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Leslie Dewan

, 28


Roozbeh Ghaffari

What if we could build a nuclear reactor that costs half as much, consumes nuclear waste, and will never melt down?

The nuclear power industry has a reputation for resisting innovative changes. But Leslie Dewan and a colleague have dared to invent a new type of nuclear reactor. “We were feeling on top of the world. We just passed our qualifying exams for our PhDs,” she says. “We thought, ‘We’re the smartest we’ve been in our lives. We can do anything. Let’s change the world with nuclear.’” Two years later, she’d designed a reactor that solves the main problems facing nuclear power. To commercialize it, she’d cofounded a startup, Transatomic Power.

For decades the nuclear industry has built one type of reactor, called a light-water reactor, almost exclusively. There are significant problems with that technology, which uses ordinary water to cool the fuel rods in which the nuclear reaction takes place. It requires expensive safeguards against a radiation-releasing meltdown if the fuel rods overheat; it produces waste products that are dangerous for 100,000 years. Dewan and a fellow graduate student, Mark Massie, designed an alternative based on molten-salt reactors that were originally proposed in the 1950s as a way to power aircraft. Though nuclear planes never became a reality, the reactor design has several key advantages. For one thing, it can be readily modified so that rather than producing large amounts of waste, it reuses much of the spent nuclear material as fuel.

It is also far safer than the light-water reactors, which require a constant source of electricity to pump in cool water and prevent the runaway nuclear reactions that lead to meltdowns. Molten salt serves as the coolant; it’s mixed with the nuclear materials, so the reactions take place right in the liquid. The heat of those reactions keeps the salt molten. A plug at the bottom of the reaction vessel is made of the same salt, kept solid by cooling it; if the plant’s electricity supply is lost, the plug warms up and liquefies, allowing the contents of the reactor to drain into a large containment tub and spread out so that the nuclear chain reactions come to an almost complete stop. The nuclear material and molten salt then cool down and turn into a contained solid that poses no danger of a meltdown.

The technology had one glaring problem, though: the reactors were large and, thus, expensive for the amount of power they produced. Dewan found a solution. “We realized that with some relatively modest changes to molten-salt reactors we could make them much more power dense and therefore a lot cheaper,” she says. She introduced new materials and a new shape that allowed her to increase power output by 30 times. As a result, the reactor is now so compact that a version large enough for a power plant can be built in a factory and shipped by rail to a plant site, which is potentially cheaper than the current practice of building nuclear reactors on site.

The reactor also makes more efficient use of the energy in nuclear fuel. It can consume about one ton of nuclear waste a year, leaving just four kilograms behind. Dewan’s name for the technology: the Waste-Annihilating Molten-Salt Reactor.

So far, the design exists as a 180-page document, computer simulations, and patent filings. Dewan has designed five experiments, each of which will cost about $1 million, to prove key aspects of the design. If those go well, she’ll still face a decade or more of further tests and U.S. federal certifications that could cost hundreds of millions of dollars. And she suggests that the future for new nuclear-power technology might not be in the United States. She points in particular to China, which is spending far more on new reactor designs and on the construction of nuclear plants.

But though it will be a long and uncertain route to commercialization of the technology, Dewan is driven by what is at stake. She’s part of a new generation of young researchers who see nuclear energy as one of the best hopes for averting disastrous climate change. Dewan originally looked to solar and wind power as ways to reduce carbon dioxide emissions, but “then I looked at the numbers,” she says. “I realized that nuclear power is the best low-carbon energy source that’s available and scalable.”

Kevin Bullis

Credits: Illustration by Golden Cosmos, photograph courtesy of Leslie Dewan; data from European Nuclear Society

Tagged: Energy, EmTech2014

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