Here on Earth, the ability to generate electricity is something we take for granted. We can count on the sun to illuminate solar panels, and the movement of air and water to spin turbines. Fossil fuels, for all their downsides, have provided cheap and reliable power for centuries. No matter where you may find yourself on this planet, there’s a way to convert its many natural resources into electrical power.
But what happens when humans first land on Mars, a world that doesn’t offer these incredible gifts? Solar panels will work for a time, but the sunlight that reaches the surface is only a fraction of what the Earth receives, and the constant accumulation of dust makes them a liability. In the wispy atmosphere, the only time the wind could potentially be harnessed would be during one of the planet’s intense storms. Put simply, Mars can’t provide the energy required for a human settlement of any appreciable size.
The situation on the Moon isn’t much better. Sunlight during the lunar day is just as plentiful as it is on Earth, but night on the Moon stretches for two dark and cold weeks. An outpost at the Moon’s South Pole would receive more light than if it were built in the equatorial areas explored during the Apollo missions, but some periods of darkness are unavoidable. With the lunar surface temperature plummeting to -173 °C (-280 °F) when the Sun goes down, a constant supply of energy is an absolute necessity for long-duration human missions to the Moon.
Since 2015, NASA and the United States Department of Energy have been working on the Kilopower project, which aims to develop a small, lightweight, and extremely reliable nuclear reactor that they believe will fulfill this critical role in future off-world exploration. Following a series of highly successful test runs on the prototype hardware in 2017 and 2018, the team believes the miniaturized power plant could be ready for a test flight as early as 2022. Once fully operational, this nearly complete re-imagining of the classic thermal reactor could usher in a whole new era of space exploration.
A Revolutionary Reactor
Any humans looking to spend more than a few days on the surface of the Moon or Mars will need to bring along a power source that checks an unreasonable number of boxes. It needs to be small and light enough to put into a spacecraft, while at the same time robust enough to survive the rigors of space travel. The lives of the crew will depend on it being infallible, but it will also need to be so simple and safe that it can operate autonomously for years. Most of these are traits not commonly associated with nuclear reactors.
Typically, the incredible energy released by nuclear fission inside the reactor’s core is used to heat water, which generates high pressure steam that powers turbines connected to electrical generators. It’s a relatively low-tech method of harnessing fission energy, but it has the advantage of being a simple and well understood process. Unfortunately there are far too many moving parts, figuratively or otherwise, to make such a system practical and safe on a small scale.
Which explains why the Kilopower bears little resemblance to traditional nuclear reactors. In fact, it’s more like an evolved version of the radioisotope thermoelectric generators (RTGs) which NASA has used to power everything from the Voyager missions to the Curiosity rover. There’s no dangerous high pressure steam, finicky turbines to spin, or coolant pumps to fail. Thermal energy is passively carried away from the reactor core using sodium-filled heat pipes, which lead to the “hot” side of a Stirling engine array. With a large deployable radiator on the other side, the Stirling engines would use the temperature differential to produce reciprocal motion that can drive a small generator.
The Kilopower has been designed as a self-regulating system where everything happens automatically and without the need for external control. There would naturally be sensors for basic diagnostics, for example checking temperatures at key points in the system, the RPMs of the Stirling engines, and the output of the generators. But outside of monitoring for these possible signs of trouble, the human crew could largely ignore the Kilopower and go about their mission.
Smaller and Safer
The use of passive heat pipes and Stirling engines rather than steam-driven turbines results in an incredible reduction of overall system complexity and size. But those improvements are only half the equation. An equally important aspect of the Kilopower design is the vastly simplified reactor core: a cylinder of uranium-235 that’s about the size of a paper towel roll and weighs just 28 kilograms (62 pounds). Despite its diminutive proportions, the core is designed to run at 850 °C (1,560 °F) for as long as fifteen years.
A single boron carbide control rod in the center of the core is used to control the rate of fission, which in turn can be used to adjust its heat output. When launched, the core would be in a “cold” state, where the control rod is fully inserted and no fission is taking place. Launching in an inert state means that the nuclear fuel won’t be consumed until the reactor is actually ready to be used at the destination location, further extending its useful lifespan. Once the Kilopower touches down on the Moon or Mars, the control rod will be removed and the nuclear reaction will begin.
The fact that the core is not going through active fission while here on the Earth also means it’s far less radioactive than one might expect. According to an interview that lead Kilopower engineer Marc Gibson gave to Power-Technology.com, it was riskier to launch the old-style RTGs:
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.
Ready for the Future
While the Kilopower has yet to leave the lab, much less slip the surly bonds of Earth, the team is already looking at scaling the system up for the sort of output that would be required for a large Mars base. The prototype reactor uses eight Stirling engines which can output approximately 125 watts each. That 1000 watts of total output is, incidentally, where the Kilopower gets its name. But with larger Stirling engines and the use of a 43.7 kg (96 lb) core, NASA believes the output could be increased to 10,000 watts. Cluster a few of those together, and you’ve got enough power for the first Martian neighborhood.
But increased energy output isn’t the only reason NASA is excited about the Kilopower. The U-235 fuel it consumes is far cheaper and easier to obtain than the Pu-238 used in radioisotope thermoelectric generators. The limited amount of Pu-238 the agency can get their hands on each year has made the logistics of planning deep space missions even more difficult than they already are, so a power source that uses a more common nuclear fuel would enable robotic missions that simply weren’t practical in the past.
At this point, NASA hasn’t announced when they intend to launch the first Kilopower reactor. But with the push to return to the Moon by 2024, it’s not unreasonable to think a small space-rated nuclear power plant might end up on our nearest celestial neighbor in the relatively near future.