“We have finished the debris cleanup and gotten the plant under control,” said the guide, Daisuke Hirose, a spokesman for Tepco’s subsidiary in charge of decommissioning.”
In September, the prime minister’s office set a target date of 2021 － the 10th anniversary of the disaster － for the next significant stage, when workers begin extracting the melted fuel from at least one of the three destroyed reactors, though they have yet to choose which one.
The government acknowledges that cleaning up the plant will take at least three to four more decades and tens of billions of dollars. A $100 million research center has been built nearby to help scientists and engineers develop a new generation of robots to enter the reactor buildings and scoop up the melted fuel.
At Chernobyl, the Soviet simply entombed the charred reactor in concrete after the deadly 1986 accident. But Japan has pledged to dismantle the Fukushima plant and decontaminate the surrounding countryside, which was home to about 160,000 people who were evacuated after the accident.
Many of them have been allowed to return, as the rural towns around the plant have been decontaminated. But without at least starting a cleanup of the plant itself, officials admit they will find it difficult to convince the public that the accident is truly over.
They also hope that beginning the cleanup will help them win the public’s consent to restart Japan’s undamaged nuclear plants, most of which remain shut down since the disaster.
Tepco and the government are treading cautiously to avoid further mishaps that could raise doubts that the Fukushima plant is under control.
“They are being very methodical － too slow, some would say － in making a careful effort to avoid any missteps or nasty surprises,” said David Lochbaum, director of the nuclear safety project at the Union of Concerned Scientists, who was a co-author of a book on the disaster. “They want to regain trust. They have learned that trust can be lost much quicker than it can be recovered.”
To show the course followed by the Manbo, Tepco’s Mr. Hirose guided me inside the building containing the undamaged Unit 5 reactor, which is structurally the same as two of the destroyed reactors.
Mr. Hirose pointed toward the spot on a narrow access ramp where two robots, including one that looked like a scorpion, got tangled in February by debris inside the ruined Unit 2.
Before engineers could free the scorpion, its monitoring screen faded to black, as its electronic components were overcome by radiation, which Tepco said reached level of 70 sieverts per hour. （A dose of one sievert is enough to cause radiation sickness in a human.）
Mr. Hirose then led me underneath the reactor, onto that is called the pedestal.
The bottom of the reactor looked like a collection of huge bolts － the access points for control rods used to speed up and slow down the nuclear reaction inside a healthy reactor. The pedestal was just a metal grating, with the building’s concrete floor visible below.
“The overheated fuel would have dropped from here, and melted through the grating around here,” Mr. Hirose said, as we squatted to avoid banging our heads on the reactor bottom.
The entire area around the reactor was dark and cluttered with pipes and machinery.
To avoid getting entangled, the Manbo took three days to travel some 20 feet to the bottom of Unit 3.
Extracting the melted fuel will present its own set of technical challenges, and risks.
Engineers are developing the new radiation-resistant robots at the Naraha Remote Technology Development Center.
It includes a hanger-sized building to hold full-scale mock-ups of the plant and a virtual-reality room that simulates the interiors of the reactor buildings, including locations of known debris.
