Leslie Dewan, Nuclear Engineer

Picture of Leslie Dewan standing in front of a power plant
Photograph by Lynn Johnson, National Geographic Photography Fellow

Leslie Dewan, Nuclear Engineer

Building a Better Nuclear Reactor to Combat Climate Change

Leslie Dewan is helping revolutionize the nuclear power industry by designing a safer, more efficient alternative to today's reactors.

Talking from Cambridge, Massachusetts, Dewan explains how a high school teacher inspired her to become a physicist, how molten salt reactors annihilate their own nuclear waste, and why she believes nuclear power is a tool we need if we are going to reduce climate change. —Simon Worrall


How did you get interested in nuclear engineering?


I had two really wonderful physicist teachers in high school, one of whom was the first female graduate in nuclear physics at MIT, and they both really inspired me. I ended up going to MIT myself for undergrad. I started out wanting to major in physics. But I realized I liked building and making things people could use, so I ended up switching to nuclear engineering and mechanical engineering.


You have said that nuclear power is "elegant." Explain what you mean.


What I like about it is that it is very compact and power dense. It's a new way of generating electricity that didn't exist in the world prior to 60 or 70 years ago. For the first tens of thousands of years of human history, people generated power through fire. This gets into the very heart of the atom and the nucleus, and being able to manipulate that is fascinating.


Since the Fukushima disaster, nuclear power has been under a dark cloud. How would you respond to critics who say it is too dangerous?


If you look at the numbers, it's actually thousands of times safer than coal and tens of times safer than solar or wind. There are about a hundred deaths per terawatt-hour of generation with coal. There are also a significant number of deaths every year from people installing rooftop solar panels or installing and maintaining wind turbines. The rate for nuclear is 0.04 deaths per terawatt-hour. The next-generation reactors will have all the benefits of conventional designs but be even safer, as they can consume their own nuclear waste.


You're conducting research that sounds like sci-fi. What are waste-annihilating molten salt reactors and how do they differ from current nuclear technology?


Our design is a version of what's called a molten salt reactor. Molten salt reactors actually date to the very beginning of the nuclear power industry. They were first evaluated in the 1950s and '60s at the Oak Ridge National Lab in Tennessee. They showed it was very safe, but it was also bulky and expensive and had a low power density. This was before there had been any significant commercial nuclear accidents. So they said, Why pay so much more for a safer reactor when the ones we have are safe enough? As a result, research slowed to a crawl for many decades.

The main advantage of molten salt reactors is that they use liquid rather than solid fuel. Conventional reactors require a continuous supply of electric power so they can constantly pump water over the core to keep it from overheating. What happened at Fukushima is that the core started heating up and caused a meltdown. In molten salt reactors, if you lose electric power, the fuel automatically drains into an auxiliary tank and freezes solid in a few hours.


Nuclear waste is a huge problem. Can these new reactors solve the headache?


Conventional reactors consume only about 3 percent or 4 percent of the energy from uranium. That's why conventional nuclear waste is so dangerous, because there's so much energy left in it. We use liquid rather than solid fuel, so we're able to extract 96 percent of the energy. By extracting more of the energy we can reduce the radioactive lifetime of the waste. Conventional nuclear waste is radioactive for hundreds of thousands of years. The majority of the waste coming out of our plants is radioactive for only a few hundred years. A few hundred years is still a long time. But it's a human time scale, whereas hundreds of thousands of years is a geological scale.


What inspires you in your work?


At the most fundamental level I'm an environmentalist. I'm doing this because I think nuclear power is the best way of producing large amounts of carbon-free electricity. I think the world needs nuclear power, alongside solar, wind, hydro, and geothermal, if we want to have any hope of reducing fossil fuel emissions and preventing global climate change.

In Their Words

The world needs nuclear power, alongside solar, wind, hydro, and geothermal energy.

—Leslie Dewan

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