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Comment by jacobush | Hacker News Reader

One nice thing which didn't automatically occur to me, is that a nuclear fission reactor on Mars would be comparatively cheap to build. You don't need any radiation shielding at all. Just walk away and turn it on. Have it produce methane and oxygen or electricity or both.

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That would require really good automation. I think most nuclear reactors still require a lot of manual labor relatively close by.

But yes, some kind of fission or fusion reactor would be on a colonization plan, if it is easier/cheaper than solar. One problem though would be getting fissile material. You can probably find some on Mars or its moons or some asteroids, but it would require lots of heavy industry to get enough of it for reactors.

Bringing significant amounts of nuclear fuel from Earth would be really unpopular, what with the risk of rockets blowing up. That risk or fear is also what prohibits the use of nuclear propulsion for space travel...

Yeah... politically it would be hard, I agree. But I don't think it would be a show stopper technically to bring fissile material from earth. For one, automated nuclear power plants are being designed for use on here on Earth. The fissile material itself does not weigh very much. It's also not (very) radioactive until you start the process.

The bulk mass of a (normal) reactor is shielding and cooling plumbing. You don't need any shielding on Mars, and cooling could maybe (probably?) be some kind of radiator/convector to the surrounding air. Maybe you could even crack the CO2 with the heat directly?

Fusion needs a lot plumbing and containment. Fission is just basically a pile of fissile material in proximity. Not very hard if you can skip on all the protection.

You still need a heat sink of some sort to extract usable energy from any reactor, nuclear or otherwise; this is a universal property of engines [1]. Radiators are very inefficient for this and would be far heavier than the equivalent water cooling method. Same for air-cooling (especially in mars's thinner atmosphere). The cooling systems are not just there to prevent meltdowns; you need heat to flow, in large quantities, from a hot place to a cool place in order to extract energy.

You're right that, if you could find some useful endothermic reaction like vaporizing CO2, you could use it as a heat sink. The problem is that distributing heat to arbitrary places in the martian ground (remember, this CO2 is not magically flowing to you) requires an even more complex heat exchanger than that used by a simple earth-based reactor.

Not to mention that having no shielding would be hell for your robots; radiation hardening of automated systems remains non-trivial. But the fundamental thermodynamics strike me as harder.

Maybe there are niches where this would make sense, but the thermodynamics is the elephant in the room.

[1] https://en.wikipedia.org/wiki/Thermal_efficiency

Actually, the CO2 is flowing, it's in the atmosphere. :) But maybe it's not enough.

Right, that's the point! The low pressure would make it very very inefficient, and you would actually need a heat exchange system far larger than earth reactors. We use liquid cooling in our PCs because a smaller heat sink can pump out more heat if it's going into liquid (for a fixed flow rate). This is why liquid cooling for CPU/GPU setups is quiet and effective: the liquid flows quietly and carries heat to a BIG, slow fan. Your fan and heat sink would be even bigger/faster at Mars atmospheric densities just to cool your puny CPU. Doing that at scale with no water cooling for your reactor would be absurdly materials intensive.

That's all for a typical high-power reactor setup. You can, of course, have a low-power setup with nuclear power. RTGs [1] use this approach, though they mainly rely on decay heat rather than self-sustaining reactions, which is just as well because they are also thermodynamically limited by the rate at which they can radiate heat away.

One addendum I should have noted: the efficiency of a thermal engine on mars would actually be perfectly reasonable for a small engine (it's nice and cold there, perfect for high efficiencies). The issue is that you can't dump the heat fast enough, so your heat sink will get hotter, trashing your efficiency and reducing your cooling capacity (both of which exert downward pressure on your final power output in a serious way).

[1] https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_ge...

I was thinking about cracking CO2 into methane and oxygen - in that case you would need a high temperature. But I get the general drift - no good cooling. Maybe on could be plunked into the ice cap. You would need a lot of plumbing then though...

Ah, yes, that would be one good place to put a lot of the energy (if you could also get some water for the hydrogen).

The low atmospheric pressure doesn't really affect the cooling because it could be compressed arbitrarily.

Shielding on Mars could just be made from surface rock. or you bury the reactor in the first place. Radioactive waste (like the contaminated shielding) is probably a non-issue on Mars because there already is too much uninhabitable space anyway.

Anyway, such things are decades or centuries in the future. In the next decades, photovoltaic or other solar energy generation will rule on Mars.

Fortunately, a nuclear reactor isn't radioactive until the reactor is turned on. Enriched uranium on its own is hardly radioactive at all (U235 t1/2=~700Myears); depleted uranium has meaningful safe non-nuclear applications[0]. There are orders of magnitude more radioactivity in the RTGs already used on deep space missions. Those are very carefully protected, but the risks are deemed acceptable. So long as you don't turn your reactor on until you're well away from earth, I see very limited risk of exposure to the public, far less than from a RTG.

[0]https://en.wikipedia.org/wiki/Depleted_uranium#Civilian_appl...

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