Building A Safer, Cleaner Nuclear Reactor

Leslie Dewan and Mark Massie are reviving the nuclear dream

Marius Bugge

One day in 2009, Leslie Dewan and Mark Massie were rummaging through old articles in an MIT library. There, they found two things the nuclear industry had abandoned a half century before. One was a sense of urgency: The first fast-neutron reactor had gone from scribbles on a napkin, in 1945, to a working prototype in only 18 months. The other was something called a molten salt reactor.

In the 1960s, scientists had operated one at Oak Ridge National Laboratory in Tennessee. But then funding dried up, light-water reactors became the industry standard, and molten salt was all but forgotten. That seemed absurd to Dewan and Massey. Molten salt reactors, they learned, are safer than what’s used today. They burn liquid uranium fuel, rather than solid fuel, which reduces the chance of a meltdown. As part of a generation of young nuclear scientists motivated by climate change, and willing to take a fresh look at nuclear power, Dewan and Massey wondered why the world wasn’t using this. They knew that to beat out cheap coal and natural gas, new reactors needed to be safer and more efficient. So they dusted off the Oak Ridge design and got to work.

Today, their start-up, Transatomic Power, is poised to build a new, even better molten salt reactor. Their reactor will burn up to 96 percent of its fuel, compared with only four percent used by light-water reactors, and generate 75 times the electricity per ton of uranium. It’s virtually accident-proof and can run on the spent fuel of other reactors. With nearly 80,000 tons of radioactive waste in the U.S. (and with 2,000 tons added every year), it could turn something toxic into something useful. “That’s what sets Leslie Dewan and Mark Massie apart,” says Richard Lester, head of the nuclear science and engineering department at MIT. “Their design addresses radioactive waste, which is huge.”

Their reactor will generate 75 times more electricity per ton of uranium

So far, Transatomic has obtained $6 million in funding, including $2 million from venture capital firm Founders Fund (backers of Spotify, Airbnb, and SpaceX). Dewan and Massie have used some of this seed money to set up a lab in Cambridge, Massachusetts, where, for the next three years, they will test materials for their future reactor under the extreme conditions created by nuclear fission. But their road to an actual reactor will be a very long one.

Consider America’s relationship with nuclear power. During the Cold War, the U.S. wanted to build a commercial nuclear power industry before the Russians. The U.S. Navy already used light-water nuclear power on its submarines, providing an easy blueprint to expand upon. The early and technically expedient decision to commercialize light-water reactors, “locked the U.S., and the rest of the world, into one type of technology,” Dewan says.

All 99 U.S. commercial reactors in use today are light-water reactors. That uniformity is not a problem until something goes wrong—and then the entire nuclear industry gets tarred with the same brush. The meltdowns at Three Mile Island and Chernobyl turned an entire generation away from nuclear power. The meltdown at the Fukushima Daachi plant in 2011 deterred another, despite the fact that nuclear is easily scalable, carbon-free, base-load power. “When nuclear fails, it fails spectacularly,” Massie says. “And it gets a lot of coverage.”

When I visit their offices in Cambridge, Dewan and Massie clutter up a whiteboard to show just how safe and efficient their reactor will be. In light-water reactors, fuel rods packed with uranium pellets are submerged in water, which slows neutrons to a speed that induces fission in the uranium and heats the rods. The hot water then powers a steam turbine that generates electricity. The system works well, except for two problems.

The first is that fission product poisons, like krypton and xenon, accumulate in the rods. These gases consume neutrons, which ultimately stops the fission reaction. Every four years or so the rods need replacing, even though 96 percent of their energy remains untapped, rendering them highly radioactive waste products that have to be stored in a secure location forever. The other problem is the reactor’s constant need for electric power to pump cold water over the core, which holds the fuel rods. This water prevents it from overheating. Lose power, like at Fukushima, and the core melts down.

In molten salt reactors, uranium salt serves as the fuel. When heated above 500°C, the fuel salt changes from a solid to liquid state. It flows past zirconium hydride, which slows its neutrons and induces fission, generating heat. Because there are no rods to trap krypton and xenon, they are continuously off-gassed. “You basically simmer the reactor like a Crock-Pot for decades,” Dewan says. “That’s how we achieve a 96 percent burn rate. We’re able to leave the uranium in and constantly remove the poisons that would otherwise shut it down.” The fuel salt flows through a loop with a drain that’s blocked by a freeze plug, a chunk of electrically cooled frozen salt. If the reactor loses electricity, the plug melts, and the fuel drains into a tank where it cools and solidifies.

“You basically simmer the reactor like a Crock-Pot for decades. That’s how we achieve a 96 percent burn rate.”

While the nuclear physics of molten salt reactors were proved at Oak Ridge 50 years ago, Dewan and Massie need to rigorously test the reactor’s materials. Down the street from their Cambridge offices, Dewan shows me a lab where they’ll be exposing metals and ceramics—components that could be used in the reactor’s pipes, pumps, and valves—to salt and extreme heat using a furnace the size of a refrigerator. Their next step will be to design and build a 20-megawatt demonstration reactor that they hope to have running by 2020. Their ultimate goal is a 520-megawatt commercial reactor, which they say can be built for $1.7 billion­—half the cost of a light-water reactor. Despite the favorable economics, the U.S. Nuclear Regulatory Commission is not focused on licensing advanced reactors. “There’s no regulatory framework that allows a developer to say, ‘This is what we need to get approval,’” says Lester. That, in turn, deters investment. “Investors want to see a regulatory pathway,” Lester says. So companies like Bill Gates’s TerraPower, which has developed a next-generation traveling-wave nuclear reactor, are looking to build abroad. Massive energy needs and pollution problems have driven China to investigate many types of reactors—molten salt, sodium-cooled fast reactors, and high-temperature reactors. China may commission multiple models, since it plans to build 45 reactors over the next decade.

To Dewan, sending their technology to China would be a missed opportunity. If the U.S. wants to lead the world in clean, safe nuclear energy again, she says, it urgently needs to broaden regulations. But there’s resistance from the coal and natural gas industries, which see nuclear energy as a bottom-line threat, and from some environ­mental groups that argue nuclear will always pose risks. But Dewan is undeterred. She doesn’t want to build a molten salt reactor in China. She wants to keep Transatomic’s operations in the U.S., where the promise of nuclear energy was born. “It’s American technology,” she says. “It’s American engineers. We want this country to have the benefits of it first.”

How It Works

  1. Rather than use solid fuel, a molten salt reactor heats uranium salt to more than 500°C, liquefying it.

  2. The liquid fuel flows past a moderator that slows down neutrons and induces fission, generating heat for steam turbines.

  3. If a reactor loses power, a freeze plug melts, draining the radioactive fuel into a tank where it cools to a solid.

This article was originally published in the June 2015 issue of Popular Science as part of our “New Faces Of Energy” feature.