Earlier robots had failed, getting caught on debris or suffering circuit malfunctions from excess radiation. But the newer version, called the MiniManbo, or little sunfish, was made of radiation-hardened materials with a sensor to help it avoid dangerous hot spots in the plant's flooded reactor buildings.
The size of a shoe box, the Manbo used tiny propellers to hover and glide through water in a manner similar to an aerial drone.
After three days of carefully navigating through a shattered reactor building, the Manbo finally reached the heavily damaged Unit 3 reactor. There, the robot beamed back video of a gaping hole at the bottom of the reactor and, on the floor beneath it, clumps of what looked like solidified lava: the first images ever taken of the plant’s melted uranium fuel.
The discovery in July at Unit 3 and similar successes this year in locating the fuel of the plant's other two ruined reactors are what Japanese officials hope will prove to be a turning point in the worst atomic disaster since Chernobyl.
What happened to the fuel was one of the enduring mysteries of the catastrophe, which occurred on March 11, 2011, when an earthquake and 50-foot tsunami knocked out vital cooling systems at the plant.
Left to overheat, three of the six reactors melted down. Their uranium fuel rods liquefied like candle wax, dripping to the bottom of the reactor vessels in a molten mass hot enough to burn through the steel walls and even penetrate the concrete floors below.
No one knew for sure exactly how far those molten fuel cores had traveled. Desperate plant workers － later celebrated as the “Fukushima Fifty” － were eventually able to cool them by pumping water into the reactor buildings.
But with radiation levels high, the final location of the fuel remained unknown.
As officials became more confident about managing the disaster, they began a search for the disaster, they began a search for the missing fuel. Scientists and engineers built radiation-resistant robots like the Manbo and a device like a huge X-ray machine that uses exotic space particles called muons to see the reactors’ innards.
Now that engineers say they have found the fuel, officials of the government and the utility that runs the plant hope to public opinion.
Six and a half years after the accident spewed radiation over northern Japan and at one point seemed to endanger Tokyo, the officials hope to persuade a skeptical world that the plant has moved out of post-disaster crisis mode and into something much less threatening: cleanup.
“Until now, we didn't know exactly where the fuel was, or what it looked like,” said Takahiro Kimoto, a general manager in the nuclear power division of the plant’s operator, the Tokyo Electric Power Company, or Tepco. “Now that we have seen it, we can make plans to retrieve it.”
Tepco is eager to portray the plant as one big industrial cleanup site. About 7,000 people work here, building new
water storage tanks, moving radioactive debris to a new disposal site and erecting enormous scaffoldings over reactor buildings torn apart by the huge hydrogen explosions that occurred during the accident.
Access to the plant is easier than it was just a year ago, when visitors still had to change into special protective clothing. These days, workers and visitors can move about all but the most dangerous areas in street clothes.
A Tepco guide explained that this was because the central plant grounds ham been deforested and paved over, sealing in contaminated soil.
During a recent visit, the mood within the plant was noticeably more relaxed, though movements were still tightly controlled and everyone was required to wear radiation-measuring badges. Inside a “resting building,” workers ate in a large cafeteria and bought snacks in a convenience store.
