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.

Tokyo Electric said Friday that a remotely controlled robot investigating the interior of reactor 3 at the Fukushima No. 1 power plant has finally spotted objects that could potentially be fuel debris. The objects look like icicles hanging around a control rod drive attached to the bottom of the pressure vessel, which holds the core, Tokyo Electric Power Company Holdings Inc. said at an evening news conference. Enclosed by the huge primary containment vessel, the pressure vessel originally contained the fuel rod assemblies. But the rods melted into a puddle and pierced the bottom of the pressure vessel once the plant lost power after being swamped by the monstrous tsunami of March 11, 2011. The robot also captured images of lumps of material that appears to have melted and resolidified near the wall of the pedestal, a concrete structure that supports the pressure vessel.

The location and condition of the fuel in the three reactors hit by core meltdowns is critical information for Tokyo Electric Power Company Holdings Inc., which runs the plant. Removing the fuel debris is considered the most difficult part of decommissioning the complex. Unit 3 has the highest level of water inside at 6 meters. The fuel debris inside is presumed to have melted through its pressure vessel and settled at the bottom of its primary containment vessel. "Until today, no one has seen the situation inside reactor 3," said Tsutomu Takeuchi, senior manager at Toshiba's Fukushima Restoration and Fuel Cycle Project Engineering Department.

There's still a huge amount of radioactive waste cleanup to do at the Fukushima nuclear power plant in Japan. Some of that cleanup can be done by careful humans. And there's some that's too dangerous for humans, but not quite dangerous enough to dissuade robots. Clearing the fuel rods out of the pool in reactor 3 is one of those tasks, and Toshiba has built a robot to tackle it. If you had to pick somewhere to eat a picnic lunch in the Fukushima Daiichi nuclear power plant, inside the containment building of reactor 3 probably wouldn't be at the top of your list, but it also wouldn't be at the very bottom.