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Uranium and Diamonds

  1. Dec 27, 2014 #1
    Last edited by a moderator: May 7, 2017
  2. jcsd
  3. Dec 27, 2014 #2

    Astronuc

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  4. Dec 28, 2014 #3
    Wouldn't a durable diamond surface be another way to trap fission products if the differential thermal expansion can be solved? If I understand you correctly I think the process of coating might be able to overcome the problem as the coating only has to be a few microns thick so tiny uranium particles could be coated and then merged and coated again into ever larger particles until you have the desired pellet size . . .

    JDM
     
  5. Dec 28, 2014 #4
    I wonder how a UO2/Zr cermet would fare in this regard. (IIRC in cermets used for cutting tools metal content is usually only about 10%. Since in this case we aren't going for strength, we can use even lower percentage of Zr metal.)
     
  6. Dec 28, 2014 #5

    Astronuc

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    The differential thermal expansion is a consequence of the fission/energy distribution in the pellet, which at steady state establishes an approximately parabolic temperature distribution between the surface of the pellet and the center. The difference in temperature can be a few hundred °C to about 1000 °C depending on the pellet fission rate. The UO2 ceramic has grain sizes on the order of 10 microns to 20 microns (with some grains smaller and some larger). Adding inert material such as diamond or metal like Zr as nikkkom indicated with UO2/Zr cermet means that U would be displaced, so the enrichment would have to be increased to get to the necessary enrichment.

    UO2 pellets are fabricated in a bulk/batch process where the power is pressed into pellet in a rotary press to a density of about 55-60% of theoretical density. The pressed (green) pellets are then collected in Mo boats or on sheets and are then sintered in a low humidity H2 (from cracked ammonia) environment at 1600-1800 °C for 2 to several hours depending on the temperature.

    Modern PWR fuel cycles use enrichments up to 4.9 to 4.95% U-235 in U. The maximum enrichment is always just below 5% since 5% is the legal limit in commercial LWR fuel, and the 0.1 to 0.05% difference is there to allow for uncertainty in the manufacturing process.
     
    Last edited: Dec 28, 2014
  7. Dec 28, 2014 #6
    People have been looking at Molybdenum matrix cermet fuel:

    http://www.nrg.eu/docs/nrglib/2004/2004_nucl_techn_146_3_bakker_klaassen.pdf [Broken]

    Looks quite good, although they need to remove Mo-95.
     
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  8. Dec 28, 2014 #7

    mheslep

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  9. Dec 28, 2014 #8

    mheslep

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    As I understand the metal rod concept from Lightbridge, the idea is to replace oxygen in UO2, not uranium.
     
  10. Dec 28, 2014 #9

    mheslep

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    The reference from the OP refers to a U/Zr alloy metal.
    http://www.technologyreview.com/new...could-boost-reactors-but-also-safety-worries/

     
    Last edited: Dec 28, 2014
  11. Dec 28, 2014 #10

    Astronuc

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    Lightbridge has an interesting concept that has yet to be proven in an LWR environment. Such fuel will have potential issues with fuel-cladding chemical interaction, depending on the fuel alloy. Metal fuel is beneficial from a fissile density and thermal conductivity perspective, but there are other disadvantages with respect to reduced melting temperature and fission product mobility.

    The OP mentioned uranium pellets, and the bulk of commercial fuel is comprised of UO2 ceramic pellets clad in zirconium alloy tubing. Even most fast reactor fuel is UO2 or (U,Pu)O2, although some carbide, nitride and carboxide or carbonitride fuel has been tested.

    In addition to normal operation, a designer must also consider the off-normal and failed fuel situations, as well as accident (RIA and LOCA) situations, and the consequences of the fuel form and it's behavior in the different situations. UO2 is more chemically stable in high temperature water coolant than UC/MC or UN/MN, where M = (U, Pu).

    I'm not aware of actual fuel fabricated by LB unless it's in Russia.
     
    Last edited: Dec 28, 2014
  12. Dec 28, 2014 #11
    Carbide and nitride fuels have a problem of generating C-14, which bioaccumulates and has a very nasty half-life of 5700 years.
     
  13. Dec 28, 2014 #12

    mheslep

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    The reference suggests metal alloys have been tried in subs, thus military LWRs.


    I would think the melting point of UO2 is not the primary accident event issue, post Fukushima, but rather the hydrogen production from Zirc metal when oxidized by water, which is well under way at 700C. Fission product mobility, as you suggest, seems like an intractable problem for a system that won't well tolerate those hot isotopes in the loop.
     
  14. Dec 28, 2014 #13

    Astronuc

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    or more generally naval reactors. I believe metal fuel has been used in reactors of Russian icebreakers, e.g., KLT-40 reactor.
    http://scienceandglobalsecurity.org/archive/sgs14diakov.pdf
    http://www.iaea.org/NuclearPower/Downloadable/aris/2013/25.KLT-40S.pdf

    Certainly the rapid oxidation and disintegration of the cladding (Zr-alloy or otherwise) is a concern. There are other design considerations on the cladding as well. Not too many systems handle high temperature steam conditions, especially as the temperature exceeds 500°C for long periods, or possibly as may have been the case with Fukushima, > 1000°C.

    With regard to diamond coating of fuel or fuel particles, one has to look at the engineering feasibility and cost, and one must be familiar with the technology, much of which is proprietary or otherwise sensitive.

    There is also the neutron damage to the diamond structure, which would also change the composition through transmutation, in addition to displaced atoms.
     
  15. Dec 29, 2014 #14

    jim hardy

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    I can only speculate here..

    Interesting thought. Mix diamond right in with the fuel pellet.

    What would be effect of having so many carbon atoms in such proximity to the fuel ?
    Seems to me moderator temperature feedback would be really quick.

    But would such energetic neutrons break up the diamond crystal lattice?
     
  16. Dec 29, 2014 #15
    In the limit, you end up with just having uranium carbide fuel. Which may be not that bad, although I don't like that neutron activation will produce lots of C-14.
     
  17. Jan 5, 2015 #16

    QuantumPion

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    No it won't. Carbon does not absorb neutrons. That is why it is a good moderator. C-14 in the environment is produced from nitrogen.
     
  18. Jan 5, 2015 #17

    jim hardy

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    The absorbtion cross section for C13 is small but finite, around a millibarn.
    Nitrogen's is a bit shy of 2 whole barns.
    http://www.isis.stfc.ac.uk/learning.../sears---neutron-cross-section-table10655.pdf

    useless and boring trivia -
    That's why we purged our incore neutron detector tubes with CO2 not air. Flux in there was around 10^14nv.
    We lubricated the steel drive cables with graphite. As best i recall the prominent activation product on the drive cables was cobalt. Of course the little fission chambers themselves (Oralloy) got the hottest.

    old jim
     
  19. Jan 5, 2015 #18
    I know a Czech researcher working with a group studying diamond coatings who is studying coating the outside of the cladding material to reduce oxidation during accidents. Another research group is looking at using diamond coating on the inside of the cladding to mitigate some chemical interactions between fission products and the Zr. Neither of these are trying to take advantage of the thermal conductivity but instead the chemical and mechanical stability. The coating in question is a mix of diamond and graphite (to give flexibility and toughness).

    The biggest issue with UO2 fuel is that it performs best at low temperatures (thermal expansion, fission product retention, margin to melting ect) but it has a poor thermal conductivity. To improve this property people have suggested doping UO2 to produce heterogeneous materials with better macroscopic heat transfer properties. A number of materials are being looked at mostly BeO, SiC, diamond and nano-tubes. In these cases it isn't a coating, but instead bulk material added to the pellet. You could imagine a pellet with many strings of high thermal conductivity material carrying heat from the center of the fuel to the surface.
     
  20. Jan 5, 2015 #19
    At high neutron flux figures typical for power reactors at 100% power, some carbon in carbide _will_ absorb neutrons. Fuel is exposed to this flux for at least one year, typically more.
     
  21. Jan 5, 2015 #20
    Yes, that's why cermet (a ceramic sintered with about 10% of metal powder) immediately pops into my mind. Its thermal conductivity is higher than of pure ceramic.

    It is a well-known class of materials, used for cutting tools and other high tech applications. There are tons of research on it - for example, how different sizes of initial powders affect the resulting material, what happens if you add a powder (possibly of a different metal) which consists of tiny needle-like particles, not spheres (some tests show that it tends to reinforce material, like microscopic rebar)...

    As a starting point, I'd try a cermet with uranium metal (added benefit of more uranium in the fuel, by weight). Also l'd try depositing a thin layer of Zircalloy on the cermet pellet's surface - there's hope this can eliminate chemical interaction with cladding tube.
     
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