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Comment by wongarsu | Hacker News Reader

"Rolls-Royce plan to have a reactor ready to send to the Moon by 2029". With that deadline I would expect something closer to a proven submarine reactor adapted for an environment without abundant water and fewer convenient ways to get rid of heat.

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Once you're on the surface you can sink a fair bit of heat into the ground. The issue in space is that you can't do convection, and there's nothing to conduct to, so you're really just left with giant, inefficient radiators.

On the moon, wouldn’t heat be a useful byproduct? (At least during the night) so a combined heat and power station would put those radiators where the people are

Up to a point, but you'd still need a story for dealing with the surplus.

Also, if you want to actually turn heat into electricity with a turbine, then you need the ability to heat and cool your medium repeatedly [1]— locating power plants on shorelines is very convenient for this, since you have a heat source from your combustion, nuclear reaction, whatever, and then a functionally bottomless cooling source in the water.

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

Nitpick: Are radiators very inefficient? I assume they're 100% efficient, just not very effective due to the limits of physics.

Not a physicist, but I'd say it depends on what you define as parameter optimized against. For space, Id be interested in heat energy dissipated per unit time per kimogram per cargospace for exanple, where radiators suck compared to convection coolers

Depends on what you mean by efficient. I assume the previous commenter meant it isn’t efficient in terms of cost and space to build large radiators that aren’t very effective.

Yes, efficiency here is in the sense of the mass and volume of device required to dissipate X amount of heat energy, particularly in an environment where getting a lot of mass and volume up there is fabulously expensive.

NuScale also still plans on a first reactor by 2027 even though are ahead in the small nuclear reactor space

At incredibly high cost.

https://www.wired.com/story/the-dream-of-mini-nuclear-plants...

Small nuclear reactors are not proven though and are quite a risky idea since nuclear benefits significantly from scale.

If your goal is to produce something that's useful in a short time span, it's better to use proven tech.

The moon is being bombed routinely with space debris so technically is an interesting engineering problem.

The same meteorites that create craters in the moon would fall over a nuclear plant so I assume that would need to be put in the face of the moon that is closer to Earth. Otherwise this would mean having several local areas of the moon contaminated and forbidden forever in a few years. Not wind, animals or water in the moon, so apart of the ejected dust, radioactivity should remain in the same place at least

Given the complete lack of shielding that we get from the magnetosphere and atmosphere I am guessing that a few kg of scattered enriched uranium is going to be background noise on the moon. The target is small, the moon is (relatively) big and we can bury the reactor; a meteor strike on the reactor is probably less of a concern than a similar strike on any habitat to which the reactor it meant to provide power.

A recent Chinese probe has shown that lunar regolith is radioactive just from cosmic ray bombardment, so as a general rule I am guessing that future lunar operations will treat the entire lunar environment the same way we treat places like Chernobyl. Shield everything, watch your dosage and exposure, and do everything possible to prevent inhalation/ingestion of particles from the external environment.

I'd think a similar design, or at least at a similar scale, would get ruled out by the necessary shielding. That would be quite a lot of mass to land on the moon.

Some innovation is in order. Mass reduction and radiation safety are among the highest design priorities I'd say.

In flight, it can be shielded with water and polyethylene, which would be needed anyway by colonists. On the surface, as someone else noted, you want good ground contact for heat dissipation so you might as well throw it into a hole and bury it, dirt being plentiful.

Since RR make submarine nuclear reactors for the UK's fleet already, yes.

Heat , what heat ? I've heard that the moon is cold.

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