It was hiding in plain sight in a side room at MIT’s Nuclear Reactor Lab, covered with a thin layer of metal panels that made it look like an oversize storage cabinet. But inside that plain metal box was an important piece of history—and a potential boon for MIT’s research and education programs in nuclear engineering.
The box contained a graphite exponential pile—a smaller-scale version of the device physicist Enrico Fermi built under the stands of the University of Chicago’s football stadium to usher in the atomic age. Fermi’s 1942 experiment used graphite to slow down neutrons emanating from a radiation source by a factor of more than a million. That got them to interact with atoms in rods of uranium inserted into the pile, initiating the world’s first controlled nuclear fission chain reaction. It proved the theories that led to the first atomic bomb and, not long after, the first nuclear power reactors.
In the following years, after the secrecy of the wartime research was lifted, universities and research labs around the country rushed to build their own graphite piles—smaller versions that wouldn’t produce chain reactions but could advance research in this nascent technology. At least 25 were built, says Kord Smith, NUE ’79, SM ’79, PhD ’80, MIT’s KEPCO Professor of the Practice of Nuclear Science and Engineering. But by the early 1960s, the nuclear power industry had settled solidly on a different approach to designing fission reactors, using either ordinary water or heavy water (made from oxygen and deuterium, the heavy form of hydrogen) instead of graphite to slow down neutrons and facilitate fission. Within a few years, most of the remaining graphite piles were dismantled or mothballed.
MIT had been one of the first to build a graphite pile, in 1957, and that device was the largest built after Fermi’s, at half the width of the original. It was shut down like the others, apparently in the early ’60s, and there it sat, forgotten, until Professor Michael Short of NSE wondered what was inside the mysterious box.
“It was hard to believe that I was unaware of the existence of this unique device, despite having been in Nuclear Science and Engineering as a student and having maintained a long-running association with NSE,” says Smith, who has taught in the department since 2011. But once he knew it was there, he was off and running with plans to get it back into operation—partly because he knew, from using one at Kansas State in the 1970s, how useful it could be for research and education, and partly to celebrate the 75th anniversary of Fermi’s experiment.
The device is essentially a large cube-shaped pile of blocks made of pure graphite—the same material found in pencil “lead”—with holes drilled through to allow insertion of the uranium rods. Made from natural, unenriched uranium, they emit such low levels of radiation that they can be safely handled with bare hands, Smith says, which is what Fermi and his collaborators did in 1942.
This is not just a bit of nostalgia or a museum piece, however. Many newer reactor designs, such as pebble-bed reactors designed to be inherently safe and meltdown-proof, rely on uranium fuel pellets with graphite cladding. But there are very few places today to carry out basic research on graphite’s behavior in a nuclear fission environment.
MIT’s graphite pile, made of 30 tons of pure graphite bricks and 2.5 tons of uranium, has now been brought back into working condition, and NSE members conducted a ceremonial re-creation of Fermi’s historic experiment at the exact moment of the event’s 75th anniversary, on December 2, 2017. That marked the beginning of the pile’s new life.
Whereas experiments in major national nuclear research facilities require months of planning and stringent application processes, students will be able to build, test, and get results from experiments in the graphite pile within days, Smith says. It will also give them hands-on experience with operating a reactor.
Students were slated to use the pile in May to make radiation measurements in situ; in the fall, two NSE courses will use it. The students’ excitement is tremendous, Smith says: “They all want to load fuel themselves!”