Any problem with a nuclear reactor in space?

GeorgeK said:

On an orbiter, near the center, so not in zero-G but very low gravity?

Also would asteroids contain uranium or some other fissionable material?

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It used to be upseting when nuclear reactors fell out of orbit.

http://www.time.com/time/magazine/article/0,9171,957052,00.html

Getting rid of the reactor heat is a huge problem for any nuclear-powered spacecraft. Most so-called nuclear-powered satellites and unmanned exploration craft don't have to worry about it too much since they don't have living creatures onboard that will be bothered by being broiled alive and they have lots of surfaces which can radiate the heat away. In addition, most of those launched don't really have nuclear reactors onboard, they have nuclear fuel cells. A small lump of radioactive material is held inside a specially-designed container which uses the heat if radioactive decomposition to generate electricity. An actual nuclear reactor does things in a fairly different manner.

A real nuclear-powered spacecraft is going to have to deal with waste heat from the reactor, contaminated coolant, waste heat from the inhabitants, contaminated waste from their "coolants", waste heat from all the other equipment, and then some.

Against the background of space an inhabited nuclear-powered spacecraft is going to glow like a star.

Now, I'd suggest you go look up how nuclear submarines manage the task and then imagine what happens if you're not surrounded by a perfect radiator, but by a perfect insulator.

I think the biggest problem would just be getting rid of waste heat. Space is cold, but it's also a vacuum so there's nothing to carry off waste heat. You just have to radiate it off.

As far as I know, asteroids don't contain radioactive elements. They do contain precious metals (like iridium) but none of the heavier elements like uranium.

You could make it a breeder reactor: http://en.wikipedia.org/wiki/Breeder_reactor

For a civilian structure, waste heat isn't so much of a problem. Just put some big freaking aluminium foil paddles on it and everything's fine.
Placing the reactor in zero g would be advantageous for handling, since there is some rather heavy machinery involved in nuclear reactors. Safety isn't much of a problem if the reactor is properly designed. A more interesting question though is whether solar cells wouldn't have a better power/weight ratio.

Asteroid should have in general, a composition similar to planets. I.e. radioactive material can be found on them, in traces. Since there's no tectonic activity though, i don't know how likely it is to find actual veins of fissionable stuff.

GeorgeK said:

Would copper be better (In cooking it is for radiating heat, but then that's also inside Earth's atmosphere. I don't know if that affects the equations.

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Copper conducts heat better, but that's usually not the limit in space. It's more about getting as big a surface area as possible. Heat conductivity only start to matter if the radiator is so big that a heat differential builds up, i.e. it radiates the heat on the outside significantly faster, so that the heat from the inside isn't conducted fast enough. Rather unlikely, unless the radiator is at very high temperatures.
Just for kicks though, diamond or graphene (not to be confused with raphite) make for extremely good conductors. Those might actually be interesting for the internal heat pipes of the system, i.e. those parts that get the waste heat from the reactor to the hull, where the actual radiators are.

So in my truck tire analogy. if I put a couple Eifle towers at the center of the hubcaps and the reactors are at the tops of the towers, that'd be an improvement?

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I don't see a particular need for the towers, but sure, placing the reactor anywhere on (or very close to) the axis of rotation puts it in low to zero gravity.

They're pretty far out. I'm not sure if they are in a trans-Neptunian highly inclined orbit (the closest possibility) or out in the Ort Cloud.

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That'd probably make solar cells less useful, yes. Although solar cells can have very low mass requirements.

So it's there, just difficult to find? Would a Geiger Counter work in space, or does it require working inside an atmosphere?

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No atmosphere required. The problem i see wouldn't be solved by a geiger counter though. Unless there are asteroids with actual ore veins to be found (maybe remnants from some collision where the material melted and separated, or chunks of a planet or something) it's not about finding the fissile materials, it's about refining them. Fissile metals are very rare, only a few parts per billion of the solar system's mass, if there are no naturally high concentrations to exploit you'd have to mine an awful lot of asteroid to get a little fuel.
cuallito's idea with Breeder reactors is pretty good. Finding material to use in a breeder reactor is easier, though it's still not exactly abundant.
Another thing to look at are advanved reactor designs. Pebble bed reactors or liquid core reactors for example are significant improvements over light water reactor designs.

Maxx said:

It used to be upseting when nuclear reactors fell out of orbit.

http://www.time.com/time/magazine/article/0,9171,957052,00.html

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Was that what prompted people to get upset every time someone LAUNCHED space probes containing nuclear materials, or were they already getting upset over that?

But yes, regardless of when it started, some people get upset at sending up nuclear materials from concern that the rocket may malfunction during launch, the materials will come back to earth, and the container will break open and cause radioactive contamination wherever it lands.

As indicated, unless we get lucky and find a highly radioactive asteroid, it seems easier to launch the materials than harvest it elsewhere in the Solar System.

Iridium is not fissile. Thorium is probably the most common element which can be used in reactors (specific types of breeders). It's still pretty rare. (as are all elements heavier than iron)
I'd say one important question is the environment of that space station. If there's a lot of asteroid mining going on, getting enough fissile material won't be much of a problem. It'd be a waste product of the refineries. Since nuclear fuel isn't burned nearly as fast as chemical fuel, any decent-sized mining operation should produce enough thorium as a by-product to feed a breeder reactor which powers the mining operation and has excess capacity.
On the other hand, if you mine the asteroids exclusively for the fissile fuel, you'll end up with amounts of by-product (iron, carbon, silicon mostly) orders of magnitude above the amount of fuel you get.
One advantage of nuclear fusion is that fuel is so much easier to find.

GeorgeK said:

So then, when they talk in Geology about the Iridium layer marking the (I think it was) KT boundary proving an asteroid impact, was that a freak coincedense that luckily it was that one very rare type of asteroid that they might use to prove the theorem? If it is common in asteroids, is it not fissionable, but maybe useful in its thermal value?

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Iridium is not fissionable. Nor is its most common isotope radioactive. It is a very heavy element, and the thinking is that, being so heavy, most of the iridium in the earth has become concentrated in the core, from back in the formational days when the earth was entirely molten.

Iridium is known to be more abundant by percentage (though still very rare) in meteorites than it is in the Earth's crustal rocks. Elemental iridium is the second-densest element (after osmium), is hard and takes a high polish. It has no value as an energy source, and its major use is in jewelry amalgams with more abundant elements like platinum.

Lhun said:

Iridium is not fissile.

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Grammatical correction: "fissile" is an adjective referring to the physical property of some materials to be stretched or drawn into thin wires or sheets. The term does not refer to any form of radioactivity. The term you are looking for is "fissionable".

Even most radioactive isotopes do not undergo nuclear fission. The most common fissionable isotopes are uranium-235, thorium-232 and plutonium-239:

http://hyperphysics.phy-astr.gsu.edu/hbase/nucene/fission.html

blacbird said:

Grammatical correction: "fissile" is an adjective referring to the physical property of some materials to be stretched or drawn into thin wires or sheets. http://hyperphysics.phy-astr.gsu.edu/hbase/nucene/fission.html

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That's 'ductile'. Fissile means 'easily splittable'.

FOTSGreg said:

Now, I'd suggest you go look up how nuclear submarines manage the task and then imagine what happens if you're not surrounded by a perfect radiator, but by a perfect insulator.

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How does a lack of atmosphere insulate?

blacbird said:

Grammatical correction: "fissile" is an adjective referring to the physical property of some materials to be stretched or drawn into thin wires or sheets. The term does not refer to any form of radioactivity. The term you are looking for is "fissionable".

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http://lmgtfy.com/?q=fissile

LOG said:

How does a lack of atmosphere insulate?

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How doesn't it? If there's nothing around something, it's insulated, pretty much by definition. If you ever broke a thermos, you might have noticed a louder than expected "pop", that's when the evacuated bottle imploded.

LOG said:

How does a lack of atmosphere insulate?

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You've got three ways to get rid of heat:

Conduction via physical contact, like a skillet on the stove.

Convection via movement of a fluid, like heat flowing out of an oven or water running through a nuclear reactor.

Radiation via the emission of blackbody energy, otherwise known as light.

A lot of cooling in every day life on Earth takes place through the first two processes and we take it for granted. Vacuum insulates because there's nothing to physically remove the heat besides blackbody radiation -- and that's pretty slow compared to the other two (not to mention blindingly obvious in any sort of space-combat detection scenarios).

If you're on a big hunk of ice or even asteroidal metal, you could dump quite a bit of heat into that. Same if you're willing to toss water out the airlock on a spacecraft or installation, though this is obviously a short-term solution due to having to carry that mass around. Otherwise you're left with glowy radiator panels, barring magic-tech solutions.

Albedo said:

That's 'ductile'. Fissile means 'easily splittable'.

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You are correct, and I stand corrected. Well, sit corrected, actually, as i am at my computer. But the term still has nothing to do with nuclear reactions.

The reactor could be on the spin axis of the colony and still be very far away - say, attached to the colony by a mile-long set of girders that hold it in place and conduct energy cables back to the colony.

Hi George,

Don't know if it helps, but back in the 50s/60s the Americans and the Soviets both experimented on nuclear powered planes.

But if your story is set away in the future, why bother with 20th Century technology? Couldn't your scientists develop a new element? A new form of power generation? Even have wireless electricity, doing away with the need for power lines etc.

In saying that, I think that mining asteroids may be rather difficult.

Regards

Hi GeorgeK,

If your story involves a space colony of that size, and so far from Earth, I doubt a reader would have a problem with fusion power.

Unlike fission power, fuel is pretty easy to come across, even if you make your characters use hydrogen isotopes (e.g. deuterium).

There is still the waste heat problem, although that is easily overcome if you don't mind being really visible.

I definately agree that you want the reactor(s) far from inhabited sections, but I don't see a problem with them being in the centre if that is reasonably far from the inhabited ring. Probably best if they only have a few connections too.

Messing up a reactor in a way that makes it go critical is virtually impossible, accidents are only likely to be a small (compared to a nuclear blast) explosion and some contamination which is easy to deal with in a closed environment.

Don't know if your story needs/would benefit from this, but the fusion reactor in the centre of the torus could actually be a fusion rocket, giving you propulsion and possibly enough power just from 'waste' energy.

George, is there a reason it needs to be a fission reactor, and not fusion? I don't know the isotope ratios of deuterium to H1 in the asteroids, but I'd bet that deuterium is more plentiful in asteroids than fissionable elements are.

Of course, if you're planning on, say, employing a reactor meltdown as a plot point, then fusion is definitely out of consideration.