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Problems associated with ex-core detectors ?

  1. Jun 25, 2012 #1
    Hi all

    What are the problems associated with ex-core detectors in general?
    Particularly for fast reactors.

    Also, why they are needed ( why in-core is insufficient) ?

  2. jcsd
  3. Jun 25, 2012 #2


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    Excore detectors are used to measure the average total reactor power and axial offset continuously. They are typically eight - one on each quadrant, upper and lower. They only detect neutrons coming from the outside edge of the core. They are calibrated based on calorimetric power readings (e.g. steam flow rate).

    The incore detectors are used to determine the power distribution within the core to determine how well the reactor behavior matches the computer models and to ensure peaking factors are within analyzed safety limits. This is done on a periodic basis, e.g. once a month. The incore detectors only measure relative power and axial offset.
  4. Jun 25, 2012 #3


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    Ex Core detectors were considered as a new requirement for Post Accident Monitoring Instrumentation for all US reactors after TMI2 (see original issue of RG 1.97) as a means to assess core damage. This was thought to be necessary due to the failure of in-core detectors during core melt. The idea was dropped due to proposals to use the containment radiation detectors. No accurate measurements are possible even with ex-core detectors because the geometry of the core is unknown once damage begins. For example, How much of the measurement is due to gaseous fission products outside the reactor vessel and how much due to core-on the floor?

    The issues of the estimation of core damage at Fukushima is still pretty much guesswork. I have no way know if that would be any different if they had ex-core detectors, but I think it is a valid question.
  5. Jun 25, 2012 #4


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    What? I'm pretty sure ex-core detectors have been standard equipment since the earliest LWR's.
  6. Jun 25, 2012 #5


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    One would have to go back 40 years and some PSARs/FSARs of early plants.

    Westinghouse had ex-core detectors as standard, and I believe In-core (with or without thermocouple) were more or less optional (or rather ex-core were primary, and in-core were secondary). Some In-core systems were fixed, other movable.

    I believe B&W used in-core for core monitoring (primary) and ex-core were optional or secondary.

    I'm not sure about CE units, but it appears that they relied on ex-core detectors for their core protection calculator (CPC).

    GE (BWRs) has used in-core detectors (local power range monitors, LPRMs) and average power range monitors (APRMs) from the beginning.

    Some examples from old plants. Beznau is an old W 2loop design.
    http://www.ktg.org/documentpool/ktg/fg-bet-rph-core-monitoring-gardel.pdf [Broken]


    NRC Training - Excore Core Neutron Monitoring System
    Notes - http://pbadupws.nrc.gov/docs/ML1125/ML11251A025.pdf
    Presentation - http://pbadupws.nrc.gov/docs/ML1125/ML11251A124.pdf

    CE CPC and ex-core detectors

    I think in-core detectors are better for monitoring local effects.

    I'm also curious about the statement: "Also, why they are needed ( why in-core is insufficient) ?" What is the source of that statement.

    There are certainly issues of pressure vessel penetration, which is not such an for lower pressure fast reactors, as it is for LWRs, and of irradation effects including performance degradation.
    Last edited by a moderator: May 6, 2017
  7. Jun 26, 2012 #6

    jim hardy

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    Problems with excore detectors ?

    I was never around a fast reactor.

    In PWR maintenance,

    it's mundane things like
    they must be positioned carefully so nothing shields them from neutrons exiting vessel,
    you'd like them all same distance from vessel and pretty close to it,
    yet they must be accessible for replacement, which often dictates a movable mount and mechanical means to control where it is placed, see page 29 fig 9.1-2 of this link (i hope it works - darn google for sticking their nose in the path)

    they must be provided ventilation lest they overheat,
    the signal cables to them can embrittle and deteriorate at elevated temperature,
    since they operate at high voltage a trace of moisture or dirt in the connections ruins the signal,
    the power range detectors are nearly 12 feet tall and have a small internal wire which is so delicate a good bump durng installation will break it, so they must be handled gently,
    see page 44 fig 9.1.17

    calibration mentioned above by Q'pion must be repeated frequently because neutron leakage changes with burnup and boron,

    signals from source range are millivolt pulses , wires to control room are perhaps 100 or 200 feet long so it takes a LOT of work to keep stray electrical noise down,
    signals from intermediate range at low power are just tens of picoamps so condition of interconnecting cables must be kept pristine (10^13 ohm-ft insulation resistance)

    but with good maintenance they work very well.

    old jim
  8. Jun 26, 2012 #7

    jim hardy

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  9. Jul 31, 2012 #8
    Thanks all for enlightment on the topic.

    The main reason for which this question was raised is:
    if the ex-core ( which happens to monitor reactor power under normal operation in a fast reactor) is flooded in case of water and surrounded by water, then my question is , the no. of neutrons reaching the detectors would gradually decrease ( absorption by water ), though the reactor power is decided by the neutrons leaking out of vessel and being detected.
    So, what to do in such a situation ?
    Anyways, if water is flooding means reactor is tripped down.
    But what would be its impact on the detector readings ?

  10. Jul 31, 2012 #9


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    Ex-core detectors are in contact with the reactor vessel enclosed in containers. It would not matter if the entire cavity was flooded as far as detector performance is concerned.
  11. Jul 31, 2012 #10

    jim hardy

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    i'd say you would have to consider the neuton spectrum at the detector's location, and the response of the detector itself.

    Detectors in my experience measure primarily thermalized neutrons (BF3,
    B10 lined and fission chambers)

    and water might well moderate a hard spectrum. Our PWR source and intermediate range detectors were surrounded by polyethylene to intentionally moderate neutons that reached them.
    We did some experiments with cadmium to measure fast to thermal ratio out where detectors were located but i do not remember results.

    So - flooding might make indication go up
    first by moderating flux there making the detector see fast neutrons it would have missed
    second by improving reflection actually increasing reactor power.

    that's my guess...

    there's an interesting pdf here that opened my eyes to newer detectors
    again google interposed themselves,
    so if this won't work look for Berkeley's course Neutron-Detection_NE104_Spring11


    vapor deposited diamond? What'll they think of next!
    try searching on "fast neutron detectors"
  12. Jul 31, 2012 #11
    ok. But, increased thermal neutrons(in case of flooding) means the detector will exhaust rapidly.
    And the performance would degrade soon.
    That too, if the reactor power is very high, means higher no. of neutrons being thermalized.

    What about the thermal(heat) limitations of these detectors ?

    I read the nice materials at links provided by Astronuc and Jim Hardy, and my many doubts were clear.

    Actually, I am trying to visualize as much as possible the damages/degradations/effects induced in ex-core detectors in case of being water surrounded, and I believe in some way the flooding may pose serious threat to correct neutron detection ( keeping the other effects of flooding aside, and only concentrating on the effect on detector).
  13. Jul 31, 2012 #12


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    What? This is ridiculous for multiple reasons. You have some serious misconceptions about how ex-core detectors operate and what they are used for.

    1) as I said before, detectors are not exposed to water
    2) detectors do not burn out or exhaust
    3) detector performance does not degrade in that way
    4) reactor would not be at full power during catastrophic accident scenario
    5) you don't need ex-core detectors for core monitoring during accident scenario, they are only for normal at-power operation
    6) how would a fast reactor become flooded anyway? Where is the water coming from? And why do you care about power range monitoring while a catastrophic beyond design basis accident is occurring?
  14. Jul 31, 2012 #13

    jim hardy

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    well actually our source range detectors, BF3 type, are a sealed unit with about eighteen feet of metal jacketed cable attached by weld to top of cylindrical measuring chamber.
    They work fine underwater, in fact for a backup indication at refueling time we hang one or two with longer cables down into the reactor vessel , under perhaps thirty five feet of water..
    It's affectionately called 'dunking chamber' or 'rubber ducky' after the bath toy.

    Water causes trouble in source range when it gets into the connections between the metal sheathed detector cable and the ordinary plastic cables going up to control room. That connection is just below floor of refueling cavity so leaks on the hatch covers are a leading cause of that trouble.
    But that's a good fifteen feet above detector location.

    Moisture in the source range cable connector lets the high voltage sputter and arc, we call it "corona", and it gives a false high reading that masks true neutron count. An insidious failure, and you can't get to it during refueling for repair because the hatches are under thirty feet of water...

    Water in the intermediate range detector cables interferes with the miniscule currents you are trying to measure. It degrades cable insulation and the current meanders away .

    Water on power range detectors , meh, it's a miliamp or so of signal hence pretty tolerant of degraded cable .

    The detectors themselves and their affixed cabled are sealed aluminum or stainless steel, so surrounding them with water would keep them cool.
    It's the other stuff in your measurement system like wiring that's sensitive to water.
    As with all electrical equipment - just keep it clean and dry

    hope this helps.

    why invent new troubles? It's the old ones that need attention.

    Is this a research project?

    Why not visit a facility and spend some time with the guys who fix the equipment?
    You'll find they really resent spending effort on imaginary problems.

    old jim
    Last edited: Jul 31, 2012
  15. Aug 1, 2012 #14
    "why invent new troubles? "

    We are living in a world which is more uncertain than ever before (positively).
    Most of the reactors are near large water bodies, either sea or river.
    For those near sea:
    biggest threat is Tsunami (Earthquake + Flood + and possibly fire). How can we deny water flooding ? No need to give examples ( Fukushima).

    For those near rivers:
    biggest threat ( and long prevailing) is Ice melting due to global warming and unexpected rains, which in turn increases the water level in rivers by several meters. How can we deny?
    Though, this problem may take a longer time to appear.

    And for those neither near sea nor rivers or any other water bodies :
    Are they really safe ???

    I am not too negative about all these aspects, but the possibility of such accidents exists.

    Moreover, considering a fast reactor ( where u cant use a 'dunking chamber' or 'rubber ducky' kinda measure), the dependence on ex-core detectors increases several folds.

    And yes, its a part of a research. We try to devise a technique in which thermal power can be correlated with the "fluctuations" in temperature inside a reactor. Though these fluctuations depends on geometry of reactor, coolant flow rate. But for a constant flow, there exists a minimal fluctuation level at each power level.
    So, if a technique is devised , where the power is correlated with pure fluctuations in temperature, we have an extra alternative, and helps in case of any problem(if any) with ex-core detector.


    and yes , for "detectors do not burn out or exhaust " :
    by exhaust, i mean depletion of radioactive layer in the detector.
    Why cant detectors radioactive layer finish ? It can finish, and thats the end of their use.
    Last edited: Aug 1, 2012
  16. Aug 1, 2012 #15

    jim hardy

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    That sure works well on a PWR. I'm sure you will succeed, at power levels where there's enough temperature difference to measure.
  17. Aug 1, 2012 #16


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    They are designed and analyzed to be safe enough. The possibility of a large asteroid collision destroying all life on earth is greater, yet we spend almost no resources on asteroid detection or interception methods development. Unfortunately, we as a society do not have unlimited resources to reduce or prevent risk from all sources. We have to use our resources in an efficient manner to prioritize the most dangerous and most likely risks.

    No it doesn't. I don't know where you got this notion from. Reactors have no dependance whatsoever on ex-core detectors except to monitor core conditions during normal operating conditions. There is no difference with fast reactors in this regard. I don't know what you are referring to by "dunking chamber" or why you think ex-core detectors are required for safety reasons.

    Thermocouples are the primary method used to determine power level and core cooling for every nuclear reactor ever made.
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