More than six years after the core meltdowns triggered by March 2011 mega-quake and tsunami, Tokyo Electric said Saturday that the robot probing reactor 3 at the defunct Fukushima No. 1 power plant had likely spotted fuel debris for the first time at the bottom of its primary containment vessel.

Some of the debris looks like rocks and sand, and was accompanied by scaffolding and other objects from the reactor that had formed a pile about a meter high.

On Friday, Tokyo Electric Power Company Holdings Inc. announced that the same robot had found what appeared to be melted fuel at the bottom of the pressure vessel, which holds the core.

On Saturday, images released by the utility showed black, lava-like objects discovered by the so-called mini sunfish robot.

“It’s natural to assume that the debris melted and dropped,” said a Tepco official who briefed reporters.

Saturday was the last day of a four-day probe into reactor 3 that began on Wednesday. After starting at around 5 a.m. and working until noon, the robot’s operators managed to retrieve the robot as well.

The debris was clearly identifiable to at least one nuclear expert.

“The images that appear to be melted fuel debris match those found in the (1986) Chernobyl crisis,” said Tadashi Narabayashi, a specially appointed professor of nuclear engineering working at Hokkaido University. “It’s definitely fuel debris.”

Narabayashi praised the investigation, saying the findings are likely to give Tepco a major boost in figuring out how to decommission the plant’s three damaged reactors.

“It will give them ideas on robots they need to develop and ways to remove the debris,” he said. “It’s an epoch-making event.”

Decommissioning is progressing slowly because the radiation in the reactors remains extremely high.

Cooling water injected to keep the molten fuel cool has filled the bottom of the containment vessel to a depth of about 6.4 meters, forcing Tepco to develop a cylindrical waterproof robot with a diameter of 13 cm that has been dubbed the “mini sunfish.”

On March 11, 2011, huge tsunami swamped the poorly protected six-reactor plant, which was 10 meters above sea level, and flooded its power supply, causing a station blackout that crippled the reactors’ cooling systems.

This caused core meltdowns to occur in reactors 1 through 3, sparking the world’s worst nuclear disaster since Chernobyl.

Tepco conducted similar robot surveys from January to March at reactors 1 and 2, which had lower water levels, but they failed to ascertain what had happened to the fuel rods.