Tokyo - Images captured by an underwater robot showed massive deposits believed to be melted nuclear fuel covering the floor of a damaged reactor at Japan's crippled Fukushima nuclear plant.
The robot found large amounts of solidified lava-like rocks and lumps in layers as thick as 1 meter on the bottom inside of a main structure called the pedestal that sits underneath the core inside the primary containment vessel of Fukushima's Unit 3 reactor, said the plant's operator, Tokyo Electric Power Co (TEPCO).
On Friday, the robot spotted suspected debris of melted fuel for the first time since the 2011 earthquake and tsunami caused multiple meltdowns and destroyed the plant. The three-day probe of Unit 3 ended Saturday.
Locating and analysing the fuel debris and damage in each of the plant's three wrecked reactors is crucial for decommissioning the plant.
The search for melted fuel in the two other reactors has so far been unsuccessful because of damage and extremely high radiation levels.
During this week's probe, cameras mounted on the robot showed extensive damage caused by the core meltdown, with fuel debris mixed with broken reactor parts, suggesting the difficult challenges ahead in the decades-long decommissioning of the destroyed plant.
TEPCO spokesperson Takahiro Kimoto said it would take time to analyze the debris in the images to figure out debris removal methods.
Tepco detected black-coloured material that dangled like icicles that could be nuclear debris near the bottom of the reactor's pressure vessel that contained the fuel rods, the report said, citing unnamed sources.
Finding the highly radioactive melted fuel rods may pave the way for Tepco to develop methods to remove the melted fuel.
The company in January spotted possible nuclear fuel debris below the damaged No. 2 reactor at the plant.
In the world's worst nuclear calamity since Chernobyl in 1986, three reactors at the Fukushima Daiichi 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.
The utility has been developing robots that can swim under water and negotiate obstacles in damaged tunnels and piping to search for the melted fuel rods.