IEEE Spectrum - North American - March 2014 - 51

tePco (3)

out oF the pool: spent fuel pools inside the
damaged reactor buildings contain hundreds
of nuclear fuel assemblies, some of which are
covered with debris [above] from the accident's
explosions. tepCo is emptying reactor 4's pool
[top left] first. in the extraction process, a cask is
lowered into the pool and filled with radioactive
fuel assemblies. then the cask is transported
to a safer location, lowered into another pool
[bottom left], and unloaded. it will take 70 trips
to empty the reactor 4 pool.

improved survey bot named Sakura. To guard
against future tangles, Sakura not only unspools cable behind, it also automatically
takes up the slack when it changes direction. It's waterproof enough to roll through
puddles, and it can carry a heavy camera
capable of detecting gamma radiation. The
bot can tolerate that radiation: Yoshida's
team tested its electronics (the CPU, microcontrollers, and sensors) and found that
they're radiation-tolerant enough to perform about 100 missions before any component is likely to fail. However, the robot
itself becomes too radioactive for workers
to handle. Sakura must therefore take care
of itself: It recharges its batteries by rolling
up to a socket and plugging itself in.
The second step in the Fukushima decommissioning is decontamination, because

only when that is complete will workers be
able to get inside to tackle more complex
tasks. The explosions that shattered several
of the reactor structures sprayed radioactive
materials throughout the buildings, and
the best protective suits for workers in hot
zones are of little use against the resulting
gamma radiation-a worker would have to
be covered from head to toe in lead as thick
as the width of a hand.
After the accident, the Japanese government called for robots that could work on
decontamination, and several of Japan's
leading companies rose to the challenge.
Toshiba and Hitachi have designed robots
that use jets of high-pressure water and dry
ice to abrade the surfaces of walls and floors;
the robots will scour away radioactive materials along with top layers of paint or con-

crete and vacuum up the resulting sludge.
But the robots' range is defined by their
own communication cables, and they can
carry only limited amounts of their cleaning agents. Another bot, the Raccoon, has
already begun nosing across the floor in
reactor building 2, trailing long hoses behind it to supply water and suction.
To clear a path for the robotic janitors,
another class of robots has been invented
to pick up debris and cut through obstacles. The ASTACO-SoRa, from Hitachi, has
two arms that can reach 2.5 meters and lift
150 kilograms each. The tools on the ends
of the arms-grippers, cutting blades, and
a drill-can be exchanged to suit the task.
However, Hitachi's versatile bot is limited to
work on the first floor, as it can't climb stairs.
Removing spent fuel rods is the third step.
Each reactor building holds hundreds of
spent fuel assemblies in a pool on its top floor.
These unshielded pools, perfectly safe when
filled with water, became a focus of public
fear during the Fukushima Daiichi accident.
After reactor building 4 exploded on 15 March,
many experts worried that the blast had damaged the structural integrity of that building's pool and allowed the water to drain out.

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Table of Contents for the Digital Edition of IEEE Spectrum - North American - March 2014