning the primary containment is likely to be the final step, if it is technologically feasible.

Investigators castigated Tepco for its safety failures after the 2011 disaster. The total decommissioning bill for Fukushima is unknown but is likely to exceed $100bn.

In a recent interview, Tomoaki Kobayakawa, the company’s new president, vowed to shake up a company that he said still suffered from a silo mentality, poor internal communication and ambiguity about who was responsible for performance.

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