An underwater robot has spotted some lava-like rocks which are believed to be melted nuclear fuel, inside Japan’s stricken Fukushima reactor.

Large amounts of the solidified lumps and deposit were spotted for the first time by the robot on the floor of the primary containment vessel underneath the core of Fukushima’s No. 3 reactor, the Tokyo Electric Power Co (TEPCO) said.

“There is a high possibility that the solidified objects are mixtures of melted metal and fuel that fell from the vessel,” a TEPCO spokesman said, adding that the company was planning further analysis of the images.

The three-day investigation using the small, remote controlled underwater robot, which is about the size of a loaf of bread, ended Saturday, the spokesman said.

TEPCO said the images were the first ‘highly likely’ sighting of melted fuel since the 2011 disaster.

The disaster which saw a massive undersea earthquake send a huge wave barrelling into Japan’s northeast coast, killed more than 18,500 people.

Consequently three reactors were sent into meltdown at the plant in the worst such accident since Chernobyl in 1986.

A long-term job

Locating the fuel debris is a key part of the decommissioning process for the plant, which is expected to take decades.

In February, TEPCO sent another robot into one of three damaged reactors where radiation levels have hit record highs.

But the mission at the No. 2 reactor was aborted as the robot had difficulty moving and could not reach its target destination beneath the pressure vessel, through which nuclear fuel is believed to have melted.

The Japanese government said in December that it expects total costs including compensation, decommissioning and decontamination to reach 21.5 trillion yen ($192.5 billion) in a process likely to take at least four decades as high radiation levels slow operations.