United States Nuclear Regulatory Commission - Protecting People and the Environment

U.S. Nuclear Regulatory Commission

Corrections for Sample Conditions for Air and Gas Monitoring

HPPOS-088 PDR-9111210244

Title: Corrections for Sample Conditions for Air and Gas


See IE Information Notice No. 82-49 entitled as above and

dated December 16, 1982. Calibration of monitoring systems

for noble gases, particulates, and iodine must include

correction for operation at reduced pressures. Newer

systems provide built-in compensation but older analog

systems may require the use of manual correction factors.

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 about 12 inches

of Hg below atmospheric pressure (30 inches of Hg). This

resulted in a reduction in density of the sample chamber by

about 40 percent. As a result of this reported sampling

deficiency, each NRC Region conducted a survey of selected

operating LWRs to determine whether licensees were making

the necessary differential corrections for effluent

monitoring. Results of these Regional surveys indicated

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. Particulate and iodine effluent

release determinations are also sensitive to sample flow

rate which may be affected by system pressure variations.

Errors on the order of 10% to 50% 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. Operating variables such as

the length of sample run, and variation in the pressure

differential across a particulate filter can also 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.

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, Health Physics 21, 328-332 (1971)]. Pressure

correction factors for specific rotameters are available

from the various manufacturers as part of the instruction

manuals supplied with the equipment. Manufacturers of

sampling / monitoring systems are aware of potential

discrepancies in flow rate measurements. 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 factors. Instruction

manuals provided to licensees by the vendors of older

sampling / monitoring systems should describe the procedures

for making the necessary corrections.

Independent verification of the 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 correction 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

conditions of 70F is less than 5% for the temperature

range of 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.

Regulatory references: 10 CFR 20.103, 10 CFR 20.106, 10

CFR 20.201, 10 CFR 20.1204, 10 CFR 20.1302, 10 CFR 20.1501

Subject codes: 6.4, 6.9, 7.2, 7.3

Applicability: All

Page Last Reviewed/Updated Thursday, March 29, 2012