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Question about LFTR?

  1. Jul 17, 2014 #1
    So I have been reading about LFTR and I understand the concepts but there are a few things I have questions about, mainly it's possible use of uranium 233. Some articles I have read and diagrams I have seen make mentions if using uranium, however other articles and diagrams make no mention of uranium at all, so is it used in a lftr or maybe just in some lftr designs? If it is used then how is it used and how is it prevented from emitting gamma rays because from what I understand a lftr only emits alpha radiation. Thanks for any help you can give!
     
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  3. Jul 17, 2014 #2
    A LFTR converts fertile Th-232 into fissile U-233. This reactor will emit gamma because of U-232 poisoning of the U-233. U-232 is a very strong gamma emitter that is generated as a small fraction of that decay chain. The articles you read that claim that LFTR reactors are proliferation resistant, that resistance is because of the U-232. The U-232 is very hard to separate from the U-233 and handling it tends to kill people. And also a portion of that proliferation resistance is probably because U-233 is prone to spontaneously exploding when it is fashioned into a fission bomb.
     
  4. Jul 18, 2014 #3

    QuantumPion

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    The half-life of the U-233 precursor, proactinium-233, is 27 days. You could chemically separate protactinium from irradiated thorium to produce pure U-233.

    I've never heard this before. The SF rate of U-233 is lower than U-235 (and much lower than Pu-239) so why is this so?
     
    Last edited: Jul 18, 2014
  5. Jul 18, 2014 #4

    QuantumPion

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    Thorium is not fuel, only breeder material. LFTR's convert thorium into U-233 to produce energy. They require an initial loading of U-233, U-235, or Pu-239 in order to begin the breeding process. All nuclear reactor types and fuel cycles will produce gamma radiation from irradiated impurities and fission products.
     
  6. Jul 18, 2014 #5
    So is it also not true that they are safer that a common nuclear reactor? I had heard that they were safe to be built in a higher density population area but if they met gamma rays then that wouldn't be true, also is this video inaccurate then?
    https://m.youtube.com/watch?v=nYxlpeJEKmw
     
  7. Jul 19, 2014 #6

    Astronuc

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    The video is simplistic and misleading. It does not address the various engineering/technical challenges associated with a LFTR system.

    It is the engineering that determines and assures safety.

    Any fission system inherently produces fission products which are beta and gamma emitters. The fission products would have to be separated and processed, then ultimately deposited somewhere isolated from the environment. Each LFTR plant requires a processing system to extract/separate U-233 and recirculate it into the reactor system. At startup, a fissile inventory (U-235 or Pu-239) is required.

    The plant efficiency will be determined by the peak temperature of the working fluid, and as temperature increases materials are challenged. It may be possible to use a Brayton cycle for power generation, otherwise, a steam Rankine cycle would be employed. Steam cycles introduce corrosion and erosion issues associated with the water interaction with the structural alloys.
     
  8. Jul 19, 2014 #7

    mheslep

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    Agreed w one exception. If a molten salt reactor design must fall back to a Rankine cycle, steam will be obtained via heat exchangers. The reactor vessel itself and fission products will always be at a couple atm at most. An MSR is not a PWR.
     
  9. Jul 19, 2014 #8
    Yes but that's not the problem the problem is separating the resulting U-233 and U-232. It requires the same equipment as separating U-235 and U-238. In addition it is far more radioactive than natural uranium.

    Sorry I was incorrect as to the requirements for premature destination. The main proliferation negator is the gamma emission. See the response on this article http://www.americanscientist.org/issues/pub/2010/5/a-thorium-future
     
  10. Jul 19, 2014 #9
    The gamma can be stopped by materials such as lead or depleted uranium. What makes them safe for high density population centers is the salt plug that holds the fuel in. If the fuel gets to hot the salt plug melts draining the fuel into a separate area separating it from the external neutron source.
     
  11. Jul 19, 2014 #10

    QuantumPion

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    No, U-232 is generated by neutron absorption in the reactor. If you chemically separate the proactinium outside of the reactor and wait for it to decay to U-233 there will be no U-232 contamination.
     
  12. Jul 19, 2014 #11

    QuantumPion

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    It's difficult to compare the safeness of a hypothetical reactor design to real world power plants. While LFTR's have some potential advantages, there are also challenges, both known and unknown, which have to be analyzed. An LFTR would certainly still require a containment building.
     
  13. Jul 19, 2014 #12
    Ok true however you still encounter the same problems with getting the proactinium out as you do the U-233. The U-232 would irradiate everything in the vicinity until the proactinium was separated out. Also due to the short half life of proactinium I doubt there would be enough at any one time to generate a usefull amount of U-233 outside the reactor. You would have to break open the reactor multiple times and separate out proactinium each time.
     
  14. Jul 19, 2014 #13

    QuantumPion

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    The half life of proactinium-233 is 27 days which is plenty to reprocess. Chemically separating protactinium from the fuel is not very complicated. And since we are referring to a LFTR, it would be easy to get out since the fuel is already liquid and presumably reprocessing equipment is already part of the plant design. Additionally, the process can be made more efficient by having external breeding blankets.
     
  15. Jul 19, 2014 #14
    Ok just focusing on the separation part of it though. You would need a remote handling facility due to the high radioactivity of the U-232. Anybody who had that kind of facility would have been better off building an enrichment facility for natural uranium. They would get a higher yield bomb without attracting unwanted attention by pilfering a LFTR for its proactinium.
     
  16. Jul 19, 2014 #15
    But the point of a nuclear reactor is to generate energy, nt to create bombs, well I mean the government doesn't see it that way but if you exclude the bomb making would lftr be more efficient?
     
  17. Jul 19, 2014 #16
    That really depends on the design of the LFTR itself.
     
  18. Jul 19, 2014 #17

    QuantumPion

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    Well no. By design, LFTR's use liquid fuel which is re-processed on site. This is completely different from enriching natural uranium, which requires extensive facilities and specialized equipment. It would not be hard to add capability for U-233 breeding and separation. The operation would be similar to that of plutonium production. The advantage of plutonium breeding vs. thorium is that plutonium breeding does not require high enriched fuel to seed the breeder, and the physics of Pu-based bombs is much more well tested and understood.
     
  19. Jul 19, 2014 #18

    QuantumPion

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    More efficient in what sense? Cost? Fuel efficiency? Thermodynamic efficiency?
     
  20. Jul 20, 2014 #19

    mheslep

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    I don't believe the seed need be HEU, but only enriched sufficiently to sustain a reaction for some time.
     
  21. Jul 20, 2014 #20
    It's ability to convert the fuel into energy, we convert almost none of uranium into energy, but from what I've read we can convert a large ammount if the thorium into energy but from what people on this post have said I'm starting to doubt everything I've read haha.
     
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