nuclear reactors in space

ohiovr · Member

nuclear reactors in space

G R L Cowan · Veteran Member

ohiovr wrote:

I'm having a bit of trouble finding technical information on the topic of fission reactors in space for electrical power generation. From what I've read most of these schemes have relied on the thermal electric effect and have power in the range of kilowatts. Thermal power in space is a trick, theres no cold air to exchange with and the potential is limited to black body radiation. Any chance of getting multi mega watt power levels with tens of tons of reactor and black body radiators?

I think for high power-to-mass ratios what you end up having to do is throw and catch pieces of carbon, or maybe boron carbide; something with low vapour pressure and low atomic weight. Once you have picked them out of vacuum, cool, you have to bring them into pressure so that, inside the heat engine, waste heat can be put into them by fluid contact.

If there were some ~~fluid~~ liquid that could do it all, cool the turbine effluent and then go outside and heat the universe without evaporating, that would be keen, but there doesn't seem to be one. Not with low atomic weight; not unless you accept, as above said, some lumpiness in the ~~fluidity~~ liquid.

(How fire can be domesticated)

-- Edited by G R L Cowan at 19:46, 2009-03-05

GoogleNaut · Guru

You could use a complete gas phase cooling package using helium in a Brayton Cycle turboalternator package--power levels from 100 kilowatts to a hundred megawatts are possible. Overall thermodynamic efficiencies approaching 50% are possible, however the system pressure tends to be pretty high...this makes piping pretty heavy especially where the radiator is concerned...this makes the radiator especially vulnerable to damage from micrometeroid impacts.

I once read something about a moving solid state phase radiator package that utilized small ball bearings pushed through tubing exposed to vacuum--I can't seem to find a reference to it at all...but it is interesting from the thermodynamics standpoint. Probably not terribly efficient from the standpoint of heat transport..but interesting!

John · Senior Member

Wouldn't the high pressure pipes also be thicker and more resistant to micrometoroids the lower pressure pipes? If one punctured a radiator pipe that would be bad news for an interplanetary plasma propelled ship.

Philipum · Veteran Member

I have found at least one example of a concept which was seriously studied in the context of the project longshot , looking at the reference quoted in Wikipedia :
http://en.wikipedia.org/wiki/Project_Longshot

On page 30 of the pdf file the system is described. It has only 300 kW of power but is it a scale-down of a multi-MW system proposed in the following conference proceedings :
http://openlibrary.org/b/OL21188599M/Space-nuclear-power-systems,-1984

I hope this helps...
Philippe

John · Senior Member

This seems like some ideas that I had back in the early 1970s (the propulsion system). I think that some technological development is a bit of an understatement. That's a lot of He3 to make! The other idea would be to make tritium by neutron bombardment of Li6 and then wait for it to decay to He3 (12 year half life).

John · Senior Member

I just had a thought on that radiator being punctured by a micrometeoroid. Could the radiator be segmented into many parts and if a punture occurs in a given part it could be shut off from the rest of the system? If there was a certain about of redunancy the problem would be minimized.

G R L Cowan · Veteran Member

John wrote:

I just had a thought on that radiator being punctured by a micrometeoroid. Could the radiator be segmented into many parts and if a punture occurs in a given part it could be shut off from the rest of the system? If there was a certain about of redunancy the problem would be minimized.

The vacuum-exposed surface of the radiator must be made of something with very low vapour pressure. What if we segment it into small pieces, and bring each piece through a tunnel that it almost completely blocks, into the hot fluid that it is supposed to cool? Leakage of the fluid past the incoming radiator piece can be made small because the gap is small and the piece is dragging -- pumping -- the fluid in, compensating to some extent for its tendency to go towards vacuum.

Now not just the inner surface, but all surfaces, of the piece are absorbing heat.

Take it back out to vacuum, again through a close-fitting tunnel, and let it fly through vacuum and radiate from all surfaces. If something happens to it, it doesn't get caught and brought back in, but as above said, it's one of many.

How fire can be domesticated

GoogleNaut · Guru

The radiator material isn't really the issue--it is the coolant that circulates through the radiator. It is possible to use a low vapor pressure coolant--like sodium, or lead--but leakage from micrometeoroids punctures will be a problem for a long duration space application. I think that with clever engineering and a willingness to accept a weight (mass) penalty it should be possible to shield the radiating elements from direct exposure to space environment by using mirrors...

Although the benefits of shielding from possible micrometeoroid impacts may not be worth the penalty of extra mass and complexity, a mass cost/benefit analysis could be done on a design to see if persuit of this concept is even worth it...

What I envision is a radiating element that radiates orthogonally behind a meteoroid shield to a pair of mirrors which reflects the heat radiation to space. By keeping the meteoroid shield wide enough at the top and the mirrors deep enough, the radiator is completely 'hidden' from space--and thus can be completely shielded from direct impact by micrometeoroids. Whether such a design is practical or not is dependent upon mass and performance cost of such a shielding system versus the combination of the flux of micrometeoroids, the combined area of the radiator, and the typical expected exposure time over the operational life of the system.

G R L Cowan · Veteran Member

GoogleNaut wrote:

The radiator material isn't really the issue--it is the coolant that circulates through the radiator. It is possible to use a low vapor pressure coolant--like sodium, or lead--but leakage from micrometeoroids punctures will be a problem for a long duration space application. I think that with clever engineering and a willingness to accept a weight (mass) penalty it should be possible to shield the radiating elements from direct exposure to space environment by using mirrors...

Although the benefits of shielding from possible micrometeoroid impacts may not be worth the penalty of extra mass and complexity, a mass cost/benefit analysis could be done on a design to see if persuit of this concept is even worth it...

What I envision is a radiating element that radiates orthogonally behind a meteoroid shield to a pair of mirrors which reflects the heat radiation to space. By keeping the meteoroid shield wide enough at the top and the mirrors deep enough, the radiator is completely 'hidden' from space--and thus can be completely shielded from direct impact by micrometeoroids. Whether such a design is practical or not is dependent upon mass and performance cost of such a shielding system versus the combination of the flux of micrometeoroids, the combined area of the radiator, and the typical expected exposure time over the operational life of the system.

And what happens when meteoroids hit the mirrors. You are making an error analogous to the homuncular theory of mind: people are aware because there is a little person in their heads, watching the screens and pressing buttons.

The radiator material, and the coolant that circulates through the cosmos and warms it, are one and the same. Stuff that flows though vacuum doesn't have to be fluid.

--- G.R.L. Cowan, ('How fire can be domesticated')
http://www.eagle.ca/~gcowan/

John · Senior Member

Isn't my idea better...at least for large systems? By segmenting cooling tubes in some patter you could have a system that monitors for leaks and selectively cuts out the portion that is damaged. By have some excess capacity you can have the system "gracefully degrade" if a few hits occur without having it knocked out. Also, just how bad is the micrometeoriod problem anyway?

GoogleNaut · Guru

Segmenting the system with isolation valves is probably the way to go, John. I think that your way preserves the integrity of the radiator as a whole despite a couple of hits with the minimum of weight penalty.

The thing I envisioned is just going to be too ungainly and not practical, especially for a flight weight article.

Also, in reply to G. R. L. Cowan:

The mirrors could be polished metal backed with multilayer (hard and then soft) shielding to break up and absorbe impactors. Most micrometeoroids will be too small to effect the mirror very much. Of course if it get's whacked with something big enough, all bets are off. The debris and ejecta will ricoche around enough to shred the whole works. Sometimes it is better just to lett the little buggers zip right through. You maybe lose a single coolant tube or two, but properly installed isolation valves on each tube will prevent catastrophic loss of coolant for the whole radiator. It will still work until either the mission is over, or you can make repairs...

-- Edited by GoogleNaut on Sunday 28th of June 2009 06:30:14 AM

GoogleNaut · Guru

Here's a nice little development regarding nuclear fission power systems on the moon:

http://www.space.com/businesstechnology/090806-moon-nuclear-power.html

They're looking at 40KW(e?) power systems to power a future moon base.

A baby step in the right direction I think.

NUKE ROCKY44 · Senior Member

Nuclear fission power plants work by splitting the nuclei of atoms in a sustainable, controllable reaction that releases heat, which can then be funneled through a power converter to transfer that energy into usable electricity.

Uh...think this was discussed several years ago, energy transfer is still in engineering competition based on mission mass inventory assessments-if I'm not mistaken.

I think what you're reading is a bit of engineering mission marketing.

NUKE ROCKY44 · Senior Member

Isn't there a Manifold Vacuum Canister System using particulate carbon powder circulating that whirls through the system, all that's supplied is positive pressure and the negative pressure and temperature of surrounding space vacuum to operate this coolant system with solenoid/valves to direct flows to absorb heat from reactor hot spots. You would have to shield the system from micrometorite strike.

Didn't someone say there was material on this manifold vac system?

Couldn't reactor radiator cooling systems involve all three coolants: gas, liquid and solid?

-- Edited by NUKE ROCKY44 on Saturday 8th of August 2009 11:29:10 PM

-- Edited by NUKE ROCKY44 on Saturday 8th of August 2009 11:43:17 PM

-- Edited by NUKE ROCKY44 on Saturday 8th of August 2009 11:52:38 PM

GoogleNaut · Guru

Concievably you could even use one inch diameter steel balls, circulating in cage-tracks through the reactor and radiator system--but this would probably be heavy and inefficient--but it could be done...

The nice thing about using fluids is that phase-change cooling (liquid to vapor--vapor to liquid) is a far, far more efficient way of tranferring heat because of the latent heat of vaporization/condensation. Hands down...

G R L Cowan · Veteran Member

GoogleNaut wrote:

Concievably you could even use one inch diameter steel balls, circulating in cage-tracks through the reactor and radiator system--but this would probably be heavy and inefficient--but it could be done...

The nice thing about using fluids is that phase-change cooling (liquid to vapor--vapor to liquid) is a far, far more efficient way of tranferring heat because of the latent heat of vaporization/condensation. Hands down...

Cooling through a phase change can indeed give off a lot of heat. But we were talking about space radiators.

That means you have two phase change options. (1) Let the phase change happen inside pipes, and let the outer surfaces of the pipes radiate away the heat. You have saved some mass flow inside the pipes, but the job of their outsides is the same as ever.

(2) Find a substance with a phase change such that the higher-temperature phase has very low vapour pressure, and can therefore be directly contacted with vacuum, and changes to a much lower-energy phase, so that exposing a little of this higher-'T' phase to vacuum does a lot for you.

Strictly, the lower-'T' phase should also have very low vapour pressure, but I think you can generally count on that, if the higher-'T' one does.

With your one-inch-diameter steel balls, you have set up a straw man. Now you should try to de-strawify it. What would be something like that, but designed to work? Obviously you would go to something less volatile than iron, and with much lower mean atomic weight, since hauling that weight around can be pretty mean. Look back in this thread for a hint.

(How fire can be domesticated)