Originally published in Scientific American.
March 27, 2009 |By Daniel Lovering
PITTSBURGH—A hulking four-wheeled robot stands idle, sheathed in steel and packed with hoses and cameras in the lobby of a Carnegie Mellon University research facility here. A mechanical claw extends from the end of a large boom. The device, called Workhorse, was designed for an entirely different environment: the dark, radiation-tainted confines of a building containing a failed nuclear reactor. It was never used for its intended purpose, but it reflects a burst of creativity in robotics that accompanied the cleanup of the worst commercial nuclear plant accident in the U.S.
Workhorse was the last of three robots built in the 1980s by a team at Carnegie Mellon to help gather information and remove debris from a reactor at the Three Mile Island (TMI) nuclear power plant near Middletown, Pa., that partially melted down 30 years ago on March 28, 1979. Students led by William Whittaker, a robotics professor, built the machines, one of which transmitted the first glimpses of the reactor’s contaminated basement after the accident.
“Here it is cutting a concrete stairwell apart,” says Whittaker, known as “Red” (a nickname remaining from childhood that referred to his once-red hair), pointing to a photo of Workhorse at Carnegie Mellon’s National Robotics Engineering Center. “It had the self-contained power, control, rigidity [and] strength to chop through it.”
The 1979 accident resulted in the release of radioactive gas into the atmosphere and stirred fears that a nuclear disaster was at hand. Although no one was immediately hurt—scientists from Columbia University’s Mailman School of Public Health and the National Audubon Society in 1990 found no convincing evidence that the accident caused a rise in cancer rates, confirming earlier findings by the Pennsylvania Department of Health—critics, including a nonprofit citizens’ group, have said people living near the reactor suffered adverse physical and psychological effects. The incident—along with the Chernobyl disaster in the Soviet Union in 1986—galvanized U.S. public opinion against nuclear power. It also prompted sweeping safety changes in the industry, which only recently has seen a resurgence, partly due to growing concern over the emission of greenhouse gases from coal and other power sources contributing to climate change.
The accident in TMI’s Unit 2 reactor began when cooling pumps failed in its nonnuclear secondary system. Then, a pressure valve in the nuclear primary system stuck open allowing large amounts of reactor coolant to escape. Making matters worse, plant operators initially failed to recognize what was happening. Metal tubes that held nuclear fuel pellets ruptured, and some of the pellets began to melt, according to the U.S. Nuclear Regulatory Commission (NRC) and former NRC official William Travers.
The power plant was shut down and several years elapsed before a full-scale cleanup began, partly because experts had to assess the extent and location of the radioactive contamination, according to Travers, former director of the NRC’s TMI-2 Cleanup Program Office. “It wasn’t known for years, actually, after the accident just how extensively it was damaged,” says Travers, who is now working for the United Arab Emirates government on tentative plans to launch a nuclear power program there.
Water had escaped from the reactor’s primary system and flooded the basement, mixing with radioactive materials and some water from the Susquehanna River. The water, which was roughly six feet (two meters) deep, along with a substantial amount of the radioactive material, was later removed, but it left what Whittaker and others described as a “bathtub ring” of contamination around the structure’s thick, steel-reinforced concrete walls.
Little was known for years about what lurked in the reactor’s basement, considered the most heavily contaminated part of the building. Workers entered the reactor building in July 1980, but the basement would not be explored until years later.
“There were so many unanswered questions,” says Whittaker, considered to be one of the world’s more respected robot builders. “What damage had occurred? What was the accumulation of the contaminated material? What were the species that were in the muck—the isotopic species? What were the concentrations? What could you do about it?”
Engineers from Bechtel Corp., a construction and project management firm that had been hired to carry out the cleanup, enlisted Whittaker’s help. He had assembled a team of students who worked furiously over the next six months to build the first robot, the Rover, which was sent to Three Mile Island in April 1984. The six-wheel machine carried lights, cameras and a device that unwound and rewound a ribbonlike tether, which supplied power to the robot and conveyed video images to its operators. Bechtel engineers later modified the robot and attached a radiation detector with an indicator that could be viewed by the cameras.
“The whole purpose was to go into the lower part of the reactor building,” says David Giefer, currently an assistant project manager for M2T Technologies, a wastewater systems company, and a former Bechtel engineer responsible for devices used in the cleanup. “A person couldn’t go in. That was the job for the Rover.”
The Rover was lowered into the basement through a hatch in one of the floors of the reactor building, and operators watching monitors in a control room drove it through mud, water and debris, capturing the initial post-accident images of the reactor’s basement. The Rover was used for several years, during which it was fitted with various tools that were bolted on, allowing it to scour surfaces, scoop samples and vacuum sludge, among other tasks. “It performed extremely well,” Giefer says.
A second version of the Rover carried a core sampler—a column of automated drills capable of extracting circular pieces from the walls of the reactor. Engineers attached the core sampler to the original Rover because the robot was already contaminated. “We didn’t need more contaminated equipment,” Giefer says. The tool allowed scientists to determine the intensity and depth of the radiation that had soaked into the walls.
Then came Workhorse, a far more complex robot that featured system redundancy—parts installed in pairs so the machine could continue operating if one of the components failed. Its boom could extend to reach great heights and it was internally pressurized to force out possible contamination. It was built to perform cleanup tasks, from power-washing surfaces to demolishing structures.
But Workhorse proved impractical. “They asked for a Swiss army knife, and we built them a Swiss army knife on steroids,” says John Bares, a robotics research professor at Carnegie Mellon’s Robotics Institute who worked on the project as a student. Giefer, the former Bechtel engineer, says Workhorse was never used because it had “too many complexities,” and “you’d have a nightmare cleaning it up and fixing it.”
By 1990, the bulk of the cleanup had been completed by humans and various robots, with debris from the reactor core shipped to the Idaho National Laboratory for storage. Managers of the plant, then owned by General Public Utilities Corp., and federal safety officials decided it would be less costly—in terms of money and workers’ health—to allow the remaining contamination to decay naturally than to continue removing it.
Today, the damaged Unit 2 reactor, now owned by Akron, Ohio–based FirstEnergy Corp., remains permanently shuttered, though the undamaged Unit 1 was switched back on in 1985 and continues to operate. The working reactor’s operating license expires in 2014, and its current owner, Exelon Corp., has applied to renew the license for another 20 years.
Other robots were used in the Three Mile Island cleanup, but those built by Whittaker’s group were the first to be utilized inside the reactor building. “The big thing about these machines is that they were mobile” at a time when robots typically were built to operate from fixed positions, such as those on factory floors, says Michael Pavelek II, another former Bechtel engineer, who was responsible for the robots’ operation and is now executive director of the Greater Lebanon Refuse Authority, a landfill in Lebanon, Pa. “You couldn’t put people in there safely. That was the huge advantage of these devices.”
Soon the robotic effort will be preserved for posterity. To coincide with the 30th anniversary of the accident, The State Museum of Pennsylvania in Harrisburg, some 13 miles (20 kilometers) southeast of the plant plans to open a display featuring a version of the Rover and other items used in the Three Mile Island cleanup.