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Get rid of transuranians in Liquid Fluoride Thorium Reactors?

  1. Aug 21, 2014 #1
    Liquid Fluoride Thorium Reactors might be a fantastic source of energy for the future. It looks like the world is divided into two clans and the moment: those who have heard of LFTR and are enthusiastic, and those who have hardly ever heard of it because it is hardly mentioned even in the most recent textbooks.
    At some moments I even wonder if LFTR is not a hoax along with perpetual motions or Martian technology. But the more I read, the more I am convinced about it. Everyone who does not know about LFTR should google it immediately or look for information on this very forum.
    LFTR would produce only fission products radioactive a few centuries and no transuranians radioactive for thousands of years. Among the often quoted advantages of LFTR is the possibility of eliminating transuranian waste from "archaic" (I mean "present") nuclear technology by feeding some of it into the LFTR. I have no reason to doubt that but I wonder how long it might take.
    It is claimed that about two kilograms of Thorium a day would be enough to make an average LFTR (1 GW) work. It is absolutely awesome. But if we imagine that the quantity of transuranians fed into the reactors represents a 10% fraction of the Thorium, it would be 200g a day. I read there are about 20000 tons of Plutonium on earth at the moment. It means 100 LFTRs would need about 250 years to "burn" all this plutonium. Am I in the right time scale or not at all?
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  3. Aug 21, 2014 #2


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    The properties of LFTR's are somewhat exaggerated and misunderstood. Most of the advantages come from the reprocessing of the fuel, but this can be done with any reactor type. Transuranics are only a problem in spent fuel that is not reprocessed because of their long half-lives. They are alpha emitters and not particularly hazardous by themselves.
  4. Aug 21, 2014 #3


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    Many tons of a material that is highly radioactive, has to stay hot to be liquid all the time, directly reacts with air moisture on contact, contains radioactive gases that have to be separated in a controlled way and safely stored for months to decades, contains various corrosive fluorine compounds of other elements and needs extensive chemical/physical reprocessing steps with radioactive materials, to name just a few of the disadvantages.

    It is certainly an interesting concept, but it has its own challenges.

    That looks quite optimistic, as getting a breeding ratio of 1 is a challenge on its own, so a reactor would probably be just a bit above 1.
    It is not just plutonium, we have some 100,000 tonnes of highly radioactive waste.
  5. Aug 25, 2014 #4
    Reprocessing is a quite expensive process.
    Separating transuranics from fission products would require adding a number of steps to it, making it even more expensive.

    Currently, no one is doing that. Not even French, the world leaders in reprocessing. It might end up being uneconomical.
  6. Sep 6, 2014 #5
    The difference being the fuel is already liquid and ready to be processed in LFTR's. If we want 'melt down proof' reactors (which the post Fukishima public demands) then removing things like the transuranics while in operation is required. MSR's are the only reactors I am aware of that can accomplish this.
  7. Sep 8, 2014 #6
    How removing transuranics help with that?
    Almost all decay heat load comes from fission products, not transuranics.
  8. Sep 8, 2014 #7
    I was trying to stay on topic hence the wording 'like transuranics'. I was going to edit but didn't get around to it.

    Either way you are completely correct of course, the transuranics in an MSR would stay in place and be fissioned while the fission products of these elements and U233 would be removed which in turn would create the 'melt down proof' design aspect of a LFTR.

    ***EDIT*** On a completely different note, I just noticed this is my 555th post, hooray!
    Last edited: Sep 8, 2014
  9. Sep 8, 2014 #8
    I believe another advantage of the thorium cycle with regards to waste management is that Th-232 requires many more neutron activations and beta decays before becoming Pu or Am. This part has nothing to do with the molten salt part though, just the starting fuel composition.

    As a side note, some MSR designs do not call for reprocessing, or reprocessing in batches. Thus fission product loading can still be significant in these designs. The real advantage of MSR is the ability to passively drain into high surface area tanks for passive cooling.
  10. Sep 8, 2014 #9
    The former Scientists at ORNL felt confident they were close to getting the engineering right on the Hastelloy (for a reactor that removes these products). If left in they likely will introduce some new challenges on the materials side of design. Then again leaving them in simplifies reactor design as well.
  11. Sep 8, 2014 #10


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    It is rather humorous to refer to a 'Molten' Salt Reactor as 'melt down' proof, because the fuel/core is in a molten state.

    The benefit of an MSR, assuming that the chemical processing and reprocessing are part of it, is that the equilibrium inventory is rather lower as compared to a conventional system in which the fission products and TU nuclides accumulate in the fuel.

    The U-233 is recycled into the core, ostensibly with some U-234, U-235 and perhaps U-236, which should be at lower levels than a U-235 fueled core.

    The fission products still need to be accumulated, calcined and vitrified in order to be stabilized.
  12. Sep 8, 2014 #11
    Agreed, I am open to suggestions on terminology :)

    It's unfortunate ORNL never had a chance to get this far.

    Certainly the best option for long term containment.
  13. Sep 16, 2014 #12
    That thing potentially wastes tons of lithium and perhaps beryllium (both are rare and valuable metals).
  14. Sep 16, 2014 #13


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    Major advantages obtain via:
    1. Molten fuel.
    i) Enables online reprocessing which means no build up of neutron absorbing fission products which in turn means a) no periodic shut down for refueling and b) high burn up.
    ii) Allows a natural fail safe by draining the fuel off the moderator.
    iii) Low pressure design, reducing reliance on containment, perhaps eliminating it.
    2. Thorium fertile fuel, which means a drastic reduction in the scale of the uranium fuel enrichment cycle.
    3. Near zero plutonium production.

    These can not be accomplished by starting with existing solid fuel PWR/BWRs and simply shipping their removed waste to current reprocessing centers.
    Last edited: Sep 16, 2014
  15. Sep 16, 2014 #14
    1 and 2 are possible with conventional heavy water or graphite moderated designs.

    3 is primarily a political problem if one allows reprocessing. Pu is very good fuel (higher actinides not so much).
  16. Sep 16, 2014 #15


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    Forgot about CANDU so I grant no enrichment there. But how does one remove fission product alone from *any* solid fueled reactor?

    No just reprocessing, else the growing stockpile of Pu would be not be growing. The appropriate reactor is required. Politics enters there, but so does engineering of (largely) Pu fueled reactors.
  17. Sep 16, 2014 #16
    Sure there isn't online removal of the fission products from solid fuel. However, if you have a short irradiation cycle and reprocessing you can do it. Many MSR don't plan on online reprocessing because of how difficult it is to deal with radioactivity of short-lived fission products.

    There isn't a significant technical problem to using Pu in many existing reactors. MOX fuel is a very established technology. The problem is simply economics. Right now virgin uranium and manufacturing new fuel is cheap enough that reprocessing doesn't make economic sense. Things may eventually change if there is a break through in reprocessing technology or shortage of uranium supply. Until then, reprocessing is mostly a political decision to reduce 'waste' or preserve natural resources.
  18. Sep 16, 2014 #17
    Last time I checked lithium and beryllium are easier to come by than U235.
  19. Sep 16, 2014 #18


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    For a thorium fueled MSR, as per the OP, online reprocessing is unavoidable.
  20. Sep 16, 2014 #19


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  21. Sep 16, 2014 #20


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    The point is not solely what is technically possible, but what also what is economically plausible. The major advantage of high fuel burnup *in place* is cost reduction. Adding a loop where, the fuel is removed from containment, shipped off for dis-assembly, is reprocessed, reassembled and reinserted into the reactor can not be remotely as cost effective.
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