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Stability of fast neutron reactors with liquid metal coolant.

  1. Sep 5, 2011 #1
    Is my understanding correct that the short term stability of fast neutron liquid metal cooled reactors is based primarily on the thermal expansion of the core, while the Doppler coefficient is far less significant factor, as the Doppler coefficient primarily affects the low energy neutrons? (The internet research seem to confirm)

    What is the fraction of thermal neutron fissions in some of the fast neutron reactors?

    p.s. i know about delayed neutrons, and their role is same for both the fast neutron and thermal neutron reactors. I'm interested in the other mechanisms that put negative feedback on the prompt criticality
  2. jcsd
  3. Sep 6, 2011 #2


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    Control rods offer the greatest control or negative reactivity. I'll have to dig around in my library and archives for some numbers. It's been a while since I've been concerned about FRs. Some degree of control is found with the reflector design.

    Some notes here - http://www.world-nuclear.org/info/inf98.html.

    And there are a few reports by the IAEA.


    There is essentially no thermal neutron spectrum, since the fast reactor uses a 'fast' neutron spectrum.

    See also - http://iaea.org/inisnkm/nkm/aws/frdb/auxiliary/coreCharacteristics.html
    Last edited: Sep 6, 2011
  4. Sep 7, 2011 #3


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    Your understanding is incorrect. I used to work at Argonne National Labs in the early 1980s when Argonne was developing the Integral Fast Reactor or IFR.

    The strongest feedback mechanism in the IFR was Doppler broadening.

    Many people don't understand that in a fast reactor, the peak of the neutron spectrum is in the keV region - which is right where the resolved resonace region for U-238 is. ( Too many people think the peak in a fast reactor is up in the MeV region - it's not!)

  5. Sep 7, 2011 #4

    jim hardy

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    ""Many people don't understand that in a fast reactor, the peak of the neutron spectrum is in the keV region - which is right where the resolved resonace region for U-238 is. ""

    i know little about them.
    But i read someplace that sodium moderator puts a lot of neutrons into the KEV range..
    Are the KEV neutrons the ones maintaining the reaction?
    Fissioning which nuclides?

    No need for a long explanation , a pointer to reference would be enough.
    I'm trying to understand what makes Godiva and BigTen go.

    1950's Evinrudes are more along my line. If you ever need to convert one from pressure tank to modern fuel pump - i can help you.
    Just finished a friend's 1955 Johnson 5 hp . What a treasure.

    old jim
  6. Sep 9, 2011 #5


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    U-235 and Pu-239 are fissile; so they will fission with neutrons of all energies, and that includes neutrons in the keV region.

    U-238 is fissionable, and hence there is a threshold energy for fission which is about an MeV.

  7. Sep 9, 2011 #6
    Hmm but it is not about the capture cross section, it's about difference in capture cross sections between different isotopes that are present... it may get messy without reprocessing, when there's americium and such.

    I was actually wondering what would happen in the accident at a fast neutron "travelling wave" reactor (recently much overhyped), which has very complicated fuel at the end of fuel life.
    I found a few references:

    I cant find any stability references for the travelling wave reactor, or any good technical info for that matter, though my gut feeling is that with such messy fuel stability would be much harder to ensure.

    With the IFR... did it require special design considerations to ensure strong negative doppler coefficient? Or is that typical / easy to achieve? Is it conceivable that this could be maintained through ultra long fuel life in travelling wave reactor?
    Last edited by a moderator: Apr 26, 2017
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