A tiny robot may hold the solution to one of the nuclear industry’s biggest safety challenges: inspecting the interior of underground pipes for leaks.
Three-quarters of the country’s nuclear reactor sites have leaked radioactive tritium from buried piping that transports water to cool reactor vessels, according to a recent study by the Associated Press. A key factor, experts say: limited methods for monitoring corrosion in underground pipes.
Enter scientists at the Massachusetts Institute of Technology.
Diving, Swimming Eggs
“We have 104 reactors in this country,” says Harry Asada, the Ford Professor of Engineering in the Department of Mechanical Engineering and director of MIT’s d’Arbeloff Laboratory for Information Systems and Technology.
Harry Asada/d'Arbeloff Laboratory
|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. |
“Fifty-two of them are 30 years or older, and we need immediate solutions to assure the safe operations of these reactors.”
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, according to MIT. Direct monitoring requires digging out the pipes and visually inspecting them—a costly, lengthy operation.
Asada’s team’s 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.
The team presented details of its latest prototype at the 2011 IEEE International Conference on Robotics and Automation.
The robot is a “highly maneuverable, compact vehicle for underwater precision inspection of complex structures,” according to MIT. Unlike most autonomous underwater vehicles (AUVs), it contains no visible propellers, rudders or other external appendages that can get tangled and interfere with the underwater structure in a cluttered environment.
Such external features would lodge in a reactor’s intricate structures, Asada said. “You would have to shut down the plant just to get the robot out,” he said. “So we had to make [our design] extremely fail-safe.”
Instead, the team has encapsulated a multi-axis, integrated thruster in the smooth, egg-shaped body.
The propulsion system harnesses the force of the water rushing through a reactor, the team says.
The group devised a special valve to switch the direction of the 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.
“At the end of the day, we get pipelines going in all … directions,” Asada says. “They’re really tiny.”
The researchers can close off various channels to shoot water through a specific valve, changing the robot’s direction. 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.
Hamster Ball with Camera
As the robot navigates a pipe system, an onboard camera takes images along the 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 meters.
The team is also working on an “eyeball” mechanism that would let the camera pan and tilt in place. Graduate student Ian Rust likens the concept 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 robot’s body, 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.
“The system has a simplicity that is very attractive for deployment in hostile environments,” says Henrik Christensen, director of the Center for Robotics and Intelligent Machines at the Georgia Institute of Technology.
Christensen, who was not involved in the work, said such robots could also prove useful in inspecting other confined spaces, including sewer pipes.
“One would like to have a system that can be deployed at a limited cost and risk, so an autonomous system of minimal size is very attractive,” he says.