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MOX Fuel Considerations

  1. Aug 1, 2011 #1

    swl

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    What are the differences between the use of MOX fuel and "standard" fuel?
    I've seen mention of the MOX fuel in news reports as being significant, but I've failed to understand their point of mentioning the use of plutonium in the fuel.

    Are there safety issues?

    Are there proliferation issues?

    Are there other issues?

    Thank you!
     
  2. jcsd
  3. Aug 1, 2011 #2

    Astronuc

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    There is no substantial difference between MOX and UO2, although MOX tends to have higher TU isotopics. Neutronically they are similar in terms of power generation, i.e., power density. In mixing MOX in UO2 cores, the core/fuel designers attempt to match the power between the different types of fuel.

    UO2 produces Pu239/Pu240 with irradation, i.e., spent fuel becomes MOX. There really isn't a proliferation issue.

    Depending on the Pu-isotopic vector, there is some increased activity that must be considered during fabrication and inspection of the fuel.

    Spent fuel is highly radioactive, whether it starts UO2 or MOX.
     
  4. Aug 2, 2011 #3
    Following is a translation of an answer given to the same question by the Finnish Nuclear Safety Authority (STUK) on their web page:

     
    Last edited: Aug 3, 2011
  5. Aug 2, 2011 #4

    jim hardy

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    ""I've seen mention of the MOX fuel in news reports as being significant, ...""

    here's a link to the folks who i believe made that MOX fuel for Tepco.
    http://www.belgonucleaire.be/uk/project.htm

    To me as a not-nuclear-engineer (but have drank beer with some) a little plutonium in reactor fuel is like ethanol in gasoline. Pu makes the reactor slightly more "peppy" because of its shorter delayed neutron times but in the modest proportion used it seems benign enough.

    If the MOX fuel was significant to the events i will be surprised.
    IMHO - Astro and rmatt gave good answers.


    ""I've failed to understand their point of mentioning the use of plutonium in the fuel. ""
    Mark Twain nailed that one:
    "If a spectacle is going to be particularly imposing I prefer to see it through somebody else's eyes, because that man will always exaggerate. Then I can exaggerate his exaggeration, and my account of the thing will be the most impressive. "


    old jim
     
    Last edited by a moderator: Apr 26, 2017
  6. Aug 3, 2011 #5

    Astronuc

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    I should have qualified my answer with the additional information that the enrichments in the TEPCO cores tend to relatively low by US/European standards, and the discharge exposures also tend to be relatively low, so the difference between MOX and UO2 is not significant from the standpoint of reactor performance.

    The decay heat for MOX fuel is slightly higher than for UO2 at the same exposure. However, the 32 MOX assemblies in FKI, Unit 3 were in their first cycle, so the burnup was quite low - on the order of 5 to 6 GWd/tHM by my estimate. So I don't see this as significant to the event in Fukushima.
     
  7. Aug 5, 2011 #6
    "Could it be that" Pu fissions more readily with fast neutrons than 235U does, hence a ruined or molten core would be more reactive with Pu, even if water is lost? And that the stabilizing effect of the void coefficient is less good with MOx?

    As for proliferation, new fuel isn't the same if it's MOx! Because it's not as hugely radioactive as used fuel, and Pu from MOx can be separated by chemical means, easier than isotopic enrichment. So stealing unused MOx is a way to grasp Pu. Not in military grade because of 240Pu proportion, but usable for a bomb.
     
  8. Aug 16, 2011 #7
    Bah humbug. You want plutonium, you buy a CANDU reactor.
     
  9. Aug 17, 2011 #8

    jim hardy

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    Enthalpy wrote:
    """Could it be that" Pu fissions more readily with fast neutrons than 235U does, hence a ruined or molten core would be more reactive with Pu, even if water is lost? And that the stabilizing effect of the void coefficient is less good with MOx?""

    Beyond my skills to answer that.
    I assume the reactor physics guys would provide for degraded core conditions in their fuel assembly design.
    But a question well stated is half answered.
    Does this help ?

    ....What happens inside a MOX element of say 7.8%Pu surrounded by normal elements in absence of water and control rods?
    Depleted U makes a decent reflector and i'd think wouldn't hardly moderate at all, so a 5% enriched reflector sounds like it'd be a fast reactor hot-rodder's dream.

    basis for 7.8% : http://www.belgonucleaire.be/uk/mox.htm
    35/450 =7.777%

    One would need to know something about the geometry - fuel pin diameter and spacing for starters and it'd still be quite a calc.
    Then there's that sea salt.

    Is there a BWR reactor engineer in the house?

    old jim
     
    Last edited by a moderator: Apr 26, 2017
  10. Aug 17, 2011 #9
    I have no practical experience in design of MOX cores, but I have the perception that MOX cores would have more negative void and doppler coefficients than UO2 ones due to the large resonance absorption of Pu isotopes.

    There's one report comparing MOX/UO2 properties from point of view of PWR/BWR reactor safety.
     
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