China, Russia, and the United States are racing to put nuclear power plants on the moon. China and Russia in May agreed to work together to complete a lunar nuclear reactor by 2036. In response, NASA’s interim chief Sean Duffy announced in August that the United States would fast track its lunar nuclear power program to have one ready by 2030.
But this sudden frenzy raises a few questions—such as why do we want nuclear reactors on the moon in the first place? And how would they work? To find out, IEEE Spectrum spoke with Katy Huff, a nuclear engineer and the director of the Advanced Reactor Fuel Cycles Laboratory at the University of Illinois at Urbana-Champaign. Huff previously served as the assistant secretary for nuclear energy at the U.S. Department of Energy (DOE).
Why do the world’s biggest space organizations want nuclear reactors on the moon and what would they power?
Katy Huff: There’s a growing interest in having a more sustained presence of humans on the moon for scientific discovery. Resources like helium-3, which can serve as a fusion fuel, may be part of the appeal. NASA’s planning to build this kind of lunar exploration base through its Artemis program, and China and Russia are working together to build one called the International Lunar Research Station. Any such lunar base would absolutely need nuclear power. Renewables alone are too intermittent to meet the energy needs of life on the moon. Plus, the cost of getting things into space scales by mass, so the unmatched energy density of uranium fission is our greatest opportunity.
Why is it suddenly a race? What’s the urgency?
Huff: The momentum began with the fission surface power project at NASA, which a few years ago solicited designs for 40-kilowatt lunar microreactors. Three designs were selected and awarded US $5 million each. Since then, China and Russia have announced on at least three occasions a joint effort to design their own lunar microreactor with a launch target in the mid-2030s. In response, NASA is accelerating its timeline for the U.S. reactor to 2030 and increasing the target power capacity to 100 kilowatts. Sean Duffy has said publicly that if China and Russia are the first to stake a claim for a lunar power plant, they could declare a de facto keep-out zone, limiting the United States’ options to site its base. So the U.S. aims to get there before China and Russia to claim a region with access to water ice.
Designing Lunar Nuclear Reactors
What are the considerations for designing a nuclear reactor for the moon?
Huff: In very low gravity, fluids won’t behave exactly as they do on Earth. So the circulation patterns for the reactor’s fluid coolants will need to be recalculated. And the moon’s large temperature swings, which vary hundreds of degrees from lunar day to night, will require the reactor to use systems that are more isolated from those swings. On Earth we eject waste heat easily because there are thermally stable heat sinks like water bodies available.
What kind of reactor do you expect NASA to choose?
Katy Huff previously served as the assistant secretary for nuclear energy at the U.S. Department of Energy (DOE).Katy Huff
Huff: It would make sense if NASA chose one of the three designs previously selected for the fission surface power program, rather than starting from scratch. But with the over-doubling of target capacity, from 40 kilowatts to 100 kilowatts, there will be a bit of a redesign involved, because you don’t just turn up the knob. The three awards went to Lockheed Martin/BWXT, Westinghouse/Aerojet Rocketdyne and X-energy/Boeing. Some of them are developing microreactors that are based around tristructural isotropic [TRISO] fuel, which is a type of highly robust uranium fuel, so I would expect the lunar reactor to be designed using that. For the coolant, I don’t expect them to choose water because water’s thermal properties limit the range of temperatures it can cool effectively, which constrains reactor efficiency. And I don’t expect it to be liquid salt either, because it can be corrosive and this lunar reactor needs to operate for ten years with no intervention. So I suspect they’ll choose a gas such as helium. And then for power conversion, NASA’s directive explicitly said that a closed Brayton cycle would be a requirement.
What would transport and startup look like?
Huff: The reactor would be fully constructed on Earth and ready to go, with the fuel in place. My expectation is that it would be transported with the control elements fully inserted into the reactor to prevent a chain reaction from starting during transit. Once on the moon, a startup sequence would be initiated remotely or by the astronauts there. The control rods would then withdraw from the reactor and a small neutron source like californium-252 would kick off the reaction.
A deadline of 2030 feels pretty rushed considering the United States doesn’t have a final design for the reactor, nor firm plans for a lunar base.
Huff: Right. That timeline does appear ambitious. We’ll have a hard enough time getting a reactor of this scale deployed as a prototype terrestrially in the next four and a half years. Getting one launch-ready and onto the moon by then is a recipe for eventually having to explain why we didn’t meet that timeline. And that could be a problem, reputationally, for nuclear energy more so than space exploration because people love NASA. Little kids and grownups alike wear NASA T-shirts. No one’s wearing DOE T-shirts.
Risks of Lunar Reactor Launch
What are the risks if something goes wrong with the launch?
Huff: Beautifully enough, fresh uranium fuel doesn’t present a radiological hazard the way spent uranium would. Only after it becomes the fission products is it significantly radioactive. So as long the reactor doesn’t operate before launch, the hazard is quite low. Even if the fuel were dispersed over the Earth, it wouldn’t pose a significant danger to the people around it. I literally have a sample of uranium sitting by my desk. On top of that, there’s a robust launch safety protocol already established for any radiological object. NASA has a lot of experience with this from sending plutonium thermoelectric generators, which are more like a nuclear battery, for previous missions.
Things have gone wrong with some of the fission reactors previously launched into space; what happened to those?
Huff: The biggest fission reactors anyone has launched into space were the 5 kilowatt-electric TOPAZ-I reactors that were part of the Soviet program. One of them had a serious incident and broke apart. It’s now in high orbit in pieces, including some of its sodium coolant, which is just sort of floating around up there as liquid metal spheres. But that doesn’t impact the Earth because it’s a tiny amount of radiological source material at an incredible distance from Earth. The more unfortunate incident happened with the Soviet Kosmos 954 reactor, which, after operating in orbit, experienced uncontrolled re-entry and disintegrated over a 600-kilometer swath of Canadian territory.
What happens if an asteroid hits the moon, or directly hits the lunar nuclear reactor?
Huff: A direct strike could damage the reactor and cause localized dispersion of the fuel. This might be a motivation to use TRISO fuel. It’s so robust because the fuel and fission products are housed in thousands of spherical, chia seed-size particles that are coated in silicon carbide. It can withstand incredible impacts and heat—well beyond the temperature of lava. Testing has shown that even when subjected to 1700°C heat for 300 hours, TRISO retains its fission products with no failures. So in the unlikely event that there’s a dead-on collision with a large asteroid at the reactor site, the debris of the reactor may be distributed in the dust of the moon, but all those little TRISO particles will hopefully remain intact.
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