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Nuclear pile powerplant

  1. Oct 19, 2014 #1
    I have a basic understanding about the fissioning process in atom bombs and in nuclear reactors but that's not what my question is about. It's about a nuclear pile of a radio-active material. A pile of the element to power a space station of considerable size like say 500,000 Metric Tons. Is it feasible to use neutron beta decay of some radioactive element in a pile that could be contained aboard the station? I'm open to alternative suggestions
     
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  3. Oct 19, 2014 #2
    Maybe I haven't been clear. The point is that I don't want to use any kind of reactor. Fission can be caused to melt down and Fusion can be caused to overload and detonate. Could a huge mass of radioactive material in an isolated part of a space station supply power without matching or exceeding the mass of the station proper?
     
  4. Oct 19, 2014 #3

    mfb

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    Only in science fiction.

    The mars rover Curiosity uses about 5kg of plutonium to get 2000 W thermal and 110 W electric power. Scaling this up by a factor 300 would give 1500kg plutonium for 600 kW thermal power and 30kW electric power (about the amount the solar cells give). The ISS is designed for 100 kW waste heat only, but replacing all photovoltaic cells by radiators would probably solve that issue. From the technical side, it looks fine.
    Launching 1.5 tons of plutonium to space is ... well, let's say you would get problems.
     
  5. Oct 19, 2014 #4

    Vanadium 50

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    These exist and are called RTGs. You get about 500 W per kilogram. 500,000 metric tons would be 250 GW, which is a lot of electricity. Roughly half of what the US uses.
     
  6. Oct 19, 2014 #5

    Matterwave

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    In addition, you'd have to worry about shielding if you have humans anywhere in the vicinity of this thing...and the shielding can be very very heavy.
     
  7. Oct 19, 2014 #6

    Vanadium 50

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    Actually, shielding is one thing you don't have to worry about if it weighs 500,000 tons (the same as a couple of aircraft carriers). If it's that heavy, it's mostly self-shielding.
     
  8. Oct 19, 2014 #7

    Matterwave

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    Hmm, but what about it's surface? Nothing is shielding the surface of all this material...o.o
     
  9. Oct 20, 2014 #8

    SteamKing

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    That's more like 5 aircraft carriers, thank you very much.

    http://en.wikipedia.org/wiki/Nimitz-class_aircraft_carrier
     
  10. Oct 20, 2014 #9

    Vanadium 50

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    Five Nimitzes. Twenty-five Yorktowns. You know, a "couple'. But that's what you want to discuss? The idea of half of the US electricity needing to be produced doesn't bother anyone?

    Assuming you use plutonium, most of the energy is in alphas. Whatever casing you use is plenty.
     
  11. Oct 20, 2014 #10

    SteamKing

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    A giant atomic pile of half a million tons is silly on so many levels, yet here we are, discussing it.
     
  12. Oct 20, 2014 #11
    I think the question was asking about whether it's feasible that a pile of some size, would be able to power a space station of the size mentioned. Basically, could a pile of nuclear waste be used to power five aircraft carriers, and if so, would it be a "feasible" amount, or would you basically need a similar mass of waste.
     
  13. Oct 20, 2014 #12

    Ryan_m_b

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    To be fair the OP specified that the station was half a million tonnes, not the pile. To answer the OPs question he'll need to provide a number of how much power he expects this station to require.
     
  14. Oct 20, 2014 #13
    What's the critical mass of terrestrial isotope mix of plutonium?
     
  15. Oct 20, 2014 #14

    mfb

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    There is natural "terrestrial isotope mix of plutonium" as all relevant quantities are bred in reactors, and the fractions depend on the reactor type and operation.
    Sure, making larger chunks will lead to some additional energy release from fission. I think taking the Curiosity values is a good approximation for a purely radiothermal energy source. For such a large space station, I would suggest to use an actual nuclear reactor as they are much more effective.

    The ISS has a mass of about 500 tons and uses ~30 kW of power, scaling that up by another factor of 1000 we get 30 MW. This would need 1500 tons of plutonium with a radiothermal source - in thousands of individual elements to avoid a critical mass. A nuclear reactor is certainly easier.
    Shielding is no problem. The station has to survive micro-meteorites, so the habitable areas have a good shielding against alpha + beta radiation anyway. The gamma radiation also has a reasonable shielding from all the material.
     
  16. Oct 20, 2014 #15

    Doug Huffman

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    The critical mass of Pu-238 is about 10 kg, a sphere about 10 cm diameter. Pu-242 100 kg, 20 cm diameter.
     
  17. Oct 20, 2014 #16

    Vanadium 50

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    Yes, but mixing in a neutron poison solves the criticality problem without impacting use as an RTG.
     
  18. Oct 21, 2014 #17

    Ryan_m_b

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    How long could an RTG be expected to run for? 1500 tonnes sounds like a lot but IIRC the half life is shy of 100 years, doesn't seem that if you're planning for the long term it would be very efficient. Especially as in a nuclear reactor you can get terajoules from a kg of uranium fuel.
     
  19. Oct 21, 2014 #18

    mfb

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    Yes, that is another issue. An RTG is great for small unmanned missions because it is very robust and works without maintenance. For larger projects, other concepts are much better.
     
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