Tokyo Electric Power, Tepco, the operator of Japan's wrecked Fukushima nuclear plant, said on Monday it may have found nuclear fuel debris below the damaged No. 2 reactor, one of three that had meltdowns in the 2011 disaster.

Should the finding be confirmed, it would mark a significant breakthrough in attempts to clean up the nuclear plant, after years of delays, missteps and leaks of radioactive water.

Finding the highly radioactive melted uranium rods may pave the way for Tepco to develop methods to remove the melted fuel.

Tepco detected a black lump of material directly below the reactor in an inspection by camera on Monday and could not rule out the possibility it was melted fuel, an official told a news briefing, adding it was “a big step forward.

The company will analyze the data to decide whether it could send a robot into the reactor for further investigation, he said.

In the world's worst nuclear calamity since Chernobyl in 1986, three reactors at Tepco's Fukushima plant melted down after a magnitude 9 earthquake struck off the coast of Japan in March 2011, triggering a tsunami that devastated a large area and killed more than 15,000 people.