A giant leap for nuclear as reactors head to the Moon and Mars

The U.S. space agency NASA has used nuclear-driven power systems in its missions since the sixties, but a new directive to establish base camps on the Moon and Mars will see nuclear play a larger part.

Illustration of a nuclear fission power system concept on Mars (Source: NASA)

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The administration’s Space Policy Directive-6 (SPD-6), a presidential memorandum on the national strategy for space nuclear power and propulsion (SNPP) released early December, aims to ensure the development and use of SNPP systems to enable the scientific, exploration, national security and commercial objectives of the United States, the memorandum states.

“This memorandum outlines high-level policy goals and a supporting roadmap that will advance the ability of the United States to use SNPP systems safely, securely, and sustainably,” it says.

The directive has four main goals: 

• Develop uranium fuel processing capabilities that enable production of fuel that is suitable to lunar and planetary surface and in-space power, nuclear electric propulsion (NEP), and nuclear thermal propulsion (NTP) applications, as needed.

• Demonstrate a fission power system on the surface of the Moon that is scalable to a power range of 40 kilowatt-electric (kWe) and higher to support a sustained lunar presence and exploration of Mars.

• Establish the technical foundations and capabilities — including through identification and resolution of the key technical challenges — that will enable options for NTP to meet future Department of Defense (DoD) and National Aeronautics and Space Administration (NASA) mission requirements.

• Develop advanced radioisotope power systems capabilities that provide higher fuel efficiency, higher specific energy, and longer operational lifetime than existing RPS capabilities.

In 2018, Nasa and the Department of Energy’s (DOE) National Nuclear Security Administration (NNSA) demonstrated a new nuclear reactor power system that could enable long-duration crewed missions to the Moon and Mars called the Kilopower Reactor Using Stirling Technology (KRUSTY) experiment. 

Kilopower is a small, lightweight fission power system capable of providing up to 10 KW. Nasa has established that four such units would provide enough power for an outpost. 

Visionary view of a Nuclear Thermal-Propulsion enabled spacecraft mission 

(Source: NASA/Marshall Space Flight Center) 

New Chapter

The memorandum points to a new chapter in the establishment of nuclear power as part of space exploration.

“What’s changed is that the agency has mandated for a sustained lunar presence then exploration on Mars,” says Chief engineer of NASA’s Space Technology Mission Directorate Jeff Sheehy who had a part in shaping the directive. 

This requires the development of more capable and longer duration, higher-power systems, both to power assets on the surface of the Moon or Mars and, in the case of transporting humans to Mars, to use the fission system as a basis of propulsion to and from the red planet, Sheehy says. 

“A lot of what the agency wants to do on the moon will lay the groundwork to develop and prove out some of the technologies that can be used on Mars. But there’s a lot of interest in the space economy and what role the Moon can play in that.”

In October, for the first time, NASA confirmed that water was present on the sunlit surface of the Moon, providing an important commodity that will mean propellants could be produced there and delivered to those in lunar orbit, turning Earth’s satellite in to a refueling station for spacecraft traveling back or heading further afield. 

“The moon is an excellent place for refueling. If we’ve got water, we’ve got oxygen and hydrogen. That provides somewhere you can go after you escape the gravity of earth,” says Steve Johnson, director of Idaho National Laboratory’s (INL) Space Nuclear Power and Isotope Technologies Division. 

INL is managing the commercialization contract for NASA and will shortly publish the draft request for proposals from more than 20 companies interested in taking part. The laboratory expects to fund three of four teams to work together on the project. 

Solar power is also under consideration, though any given point on the moon sees continuous sunlight for around 14 days followed by some two weeks in total darkness, making solar technology only a partial solution to the problem of power generation.

Solar can also be relatively bulky and the mass of the technology under consideration plays an important role in which will be adopted. 

“Every kilo of payload you launch costs quite a bit of money. In space flight everything is complex and space flight is difficult, but we’re always looking for the least massive, least complex and least expensive solution,” says Sheehy. 

Built for the job

Civil nuclear power is under going its own revolution, with cutting edge technology producing ever smaller, modular reactors that claim to be safe straight off the factory production line, but the reactors that INL and NASA have in mind must survive conditions much more extreme than anything being built today. 

“To get to Mars, you’re going pretty fast and you’ve got some pretty large G-forces so it needs to be able to survive that,” says INL’s Johnson. 

The reactor also must fit in the launch vehicle as well as have its own landing pad from which it will run once on the lunar or Mar's surface. The reactor proposal is looking for a machine that is no more than 3.5 meters by 3.5 meters by 6 meters and is limited to 3,500 kilograms. 

“That mass has to include your shielding as well, because ultimately you’re going to want some guys on the moon and even if they’re a kilometer away, if that radiation is too high, there’s not a lot of atmosphere there to attenuate your radiation field,” says Johnson. 

The machine, which is likely to use a kind of TRISO (TRi-structural ISOtropic) particle fuel, also needs to be relatively foolproof as any operations and maintenance will be, quite literally, a world away. 

“There’s going to be no repairman, so it has to work. Once it’s there, it’s there. If it stops working after a year then all you’ve got is a big, radioactive machine that doesn’t work, and that’s not good,” says Johnson. 

By Paul Day