NASA is planning to put astronauts on Mars one day and since the Red Planet is about as off the grid as you can get, the space agency is developing a new generation of modular nuclear reactors to power manned outposts. Under funding from the Space Technology Mission Directorate (STMD), the Kilopower project is a multi-year effort to build simple, inexpensive reactors that can be used for a wide variety of planetary and deep space missions.

One of the primary problems with almost any space mission is how to provide it with power. Depending on the goal of the mission and its duration, there are any number of options. The very first satellites used batteries that supplied them with electricity for a few days. Soon, solar panels were added that extended the mission life to years. Fuel cells provided manned missions with not only power, but drinking water as well as hydrogen mingled with oxygen to create electricity and a potable waste product.

Unfortunately, all of these options turned out to be very limited in application. The most successful of them, solar power, only works when sunlight of sufficient brightness shines on the panels. This means that it's a system largely confined to the inner Solar System with Jupiter as the extreme limit, doesn't provide much in the way or power density, is bulky, and is useless at night or on planetary surfaces that may be obscured by dust and clouds.

The most practical alternative to solar panels is nuclear power. It was first considered for spacecraft almost as soon as the first reactor came online 75 years ago, and has been used as a practical source since 1965 when the US SNAP-10 experimental reactor was launched into orbit.

The main nuclear power source used by US space missions is the Radioisotopic Thermoelectric Generator (RTG). A solid state device, an RTG uses the heat from plugs of plutonium 238 as either a way to keep electronics warm, or to generate electricity using thermocouples. It's a mechanically simple system that has been used for over 50 years for powering the Apollo mission lunar experiment packages, the Viking and Curiosity Mars landers, and the Pioneer, Voyager, Galileo, Cassini, Ulysses, and New Horizons deep space missions. It could also be used in Earth orbit, but, for political as well as engineering reasons, its use has been largely restricted to deep space missions.

Another problem with RTGs is that they don't produce much more than 300 Watts of electricity. This is fine for missions like Voyager where the emphasis is on longevity rather than brute power, but for planetary surface missions, RTGs can't handle anything much larger than the Curiosity rover. Worse, the plutonium 238 was a byproduct of Cold War weapon programs and is now in very short supply – it would require reopening long-closed production lines to create more.

If an RTG can be compared to an atomic battery, then a fission reactor is an industrial-scale gas turbine on steroids. Operating on the same principle as the civilian and military nuclear reactors, the space versions can produce enough power to act as a propulsion system with 30 percent greater thrust than the most advanced liquid fuel rockets. Needless to say, more modest reactors can provide power in any quantity needed. And the limiting factors? Cost, complexity, and safety.

The US only sent up the one reactor in 1965, which will remain in orbit for the next 4,000 years, but the Soviets sent up around 40 reactor-powered satellites during the Cold War to run high-powered radar systems for surveillance. The West has avoided using reactors due to having access to more advanced electronics, as well as the fear of political opposition.

The poor public image of space reactors wasn't helped when in 1978 the reactor-powered Soviet Cosmos 954 satellite made an uncontrolled re-entry and broke up over Canada, scattering radioactive debris along a 600 km (370 mi) line in the northern wilderness. This prompted Russian engineers to incorporate a mechanism to jettison the reactor core in future designs and send it into a safe orbit before the satellite made re-entry.

But now, as NASA missions in deep space become more ambitious, the agency's Marshall Space Flight Center in Huntsville, Alabama, along with the Department of Energy's (DOE) Nevada National Security Site and Los Alamos National Laboratory, has begun testing a new reactor design for Mars and beyond. Currently in the demonstrator stage, the Kilopower reactor is being checked against analytical models for hardware verification.

A reactor is very attractive to NASA because, like the RTG, it is compact, independent of any outside source of power, and can operate in extremely harsh environments. However, unlike the RTG, it puts out significant power per unit of weight.

"The reactor technology we are testing could be applicable to multiple NASA missions, and we ultimately hope that this is the first step for fission reactors to create a new paradigm of truly ambitious and inspiring space exploration," says David Poston, Los Alamos' chief reactor designer. "Simplicity is essential to any first-of-a-kind engineering project – not necessarily the simplest design, but finding the simplest path through design, development, fabrication, safety and testing."

Rated at 10 kilowatts, the Kilopower reactor puts out enough power to support two average American homes and can run continuously for ten years without refueling. Instead of plutonium, it uses a solid, cast uranium 235 reactor core 6 inches (15 cm) in diameter. This is surrounded by a beryllium oxide reflector with a mechanism at one end for removing and inserting a single rod of boron carbide. This rod starts and stops the reactor while the reflector catches escaping neutrons and bounces them back into the core, improving the efficiency of the self-regulating fission reaction. Until activated, the core is only mildly radioactive.

The heat from the reactor is collected and transferred using passive sodium heat pipes. These feed the heat to a set of high-efficiency Stirling engines. These are closed-loop engines that run on heat differences that cause a piston to move back and forth similar to the piston in an internal combustion engine, though with a compressible gas medium instead of an exploding mixture of petrol and air. This cools the reactor via a radiator umbrella as well as powering a dynamo to generate electricity.

The design is modular, so the self-contained reactor units can be hooked together to provide as much power as needed, whether it's a deep space probe or a Martian outpost. According to Lee Mason, STMD's principal technologist for Power and Energy Storage at NASA Headquarters, the technology is "agnostic" to its environment, allowing it a wide range of applications.

The Kilopower project is currently working toward a full-power test lasting about 28 hours. From there, NASA hopes to move to a test in space, but the Nevada tests are more of a breadboard test in a vacuum to show that the technology is feasible.

"What we are striving to do is give space missions an option beyond RTGs, which generally provide a couple hundred watts or so, says Mason says. "The big difference between all the great things we've done on Mars, and what we would need to do for a human mission to that planet, is power. This new technology could provide kilowatts and can eventually be evolved to provide hundreds of kilowatts, or even megawatts of power. We call it the Kilopower project because it gives us a near-term option to provide kilowatts for missions that previously were constrained to use less. But first things first, and our test program is the way to get started."

The video below describes the design of the Kilopower reactor.

Source: NASA

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