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Following the meltdown, nearly 165,000 people had to evacuate the area surrounding the Fukushima plant to avoid radioactive exposure. Today, even after extensive cleanup efforts, 50,000 people still can't go home.

In the first chaotic weeks after the meltdown, with radiation levels far too intense for anyone to work inside the reactors, Tepco scrambled to deploy robots to assess and contain the damage. Tractor-treaded bots from iRobot, drones from Honeywell, and a prototype disaster-­response mech from Tohoku University scouted the rubble-­strewn facility and tried to measure the intensity of the radiation. A remote-­controlled concrete pumping truck was adapted so that its extendable spout could pour water into the reactors, cooling and stabilizing the overheated chambers.

In the months and years that followed, Fukushima became both a market and a proving ground for ever-advancing robot technologies designed to operate in hazardous conditions. Remote-controlled front-end loaders, backhoes, and other heavy equipment were put to work breaking up radioactive debris and loading it onto remote-controlled dump trucks. A four-legged walking robot investigated the reactor buildings. Robots with 3-D scanners were sent in to gather imagery and map radiation levels. Swimming robots inspected pools where spent fuel rods were stored, taking pictures.

But none of these robots were capable of penetrating the innermost areas of the reactors. In August 2013, the Japanese government assembled a consortium of public utilities and private companies, including Mitsubishi, Hitachi, and Toshiba, to create robots specifically for the most challenging environments. Dubbed the International Research Institute for Nuclear Decommissioning, it has developed some 20 machines that have been deployed onsite. Their ranks include a snakelike bot that crawled through a tiny accessway into Unit 1, then bent itself into a more stable U-shape to explore inside. Then there was the Scorpion, a tank-tread-driven machine with a camera mounted on an elevating "tail" that was sent into Unit 2. The Japanese government is bankrolling a $100 million, state-of-the-art R&D center near the nuclear plant where robot operators train on digital models of the reactors in a giant 3-D Holo Stage and on life-size physical mock-ups.

But even with the massive government investment, many of the new robots still couldn't hack it inside the reactors. The camera on one of them, sent to clear a path for the Scorpion, was shut down by radiation; the Scorpion itself got tripped up by fallen debris. The first version of the snakelike bot got stuck; the second did better but failed to find any melted fuel. "It's very difficult to design a robot to operate in an unknown environment," says Hajime Asama, a professor at the University of Tokyo who was one of the first roboticists the government turned to for help. "Until we send the bot in, we don't know what the conditions are. And after it's sent, we can't change it."

Kenji Matsuzaki has worked in Toshiba's nuclear technology branch for more than 10 years, and by May 2016, when he was assigned to the team developing a robot to explore inside Unit 3 of Fukushima, he was familiar with the plant's basic architecture. All six of its reactors are boiling-water reactors, a type designed in the late 1960s and early 1970s and found all over the world, including in the United States. They generate electricity by circulating water through their infernally hot cores, converting it to steam that is used to turn turbine generators. Each reactor has three containers set one inside another like Russian nesting dolls. The smallest container, a steel capsule about the length of a tennis court, is called the reactor pressure vessel. That's where the nuclear fission reaction takes place, powered by fuel composed of uranium dioxide baked into ceramic pellets. This capsule is enclosed inside a primary containment vessel, a concrete and steel structure shaped like a massive light bulb, designed to capture any radiation that might accidentally escape. The containment vessel in turn is housed inside the reactor building, a concrete and metal rectangle that offers only minimal protection from radiation.

Technicians in protective gear can work for short periods inside the reactor building, but they can't enter the far more radioactive containment vessel, which is where they were likely to find at least some of the missing fuel. Building a robot that could get inside and maneuver around the containment vessel presented several unique challenges. First, the containment vessel was only practically accessible through a 5.5-inch circular maintenance opening about 8 feet above the floor of the reactor building, so the robot would have to be small. Second, because the containment vessel had been pumped full of water to cool it down, the robot would have to be able to swim. Third, since the water and thick walls would defeat wireless signals, this small, swimming robot would need to be powerful enough to move under­water while dragging as much as 65 yards of electric cable behind it.

It took months of research, experimentation, and testing in Toshiba's labs and in an enormous simulation tank at the government-run Port and Airport Research Institute to balance all these capabilities inside the little machine. Matsuzaki's team had to try different configurations of propellers, cameras, and sensors, boost the power of the propeller motors, develop a new type of coating to make the cable move more smoothly, and ensure the whole package could withstand a blistering level of radiation.

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