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Smallest Possible N-Fuel Elements?

  1. Apr 6, 2008 #1
    This is actually about nanoparticles for better batteries:


    But my point is that nanoparticle fabrication is becoming a known, industrially mass-produceable manufacturing technique.

    So why can't nuclear fuel elements be made nano-sized? What is the lower limit on the size of nuclear fuel elements?

    TRISO fuel elements used in various advanced reactors are supposed to be mere hundreds of microns in diameter. What prevents similar fuel elements from being made hundreds of nanometers in diameter?

    The advantage? Well, you'd need less fuel to achieve a given power output, and you'd achieve a higher burnup fraction, with less unused fuel sitting in the waste. There'd be less need for reprocessing, and more efficient consumption of that fuel.

    As a result, you could then use such nuclear fuel to power spacecraft, mars rovers, etc, or even perhaps to power certain vehicles here on Earth, to reduce consumption of greenhouse fuels.

    So what then is the lower limit on the size of nuclear fuel elements, for practical purposes?
  2. jcsd
  3. Apr 7, 2008 #2
    Oh yeah, and does anybody remember these recent announcements about using nanometer size quantum effects to force unidirectional heat flow, and prevent heat from traveling back to the source? Couldn't that be likewise used to more efficiently harvest heat from nuclear fuel? Here, look at this:


    Nano-sized grains or structures in this thermoelectric material block heat transmission in the wrong direction, to prevent backflow of heat against the direction of cooling.

    So anyway, can't something like this be done for nuclear fuel materials? Use similar phenomena to maximize the outflow of heat from the fuel, and minimize any backflow of heat into the fuel. Why can't that be done? This would harvest heat more efficiently.
  4. Apr 7, 2008 #3
    I don't think making fuel elements smaller would be useful, you will just need to load more of them into a reactor since you need a certain amount of fuel before you reach criticality (i.e. there is a required mass of fuel before your reactor can run on its own). You could increase the enrichment of the fuel, that could make fuel elements smaller.

    TRISO particles are made small, but each element has a lot of them inside (up to 10,000 I think), and with the protective layers around them, they end up being about the size of a tennis ball.

    That material used to increase conductivity is a semiconductor. I'm not completely sure, but I don't think semiconductors do well under neutron irradiation. The current cladding used for nuclear fuel (the "wrapping" on the uranium) is zirconium, which has a pretty small thermal neutron cross section.
  5. Apr 7, 2008 #4
    The point of making the fuel particles smaller is to create more surface area for higher power output. If energy can be drawn from them faster, then less fuel is required to achieve a given power output. So just as TRISO particles allow more power output than larger fuel elements, therefore nano-sized fuel particles would allow for even higher power output, and also for higher burnup fraction.

    This could allow nuclear power to be used for mobile/transportation applications, such as spacecraft, quiet intercontinental hypersonic airliners, etc.

    I just want to know what factors impinge on the lower limits of fuel particle size.
    Why can't fuel particles be made nano-sized, for example? What practical constraints prevent this?
  6. Apr 7, 2008 #5


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    Yes there would be more surface area, but then the nanoparticles would be sufficiently small that the working fluid carrying the heat away from the mass of nanoparticles would be able to transport those particles away - unless they are fixed. But to be fixed the particles have to be held in a porous structure, otherwise the particles would have be dispersed in a metal matrix of some kind, with the fluid outside.

    Nuclear propulsion would not be practical on an airliner or terrestrial vehicle because one needs a certain mass (and volume) to maintain a critical system. In addition, the smaller the core, the higher the enrichment, and it is not practical to have small reactors, with highly enriched U or Pu. Sheilding would be fairly massive. Even nuclear propulsion in space is very challenging. Besides - hypersonic aircraft are not quiet given the high velocity propellant and interaction with the atmosphere.

    Nanosize fuel particles are made and dispersed in cermet fuels. The limitation on size comes from safety matters. The fuel must not release fission products, e.g. radionuclides of Kr, Xe, I, Br, Cs, Sr, . . . to the environment, nor are the fission products to be released into the power plant systems.
  7. Apr 7, 2008 #6
    Well, suppose they were bonded onto the surface of a metal matrix, with a convective fluid flowing over them? Wouldn't that keep them from flowing away, while also staying in contact with the fluid?

    Plasma envelopes have been shown to greatly reduce friction between an aerobody and the atmosphere. Nuclear electricity could help create such plasma. Many of the atmospheric constituents in the upper atmosphere / ionosphere are already more amenable to ionization.

    The smaller your fuel particles, the higher their heat transfer rate, and allowable power output. The higher the power output, the less overall fuel you need, and thus the lower the overall shielding requirements. I'm not saying this would be suitable for a small aircraft, but for a long-distance hypersonic airliner/transport, the vehicle would be expected to have a larger size and payload capacity, and thus be more accommodating to the mass requirements of a nuclear reactor.

    Perhaps such a nuclear-powered aircraft could take on water at the airport, with the higher temperature particle-bed reactor splitting it into hydrogen, and that compressed hydrogen would then later be used as propellant, whose thrust would be greatly amplified by the nuclear reactor during flight. That same compressed hydrogen would be used to cool the particle bed reactor, and draw power from it. I'd even wonder if a monoatomic gas like helium could be used as a heat exchange fluid between the particle bed and the hydrogen, since helium is non-corrosive and should have better flow characteristics as a monoatomic gas.

    Btw, wouldn't liquid hydrogen be a superior moderator against fast neutrons?
    Last edited: Apr 7, 2008
  8. Apr 7, 2008 #7


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    Bonding to a surface kind of defeats the purpose of increasing surface area. Then how small would one make the metal matrix, because a foil would not be strong enough to stand shear forces, and then there would be vibration issues.

    I'm not sure about a plasma envelope around a craft. The idea is to keep the craft cool, which is already challenging enough.

    BTW - think about the fact that increasing the surface area increases the drag on the working fluid, which means more work to force/pump it, and a greater pressure drop across the system.

    Liquid hydrogen is a decent moderator of fast neutrons, but it is also of relativley low density. Furthermore it is a significant safety issue and cyrogenic systems like that in the shuttle require a lot of maintenance. One cannot afford a failure, which would likely be catastrophic.

    Helium is rather expensive.

    There are other considerations such as the thermal-hydraulic conditions, thermophysical properties and structural issues that come into play, besides the heat transfer surface of the fuel.
  9. Apr 8, 2008 #8
    The title of the thread is "Smallest Possible N-Fuel Elements?", where sanman is asking about nano-particles for batteries. He's talking about smaller, more powerful, or longer lasting batteries, but I think he's a bit confused about whether atomic batteries are running as a self-sustaining nuclear reaction, or standard radioactive decay; as he mentions fuel elements in nuclear reactors. Atomic batteries are generally too small to be able to reach critical mass for a self-sustaining nuclear reaction. Atomic batteries therefore use standard decay to generate electricity.

    Nano-construction techniques should increase the efficiency for an atomic battery; regardless of whether it of a thermionic or non-thermal type. For a brief overview, you can find 7 different types of atomic batteries described in the Wikipedia site.
  10. Apr 8, 2008 #9
    Well, actually I was using the article on batteries to point out that technology is now adept at producing nano-sized particles for various industrial purposes. So then likewise, why couldn't nuclear fuel be fashioned into nanoparticles, for portable applications requiring high power output.

    I'm not sure what the minimum mass is for a nuclear reactor, but I would think that in general, the small the fuel amount, the lower the overall mass of the reactor.
  11. Apr 9, 2008 #10


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    The smallest sustainable nuclear reaction systems are the basis for nuclear weapons, and they contain many kg's of highly enriched (>90%) U-235 or Pu-239. As size decreases, enrichment must increase, so small nuclear batteries based on fission are not practical.

    Nuclear energy by decay is ongoing, i.e. it cannot be turned off. A fission-based system can be adjusted in power level or shutdown, but the control becomes more difficult as enrichment is increased with decreasing size.
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