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What is the feasability of a Neutron Piston?

  1. Apr 10, 2012 #1
    Hello all, this is my first post so hopefully its not overly foolish.

    I propose a mechanical piston driven by a cylinder that is filled with Uranium Hexafluoride and an inert gas (Nitrogen?). As the piston descends, the the UF6 is compressed and the neutron flux spikes. The increased temperature would expand the nitrogen and push the piston, allowing the UF6 to expand and decrease the flux again.

    Assuming a proper cooling system for equalizing the temperature after each piston stroke, and ignoring wear and tear, it seems to me that the piston could run continuously. Is there any merit to this idea or am I missing an important piece of the puzzle?

    Honestly it seems too simple. I cant find the hole :)
     
  2. jcsd
  3. Apr 10, 2012 #2

    etudiant

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    I'm not nuclear savvy, but I think that the UF6 neutrons need to be slowed by something, such as water or graphite, to be effectively absorbed by UF6 molecules.
    Separately, the reaction speed of exponential fission is a lot greater than mechanical inertia driven motion. The design, if implemented using some suitable radioactive fissile gas, might achieve an excursion on the first compression, rather than the desired expansion..
     
  4. Apr 11, 2012 #3

    QuantumPion

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    An easier method for creating a reciprocating nuclear engine is to just make the cylinders out of uranium metal alloy and fill the cylinders so that the maximum volume is critical. You could make the working fluid the moderator or make the pistons neutron absorbers and configure the system accordingly. Then you don't have to worry about storing, shielding, and containing gaseous fuel and fission products.
     
  5. Apr 11, 2012 #4

    jim hardy

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    it's interesting to tinker with a near-critical arrangement surrounded by steam.

    Increasing pressure compacts steam increasing moderation...

    read up on " undermoderated " you could get yourself setup for a runaway i think.
     
  6. Apr 11, 2012 #5

    QuantumPion

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    No, runaway reaction occurs when you are overmoderated, not undermoderated. You want it to be undermoderated - that is what makes it stable. When the water flashes to steam causing the piston to expand, the "core" becomes subcritical.
     
  7. Apr 11, 2012 #6

    QuantumPion

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    Thinking about it further, you have moving parts with tight tolerances under high neutron flux. Creep would be a significant problem. The pistons, seals and block would have to be very robust or limited to low burnup.
     
  8. Apr 11, 2012 #7

    mathman

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    Naive question: where did the neutrons come from?
     
  9. Apr 11, 2012 #8
    It seems like a primitive nuclear stirling engine. The thermal efficiency is probably lower than that of an ordinary pressurized/boiling water reactor.

    One problem is that the reaction might be stable and the piston will just hover halfway. There have been a number of criticality accidents where a stable fission reaction developed because thermal expansion made the reaction subcritical again.

    Fission products that absorb neutrons will make creating a stable oscillation very hard. some of them will precipitate on the walls of the cylinder.

    Without a modereator, you'll need a really large volume of UF6 to get a critical mass with low-enriched uranium

    Fission products will continue to produce heat when the cylinder should be cooling, this heat will have removed by cooling and won't help with energy production.
     
  10. Apr 11, 2012 #9
    I think the biggest "hole" in the idea was just mentioned, that of fission products absorbing neutrons and also continuing to produce heat afterwards. I hadn't considered that.

    I also hadn't considered the possibility that it would be less efficient than a standard boiler setup, although it does make sense.
     
  11. Apr 11, 2012 #10

    QuantumPion

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    It would be less efficient but there are advantages to a reciprocating engine - compactness, and the ability to change load on demand. Such an engine might be well suited for a large land vehicle like a tank, mechanized artillery, or possibly airplane.
     
  12. Apr 12, 2012 #11
    A really big problem is the low density of the UF6 gas. You might need a very big cylinder, that's as large as a tank, and you'll need a moderator, or highly enriched uranium.
     
  13. Apr 12, 2012 #12

    QuantumPion

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    As I mentioned previously it would be impractical to try to use a gaseous fuel for multiple reasons. For one, the UF6 would have to be very high enrichment. This leads to many issues in preventing criticality where you don't want it. Then you would need a system for recycling and cooling the highly radioactive exhaust to reuse, because you aren't going to be ejecting a million dollars worth of fuel on every stroke.
     
  14. Apr 12, 2012 #13

    etudiant

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    Your idea has precedents.
    There was a suggestion from satellite observations during the waning days of the Soviet Union that they had built a couple of enormous pressure spheres, connected with a duct festooned with cabling.
    The suggestion was a nuclear pulse device such as you suggest, with the expansion on one side compressing the other, with the plasma power tapped off in the middle.
    I have no idea if this was the actual purpose or what happened to that installation, but it is still in the records in the Aviation Week issues of the period.
     
  15. Apr 13, 2012 #14
    Why would you even have a desire to do this? A nuclear piston seems utterly ridiculous and unneeded.
     
  16. Apr 13, 2012 #15
    Have you filed for a patent for your idea yet? If not, you should search espacenet.com to check whether others have filed for it.
     
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