Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
SSINS No.: 6835
IN 82-49
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF INSPECTION AND ENFORCEMENT
WASHINGTON, D. C. 20555
December 16, 1982
Information Notice No. 82-49: CORRECTION FOR SAMPLE CONDITIONS FOR AIR
AND GAS MONITORING
Addressees:
All nuclear power plant facilities holding an operating license (OL) or
construction permit (CP), research and test reactors, fuel facilities, and
Priority I material licensees.
Purpose:
This information notice is provided as notification of errors in radioactive
gaseous effluent monitoring. Regional surveys of equipment and practices at
selected light water reactors (LWRs) revealed that a number of LWRs were not
routinely correcting for pressure differentials between main vent effluent
streams and associated offline sampling systems. Failure to correct for this
pressure differential can introduce errors (both in direct-reading gas
monitoring and in flow indication) in monitoring gaseous effluents with
offline sampling systems. These sampling errors can cause a significant
underestimation when quantifying effluent releases.
The potential also exists for systematic errors in occupational air
monitoring programs. Failure to apply pressure correction factors for flow
measuring devices when calibrating air samplers could lead to erroneous
determinations of airborne radioactivity levels.
It is expected that recipients will review the information for applicability
to their facilities. No specific action or response is required at this
time.
Description of Circumstances:
A problem of pressure differentials in gas monitoring systems was identified
by the licensee at the Diablo Canyon nuclear power plant. At Diablo Canyon
the gas monitor takes suction through an isokinetic sampling head about 100
feet up the plant vent stack. In maintaining a flow of 10 cfm, necessary to
ensure isokinetic sampling, it was found that the gas monitor chamber
pressure was approximately 12 inches of Hg below atmospheric pressure (30
inches of Hg). This resulted in a reduction in the density of the sample
chamber gas by approximately 40 percent.
The licensee has found that the reduced pressure phenomenon is most
pronounced in systems with long sample lines and is less noticeable where
sample lines are short. The licensee plans to incorporate pressure or AP
gauges on the gas monitoring equipment and to include corrections for
pressure variations in calculation of gaseous activity.
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IN 82-49
December 16, 1982
Page 2 of 3
As a result of this reported sampling deficiency, each Region conducted a
survey of selected operating LWRs to determine whether licensees were making
the necessary pressure differential corrections for effluent monitoring.
Results of these Regional surveys indicate that a generic deficiency does
exist. Twenty plants were surveyed and eleven facilities reported they made
no pressure differential corrections.
Discussion:
Since calibration of normal range noble gas detectors (sensors) is usually
done at atmospheric pressure using Kr-85 gas, it is essential that
calibration and operational readouts be automatically corrected for the
reduced pressure conditions encountered in system operation, or procedures
specify the application of appropriate correction factors. Current models of
effluent air monitoring systems provided by major vendors usually
incorporate such correction factors.
Particulate and iodine effluent release determinations also are sensitive to
sample flowrate which may be affected by system pressure variations. Errors
in the order of 10 percent to 50 percent in the calculation of particulates
and iodine can result if no compensation is provided for measurement of
actual gas flow in the sampling system at reduced pressure.
One of the simplest and most commonly used gas flow measurement devices is
the variable area flow meter, commonly known as the rotameter. A rotameter
calibrated at atmospheric pressure will not read correctly at either higher
or lower pressure, unless properly compensated. (D. K. Craig, See Attachment
1). Pressure correction factors for specific rotameters are available from
the various manufacturers as part of the instruction manuals supplied with
the equipment.
Operating variables such as the length of sample run, and variations in P
across a particulate filter can affect operating pressure. In addition to
long sample runs, another significant factor is the increase in pressure
drop across a particulate filter caused by dust loading. Craig cites an
example involving dust buildup on a filter where P increased from 5.9 inches
of Hg to 10.7 inches of Hg while the rotameter float reading was kept
constant. The initial flow rate was measured at 5.08 liters/minute and the
end flow was 4.02 liters/minute.
Assuming linear change in flow rate, the true mean value would have been
4.55 liters/minute. A determination of total volume flow made on the
assumption that the 5.08 liters/minute initial value prevailed over the
entire sampling period would have been 11.7% too high, and air contaminant
concentrations obtained using the initial flow rate would have been too low,
by the same percentage.
Manufacturers of sampling/monitoring systems are aware of the
flow-measurement discrepancies just discussed. Current systems provide
built-in compensation of air flow rate indication for operation at
less-than-atmospheric pressure through the use of pressure and temperature
transducers and computer software algorithms. Older analog systems may
require application of manual correction
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IN 82-49
December 16, 1982
Page 3 of 3
factors for given conditions of P and flow. Instruction manuals provided to
licensees by the vendors of older sampling/monitoring systems describe the
procedures for making the necessary corrections.
Independent verification of calibration of a flow rate measurement system
can be accomplished by placing a calibrated rotameter in series at the
sample intake end of the system and comparing readings of the system
rotameter under various system pressure conditions with those of the
calibrated rotameter. Since the verification rotameter operates at ambient
pressure, the only corrections needed for the calibration procedure are the
correction for ambient pressure (relative to standard) and a small
correction for temperature (the latter is only necessary for high precision
work--the error in assuming a standard condition of 70F is less than 5%
for the temperature range 24F to 116F which encompasses most plant
effluent streams).
Existing NRC regulations require the control of radioactive releases from
nuclear facilities and require measurements of radioactive materials in
effluents. It is implicit in all requirements for effluent monitoring that
these measurements be reasonably accurate. Licensees are expected to review
their facility's effluent monitoring program to determine the applicability
of the information provided in this notice.
No written response to this information notice is required. If you need
additional information about this matter, please contact the Regional
Administrator of the appropriate NRC Regional Office or this Office.
Reference:
Craig, D. K., "The Interpretation of Rotameter Air Flow Readings," Health
Physics. Pergamon Press 1971. Vol. 21 (August) pp. 328-332.
Richard C. DeYoung Director
Office of Inspection and Enforcement
Technical Contact: J. E. Wigginton
301-492-4967
P. G. Stoddart
301-492-7633
Attachments:
1. Article by D. K. Craig, "The Interpretation
of Rotameter Air Flow Readings."
2. List of Recently Issued IE Information Notices
