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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IN 90-44
June 29, 1990
Page 2 of 3
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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IN 90-44
June 29, 1990
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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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