Re: Neutron Measurements at Nuclear Power Plants

["Eric L. Darois" <daroiel@naesco.com>]

     We have a good deal of experience with Tissue equivalent Proportional 
     Counters (TEPC) for neutron surveys in containment.  Also during 
     start-up, we performed spectral measurements using a H-3 spectrometer, 
     modeled after the development work done at Battelle.
     
     Far West Technologies Inc. developed a hand-held neutron survey meter 
     several years ago that uses a TEPC.  This device has a built-in 
     MCA/microprocessor.  The energy dependence is excellent and it 
     compares very well with the more sophistcated TEPC system used during 
     our start-up study.   We performed comparisons at NIST using moderated 
     and unmoderated Cf-252, all within 30%.  The instrument is only about 
     5 lbs !!!  
     
     We use this meter exclusively for power entry work.  We havel also 
     used this as a calibration standard for site specific TLD neutron 
     correction factors.  HP technicians like this much better than the 
     rem-ball.
     
     
     
     Eric Darois: DAROIEL@naesco.com
     Jay Tarzia: TARZIJP@naesco.com
     Seabrook Station
     


______________________________ Reply Separator _________________________________
Subject: Neutron Measurements at Nuclear Power Plants
Author:  radsafe@romulus.ehs.uiuc.edu at Internet
Date:    6/6/96 7:32 PM


I would like to expand on the good discussion provided by Bob Flood on 
measurements of neutrons at nuclear power plants. 
     
In TVA, we use both the Eberline PNR-4 and Ludlum 12-4 rem-ball to measure 
dose rates from neutrons.  These instruments are calibrated on a bare PuBe 
source.  In 1991, we sent some of these calibrated instruments to Battelle 
Northwest Laboratories for "as found" measurements to determine the accuracy 
of the calibrations performed in TVA.  The results were:   
     
  Battelle Source  Relative Instrument Reading 
     
     bare PuBe        10% low 
     bare Cf-252        40% high 
    Deuterium moderated Cf-252      100% high 
     
During the start-up of Watts Bar-1, we made Bonner sphere measurements just 
inside a door to lower containment.  The data were analyzed with a code that 
provided the spectrum and neutron dose.  The spectrum indicated that the most 
abundant neutron energy was approximately 0.1 MeV.  This agrees with previous 
investigations (NUREG/CR-2233).  Neutron dose rate measurements were made at 
the same location using a rem-ball.  In addition, Panasonic UD-802AS TLDs were 
taped to the 12 inch Bonner sphere and exposed at the same location for one 
hour.  The Bonner sphere is not a true humanoid phantom.  The results were: 
     
  Instrument Response Relative to Spectral Measurement 
     
   Rem-ball     130% high 
   TLDs   approximately 80% high 
     
The calibration on the TLDs is traceable to Deuterium moderated Cf-252 which 
has a spectrum with peaks at approximately 3 and 0.01 MeV. This is harder 
than the spectrum measured at the plant.  Since the TLDs are thermal neutron 
detectors, you would expect the TLDs to over-respond to the softer plant 
spectrum. 
     
The energy response spectrum supplied by Eberline for the PNR-4 also indicates 
that the rem-ball would over-respond to the plant spectrum when it is 
calibrated on the harder PuBe spectrum. 
     
Since neutrons provide such a small fraction of the total person-rem dose 
received, no corrections have been made to measured personnel doses.  If the 
fraction was larger, corrections could be made to make the documented 
personnel doses as accurate as possible. 
     
All these thoughts are my own. 
     
John Lobdell, Ph.D., C.H.P. 
lobdell@HiWAAY.net