The nuclear stargazer | Ep 99

Intro Voices

Coming to DARPA is like grabbing the nose cone of a rocket and holding on for dear life.

DARPA is a place where if you don't invent the internet, you only get a B.

A DARPA program manager quite literally invents tomorrow.

Coming to work every day and being humbled by that.

DARPA is not one person or one place. It's a collection of people that are excited about moving technology forward.

Tabitha Dodson: If you think back to when people invented fire—of course we weren’t there, but it was an amazing discovery, I’m sure—when human beings discovered how to rub sticks together and how to harness the energy of a combustion reaction. That was a very fundamental scientific problem that the primitive human was able to dominate. However, in my opinion, we’re at the same level of primitive understanding with nuclear science. We can take two sticks, which is the U-235 and a neutron, and we can kind of rub them together and make fission. But beyond that, we can’t really do anything more sophisticated.

Tim Haynes: What will it take to move beyond the nuclear equivalent of rubbing sticks together, and harness nuclear energy wherever enduring power is needed? For today’s guest, that question sits at the intersection of her work at DARPA and a fascination she’s carried since childhood: the desire to reach further into space. Hello and welcome to Voices from DARPA. I’m your host, Tim Haynes. On this episode, we’re talking with Dr. Tabitha Dodson, a nuclear plasma physicist and aerospace engineer.

Tabitha Dodson: I am Tab Dodson. I’m a program manager in the Defense Sciences Office, and I run a portfolio of nuclear space programs.

Tim Haynes: Tab’s story doesn’t begin with a reactor. It begins with a deep sense of wonder and curiosity.

Tabitha Dodson: I think I was in third grade. There’s a book by—I think it’s by Kip Thorne—called Black Holes and Time Warps. They just talked about these extreme phenomena that were just out there, far beyond our reach. We couldn’t get to them, but we could measure them and study them from Earth. And we only just scratched the surface with respect to how we understand how these things occur. They’re so extreme as compared to how we kind of run our everyday, mundane, boring lives on this planet. We don’t have a magnetic field that’s so strong that it rips the orbit of an atom into a cylinder instead of a sphere. You can only get that if you’re pushing the theoretical limits of the magnetic field in space.

Tim Haynes: That interest in the unreachable stayed with her. But the path from a childhood curiosity about black holes to a career at DARPA wasn’t a straight line. 

Tabitha Dodson: I was raised in West Virginia, and I would say everyone around me had a very similar upbringing. Nobody in my family went to college. We also lived on a farmstead in the middle of a huge cornfield in a house that was over 100 years old. And we didn’t have what you would call modern plumbing, so we had an outhouse. We didn’t have heat in the winter.

The way I was raised was that your value and your output was measured by your productivity—something that someone could see. So if you said you were a hard worker, then you could sort of measure that by the amount of hay bales that you piled or the amount of crops that you harvested.

Tim Haynes: After high school, Tab worked on a horse farm. While there, her grandmother encouraged her to try college to see where it might lead.

Tabitha Dodson: I entered physics because it was something I was truly passionate about and interested in, and there are so many unanswered questions that span the entire gamut from particle physics all the way up to space travel.

Tim Haynes: A coding hobby from high school gave Tab a way to create that same kind of measurable value, and helped her pay her way through college.

Tabitha Dodson: I consider myself really lucky when I was a kid because it was around the time when computers started to be put in schools, and I was fascinated. I had not only a part-time job in high school kind of doing minimum-wage stuff, but I also got a part-time job at the high school as the assistant to the computer science teacher. And so, I became such an asset that I got the keys to the computer lab, and I got to go in there whenever I wanted. I spent a ton of time in high school in the computer lab writing shell scripts, writing code. It became my passion as a young person.

Fast forward to when I was going to enter college—and keep in mind that I didn’t go straight to college after high school, I was working on a farm for a while. And again, in order to afford college, I had to work part-time jobs and take out loans, compete for scholarships, the sort of normal stuff, but it still wasn’t enough money. So, I went to the astrophysics department, and I said, ‘I want a job. What can I do?’ And they realized that I was a really gifted coder. I could churn out hundreds of lines of code, which is what you need to do in order to do astrophysics. All astrophysics is, is pulling in terabytes of data from space across the spectrum—radio, X-ray, visible, whatever—and you’ve got to code all the math depending on what theories you’re running with of how you think these phenomena are behaving, and just churn through tons and tons of data.

Tim Haynes: Tab’s early coding work for the astrophysics department helped launch her career. She went on to earn bachelor’s degrees in physics, sociology, and anthropology, followed by a master’s and two doctorates in fields ranging from space policy to applied physics. Across her academic career, and in her work with the U.S. Naval Academy, Air Force, Space Force, NASA, and other government agencies, her fascination with space remained the threadline.

Tabitha Dodson: This is going to sound really silly, but have you ever seen the movie, Moana? When she’s looking across the ocean and saying, ‘It’s calling her.’ I saw that movie and I was like, ‘That’s how I feel for space.’ It calls me. And then she gets in the boat and goes—but we can’t do that; we’re stuck on the island. The thing that generates the most energy and which has the most potential to revolutionize our ability to get into space is nuclear, and so that’s why I happened to fall into that.

Tim Haynes: Nuclear technology may be our ticket to the stars, but it’s been slow to mature into new capabilities. Tab’s desire to bend the development and adoption curve for nuclear technology made DARPA an obvious place to work.

Tabitha Dodson: This might be hard to believe, but I’ve actually been reading books about DARPA since I was a kid, and have always thought to myself, ‘Wouldn’t it be awesome to work there at DARPA? That would be the greatest place to work. You could really drive some life-changing, world-changing technologies.’ And I actually didn’t think it’d be possible to work at DARPA. But then one day, in 2018, I heard a rumor that the Pentagon was interested in nuclear propulsion. And so, it actually gave me the confidence to apply to come to DARPA because I saw that as, finally, this is my chance to go somewhere where it was my dream to work, and I finally have the ticket to say that I’m worthy to be there.

Tim Haynes: DARPA brings in program managers with deep expertise in a field. Their job is to identify a critical problem, define a bold but testable technical goal, and then drive towards a demonstration that can change what comes next. For Tab, one of those world-changing ideas is Fat Cat, a program developing and validating groundbreaking models to help predict and control the physics of small nuclear reactors.

Tabitha Dodson: Fat Cat [Fission Autonomous Control / Fast Capacitance], is meant to be a first-of-a-kind demonstration of 100% reactor autonomy. Right now, we have reactors—either in our commercial grid power fleet or in naval reactors—which have some level of automation, but nothing which is 100% autonomous without a human in the loop. There are a couple of things that we’re going to be overcoming in this program. One is an extreme aversion from a safety culture perspective. So this is kind of ‘DARPA-hard’ in a sense that we’re breaking through this fear. It’s kind of a big fear factor when you say, ‘Okay, I’m going to make a reactor where it can control itself.’ Of course, nobody does that, although they say that they can. And then, of course, we have our technical challenge, which is actually making a controller that has an algorithm which is capable of keeping up with the changing neutronics of the reactor such that it can predict and control that reactor.

Tim Haynes: To understand the technical challenge, we have to start with the basic physics of fission. A nuclear reactor works by harnessing the instability of an element like uranium. When a loose neutron strikes a uranium atom, the atom splits apart, releasing more neutrons and starting a chain reaction. That splitting also creates an enormous amount of heat, which we then use to generate electricity. 

Tabitha Dodson: We are starting with a very small fast-spectrum reactor. Fast-spectrum reactors are unmoderated, and they’re simple in a physics sense because our fast-spectrum reactor just has a chunk of uranium-molybdenum, and that’s basically it. And we have a B4C [boron carbide] rod that goes in the middle and beryllium that goes on the outside of the reactor.

If you look at basically any other reactor in this country—commercial grid power, naval reactors, or even some of the designs for some of the space reactors right now—those reactors are made up of hundreds or thousands of fuel elements. Those fuel elements have cladding, insulation. They have thousands of holes in them. They have around the outside of the core at least four or five layers, and every single material has an impact on the statistical distribution of the energy of the neutrons and how they are actually able to make power and so on.

Tim Haynes: Without moderators, it’s possible to make reactors that are a fraction of the size, which creates new challenges and opportunities. Neutrons in a fast reactor move roughly 10,000 times faster than those in a conventional reactor. With fast neutrons, it’s possible to quickly throttle the amount of power being generated, enabling a much wider array of use cases.

Tabitha Dodson: We are going to take an existing core to prove that the algorithm can estimate the power levels of the reactor over time and then, obviously, power a controller that will control the power of the reactor over time. So, this will be the first-ever demonstration of autonomous control and also using an algorithm that can, with high certainty, predict the power over time of that reactor.

Tim Haynes: Choosing a small reactor with relatively straightforward physics doesn’t mean the challenge is easy. It means DARPA is selecting a manageable place to demonstrate a new approach. A reactor controller must measure how power is changing, anticipate what happens next, and respond almost instantly to keep the system safe. Autonomy appears to be a viable solution to build these small, safe reactors. And while Fat Cat aims to demonstrate that autonomy, its greater value may lie in the powerful new tools that it produces along the way.

Tabitha Dodson: A big thing that’s going to be coming out of this program, and that I would like to be able to pass on to the broader nuclear enterprise, are things like fast-acting neutron sensors, fast-acting thermocouples, and also rad-hard controllers and electronics that the autonomous control actually utilizes in order to control the reactor.

Tim Haynes: Rad-hard, short for radiation-hardened, describes electronics designed to operate in high-radiation environments.

Tabitha Dodson: In particular, if we want small, compact systems—so Fat Cat is a very small reactor, it could be used in a small platform—you need to put the controller really close to the reactor, maybe even closer than a couple of meters away. And in that sense, it has to not only be computationally very high quality, but it also has to be able to survive the radiation environment.

Tim Haynes: Tab aims for the lessons learned from Fat Cat to accelerate nuclear innovation writ large. By focusing on understanding and proving out the difficult physics for fast reactors, the program will create models to allow the broader nuclear power ecosystem to safely design and gain approvals for new types of reactors. And by pioneering autonomous control systems, Fat Cat will enable small reactors to power mission-critical systems in remote environments, revolutionizing energy solutions for space exploration, undersea operations, and beyond.

While Fat Cat explores compact reactors, another program in Tab’s portfolio, called Rads to Watts, takes a completely different approach to enduring power.

Tabitha Dodson: At a very high level, Rads to Watts is using nuclear radiation to drive electricity with direct energy conversion methods. That’s as compared to indirect energy conversion, which is what we use now. Think, for example, a nuclear reactor which generates radiation, but the radiation generates heat, and then the heat goes on to heat water, and then we use the water and the steam to drive electricity. We don’t directly take the radiation and sink it into something that pushes electrons.

That’s what Rads to Watts seeks to do, and so we are taking advantage of some of the more advanced rad-hard electronics that are out there right now. We have seen in the nuclear radiation-hard electronics community some breakthroughs in voltaic device technology.

Tim Haynes: One established type of nuclear battery is an RTG, or radioisotope thermoelectric generator. RTGs convert heat from decaying radioactive material directly into energy. They’ve powered many space probes, including NASA’s Curiosity and Perseverance rovers on Mars, because they’re able to provide reliable power for decades. But RTGs are highly inefficient.

Tabitha Dodson: They take advantage of the heat to drive the radiation conversion, and so, in that sense, there is only less than 5% efficiency for those things. There are thousands of watts of untapped energy in the nuclear material which could otherwise be converted to electricity. Efficiency is one element of bringing down the size or the volume of the device, but also increasing the overall watts per kilogram. So, for example, think of an RTG which, in its current form factor, creates about 100 to 300 watts. If you can have the exact same size, but generating on the order of 1,000 watts, 2,000 watts—that’s kind of what we’re aiming for.

Tim Haynes: Rads to Watts aims to deliver 10 times the power efficiency of RTG systems. While RTGs rely on heat, the radiovoltaics being developed in Rads to Watts bypass heat entirely and work more like solar panels, but powered by nuclear particles instead of photons of light. Rads to Watts has seven performer teams designing radiovoltaic cells for enduring power in extreme high-radiation environments.

Tabitha Dodson: There have been past radiovoltaic programs, even at DARPA, but those programs only focused on, say, one material, like the charge generation region, which is kind of the most important—that’s like the sandwich layer that stops a particle, where the radiation particle creates all the electrons into the charge generation region, and then the electrons come out.

But in this case, the device is the most important thing. That’s the thing that you put into your platform to make power. And in order to make a device, you need something to collect the electrons, so you need the charge collection region. That’s the electrode. And then, of course, you need the radioisotope itself, the source, that kind of goes on top. They have diffusion layers; voltaics, like photovoltaics, in a lot of cases might have four or five different layers depending on the different dopant levels and things like that in order to make them truly high efficiency. The multilayer approach is the other innovation that we’re seeking to overcome.

Tim Haynes: Fat Cat and Rads to Watts face different challenges. But they point to the same larger question: What becomes possible when systems carry reliable power with them, instead of depending on a nearby grid or a favorable environment? Beyond her portfolio of DARPA programs, Tab is also thinking about how society can move nuclear technology from promise to practical use, and how we can accelerate solutions with global impact.

Tabitha Dodson: Nuclear science is like a city which kind of was started. There’s a big blueprint for the city, and maybe there were a few bricks laid, but the city hasn’t been built yet. The fact that we’re using steam-age technology to create energy out of something as simple and primitive as what’s basically like rubbing two sticks together, in my opinion, is ludicrous. There’s this chasm that exists between what we know and what we want.

Sometimes, nuclear enthusiasts would say that if you could truly proliferate, socialize, normalize, and truly master nuclear power, then you could bring unlimited energy to society. Energy is used for everything from medicine to farming to anything that involves resources. And so, from a Department of War perspective, it would enable us as a nation to push off the surface of the Earth, to secure our assets, to ensure that we could move faster, be less vulnerable, and have more energy to do a greater variety of missions in space. And this is something where I feel so lucky to be at DARPA.

Tim Haynes: Big ideas require more than just enthusiasm. They require people who understand the field so deeply they can distinguish what’s possible today, what is still unknown, and what it will take to bridge that gap. And they require a place like DARPA, which brings the right people together to turn promising ideas into world-changing innovation.

Tabitha Dodson: In my case, I’m a nuclear space person. I have expertise in that area. I can see what’s real, what’s crazy, and what’s fantasy on paper, and I can kind of sift through the nonsense and pull out the reality of what’s possible to be done. DARPA does give individuals who have big ideas—who say, ‘Okay, I need X, Y, Z materials or resources or time to do a program or solve a problem in a short amount of time’—it gives people those resources to do those things.

Tim Haynes: At DARPA, the distance between aspiration and reality is closed with experiments, models, and real-world demonstrations. For Tabitha Dodson, it’s also personal. The same curiosity that began with books about the mysteries of space continues to drive her to create the power systems that could one day take us there.

Tabitha Dodson: As a species, we seek to explore and to travel the unknown, and I think that nuclear would really open up the door to possibilities to enable us to do that.

Tim Haynes: That’s all for this episode of Voices from DARPA. To learn more about Dr. Tabitha Dodson and her programs, including Fat Cat and Rads to Watts, check the show notes or visit www.darpa.mil. Thanks for listening.