Nuclear reactors tend to produce quite a bit of heat. In a spaceship or space station, instead of trying to transfer that to heat transfer devices to radiate away from the ship, could we do it by just having the core itself in open space tethered to the spacecraft so most of the heat just radiates away directly, and just use what we need from it? This way the core doesn’t heat up the spaceship (much). Sort of like towing with you a very small but fission based ‘star’ for power.
IIRC, in space there’s nothing to convect the heat into (i.e. no medium of air or water) - vacuum has almost no cooling properties. I think it would directly radiate heat into the vacuum, but very, very slowly. Almost certainly not enough to cool entirely.
On a space reactor, the heat is the useful output. You don’t want to radiate it away; you want to use it.
As iiandyiii notes, space is not a very good conductor of heat, so having the reactor out in open space doesn’t solve the problem, and in fact makes it worse. Nuclear reactors by their nature tend to be fairly compact, so to cool them using empty space as a cooling medium you need to connect them to some huge radiators. The reactor+radiators will heat up close to the same temperature as the reactor, having the same “cooling in space” problem as the original reactor, but at least the overall system has a much larger surface area to inefficiently radiate heat from.
The spaceship from Avatar was designed to suggest these issues. Engines connected to big radiators that glow red hot from cooling the engines, and a skinny tether connecting to the human-occupied parts of the ship hoping to conduct as little heat as possible that way.
Well, the usable heat that can be converted into work or electricity is useful; the residual heat that isn’t converted needs to be rejected, as does all of the ‘waste heat’ from doing work within the system (or for a crewed spacecraft, from the people themselves who produce ~100 W of basal thermal energy, and more when in exertion).
@iiandyiiii is sort of correct; vacuum out to space is a ‘cold reservoir’ (interstellar space is at the 2.7 Kelvin cosmic microwave background), but because there is no medium the only means of heat transfer is via radiation, and while that occurs at a rate proportional to the difference of temperature to the fourth power, it is also dependent upon the outward facing area to radiate away thermal energy. In contrast, on Earth most heat rejection to the environment is done by running a fluid (air or water) across a manifold and because the fluid is constantly being replaced (for forced air, or in the case of a moving vehicle) the heat transfer capability can be substantial. With radiation as the only means of heat rejection, you need to have large radiation panels (the accordion panels) shown below to keep it cool enough to be habitable, and the ISS gets virtually all of its power from its solar arrays so it produces minimal internal heat.
Although people often say that “Space is very cold”, in fact the biggest challenge for spacecraft is insulating from impinging solar thermal radiation and getting rid of excess internal heat. Large power generating and propulsion systems are a substantial challenge for spacecraft. And you can’t just put the reactor way out on an insulated boom and let it heat up because it will eventually accumulate enough thermal energy to cause systems to start breaking down or reach material failure.
Stranger
Yeah. As you say, there’s useful heat and there’s waste heat. I could have said that a lot better.
The OP didn’t distinguish and I was trying to but mostly failed miserably. Too much typing on my phone. Yeah, that’s a decent excuse.
I’ll label that a “waste post” that wasn’t properly radiated.
What’s the worst case scenario here? A melted down core that still is useful?
I don’t understand the question. If you have a meltdown of the reactor core (thermal failure of the fuel elements) then it certainly isn’t useful, and frankly you’ll probably have damaged the inner loop you are using to extract thermal energy long before then. Regardless, if you can’t reject enough heat to regulate the temperature, the system will just continue to get hotter until it hits the point of material failure or rupture of the coolant system. Eventually, you’ll just have a stick with a big ball of molten metal at the end where your reactor used to be.
Stranger
Yes but can’t we use such a radioactive molten metal ball as a power generator in itself? Also can’t we convert the radiation or charged particles directly into electricity instead of using the thermodynamic cycle?
You could have what’s known as a nuclear-thermal rocket, in which the reactor heats propellent to produce thrust. Since the propellent is the working fluid in the cycle and is expended no radiators are needed (in principle; actual engineering might require some way to get rid of extra waste heat).
The problem comes when you have any sort of closed-cycle power generation, such as using a nuclear reactor to drive a working fluid to generate electricity to power an ion or plasma drive. You start with a cool(ish) fluid; the reactor heats it which increases it’s pressure; the pressurized fluid drives some mechanical generator to produce electricity; you then have a depressurized, somewhat cooled fluid.
To reuse the fluid you have to get it back to the reactor somehow. But if you pressurized it enough to return to the start of the cycle you would have to do at least as much work (or more) than you got of it in the first place. The only way to reuse it is to let it cool enough that it will condense and lose pressure. This is the classic Carnot cycle. The rub is that in a vacuum the only way to cool the working fluid is to let it radiate heat away into the 2.3K° background of space. This is not easy- thermal flux falls off to the fourth power of the radiant temperature.
There are ways to do that (it’s usually called “betavoltaics”), but you need very specific sorts of materials, and it’s hard to get a large amount of power out of such a system. It only works with natural decay (which is relatively slow), not with the higher power you get out of a reactor.
You can, although with current technology the conversion efficiency is dismally low. And ultimately even that’s a thermodynamic cycle, it’s just that the radiation and/or charged particles correspond to a theoretical temperature of millions of degrees Kelvin, so the theoretical thermodynamic efficiency is quite high.
Yes that one is well known, and since you are expelling the hot stuff that helps with thermal management.
Ultimately, anything can be cast as a thermodynamic cycle. But it’s not usually relevant to do so.
How would you get power from a ball of molten slag? And the radiation from a [SUP]235[/SUP] fission is just neutrons and ‘prompt’ gamma rays; no charged particles. There are products down the decay chain that release alphas (helium nuclei) and beta (electrons or positrons) which are charged, so in theory you could surround the reactor with an electrostatic grid and get current (which is how betavoltaic batteries work) but that isn’t going to extract much power.
Stranger
First bummer about the lack of charged particles though there are other fissionable reactions.
I tried to indicate how when I said " Sort of like towing with you a very small but fission based ‘star’ for power." If you have something insanely hot that you can have in your general vicinity, it would be possible to tap into the power. Initially I was thinking about the beta particles for electricity, however even back to a thermodynamic cycle, or if white hot enough photovotalics. Basically let it go hot and gather power from that externally.
Even photovoltaics worked because they’re well below the black body temperature of the electromagnetic radiation they’re converting. As a practical matter they can’t work beyond their operating temperature (much less their melting point), and as a theoretical matter they wouldn’t generate electricity if they were so hot that they glowed as brightly as the light hitting them.
NASA did develop a unique cooling scheme that radiated heat to the blackness. It was part of the Solar Dynamic System that was dropped from consideration in favor of solar arrays. A long tube carried the hot fluid under pressure. At intervals there were nozzles that shot the fluid into space in thin, laminar streams. A suitable distance away was another long tube with small funnels that matched the spacing of the nozzles. The now much cooler fluid was captured in the funnels and returned to the system.
I don’t know how far the concept got and I don’t think the program did any testing at Zero G in a vacuum. It would have been an interesting experiment. See, sometimes those long meetings weren’t all boring.
Ah, the liquid droplet radiator concept. You can read a lot more about it here:
https://www.projectrho.com/public_html/rocket/heatrad.php#id--Radiator_Types--Liquid_Droplet
Ok if we ignore the propulsion part for now, let us say the spacecraft is up to speed and on its trajectory or orbit. It can place the radioactive core and let it go to meltdown some distance off the craft. Using that for power. Perhaps a mission to the outer planets, Kuiper belt (perhaps that sun gravity lensing position) or even interstellar.