A spherical robot equipped with a camera may navigate underground pipes of a nuclear reactor by propelling itself with an internal network of valves and pumps. Image: Harry Asada/d’Arbeloff Laboratory |
As
workers continue to grapple with the damaged Fukushima Daiichi nuclear
powerplant in Japan,
the crisis has shone a spotlight on nuclear reactors around the world. In June
(2011), The Associated Press released results from a year-long investigation,
revealing evidence of “unrelenting wear” in many of the oldest-running
facilities in the United
States.
That
study found that three-quarters of the country’s nuclear reactor sites have
leaked radioactive tritium from buried piping that transports water to cool
reactor vessels, often contaminating groundwater. According to a recent report
by the U.S. Government Accountability Office, the industry has limited methods
to monitor underground pipes for leaks.
“We
have 104 reactors in this country,” says Harry Asada, the Ford Professor of
Engineering in the Department of Mechanical Engineering and director of Massachusetts Institute of Technology’s (MIT’s)
d’Arbeloff Laboratory for Information Systems and Technology. “Fifty-two of
them are 30 years or older, and we need immediate solutions to assure the safe
operations of these reactors.”
Asada
says one of the major challenges for safety inspectors is identifying corrosion
in a reactor’s underground pipes. Currently, plant inspectors use indirect
methods to monitor buried piping: generating a voltage gradient to identify
areas where pipe coatings may have corroded, and using ultrasonic waves to
screen lengths of pipe for cracks. The only direct monitoring requires digging
out the pipes and visually inspecting them—a costly and time-intensive
operation.
Now
Asada and his colleagues at the d’Arbeloff Laboratory are working on a direct
monitoring alternative: small, egg-sized robots designed to dive into nuclear
reactors and swim through underground pipes, checking for signs of corrosion.
The underwater patrollers, equipped with cameras, are able to withstand a
reactor’s extreme, radioactive environment, transmitting images in real-time
from within.
Cannonball!
At first glance, Asada’s robotic inspector looks like nothing more than a small
metallic cannonball. There are no propellers or rudders, or any obvious
mechanism on its surface to power the robot through an underwater environment.
Asada says such “appendages,” common in many autonomous underwater vehicles
(AUVs), are too bulky for his purposes—a robot outfitted with external
thrusters or propellers would easily lodge in a reactor’s intricate structures,
including sensor probes, networks of pipes and joints. “You would have to shut
down the plant just to get the robot out,” Asada says. “So we had to make [our design]
extremely fail-safe.”
He
and his graduate student, Anirban Mazumdar, decided to make the robot a smooth
sphere, devising a propulsion system that can harness the considerable force of
water rushing through a reactor. The group devised a special valve for
switching the direction of a flow with a tiny change in pressure and embedded a
network of the Y-shaped valves within the hull, or “skin,” of the small,
spherical robot, using 3D printing to construct the network of valves, layer by
layer. “At the end of the day, we get pipelines going in all … directions,”
Asada says. “They’re really tiny.”
Depending
on the direction they want their robot to swim, the researchers can close off
various channels to shoot water through a specific valve. The high-pressure
water pushes open a window at the end of the valve, rushing out of the robot
and creating a jet stream that propels the robot in the opposite direction.
Robo-patrol
As the robot navigates a pipe system, the onboard camera takes images along the
pipe’s interior. Asada’s original plan was to retrieve the robot and examine
the images afterward. But now he and his students are working to equip the
robot with wireless underwater communications, using laser optics to transmit
images in real time across distances of up to 100 m.
The
team is also working on an “eyeball” mechanism that would let the camera pan
and tilt in place. Graduate student Ian Rust describes the concept as akin to a
hamster ball.
“The
hamster changes the location of the center of mass of the ball by scurrying up
the side of the ball,” Rust says. “The ball then rolls in that direction.”
To
achieve the same effect, the group installed a two-axis gimbal in the body of
the robot, enabling them to change the robot’s center of mass arbitrarily. With
this setup, the camera, fixed to the outside of the robot, can pan and tilt as
the robot stays stationary.
Asada
envisions the robots as short-term, disposable patrollers, able to inspect
pipes for several missions before breaking down from repeated radiation
exposure.
