TOKYO (AP) — An underwater robot has captured images of lava-like lumps inside a damaged reactor at Japan's crippled Fukushima nuclear plant, spotting for the first time what is believed to be nuclear fuel that melted six years ago.

Plant operator Tokyo Electric Power Co. says the robot found the objects Friday on its second mission inside the primary containment vessel of the Unit 3 reactor at Fukushima, which was destroyed by a massive earthquake and tsunami. Experts believe the fuel melted and much of it fell to the chamber's bottom and is now submerged by radioactive water.

In an earlier survey Wednesday, the robot found severe damage in the vessel but no signs of melted fuel.

Locating the fuel in each of the three wrecked reactors is crucial for decommissioning the plant.