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NASA plans to develop a 100 kW nuclear power plant for lunar applications Source: NASA
However, RTGs are difficult to scale. Mohammed Ziauddin, analyst at NEI parent company, GlobalData, explains: “RTGs can only produce a small amount of electrical power for their weight – typically only around 2–3 W per kilogram. This is fine for spacecraft instruments, but not enough for the kilowatt or megawatt levels needed for a lunar base or propulsion system.” Plutonium 238 – the most common power source – is also rare and expensive. Aside from RTGs, there have been two other answers
to powering lunar exploration: solar and fuel cells. The former hits an immediate snag: the US, Russia and China have their sights set on the southern polar region of the moon where its significant water ice could be used to develop lunar infrastructure for crews and to manufacture future commodities and resources. However, at these latitudes the sun barely creeps over the horizon. Alternatively, fuel cells have been fundamental in powering space exploration, used on the Gemini and Apollo missions, and more recently on the Space Shuttle. With the advantage of water as a byproduct, the technology is also lightweight and offers high energy density, making it scalable. Such a system requires regular replenishment of fuel though, and a delivery hiccup could result in a loss of power at the base. Fuel cells also degrade over time.
With these options off the table, nuclear fission has emerged as the most viable, durable solution.
The vision for fission: transporting a nuclear reactor Putting nuclear power on the moon is feasible but not straightforward. A spokesperson from the US Department of Energy (DoE) explains: “a lunar reactor will be transported fully constructed via rocket to the moon, which will create size and weight limitations. Landing, activating and operating a reactor on the lunar surface will be a novel challenge given the unique environment”. In June 2022, the DoE’s Idaho National Laboratory
awarded initial design contracts to X-Energy, Westinghouse and Lockheed Martin to develop designs for a 40 kW reactor that could weigh up to six tonnes. However, the final design will be 100 kW and must fit on a human delivery-class lander, which can accommodate up to 15 tonnes. Efficiency is therefore the name of the game. Nasa has specified that its design will use a Brayton cycle system,
including one or more loops, each of which would contain a turbine generator, compressor and heat exchanger. It may also include pumps and valves, but fewer mechanical parts mean fewer opportunities for mechanical breakdown, while the added mass and drain on the power source could impact the nuclear system’s efficiency. Once the working fluid has expanded through the
turbine, it must be cooled and compressed to restart the cycle; waste heat will likely be rejected through radiative cooling, in which the exiting gas will pass through a large radiator, dissipating waste heat into space. Most nuclear reactors have an efficiency of around
36% (although some, such as helium-cooled reactors, can reach more than 40%). Waste heat is therefore significant, unavoidable and tricky to reject, thanks to the absence of air – and therefore convection – in space. It is a significant engineering hurdle, says senior manager of business development at Lockheed Martin Space Kerry Timmons, and a key focus. “The heat rejection and the thermal management of
the system is one of the core technologies that Lockheed Martin has been focusing on, because nuclear systems do get very hot. We are converting that heat into electric energy, but we also have to reject some of it,” she says. Aerospace and nuclear engineer Ugur Guven suggests the use of a helium-cooled nuclear reactor, “because helium is an inert gas and behaves well, hence it is easy to use as a coolant. The right nuclear fuel would also have to be used or, to generate a lot of power, a gaseous core reactor with uranium hexafluoride could be used.”
The moon’s environment for nuclear power Efficiency aside, the moon’s environment presents a set of practical challenges including reduced gravity, cosmic radiation, micrometeors, and the lack of atmosphere. Guven explains that “gravity on the moon is
approximately a sixth of the Earth’s, so engineers must make the reactor work in a stable manner in a reduced gravity environment. Plus, it must be shielded properly, because the moon lacks the protection of the atmosphere, which protects against small meteorites.” Beyond these already significant threats, the reactor
must also withstand the moon’s extreme thermal cycles and abrasive dust, without regular maintenance. Protecting the reactor will likely involve in-situ regolith
shielding, which will avoid increasing the launch weight. Nasa reports that this may involve digging a hole and
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