Effluent Radiation Monitor Calibrations
See the memorandum from R. L. Baer to C. J. Paperiello dated November 13, 1985. Regulatory Guides and ANSI N13.10-1974 do not suggest multipoint calibrations are necessary beyond the initial preoperational testing for effluent monitors. Single point calibration using secondary sources are acceptable where detectors are inherently linear.
After a review of the existing Regulatory Guides (1.21 and 4.15) and ANSI industry standards (ANSI N13.10-1974) that establish relevant guidance, it is believed that these documents do not suggest multipoint calibrations are necessary beyond the initial preoperational acceptance testing for these effluent monitoring systems (sometimes referred to as "primary calibration", as used in ANSI N13.10-1974, Section 5.4.10). Section 5.4.10 further states that the primary "...calibration shall be related to a secondary source or method which will be used for periodic in-plant ecalibrations."
This suggests that routine re-calibrations can be less rigorous than the one-time, initial primary calibration. These periodic recalibrations should be viewed as ensuring that the detection system has remained stable over time. Therefore, "single-point" calibrations using secondary sources (e.g., solid sources), should be considered adequate to meet the requirements of standard Technical Specifications where detectors are inherently linear.
Assuming a licensee calibrates at a single point, the licensee should consider selecting that point at or near an alarm or action level. Routinely calibrating near an alarm point, coupled with the ongoing comparison of real-time monitor readings against laboratory analysis of periodic grab samples containing "normal" levels of radioactive effluents, seems to provide an adequate assurance of proper monitoring operability. However, calibration near an alarm point or action level is neither a requirement nor a position in the relevant guides or standards.
Region V provided input pertinent to this discussion which focused on detector saturation problems. They provided documented performance testing by a Region V licensee to determine the potential for saturation problems with the plants' effluent monitors. In general, the licensee found Geiger-Muller (GM) tubes were most seriously affected, NaI scintillator / photomultiplier (PM) tubes less affected, and plastic scintillator / PM tubes least affected.
Given the overall upgrade in effluent monitoring as a result of the NUREG-0737 requirements, each licensee should already be able to demonstrate adequate effluent monitoring capability at high ranges needed during accidents to provide meaningful information relative to a monitored "accident-type" release stream. The evidence demonstrating monitor operability at high ranges need not be verified by each licensee as primary calibrations since previous guidance provided by NRR for calibration of NUREG-0737 monitors suggests other acceptable alternatives.
In summary, "single-point" routine calibrations are adequate for scintillation monitors, given the monitors inherent stability and a thorough initial primary calibration. The use of single-point, routine calibrations for GM tubes is acceptable, given that the radiation monitor initiates a fail-safe trip function (isolates, or re-directs the effluent to another monitored pathway) below the radiation level where the initial primary calibration began to show appreciable saturation losses. To ensure that control room operators understand GM effluent monitor system limitations, emergency implementing procedures should clearly define these system limitations. For example, in the event of a steam generator tube failure, the procedures should highlight e.g., caution notes) probable invalid readings from an SJAE GM monitor (down scale response as the detector saturates, in response to a worsening primary-secondary leakage).
Regulatory references: ANSI N13.10-1974, Regulatory Guide 1.21, Regulatory Guide 4.15, Technical Specifications.
Subject codes: 6.4, 7.3