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Questions about fast reactors and nuclear waste

  1. May 11, 2012 #1
    Hello, I have a few questions regarding the transmutation processes inside fast reactors. I would appreciate your help. I am doing some work at the university about SFR and ABR.

    First of all, I know that fast reactors can operate in 3 modes: burner, breeder and converter (halfway between burner and breeder). My question is, can the same reactor operate in any of these ways, or are they only designed for one specific mode? If you put MOX fuel, does it mean that you want it to operate in breeder mode?

    Another thing is, how do you calculate the transuranics conversion ratio? I am currently doing (mass of transuranics at EOC)/(mass of transuranics at BOC) but I don't know if this is correct.

    Also, if your objective is to burn transuranics, should you put just Plutonium and no Uranium in the fuel? Because if you put Uranium, then the U-238 transmutes to U-237 through n,2n, then decays beta into Np-237 which is very inconvenient because it has a very long half life.

    Also, could you please give me some good internet bibliography about SFRs? Google is not really helpful.

  2. jcsd
  3. May 11, 2012 #2


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    The mode depends on the fuel design and core design (e.g., size of blanket), and proportions of Pu and TU isotopes, and enrichment. It's a matter of balancing Ʃf and Ʃa within the fuel assembly, among fuel assembly, and among regions of the core.

    The breeder/conversion ratio should be determined by the TU content at discharge of fuel, but not on the entire core, since some of the fuel is retained for one or more additional cycles.

    Fast reactors generally use MOX fuel with about 20% fissile content, although the fissile content may vary (+/-). Usually MOX is (U,Pu)O2, where the Pu(239,240,241,242) and perhaps some Am, Cm is diluted in a matrix of natural or depleted UO2, so one still has to deal with the transmutation of U235 and U238. Usually, for a breeder reactor, the intent is to convert U-238 to fissile Pu for future use.

    Fast reactors can also use mixed carbide (MC) fuel, mixed nitride (MN) fuel, mixed carboxide (MCO) fuel, metal fuel (e.g., (U,Pu)Mo), and combinations, e.g., cermet, or cer-cer fuel. (U,Pu)ZrC is another possibility and represents fissile material dispersed in an inert matrix.

    IAEA has some TECDOC's on fast reactor technology and experience, transmutation and fuel cycle matters. I'll try to post links later.

    Update: Look for Recent Publications on this page - http://www.iaea.org/NuclearPower/FR/
    Click on the links and one can find a link to a free pdf that one can download (use Save Target As)

    See also - http://www.iaea.org/INPRO/publications/index.html - for some reports on Innovative fuel and fuel cycles
    Last edited: May 11, 2012
  4. May 24, 2012 #3
    Ok, I just realised that my formula to calculate the transuranics conversion ratio was wrong.

    It should be:


    Where HMD is total heavy metal mass destroyed and TRUD is total transuranic mass destroyed. Or, to make it simple, it is like:

    TRU.CR.= U-238 destroyed / Pu-239 destroyed
  5. May 24, 2012 #4


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    That doesn't seem quite correct.

    The conversion ratio should be related to quantity of TRU destroyed divided by the initial quantity or the total TRU, which would include the initial and that which is produced during the process.

    U-238 does undergo fast fission as well as neutron capture. The destruction of U-238 should not be considered in TRU conversion directly, but only the portion which becomes TRU during the process, which is the TRU production part.
  6. May 24, 2012 #5
    I made a very coarse approximation in the second formula. It's an hipotethical case in which you only have U-238 and Pu-239. All Uranium transofmrs into Plutonium, and all Plutonium undergoes fission. This approximation is not so far from reality.

    The TRU.CR is, in theory = TRU atoms produced / TRU atoms destroyed through the cycle. You don't care about the initial or final amount, you just want what's been produced divided by what's been destroyed.

    My source of information is:

    Description of Transmutation Library for Fuel Cycle System Analyses
    Steven J. Piet
    Samuel E. Bays
    Edward A. Hoffman
    August 2010

    Page 16

    (sorry I can't put links)

    I know that U-238 can undergo fission, but I lack that information for my calculations, so I approximate and say that they only absorb neutrons.
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