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Slow or fast neutrons in LFTRs (Liquid Fluoride Thorium Reactors)

  1. Aug 20, 2014 #1
    Some nuclear reactors are called fast breeders because:
    1. they use fast neutrons, which means the neutrons have more energy than "thermal neutrons", that have the same energy as the surrounding material.
    2. they are called "breeders" because they "breed" fuel. U238 is not a fissile atom, but by absorbing a neutrons and spitting out two electrons out of the nucleus (so that two neutrons can turn into protons), it can be turned into Pu239, which is fissile.
    Fast breedeers are not so common. Though there is a large one in Belayarsk Russia currently working and another in Monju Japan, France stopped its own (Superphenix) some years ago.
    The trouble with breeders is you need more neutrons. If your reaction is based on U235, a naturally occurring atom, you just need one neutron to make it fission and get energy. If it is based on U238, you'll need two neutrons, one to turn the U238 into Pu239, another to fission the Pu239. The problem is solved by working with fast neutrons, because high energy fissions produce more neutrons.
    Thorium 232, an extremely abundant element, is fertile too. If it absorbs a neutron and spits two electrons out, it becomes U233, which is fissile. So I naively supposed that the possible reactor of the future, the utterly fascinating LFTR (Light Fluoride Thorium Reactor), would need fast neutrons too. But apparently, it uses thermal neutrons (for example, you can see this page, which looks serious though it is not written by a specialist: http://www.2112design.com/blog/lftr/) It seems to me there will never be enough neutrons. Do you have any solution to that?
  2. jcsd
  3. Aug 20, 2014 #2
    Instead of looking at the number of neutrons emitted in fission, its helpful to look at the number of neutron emitted per neutron absorbed by the fissile material. The difference is that the fuel can sometimes absorb neutrons and not fission.

    Lets look at this ratio for a few different fissile isotopes for both thermal and fast neutrons.

    U-235 2.08
    Pu-239 2.03
    U-233 2.31

    U-235 2.4
    Pu-239 2.9
    U-233 2.5

    A couple of things. First for all these isotopes a fast spectrum will produce more neutrons per neutron absorb by the fissile material. Second, for U-235 and Pu-239 the ratio is just a little above 2. It would be very hard to make a thermal breeder reactor with either of these 2 isotope. However, the ratio for U-233 is noticeably larger. While not as larger as the ratio for a fast spectrum, it is still large enough.

    U-233 is the fissile material in a thorium reactor. And the fact that is has such a large ratio of neutrons produced per thermal neutron absorbed, is why thorium can work as a thermal breeder.

    You also have to also consider the various neutron loss mechanisms. By looking at the ratio of neutrons produce be neutron absorbed I've accounted for one loss mechanism (neutron capture by the fissile material), but there are others. For example neutrons can be absorbed by other reactor materials (like a control rod), or they can escape out of the reactor (leakage).
  4. Aug 20, 2014 #3


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    You can breed fuel in a thermal U-235 reactor, you just can't achieve a breeding ratio greater than 1. Meaning it would not be indefinitely self-sustaining and would require refueling with fissile material in addition to fertile material.
  5. Sep 20, 2014 #4
    I think the breeding ratio for one of ORNL's designs was 1.07. They sacrificed just about every other design consideration to achieve that, however. These days, given the availability of uranium, 10 or 20% makeup fuel is not a big deal. This expands the design space nicely.
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