United States Nuclear Regulatory Commission - Protecting People and the Environment

Corrections for Sample Conditions for Air and Gas Monitoring

HPPOS-088 PDR-9111210244

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 Monday, June 08, 2015