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Need help with a topic for a sci-fi book. Basic premise is 200yr in future, mining facility on small Mars moon of Phobos. Power source we would like is an Sodium Fast Reactor breeder as we think this would be cost effective for materials and sustainability. What I need to know. Phobos has insanely low gravity and I need know how the SFR would operate in that environment and what other components would I have to add to make it function. I know without oxygen liquid sodium at high temp wouldn't react if released however is there a point the temp in an SFR reactor can get so hot it explodes vice a standard melt down?

BTW I am Gas Turbine tech and understand a good bit of tech but Space and Nuclear is a little too far in left field for me.

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As I see it you have 3 issues here:

  • Gravity.

Normally sodium circulation would occur because of a temperature/density gradient which facilitates convection to carry it through the heat-exchange cycle. In minimal gravity the circulation would be crippled without a pump, both for the core->sodium heat exchange (spinning the sodium to create a centrifugal density/temperature gradient) and for the sodium->water heat exchange.

  • Atmosphere.

Attribution: Wikipedia 2019, CCSA License

Energy extraction occurs in a temperature gradient. Whilst complete conversion of the steam to rotational energy in the turbine is the ideal, you'll still need to shed a great deal of heat energy. Perhaps a secondary set of turbines using acetone to gather a bit more useful energy instead of the condenser phase might be in order. You'd still want radiators to offset inefficiencies inherent in energy transformation. Needless to say, you don't want the sun's incident radiation to heat the thing up - a series of moving mirrors and shades would be synchronized to local "daytime".

  • Leaks.

According to renowned nuclear physicist M.V Ramana, sodium leaks are almost impossible to prevent. Fortunately, without a reactive atmosphere, sodium fires and the formation of caustics should be obviated. Any sodium can be recovered by remote handling, and fed back into the pool as necessary.

  • Caveats:

Have more than one pump, redundancy of systems is critical in a reactor like this. Parallel systems and fail-safes wherever possible. A multiplex of 3 or more independent systems would not be overkill.

Radiators: These may need to regularly have dust removed to prevent a drop in efficiency. Regular micro-meteor damage repairs need be scheduled. Again redundancy of shading is advisable.

Have plenty of spare control rods and sodium to cool the thing down as necessary.

Avoid having it in close proximity to people, just over the horizon would be nice.

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    $\begingroup$ I feel like the requirement for the pumps to avoid meltdown/explosion eliminates most of the benefit of going with Liquid Sodium in the first place. $\endgroup$ – Morris The Cat Oct 17 '19 at 19:31
  • $\begingroup$ @MorrisTheCat I agree, care to write a frame-challenge based on that? $\endgroup$ – Draft 85 Oct 17 '19 at 19:38
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There should be no serious problem with operating a nuclear reactor in zero gravity provided it has been specifically designed for that purpose. Such a reactor in zero or very low gravity would need a pump or more likely a series of pumps and back up pumps to ensure circulation of the sodium.

In fact sodium cooled nuclear reactors have flown in space: https://en.wikipedia.org/wiki/TOPAZ_nuclear_reactor

Although a fast breeder would involve far more complications these should not be insurmountable especially 200 years into the future and especially in an environment presumable far removed from human habitation. Such space based reactors would need a lot of radiator space to remove waste heat. BTW I’m not sure that putting it on or near Phobos would be beneficial. It would probably be a dust hazard.

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  • $\begingroup$ Care to explain the dust hazard. Others brought up the pumps but I think if we come up with a realistic way to spin the planet we could invert the plant and build toward the moons core. As for cooling I don't it should be a problem you could run nitrogen cooling loop on the lunar surface which is below freezing environment. $\endgroup$ – Christopher Wojtan Oct 18 '19 at 2:05
  • $\begingroup$ Yes cooling and dust are issues but not insurmountable problems. My point was that any extensive activity on the Martian moons would likely dump a lot of dust onto orbit around the moon. This could cover surfaces with dust during construction. Especially on surfaces that would best be kept clean. $\endgroup$ – Slarty Oct 18 '19 at 7:55
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Most (probably all) reactors designed for use on Earth basically require a decent gravitational field in order to stop Unfortunate Things happening like all your coolant floating away, or gravty-assisted mechanical systems failing, or crud getting into mechanical bits and so on. The lazy answer, therefore, is to use a design that is intended for microgravity use, which will probably look quite different to other designs. There have been space based reactors in the past (SNAP-10A, 1965) and designs have been made for slightly more modern ones (Megawatt Class Nuclear Space Power Systems (MCNSPS) Conceptual Design and Evaluation Report, 1988) and I don't doubt more work has been done since then.

Option B is to use spun gravity. In The Expanse, Ceres had been spun up to provide centrifugal gravity as its natural gravitational pull (about 3 centigees) is too low to be comfortable. Phobos is of course much smaller and has 100th of the surface gravity of Ceres, but could be spun up much more easily. In Red Mars, a circumferential train track was wrapped around Phobos, and a maglev train driven along it at speed to provide artificial gravity. Either approach would allow the use of more conventional reactor designs, though care wil have to be taken if the spin is ever reduced in future.

is there a point the temp in an SFR reactor can get so hot it explodes vice a standard melt down?

Maybe! Sodium boils at 1156K, and above that Interesting Things are going to start happening in your reactor chamber. It won't, however, be a pressurised chamber, so there's less scope for dangerous pressures of sodium gas to build up. It isn't immediately obvious if the temperature could reach this point however... it almost certainly exceeds the reactor's expected operating temperature range, and one might expect various dumb failsafes to trigger before that happened, even if cleverer systems did not. If a loss of coolant causes fuel rod temperatures to rise much further than 1156 you may find that bits of the assembly start to melt, so it'll be a bit of a race between a meltdown and a pressure buildup.

I think I'd err on the side of "no", but I can't be at all certain on this, and I don't fancy crawling through reactor literature to find out what more knowledgeable people have to say on the matter.

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  • $\begingroup$ I didn't read Red Mars, did they run a second maglev the OTHER way to keep the moon itself from accumulating spin? Otherwise it would... I mean, that'd be a way to spin it up Ceres style instead of using reaction mass. $\endgroup$ – Morris The Cat Oct 17 '19 at 19:32
  • $\begingroup$ @MorrisTheCat they didn't go into detail, just mentioned it in passing. Given that the spin of Phobos was not mentioned, one assumes it was sorted out in some sensible fashion ;-) $\endgroup$ – Starfish Prime Oct 17 '19 at 19:43

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