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Any other incidents/accidents like this ?

  1. Sep 6, 2012 #1

    please share information about any accidents/incidents related to the misreadings/malfunctioning/wrong interpretation of ex-core detectors ( for any reasons)
    I searched and found one incident of Calvert-Cliff Nuclear power plant in 1988.
    (Page 2, at Clueless at Calvert Cliffs )

    Though it is very old and doesn't entitle for malfunctioning of detector, but it is important to analyze, as the power measurement at higher level is based on ex-core detector.

    The detector need not be malfunctioned necessarily. But any wrong readings from it has a serious consequence.

  2. jcsd
  3. Sep 6, 2012 #2


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    Core power is measured using several indicators, all of them indirect: ex-core neutron detectors, core exit thermocouple temperature, steam flow rate, calorimetric power, and generator output. There would not be any problem due to a misreading on any one indicator, and it is extremely unlikely that all indicators could be simultaneously wrong as they are completely independent systems.

    The story in question in your link is completely normal procedure. As mentioned previously, ex-core detectors are a secondary indicator of power. They have to be calibrated to other indicators, mainly calorimetric power. When changes to core loading are made, especially changes which affect neutron signal at the edge of the core, the calibration would have to be updated, which was done.

    Also note that what the story calls the "large discrepancy" between 39% and 42% power is not at all large. The measurement uncertainty is often around 2%, although modern technologies and improvements have reduced this in recent years.

    The story in your link somehow concludes that the necessity to re-calibrate instruments, a normal operating procedure, is an indication of some sort of problem. Their conclusion, that they plant operators are faulty because they supposedly didn't know what the power level was, is completely false and is based on a total lack of understanding of plant design and operating procedures.

    Note that your document link source is the Union of Concerned Scientists, a vehemently anti-nuclear organization.
  4. Sep 6, 2012 #3


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    To answer your original question of "have there been any other incidents like this", the answer is:

    a) this is not an incident, it is normal operation;

    b) it happens with every single plant every time they complete a refueling outage or replace an ex-core detector.
  5. Sep 27, 2012 #4
    There are requirements to perform monthly calibrations on in-core monitoring equipment. This is a license requirement and is part of the station's technical specifications. Additionally the power range monitors are typically recalibrated or re-scaled whenever large power changes happen.

    Plants have a requirement to maintain their in-core monitoring equipment within 2% of actual reactor thermal power.

    In-core equipment is known to go out of cal on large changes. Sudden things like loss of a recirc pump, rapid downpower, loss of feedwater heating, can cause the power range monitors to be a little off due to large changes in axial power in the core. adjusting the scaling for the power range monitors quickly brings it back, and the next in-core probe run will correct the local monitors as well.
  6. Sep 29, 2012 #5

    jim hardy

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    I used to work on such systems. They're beautifully simple and reliable but like any system they can be misoperated.

    what is the nature of this inquiry?
    i wouldn't want any part of jaded reporting or scaremongering as in that link.
    That you had to ask makes me suspect of your objectivity.
    Who are you? Why do you ask? Why are you looking for bad news?
    Do you understand the basics of reactor instrumentation?

    If this is a legitimate research inquiry please enlighten us..
    If not, see http//en.wikipedia.org/wiki/Blinkenlights
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