Information Notice No. 82-49: Correction for Sample Conditions for Air and Gas Monitoring

                                                            SSINS No.: 6835 
                                                            IN 82-49  

                               UNITED STATES 
                          WASHINGTON, D. C. 20555 

                             December 16, 1982 

                                   AND GAS MONITORING 


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. 


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 

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. 


                                                      IN 82-49 
                                                      December 16, 1982  
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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. 


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  

                                                      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. 


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 

                    P. G. Stoddart 

1. Article by D. K. Craig, "The Interpretation 
     of Rotameter Air Flow Readings." 
2. List of Recently Issued IE Information Notices 


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