United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 90-44: Dose-Rate Instruments Underresponding to the True Radiation Fields

                                UNITED STATES
                        NUCLEAR REGULATORY COMMISSION
                    OFFICE OF NUCLEAR REACTOR REGULATION
                           WASHINGTON, D.C.  20555

                                June 29, 1990


Information Notice No. 90-44:  DOSE-RATE INSTRUMENTS UNDERRESPONDING
                                   TO THE TRUE RADIATION FIELDS


Addressees:

All NRC licensees.

Purpose:

This information notice is intended to alert addressees to potential 
problems resulting from dose-rate survey instruments underresponding to the 
true radiation fields.  It is expected that recipients will review the 
information for applicability to their facilities and consider actions, as 
appropriate, to avoid similar problems.  However, suggestions contained in 
this information notice do not constitute NRC requirements; therefore, no 
specific action or written response is required.  

Description of Circumstances:

The Department of Energy (DOE) recently issued a Safety Action Notice that 
described an underresponse of two Bicron RSO-5 dose-rate instruments by a 
factor of about 100 on the second range (0-50 mR/h).  The problem occurred 
when two radiation protection personnel were providing radiation protection 
coverage at a DOE contractor facility.  A normal reading was observed on the 
dose-rate instrument's third range (0-500 mR/h).  No personnel were 
overexposed to radiation when these instruments failed.  Other instruments 
which have the same type of magnet arm switching design as the RSO-5 can 
have this same type of problem.

The instruments were removed from service and were returned to the 
manufacturer.  Bicron's examination confirmed the effect and found that the 
failure had been caused by a change in positioning of the magnet that 
activates reed switches in the ion chamber.  This magnet moves on a 
cam-actuated arm, and the cam is attached to the range switch.  Figure 1 
illustrates typical magnet arm positions for the Bicron RSO series of 
instruments.  At the zero position (one end) of the range switch rotation, 
the arm moves the magnet over the zero reed switch and shorts the electronic 
input for zeroing.  Switching from zero to the first (0-5 mR/h) and second 
(0-50 mR/h) ranges, the cam moves the end of the magnet arm to the center 
position between the zero and high-range reed switches, and neither reed 
switch is closed.  If the magnet arm is not properly aligned with the cam at 
the point of contact, the extra room allowed (play) for movement of the 
magnet arm with respect to the cam can result in the magnet at 



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the other end of the magnet arm being placed sufficiently past the center 
position to close the high-range reed switch upon range switching between 
the first (0-5 mR/h) and the second (0-50 mR/h) of the lower two ranges even 
though the magnet arm is not supposed to move.

Because of this possible mechanical problem, severe underresponse to the 
true radiation fields for these dose-rate survey instruments can occur as a 
result of switching between these lower two ranges.  This is because when 
the high-range reed switch is in the closed position, a different electronic 
circuit is actuated which reduces the sensitivity on these ranges by a 
factor of about 100.  This reduced sensitivity requires a true dose rate of 
5,000 mR/h to produce a full-scale deflection on the second-range (0-50 
mR/h) scale or a true dose-rate of 500 mR/h to produce a full-scale 
deflection on the first-range (0-5 mR/h) scale.

The Instrument Engineering Department of Bicron has included an additional 
quality assurance (QA) test to ensure the proper alignment of the magnet arm 
with respect to the reed switch and latitude for minor position changes that 
might occur.  This QA test is to be performed during the final acceptance 
test of a new instrument and again before the new instrument is calibrated 
for shipment.

Discussion:

All licensees need to be aware of the potential for underresponse at the 
lower two ranges for all magnet arm switching dose-rate instruments.  
Failure (i.e., high-range reed switch in the open position) at the 
high-range settings (0-500 mR/h, 0-5,000 mR/h) can only result in 
overresponse of the instrument.  If, when performing a survey with the 
instrument, the operator is not sure that he/she is in a high radiation 
field, the operator can verify that the instrument is not underresponding 
during these situations by switching to the high-range settings to quantify 
the upper limit of the radiation field before switching to the lower range 
settings.

This type of instrument problem reinforces the importance of source checking 
the instrument for each range before use, consistent with good ALARA (as low 
as reasonably achievable) practices. 

It is important that licensees also be aware that use of this type of magnet 
arm switching instrument in areas with high magnetic fields can 
inadvertently actuate the reed switches in the instrument without movement 
of the magnet arm.  For more information, refer to ANSI N42.17A, 
"Performance Specifications for Health Physics Instrumentation Portable 
Instrumentation for Use in Normal Environmental Conditions."

In addition to the Bicron Model RSO-5, malfunctions may also occur for the 
following instruments; 

     -    Bicron Model RSO-50
     -    Eberline Model RO-2
     -    Eberline Model RO-2A

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Since 1982, Eberline has added a pilot hole to the switch shaft to allow the 
set screw to align the collar which actuates the magnet arm.  Additionally, 
in the Eberline design, the magnet arm is straight and the distance between 
adjacent positions is greater than in the Bicron instrument.  This feature 
reduces the probability for indirect switching of a reed switch (as happened 
with the two instruments at the DOE contractor facility).

Although the magnet arm is aligned properly at the time of production, this 
alignment can shift with time and normal field usage.  Therefore, the 
Department of Energy Safety Action Notice recommends that all magnet arm 
switching dose-rate instruments be inspected at regular intervals for proper 
alignment of the magnet arm.  Both Bicron and Eberline have prepared new 
inspection procedures for their instruments.  For copies of revised 
maintenance procedures and other technical information concerning their 
instrument, contact the appropriate manufacturer at one of the following 
addresses: 

          Bicron Corporation
          12345 Kingman Road
          Newbury, Ohio  44065
          Joseph G. Bellian
          (216) 564-8000

          Eberline Instrument Corporation
          504 Airport Road
          P.O. Box 2108
          Santa Fe, New Mexico  87504-2108

               Customer Service, Santa Fe, New Mexico
               Robert Montaya
               1-800-234-4212

               Customer Service, Columbia, South Carolina
               John Witzell
               1-800-274-4212

This information notice requires no specific action or written response.  If 
you have any questions about the information in this notice, please contact 
the technical contact listed below or the appropriate NRR project manager. 




                              Charles E. Rossi, Director
                              Division of Operational Events Assessment
                              Office of Nuclear Reactor Regulation

Technical Contact:  Joseph C. Wang, NRR
                    (301) 492-1848

Attachments:  
1.  Figure 1, Typical Magnet Arm Position, RSO Instruments
2.  List of Recently Issued NRC Information Notices
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