Nuclear Reactors in space

[Kibble]

Nuclear reactors are always discussed like magical superpower machines with no downsides except that evil politicians won't let us use them. But despite constant proposal since their invention, they have never found a real use for spacecraft. There's gotta be a concrete reason right? My guess is that they are just too complex, too much plumbing, and too little room for failure.

[cantab]

Nuclear reactors are always discussed like magical superpower machines with no downsides except that evil politicians won't let us use them. But despite constant proposal since their invention, they have never found a real use for spacecraft. There's gotta be a concrete reason right? My guess is that they are just too complex, too much plumbing, and too little room for failure.

My guess is simply that nobody's wanted to send a spacecraft with high power demands beyond Jupiter. In the inner solar system solar arrays are proven technology and plenty good enough, especially going by the figures Streetwind mentioned. And I'll take back my previous statement - while I still think some sort of "ion drive" will be used to get humans to Mars it may well be solar powered not nuclear powered. In the other solar system probes have got by with RTGs, or even solar arrays at Jupiter for Galileo IIRC.

[Kibble]

solar arrays at Jupiter for Galileo IIRC.

Juno, actually, but you're right! While solar-electric propulsion continues to be developed from Deep Space 1, onto the practical real-world application in Dawn, future probes and maybe future piloted vehicles will be powered by this proven technology, while nuclear anything remains a paper project falling further and further behind.

[sgt_flyer]

Classic uranium based reactors always had a terrible power to mass ratio because of the cooling systems. (And it's even worse in space) - because the conversion efficiency is terrible.

The problem has always been because of the working temperature of solid fuel rods, moderator and of the primary coolant after that - (in Pressurized Water Reactors, the primary coolant temperatures are around 315°C after having been heated by the reactor.) because of this kind of operating temperatures, you are limited to use low efficiency conversion cycles. (On earth, it's generally between 30 and 40% of conversion efficiency)

What we need are ways to have much more efficient direct energy conversions instead of needing to go through a thermal loop. (And if you can do direct energy conversions from nuclear reactions, you'll be able to use it as incredibly efficient means of spacecraft propulsion)

[Streetwind]

Direct energy harvesting from fission reactors seems a bit farfetched, considering they don't emit much in the way of charged particles... It's a concept I've seen mentioned exclusively together with fusion reactors so far.

Also, I've done some more googling since I don't trust single wikipedia statements, and some other sources I've found peg solar power systems at 150 W/kg in 2010/2011, with feasible paths to extend this to 500 W/kg over the coming decade. This doesn't necessarily have to conflict with the previous statement of 300 W/kg at an earlier date, it just means that that figure may not have included the entire power delivery system while the other one does? Difficult to say. But even at 150 W/kg, solar power remains competitive.

Same source also claims nuclear reactor concepts as high as 250 W/kg exist, but include a whole series of never-used-before technologies like liquid metal cooling and droplet radiators and who knows what. Basically those reactors are paper ideas that rely on other paper ideas. How quaint

I'm not sure we'll manage to reach 500 W/kg for solar power systems by 2020, but if we did, that would be pretty cool and let us go manned anywhere in the inner solar system with electric propulsion. Heck, considering we have a electric probe with early 2000's panels in the belt right now, we may even be able to go that far.

Edited September 22, 2015 by Streetwind

[Kryten]

Liquid metal cooling is standard for space based reactors, as far as I know all flown models have used it. About half of those have subsequently spewed said liquid metal into earth orbit, which is another reason people are wary of space nuclear reactors-they're major debris hazards.

[peadar1987]

What? We can't send it in two parts?

First send reactor and second fuel, just like we are sending crew with safe capsule and abort systems, and then put fuel into reactor in safe orbit?

A reactor that hasn't been run is perfectly safe. The fuel is only negligibly radioactive. The simple solution is just not to send the reactor critical before it is in orbit.

[Requia]

Liquid metal cooling is standard for space based reactors, as far as I know all flown models have used it. About half of those have subsequently spewed said liquid metal into earth orbit, which is another reason people are wary of space nuclear reactors-they're major debris hazards.

The 21st century designs (not actually flown) are water based systems. Debris is only a concern if you plan to stay in LEO.

[Elthy]

I wonder if the ISS solar arrays could be reused for e.g. a manned mars mission, according to wikipedia they provide 256kW of power. The advantage is that they are allready in space...

[Ralathon]

I wonder if the ISS solar arrays could be reused for e.g. a manned mars mission, according to wikipedia they provide 256kW of power. The advantage is that they are allready in space...

The ISS panels are getting rather worn down. Their efficiency decays over time and they get hit by micrometeorites etc. Not to mention that with current technology we could build much better solar panels. All in all, it's better to just build new ones instead of retooling old hardware that wasn't designed to be used that way.

[Streetwind]

Indeed, they're down to ~180 kW these days. The 256 kW figure is fresh after deployment IIRC.

[Alias72]

Agreed. Were not Mark Whatney, we don't need duct tape (but it's magic and should be worshipped!)