NUMBER: 9846
CATEGORY: Instrumentation
QUESTION: What is the best way to prevent radon progeny from interfering with RCA egress at commercial nuclear power plants.
ANSWER: Unfortunately, there are relatively few options available for preventing interference from radon progeny as workers are monitored upon leaving the RCA at nuclear power plants. In-plant radon concentrations vary with plant location, construction characteristics, time of year, time of day, and local weather conditions. Radon progeny can deposit on skin, hair, clothing, etc. and complicate the contamination assessment process. The necessity for wearing anti-contamination clothing, much of which uses synthetic materials, such as polyethylene or polyester, often enhances attachment of electrically charged radon progeny to clothing surfaces.
In non-power plant environments radon reduction is sometimes accomplished through increased ventilation of the work areas, and this is often very effective. While increased local ventilation might be possible in limited cases in nuclear plants, it is often not a practical option. One is then left with the possible solution of using personal contamination monitors that use detection techniques and/or appropriate software to evaluate and reject the response to radon progeny. There are some such systems available. Here are a couple of links to representative monitoring systems available through Canberra: the ARGOS-AB systems use gas flow proportional detectors sensitive to alpha and beta radiations and use specially designed software to evaluate radon progeny and to reject them in the monitoring process; the same company also makes scintillation detector-based monitors, the Argos-TPS systems, that also incorporate radon rejection software. Here is a link to another system, the Thermo Scientific PCM-2, which also uses radon rejection methodologies. The citation of the above monitors should not be interpreted as an endorsement of these systems over any others.
None of these systems are completely effective in radon progeny rejection in all circumstances, and at times when the software is not effective or when contamination monitors without radon rejection capability are being used, the contamination by such progeny may be a notable interference that must be recognized and accounted for by techniques beyond what the routine exit monitors offer. Personal air (filter) samples can be very useful in identifying and quantifying radon progeny as well as other radionuclides, although analysis may be more time consuming than desired. Measurement of simple beta-to-alpha ratios on such filters is relatively fast, however, and for plants that have high beta-to-alpha ratios, the ratio can be used for quick assessment of the presence of radon progeny. While a personal air sample provides the best indication of what radioactivity the individual was exposed to (and inhaled), area air samples may also be useful in assessment of radioactivity content and alpha-to-beta ratios.
If you are interested in an analysis and representative data associated with setup and interpretation of a contamination monitor, here is a link to a poster paper presented recently at the 2011 national Health Physics meeting that goes through many of the details involved in predicting response and assessing the impact of elevated background in a particular plant situation (Skrable, et al., Efficacy of Personal Air Samplers and Exit Portal Monitors for the Daily Detection of Significant Intakes of a Mixture of Radionuclides Present in a Contamination Event at a Nuclear Power Plant in the Presence of Radon Progeny). The link to the pdf version of the paper is at the bottom of the abstract. Note that verbal descriptions/discussions that apply to each visual are all contained after the visuals. The paper mainly considers the impact of radon progeny on personal air sample results but also presents information about determination of expected portal monitor response and determination of the alarm set point (visuals 50-56, pages 58-64). There is no quantitative discussion of the relationship between radon concentrations or degree of progeny contamination and the response of the portal monitor. The discussion uses specific plant conditions for a plant that was shut down and which had plant-produced contamination associated with significant fuel failures that produced unusually low beta-to-alpha ratios (about 32:1) and assumptions that may not apply to any other particular case, but the methodologies and many considerations shown are appropriate for numerous situations.
I wish you well in your efforts to minimize radon progeny influences on your monitoring systems.
George Chabot, Ph.D., CHP
