Neutron Measurements at Nuclear Power Plants

[John Lobdell <lobdell@fly.hiwaay.net>]

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