**2.1 Sampling point source releases of radioactive substances**

Over 40 years ago, proscriptive sampling methods were normative, with an emphasis on the isokinetic sampling of airborne radioactive material from exhaust points (ANSI, 1970). Since then, advances in sampling techniques and improved technology have yielded a new approach to representative sampling (ANSI, 2011; ISO, 2010). Because of these advances, the goal of achieving an unbiased, representative sample now results in a standards-based approach with definitive criteria to establish the sampling at a wellmixed location.

Point sources are discrete, well-defined locations (such as a stack, vent, or other functionally equivalent structure) from which radioactive air emissions originate (Washington Administrative Code [WAC], 2005; U.S. Environmental Protection Agency [EPA], 2002a). Point sources are actively ventilated or exhausted. Emissions from a point source may be captured, treated, monitored, sampled, and/or controlled. At some threshold, direct effluent sampling must be conducted to verify low emissions, and a graded approach based on potential emissions is recommended. Table 1 shows the ANSI N13.1-2011 approach to direct effluent sampling and monitoring requirements based on the U.S. limit of 0.1 mSv/yr (10 mrem/yr) (EPA, 2002a).


Table 1. Graded approach to sampling and monitoring (ANSI N13.1-2011)

Using a graded approach to determine direct effluent sampling and monitoring needs (Table 1) and to design a robust sampling system (whereby the sample is extracted at a homogeneous location within the point source) requires an evaluation of the sample environment, transport mechanisms, and collection materials. The criteria for the homogeneous sampling location includes a determination of the angular or cyclonic flow, uniformity of the air velocity profile, gas concentration profile, and particle concentration profile (ISO, 2010; Table 2). Scaled tests may be utilized to demonstrate compliance with these criteria; however, as technology improves, modeling techniques such as computational fluid dynamics may be used to validate a well-mixed location without the necessity of field tests conducted in the stack or vent (Recknagle et al., 2009).

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collection media include silica gel (Fig. 3) or molecular sieves for tritium collection, activated charcoal or silver zeolite cartridges for radioiodines, and bubblers for other gases. For collection media selection, detection criteria for the measurement of alpha, beta, and gamma radiation must also be established to meet lower limits of detection. The sample volume affects the criteria for detection limits, sample size, sampling frequency, and materials used.

Fig. 2. Fixed head radioactive air stack sampler with 47-mm diameter filter in place

Fig. 3. Silica gel columns in use for tritium sampling system; three columns are used for collecting water vapors, and then the dry gas goes through a catalyst to form the water

vapor collected using the two columns (Barnett et al., 2004)


Table 2. Summary of recommendations for a stack sampling location (ISO 2889:2010)
