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
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 ver-
sions 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 graph-
ite to slow down neutrons and facilitate fission. Within a few
years, most of the remaining graphite piles were dismantled
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 recreation 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 radia-
tion 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!” —David L. Chandler
Piece of nuclear history
springs back to life
Long-forgotten experimental device
restored for research and teaching.
Professor Kord Smith and Micah
Gale ’ 18 with MIT’s restored
graphite exponential pile.