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What is measured in a fast reactor for power calculation : fast neutron flux or

  1. May 29, 2012 #1
    What is measured in a fast reactor for power calculation : fast neutron flux or overall ( fast + thermal) neutron flux ?
    My doubt is :
    The fission chambers used for measuring neutron flux undergo chemical reactions due to which type of neutron : fast or thermal ?
  2. jcsd
  3. May 29, 2012 #2


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    In a fast reactor there is no appreciable thermal flux since there is no moderator. Fission chambers use high enriched uranium or plutonium and absorb fast or thermal neutrons. They do not rely on chemical reactions, they are ion chambers which are excited by fission.
  4. May 29, 2012 #3
    Thanks QuantumPion.
    Actually, my doubt arose because I imagined:
    If the fission chamber placed below safety vessel ( ex-core) in a fast reactor is suddenly flooded with water ( eg. in a Tsunami or flood), will there be any negative impact on the power calculation ?
    I mean, if a fission chamber is capable of measuring fast neautron flux, it will be by default also capable of measuring thermal flux. As the fission chamber is surroundded by water now ( as I imagined), does it means that there would be no major effect on fission chamber performance? it would be more or less show same power?
  5. May 29, 2012 #4


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    If there was an emergency situation, the reactor would be shut down and the power-range detectors would not be in use. Fission chambers are in-core detectors and are only used at power during normal operation. The ex-core detectors are in direct contact with the sides of the vessel and are sensitive enough to detect neutrons which must travel through the entire core, barrel, coolant and vessel. They would not be directly affected if the reactor cavity was flooded.
  6. May 30, 2012 #5
    thank you for the reply.

    So it can be fairly assumed that in case of a tsunami for eg. , and considering the full integrity of the reactor structure, if water floods to the ex-core detector, there would be not much problem. But what if there is a severe accident like in earthquake ( followed by tsunami)?
  7. May 30, 2012 #6
    and also, what are the problems associated with ex-core detectors?
  8. May 30, 2012 #7


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    If water were to flood a liquid metal fast reactor such that the excore detectors were flooded, I'd be more worried about a sodium-water interaction, if the reactor was Na or NaK cooled. If it's Pb-Bi, that's not so much a problem.

    If water got into the core, then there would be a concern about criticality.

    Fast reactors may also use incore detectors, but off-hand, I can't think of an example. I'll look later for an example.

    I would imagine that tsunami and earthquake protection policies have become more stringent since the Fukushima accident. In the US, fast reactors tend to be constructed inland, far from tsunamis, and generally in areas of low seismic activity, and historically low magnitude (< mag 5) earthquakes.
    Last edited: May 30, 2012
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