Monitoring at Nuclear Power Plants for Contamination by Radionuclides that Decay by Electron Capture
See the memorandum from L. J. Cunningham to J. H. Joyner (and others) dated May 28, 1992. The memorandum contains an enclosure with three attachments providing information concerning monitoring contamination from electron-capture emitters. HPPOS-071 contains a related topic.
Information provided by the NRC Regions did not suggest a generic health and safety problem with monitoring electron-capture emitters among nuclear power plants, but did indicate a wide range in contaminating activity. Many licensees recognized that conventional detectors used in hand frisking for beta-emitter contamination, particularly “pancake” GM detectors, have a low counting efficiency for x-rays and gamma rays emitted by electron-capture nuclides.
Some licensees have or were considering obtaining more efficient detectors (such as proportional counters filled with argon-methane) for monitoring electron-capture nuclides. However, some licensees appeared to be making improper applications of the numerical criteria in IE Circular 81-07 (see HPPOS-071) to monitoring for electron-capture nuclides and to automated personnel contamination monitors. Therefore, the enclosure to the memorandum includes the following discussion of previous NRC guidance on monitoring for contamination at nuclear power plants.
IE Circular 81-07 (IEC-81-07) provides guidance on monitoring for surface contamination by “beta-gamma” and alpha emitters. As indicated in that circular an in IE Information Notice 85-92, the numerical criteria included in that circular (e.g., a detection capability of 5000 dpm/100 cm2 for total “beta-gamma” contamination) are based on considerations of hand frisking with portable survey instruments equipped with thin-window (relatively small area) “pancake” GM detectors that respond primarily to beta radiation and that are relatively insensitive to x-rays and gamma rays. Thus, the numerical criteria were not intended for, and are not appropriate for, surveys for contamination by radionuclides (or mixtures of radionuclides) that emit photons but that emit little or no beta radiation. The staff does not plan to develop new criteria for detection of photons, whether x-rays or gamma rays, in contamination surveys. The qualitative guidance in Circular 81-07 and Information Notice 85-92 is applicable to all surveys for contamination of materials before release to unrestricted areas. However, the guidance in Circular 81-07 and Information Notice 85-92, for the detection of contamination of materials, is not intended to be applied to automated personnel contamination monitors, which are used for detection of contamination of workers. In any case, the numerical criteria of IE-81-07, which are expressed in terms of activity per unit area, are not applicable to measurements of the total activity of the contamination on either materials or workers.
The NRC staff, as noted in the “NRC Staff Perspective” included with the enclosures, is concerned with the potential for unauthorized release of detectable contamination from licensed material. Licensees should be aware of changes in contamination detection capabilities resulting from changes in radionuclide composition.
Regulatory references: 10 CFR 20.1501
Subject codes: 6.1, 6.3, 7.6, 7.7, 8.3, 8.4, 9.7