On a Fission and Fusion Mission

Most of MIT’s buildings are known not by name or by address, but by number. Building 10 is the iconic dome with the large grass courtyard stretching to the Charles river. Building 54 is the 295-foot tall, I.M. Pei-designed concrete block that looms over campus and from which pumpkins are dropped at Halloween. Building NW12 is the nuclear fission reactor.

More colleges than you might think (about 30) operate nuclear reactors. They’re not the kind with the large concrete hyperboloid cooling towers usually associated with nuclear power plants. Instead, they are small research reactors. MIT’s fission reactor is hidden in plain sight under a sky-blue dome behind a chain-link fence topped with razor wire. There’s also a fusion reactor, known as a Tokamak, squirreled away in building NW21.


Most people are familiar with nuclear fission — if not in detail, at least in principle. Fission is the splitting of the atom into smaller parts, which results in a release of energy and, potentially, a chain reaction that releases vast amounts of energy. In a controlled fashion, the chain reaction creates heat that can boil water, with the resulting steam used to spin a turbine and generate electricity. Uncontrolled, it’s a nuclear bomb.

MIT’s six-megawatt reactor is the second-largest university reactor after the University of Missouri. (Commercial reactors typically produce 1,000 megawatts.) Built in 1958, it’s also among the oldest. And all you have to do to look inside is ask.

The first sign that the nuclear reactor tour isn’t your standard show-and-tell is the distribution of personal dosimeters — small devices that measure radiation exposure. From there, the group enters a small chamber bounded on either side by enormous, foot-thick steel doors enveloped by vacuum-pressure seals. Once the door behind us closes and seals shut, and any claustrophobics start to panic, the door in front opens to reveal the containment unit.

Whatever budget the reactor has, none of it is set aside for decorating. The concrete walls of the structure are several feet thick and lined on the inside with conduit, pipes, and instruments. This is a workspace for engineers and scientists, and it shows.

Although we weren’t allowed to take cameras into the containment unit, MIT published a video tour you can see here:

In the center of the unit is the reactor itself. Encased in another 5-½-feet of concrete, the core stands roughly 20 feet tall. A metal staircases leads to the reactor’s top, where researchers can access the uranium-filled rods submerged in water.

Around the reactor itself are a variety of instruments used in experiments. For example, students insert various materials into the core to learn how they react to high-radiation environments. Such experiments can allow for the development of new materials that could lengthen the lifespan of nuclear power plants. Here are five other cool things you can do with a nuclear reactor:

Beneath the core lies a small operating room where doctors used to conduct radiation experiments on cancer patients. The empty room’s green tiled walls, mid-floor drain, and fluorescent overhead lights lend an aura of horror to what was once a room dedicated to the cutting edge of medicine.

Around the corner sits the control room. Buttons, gauges, monitors and displays envelop the operator — often a student. Aside from being smart and focused, I’m thinking operators must also be good sports; surely every person who tours the facility makes the same corny jokes and demonstrates the same awe the recent teenagers are operating a nuclear plant.

Exiting the rector requires testing our hands and feet for radiation contamination, and checking and returning our dosimeters. Yes, the threat of contamination is ever present, but there’s also a reassurance that comes with the “clean” results the radiation detector displays after scanning our hands and feet.


A few doors down from the fusion reactor is the Plasma Science and Fusion Center. This is the home to MIT’s other nuclear reactor: the fusion tokamak.

Whereas fission is all about splitting the atom, fusion is all about forcing atoms to merge. This is how the sun works, and that’s a pretty good source of energy. Despite being a form of “nuclear” energy, fusion doesn’t use radioactive elements, like uranium, and doesn’t leave radioactive waste behind. But, so far, technical challenges have relegated fusion to the old joke, “it’s the energy source of the future, and always will be.”

That said, if scientists and engineers can ever master fusion, it would be revolutionary. That’s why 35 countries are working together to build the $14 billion International Thermonuclear Experimental Reactor in France. Known as ITER (rhymes with lighter), this reactor is of a variety called a tokamak, which uses powerful magnets to contain a 150 million degree celsius plasma.

MIT’s tokamak is scaled-down version, though it, too, generates a plasma in excess of an unfathomable 35 million degrees celsius.

Fellow Rowan Jacobsen and I took a tour of MIT’s tokamak fusion reactor, which is hidden under the blue concrete shielding behind us.

Why do you need a plasma that hot, and how do you keep it from just burning a hole through the Earth?

The answer to the first question is that creating a super hot plasma under pressure is necessary to force hydrogen atoms to fuse together. The answer to the second is: magnets. Using powerful magnets, engineers are able to guide the plasma, shape it like a doughnut, and start the fusion process.

There’s not a ton to see when looking at the tokamak itself. A giant concrete sleeve surrounds the device itself. A small porthole provides access for lithe graduate students who need to get inside the device.

There’s a market for slim grad students who aren’t afraid to squeeze themselves through this 20-inch-wide porthole. Getting inside the tokamak reactor is necessary for maintenance and to swap out components.

The tokamak is impressive and the gleaming metal interior is beautiful, as this 3D animation shows. But more impressive to me was the infrastructure around it. Producing a 35 million degrees celsius plasma takes a serious energy jolt — roughly 4 million watts. To deliver that much power, you can’t rely on standard copper wire. Instead, enormous metal beams — visually similar to what you might find being used to erect a skyscraper — serve as the tokamak’s wires. Anything less substantial would melt under the heavy electrical load.

One of the MIT tokamak’s achievements was its setting of a world record for plasma pressure, which it achieved on Sept. 30, 2016. The day was notable for another reason, too. It was the day funding for the reactor ended.

Although it has sat quiet since, there may be a new tokamak on the horizon, thanks to an investment from an Italian energy firm. The goal is to have the new rector, Sparc, ready in three years. So for now, the saying remains true: fusion is still the energy source of the future.

One thought on “On a Fission and Fusion Mission

Leave a Reply

Your email address will not be published. Required fields are marked *