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Sustained nuclear criticality in liquid vortex

  1. Feb 2, 2016 #1
    In 1958, chemical operator Cecil Kelley was killed by a nuclear excursion in a mixing tank. A tank intended to reprocess trace amounts of dissolved plutonium-239 accidentally had dramatically more radioactive material dumped into it. The plutonium, being dissolved in a lower-density fluid than the rest of the solution in the tank, floated to the surface. When Kelley turned on the mixing tank, centrifugal force caused a vortex to form which concentrated the lower-density plutonium at the center. The plutonium reached prompt criticality in under a second and irradiated Kelley with about 36 Grays, seven times the adult lethal dose. He died within hours.

    In this event, the excursion was halted almost immediately by the release of energy, which rapidly heated the solution and dispersed the momentarily concentrated plutonium.

    I'm wondering:
    1. Would it be possible to build a nuclear reactor using dissolved radioactive salts in aqueous solution by generating a vortex in the fluid?
    2. Could such vortices be designed in order to self-sustain, so that the energy was released in such a way as to accelerate the rotation?
    3. If so, would it be possible to dissolve magnetic particles in the solution so that the whole rotating fluid mass produced a rotating magnetic field which could be used to directly provide electrical power by electromagnetic induction?
     
  2. jcsd
  3. Feb 2, 2016 #2
    Imagine what would happen if the vortex collapsed. A cylinder with a vortex in the middle has a higher surface area to volume ratio than a cylinder without a vortex. This means that the cylinder is a more favorable configuration for fission. If the vortex collapses, then the neutron multiplication in the reactor will increase. The power level will grow exponentially and the potential for a serious accident is huge!

    So no, it's not possible to build a safe nuclear reactor that runs in a vortex configuration.
     
  4. Feb 2, 2016 #3

    mfb

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    The heat production would have a cylindrical symmetry, so it cannot speed the vortex up.

    Extracting energy from such a vortex would be impractical as well. The magnetic particles would have to have some fixed orientation - against external fields (otherwise they do not perform work). That is not possible in a liquid, unless you add a solid structure. But then you have a bad combination of turbine and generator, where two separate devices are more efficient.

    No, it does not work.
     
  5. Feb 2, 2016 #4
    It was my understanding that in the Kelley excursion, criticality was achieved because there were two unmixed layers of fluid. At rest, the surface area of the plutonium-carrying layer was very high, but as the vortex formed, it concentrated the plutonium into the center. The bottom layer of fluid acted as a neutron reflector, and thus prompt criticality was inevitable:

    prmptcrtcl.png
    Such a design can be rendered inherently safe because at rest, it is in a lower-criticality state than it is in motion; however, if the vortex becomes too sharp and power output goes too high, the energy release will cause mixing of the two fluids and immediately disperse the fuel.

    I'd presume that the design of an intentional nuclear reactor would use some heavy immiscible fluid as the base/reflector fluid. Either that, or use a container with a shape that allows criticality only within a certain range of vortex strengths, perhaps using a neutron reflector that covers only some portion of the container:

    crtcl rctr.png

    Hmm.

    What about something like a rotary ramjet turbogenerator, but using fissile-fuel saltwater as the fuel and a neutron-reflecting ramjet?

    Nuclear rotary ramjet.png
    The fluid would pass through an impeller that sent it into narrow channels at the base. .The rotating portion would compress the fuel to near-criticality before exposing it to a neutron-reflecting "igniter" which would cause criticality; the heating and expansion of the fuel would push the rotating portion before the fuel passed out of the impeller and into a heat exchanger before being recycled back through.
     
    Last edited: Feb 2, 2016
  6. Feb 2, 2016 #5

    mfb

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    Direct steam production including the nuclear material: possible, but with many safety issues. Pressurized radioactive gas...
    The space version is the nuclear salt-water rocket without turbine.
     
  7. Feb 2, 2016 #6
    Indeed, pressurized radioactive gas is something to be avoided on the best of days. Hence the desire for generating a stable, continuous critical reaction inside a fluid vortex, and doing so using some sort of asymmetry so that the energy can accelerate the vortex.

    Is there any way to extract work from a nuclear fission reaction other than by heat?

    If the fluid is compressible, then using an arrangement like this one ought to result in asymmetric pressure on the central "turbine", causing it to rotate...

    topdown.png
     
  8. Feb 2, 2016 #7

    mfb

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    Not if you want a critical fission reaction. For beta decays, a direct conversion of the released electron (let it fly against an electric field) is studied, but no practical application so far. For fission this wouldn't work.
    Good luck getting that efficient.
     
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