For Mars, the Nuclear Option is the Only Option
As we established in Solar Power Is Never Going to Work on Mars, a Martian base operating at reasonable levels of exploration and in particular producing the propellant necessary for a return trip to Earth was going to require about 2.75 Mw. As many issues as nuclear [pwer runs into on Earth, there is a reasonably mature reactor design which can be further developed in the next five years to provide this power budget: NASA’s KiloPower reactor:
Kilopower is a simplified nuclear reactor designed expressly for space exploration, that is to say, requiring the bare minimum of human intervention combined with a great number of failure modes. In short, it’s a set-and-forget power solution that is very, very difficult to have an accident with.
In 2018, Kilopower underwent a battery of tests in the KRUSTy program (Kilopower Reactor Using Stirling Technology), a follow on from the DUFF ( Demonstration Using Flattop Fission) program in 2012. If you guessed that the team were fans of the Simpsons, you would not be wrong. KRUSTY had three primary objectives:
- Operate the reactor at steady state with a thermal power output of 4 kWt at a temperature of 800° C
- Verify the stability and load following characteristics of the reactor during nominal and off-nominal conditions
- Benchmark the nuclear codes and material cross sections using the test data
In plain English, make the reactor generate power, try to break it and see what happens, and get the performance data necessary to build the next generation. The team tested a number of failure modes, including complete loss of cooling, and in each case, the reactor shutdown or slowed to idle safely.
How Mature is this Reactor?
NASA defines readiness according to the Technology Readiness Level scale (TRL), and currently Kilopower is at level 5, which basically means, “it works, but it hasn’t been tested in space”. That’s not to underplay the effort required to take it to TRL 8, which is “ready to rock”, and then to TRL 9, which means “reliable flight hardware”, but this seems to be achievable in a timeframe of five years or so, assuming that SpaceX keeps their rate of progress apace.
Crunching the Numbers
Before we dig further into how the reactor works, let’s figure out some of the logistics involved in satisfying our power budget with Kilopower.
Our long-term budget for a propellant plant and a modest Antarctica-style facility is 3mW, or 3000kW — but we don’t have to get there by mission zero — just enough to do some basic operations and produce propellant. While our propellant manufacturing plant has a hard requirement of 1.75 Mw, NASA has settled on the figure of 40Kw as an early target for a small team of around six crewmen. That puts us at about 1800Kw for needed for the absolute first mission to Mars.
The Kilopower’s output is listed by multiple sources as 10kW, with the ability to throttle down to 1kW. Simple math yields a requirement of 180 reactors. According to the chief designer David Poston in a presentation to the Mars Society, the reactor is the size of a trashcan, with a radiator assembly 11 feet tall, about the height of a “very tall stepladder”. We’ll talk more about that radiator in a bit.
The Weight Problem
Or should I say, the “mass problem”. The unshielded version of the 10kW version of the reactor is listed as having a mass of 1500kg. 180 of these would have a mass of 270,000kg. According to SpaceX on Twitter, the target payload to Mars is “over 100t” (metric tons, i.e., 1000kg). That means that it would take two full and one partial payload for BFR. And it gets worse — Space.com cites a shielded mass of 2000kg — raising the payload to 360,000kg, unless the reactors are operated in an exclusion zone behind a (thick) hill.
So, bottom line — these reactors will have to be prestaged (though inactive) on the Martian surface via multiple missions preceding the first human landing. This is no game-changer — staging multiple Starships on Mars is so part of the plan that it’s been in the renders all along:
We can also assume that one of those Starships is a propellant production plant. Once the flags are planted, and the speeches are finished, the astronauts will begin unpacking and deploying each of those 180 Kilopower reactors from three (or four) Starships into a series of power arrays.
The reactors are not radioactive until turned on, so even if the unshielded version is in play, it will be a safe operation — at least until you need power from that particular array. What you might end up with is a single, shielded array that is deployed first, right next to the landing site. You can turn that on as soon it’s deployed and generate power while you carefully deploy the others further away. According to Poston, the time for the reactor to cool down to background radiation levels for maintenance is just a few days.
Let’s talk about why Kilopower is such an effective design for this use case.
How Kilopower Works
Kilopower is, in a few words:
A uranium-fueled reactor with a boron carbide moderator and a beryllium oxide reflector, driving free-piston Stirling engines filled with liquid sodium.
The key safety aspect of the reactor is a negative Doppler ratio, or fuel temperature reactivity coefficient — the higher the temperature, the slower the reaction rate of the fuel.
In normal operations, the Stirling engines draw heat away from the reactor — this is how the power actually gets generated. As the temperature rises, the engines draw more and more heat, while at the same time, the physical properties of the uranium fuel cause the fission rate to fall. At lower power levels, the opposite property obtains, creating a thermal sweet spot of around 800c, observed in the KRUSTy pathfinder.
Managing the Heat
The Stirling engines are only about 30–34% efficient, meaning that 66–70% of the heat produced from the nuclear reaction needs to be removed from the reactor separate from power generation (or else the intrinsic safety design of the reactor will begin to shut it down).
Since Kilopower is designed to be used in a variety of space applications, the reactor is designed for the worst case for getting rid of this heat — the vacuum of space. Without atmospheric convection to carry away the heat, Kilopower relies on a radiator array on top of the reactor.
For a 10kW reactor, we’re talking about 30kW of heat. In space, this would simply be carried away as infrared radiation, but on Mars, even with it’s tenuous atmosphere, this heat will stick around and heat the local environment. In this infinitesimal way, the terraforming of Mars will begin on mission one.
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