Jan 25, 17 / Aqu 25, 01 08:17 UTC

Re: Nuclear Power in Space  

How much heat? Gigawatts, more likely Terrawatts.

Conduction is only sensible as an option if it can radiate from that mass faster than it can conduct - or the mass is going to get hotter until the point it provides no coolant capacity. You'd then need to think about how much mass you'd need to radiate faster than it absorbs, as much as the shap of the mass - this is then bringing the argument back to the realms of "just increase surface area"... which is at best a primative solution.

That engineering toolbox link 404's BTW.

  Updated  on Jan 25, 17 / Aqu 25, 01 08:18 UTC, Total number of edits: 1 time
Reason: typo

Jan 25, 17 / Aqu 25, 01 11:52 UTC

Strange, it doesn't for me.

Jan 25, 17 / Aqu 25, 01 14:53 UTC

Okay, let's operate on the assumption that blackbody radiation is the only viable and reliable (not going to consider the number points of failure involved in a heat barge idea- it's pretty to think about, but hardly practical) method of waste heat disposal. Mathematically, the amount of heat transferred via blackbody is proportional to the temperature of the emission surface and to the emissivity of the exposed surface. We've just about reached the technically feasible ceiling on the latter, but we might be able to squeeze out a little bit more emissivity with newer materials, which would allow us to shrink the required radiator space. The former, however, provides considerable room for improvement. Historically, spacecraft have used one waste heat removal system (albeit sometimes with redundancy). However, if we are to seriously consider using nuclear power as our primary source, we should consider designing a separate radiator system for the reactor and associated systems, one designed to function at much higher temperatures than would be acceptable for habitable modules, and keeping the structural interface between the reactor module and the habitation module at a minimum to reduce heat transfer. This also has the added bonus of reducing the amount of radiation shielding needed between the reactor and the habitation modules.

Assuming we can design a system that can operate reliably at twice the absolute temperature of the habitation modules, we can reduce the required heating surface (for the reactor, anyway) by a factor of 16, which assuming EyeR's calculations are correct, would mean about 125 m^2 per megaWatt, not including redundancy. While this would still necessitate a huge amount of surface area for our needs, it is still considerably more practical than previous concepts. We may reap further benefits from using multiple reactor modules, though I'd need to look at the practicality of this.

  Last edited by:  Joshua Holzhauser (Asgardian)  on Jan 25, 17 / Aqu 25, 01 17:51 UTC, Total number of edits: 1 time

Jan 25, 17 / Aqu 25, 01 17:01 UTC

Is thickness important, or could you get away with foil?

Jan 25, 17 / Aqu 25, 01 18:09 UTC


Assuming you are talking about the thickness of the radiators, no, thickness is not important, with one caveat: The radiators need to have some sort of thermal exchange fluid to transfer heat from the reactor to the radiator surfaces, which means that they can't just be a foil.

Regards, InsanityOS

  Last edited by:  Joshua Holzhauser (Asgardian)  on Jan 25, 17 / Aqu 25, 01 18:10 UTC, Total number of edits: 1 time

Jan 26, 17 / Aqu 26, 01 10:53 UTC

Seems that link doesn't 404, I just picked up the full stop from the end. My bad.

The barges wasn't a realistic solution proposal just a vague idea. There's certainly "disadvantages" to such a method, it was just first way i'd thought to distribute coolant.

A reactor would obviously require a seperate coolant system to the environment, otherwise you'd end up cooking, not cooling the occupants. 125m2 per megawatt is a lot smaller than I would of predicted, that's smaller than the ISS use now to dissipate a couple of KW.

Jan 30, 17 / Pis 02, 01 13:56 UTC


This figure would require operating at twice the absolute temperature as current radiators, not accounting for imperfections in our system that would reduce efficiency. I'm not a mechanical engineer, but I'd imagine that there must be some kind of difficulty in designing a system that operates ~ 600 K, or more. Plus, I just Google'd the equations, I might be miscalculating due to misunderstanding the physics, but the figure I saw gave the absolute temperature a power of 4 relationship with the heat dispersed (via blackbody radiation).

By all means, if I'm mistaken, tell me! How else can I learn?

Regards, InsanityOS

Jan 30, 17 / Pis 02, 01 23:00 UTC

I've done no math, that's what computers are for - I'm far too lazy - I just looked at other systems in place or previously deployed for a real-world value and loosely extrapolated from there. Thusly my predictions of being much larger than 125㎡/mW.

That's still a significant size reduction, however, realistically - especically to consider long term mass residential facilities - the need for dissipation is likely to be measured in gigawatts if not terrawatts. Even at only 125m²/mW that's still 125000m²/gW, or 125km²/gW.

As for designing something that operates at ~600Kelvin, I'd imagine that's possible. Most metals get quite soft at these temperatures, but it's likely ceramics will work well, and various parts of the coolant system can be lent additional coolant systems.

  Updated  on Jan 31, 17 / Pis 03, 01 00:27 UTC, Total number of edits: 1 time
Reason: typo

Dec 9, 17 / Cap 07, 01 03:52 UTC


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