Designing a nuclear reactor for space requires a unique approach. A proof of concept test was completed in 2013 – the Demonstration Using Flattop Fission (DUFF) experiment, at the Nevada National Security Site. Following DUFF, the team worked to further ensure the reactor was the best fit for outer space.

“The design is unique, using heat pipes to passively move the heat from the reactor up to the power conversion,” says Marc Gibson, lead Kilopower engineer at NASA’s Glenn Research Center.

“We got the fuel from the Y-12 National Security Complex, we designed the core, sent them all the drawings and the manufacturing specs, and they supplied the highly enriched core. Then we mate it up with our power-conversion system, which we designed, built and tested electrically here at NASA. We combined the fuel and power conversion at the test site in Nevada and that became the Kilopower Reactor Using Stirling Technology (KRUSTY) experiment.”

Kilopower uses a solid, cast uranium-235 reactor core to create heat that passive sodium heat pipes transfer to high-efficiency Stirling engines. The design differs from its terrestrial counterparts in many ways, including higher temperature fuel and a fast reactor rather than thermal, but most importantly, it is on a much smaller scale than any reactor used on Earth, and the core is only the size of a paper towel roll.

“This is very low-power compared to terrestrial reactors,” says Gibson. “Terrestrial reactors are making GWs of electricity, we're making KWs. Kilopower ranges from 1KW-10KW and that would only power one to five homes terrestrially in the US, so you can try and get a feel for the scale of that.

“When you put that into space it’s a big deal because 800W or 900W is about all we've ever done with a nuclear system for space, so our low end is starting above the largest power system that we've ever put in space.”

On top of the technical challenges, Gibson also had to contend with the nuclear legacy within NASA.

“A lot of people didn't believe that we could test nuclear reactors at a low cost and in a timely manner,” says Gibson. “It's hard to overcome that scepticism. It proved a lot of people wrong, especially those that thought this was going to take hundreds of millions of dollars.”

While many within NASA were concerned, the low profile of the technology ensured that criticism in the public space was minimal. Terrestrial nuclear projects notoriously have to contend with passionate anti-nuclear groups, who question the safety of nuclear. Although launching a nuclear device into space may seem like it would provoke concerns, Kilopower is safer than previous technologies.

“What a lot of people don't know is that when you look at how many curies of radioactivity the fuel has at launch, [the fission reactor is] several orders of magnitude lower than the radioisotope systems that were launched with the Curiosity mission.”

“We can easily prove that launching a fission reactor is going to be several orders of magnitude safer than the radioisotope systems that have already launched, so it should be a slam dunk.”

Kilopower has already proved to be NASA’s most successful nuclear project in half a century, not only reaching the testing phase, but also proving itself in the KRUSTY test in March this year.

“I think [KRUSTY] was very significant in that past projects spent hundreds of millions of dollars and didn't get very far,” says Gibson. “We spent less than $20m over the three-year time period and were able to do a full nuclear ground test for roughly 28 hours.”

The KRUSTY test, which finally got NASA’s nuclear experiments off the drawing board and back into the field, had four phases. The first two tested whether each component behaved as expected without power. During the third stage, the heat was increased incrementally, before moving onto the fourth wherein the reactor underwent start-up, ramp to full power, steady operation and shutdown. The advances achieved in the KRUSTY test will ensure the Kilopower reactor’s continued development as the team moves towards creating mission concepts and performing additional risk reduction activities.

“We were able to make the fuel and test the reactor here in the United States using our existing facilities,” continues Gibson. “I think that gave NASA management a lot of confidence that this isn't just a paper study – this is real. We’ve got a team that can go and design, test and build nuclear for space. We can go and use this technology on missions. That's the piece that hasn't been there in the last 50 years – no one ever tested the things that they put on paper.”

Unlike the SNAP reactor that came before, the team hopes that Kilopower will one day provide long-term sustained power for manned missions to other planets.

“The ultimate goal could be the Moon, it could be Mars, and it could be in space somewhere,” says Gibson. “We're trying to come up with a fission reactor that we could use for multiple missions, so the same design would hold true whether it’s a power system on the Moon, Mars, or whether it's used in space.”

Gibson and his team are now working on formulating missions on which the technology can be tested. They are using NASA’s Technology Demonstration Missions Directorate to identify the best mission for Kilopower in the early 2020s.

As missions bring mankind further from Earth, reliable, sustainable power will be needed to ensure vital life support and communications systems never go out. Nuclear power could provide this.

“We've got a lot farther in the last three years than we had in the past 50,” says Gibson. “I think nuclear is really going to take off when we start colonising other planets. When you start putting humans on the Moon or on Mars, you're going to need a lot of power. And, when you start looking at how much those power systems weigh and what their capabilities are, nuclear really starts to outshine solar. I think that's going to be the emergence of when all of this really starts to come to fruition when we start colonising, or at least sending humans to the other planets.”