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Performance Evaluation of the Sampling Head and Annular Kinetic Impactor in the Savannah River Site Alpha Continuous Air Monitor
Abstract and Figures
This paper describes evaluation and testing of the Savannah River Site (SRS) impactor-type alpha continuous air monitor (CAM) to determine the particle delivery and collection efficiency in the de,ice using fluorescence-labeled, polystyrene lates spheres and liquid droplet aerosols. Particle sampling efficiency through the dome-shape sampling inlet for particles,with aerodynamic diameters of 6.2 and 10 mu m was 98% at 20 cfm (566 L/min), 90% at 32 cfm (906 L/min), and 87% at 40 cfm (1132 L/min) sampling flow rates. Internal delivery efficiency through the CAM was > 94% at 20 and 32 cfm for 0.5-, 1.1-, 2.2-, and 3.2-mu m particles and >90% for 6.2-mu m particles. For 10-mu m particles, the internal delivery efficiency was 91% at 20 cfm and decreased to 83% at 32 cfm and 77% at 40 cfm. The 50% cutoff aerodynamic diameter for the impactor was 3.2 mu m at 20 cfm, 2.6 mu m at 32 cfm, and 2.3 mu m at 40 cfm. For a typical radioactive aerosol in the workplace (activity median aerodynamic diameter of 5 mu m with a geometric standard deviation of 2), these cutoff diameters provide collection efficiencies of 74% at 20 cfm, 83% at 32 cfm, and 87% at 40 cfm. The normal grease layer of 1.5 mg that is routinely applied to the planchet of the SRS CAM was adequate to quantitatively retain all collected particles with diameters of 3.2 mu m or less at flow rates of 20, 32, and 40 cfm. For particle sizes of 6.2 and 10 mu m, approximately 80-85% of the particles were retained on the impactor planchet and 15-20% were re entrained into the exhaust airstream due to particle bounce. The delivery and collection efficiencies in the SRS CAM can be combined to give overall efficiencies for detection of airborne actinide aerosols as a function of particle size. For 6.2 and 10 mu m aerodynamic diameter particles, the total collection efficiency of the SRS CAM at 20, 32, and 10 cfm exceeds the conservative assumption of 50% efficiency for plutonium particle collection that has been traditionally used at SRS.
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... It is not unusual for different components of the same instrument to be calibrated separately. For example, when one is interested in the performance of the sampling inlet (inlet efficiency), transport line (transport loss), and detection (counting efficiency) or collection section (collection efficiency) of a high volume aerosol sampling system, a series of calibrations can be made, one for each individual component (Chen et al. 1999). Different parameters might be selected for investigation in the different components and, consequently, different test facilities with different test aerosols are often employed during the component calibrations. ...
... Another way of calibrating an instrument is to introduce the aerosol particles into an aerosol test chamber ( Fig. 21-1b) that houses the subject instrument and the test standard (Marple and Rubow 1983;Chen et al. 1999). This chamber usually has a large test section to provide a quiescent atmosphere in which the entire instrument can be exposed to the aerosol as in the real sampling environment. ...
... Polystyrene latex particles of different sizes have also been concurrently produced in an aerosol to obtain more than one data point per experimental run. Monodisperse latex particles containing fluorescent dye or radiolabeled isotopes are also used in calibrations when quantitative measurements by fluorometric or radiometric techniques are needed (Newton et al. 1980;Chen et al. 1999). Two problems arise in the generation of these latex particles: formation of agglomerates and existence of residual particles. ...
Introduction Measurement Methods and Calibration Standards General Considerations Calibration Apparatus and Procedures Test Aerosol Generation Calibration of Flow, Pressure, and Velocity Instrument Calibration Summary of Calibration Procedures References
... Traditionally, this type of study requires a meter-scale settling chamber with a generation system at the top and samplers to be tested located at the bottom (Marple and Rubow 1983;Chen et al. 1999;Kenny et al. 1999;Koch et al. 1999). However, depending on the requirements of budget, space, operation, and maintenance, it is sometimes not practical to fabricate such a massive system. ...
... During each test run, monodisperse aerosol with a given particle size was dispersed at the top of the chamber, settling into the quiescent portion of the chamber, and then collected by the reference and test samplers near the bottom. After each run, the samplers were dismantled and all filters, probes, and sampler parts were immersed or rinsed with ethyl acetate to remove the fluorescent dye from the particles using the procedure developed by Chen et al. (1999). The fluorescence intensities of the samples were then analyzed using a PTI spectrofluorometer (Photo Technology International, Model C-60, Monmouth Junction, NJ). ...
A compact, low cost, and easy to operate experimental system was developed for evaluating personal samplers in a calm-air environment. The system can provide uniformly distributed monodisperse microspheres up to 69.7 w m in a quiescent environment for determining sampler efficiencies using a fluorescence method. The reference concentration was measured using one vertical and two horizontal sharp-edged probes. The vertical sampling probe met the criterion for greater than 90% sampling efficiency for upward facing cylindrical probes given by Agarwal and Liu, except at the particle size of 69.7 w m. In addition, the consistent agreement among all three reference sampling probes at all particle sizes tested implied that representative measurements of aerosol concentrations in the test section were achieved. In this study, the RespiCon (Model No. 8522, TSI Inc., St. Paul, MN, USA) and IOM (Cat. No. 225-70, SKC Inc., Eighty Four, PA, USA) personal aerosol samplers were evaluated in the system with air velocities in the sampling region below 1.5 cm/s. The sampling efficiency of the IOM was near 100% at all particle sizes, while the RespiCon matched the conventional respirable and thoracic convention curves but undersampled the inhalable fraction.
... Black carbon is an especially useful observable for this purpose, because its origin is exclusively from primary emitters. Short-duration measurements of PM 2.5 or PM 10 mass, [21][22][23][24] total or organic carbon, 25,26 nitrate, 27,28 particle-bound polycyclic aromatic hydrocarbons, 29 and particle light scattering 30,31 also include contributions from secondary particles that form over longer aging times and tend to be more homogeneously distributed over space and time than primary particles. The methods described here for black carbon are equally applicable to other short-duration pollutant measurements. ...
A successive moving average subtraction method is developed and applied to black carbon measured over 5-min intervals at a downtown location near many small emitters and at a suburban residential site within the urban plume but distant from specific emitters. Short-duration pulses assumed to originate from nearby sources are subtracted from the concentrations at each site and are summed to estimate middle-scale (approximately 0.1-1 km) contributions. The difference of the remaining baselines at the urban and suburban monitors is interpreted as the contribution to the downtown monitor from source emissions mixed over a neighborhood scale (1-5 km). The baseline at the suburban site is interpreted as the contribution of the mixture of black carbon sources for the entire city. When applied to a 24-day period from February and March 1997 in Mexico City, the analysis showed that 65% of the 24-hr black carbon was part of the urban mixture, 23% originated in the neighborhood surrounding the monitor, and only 12% was contributed from nearby sources. These analyses indicate that a fixed-site monitor can reasonably represent exposures in its surrounding neighborhood even when many local sources, such as exhaust from diesel buses and trucks, affect the monitor.
... However, continuous monitoring systems do not necessarily perform counting and sampling simultaneously and continuously permitting the use of arbitrary sampling, waiting, and counting times for specific purposes. Sampling is often performed using filters, but an impactor may also be used (Kesten et al. 1993;Chen et al. 1999), allowing the size-activity distribution of radioactive particles to be obtained. ...
Rapid method for identifying the presence of alpha particle emitting radionuclides in outdoor air is of paramount importance should a nuclear or radiological incident occur. Minimum detectable activity concentrations of U, U, Pu, and Pu in outdoor air are calculated for two direct alpha spectrometry methods: continuous air monitoring is compared with separate sampling and subsequent alpha particle counting in a vacuum chamber. The radon progeny activity concentration typical for outdoor air and the effects for the alpha particle spectra caused by the properties of the filter and the aerosol particles are taken into account using measurements and Monte Carlo simulations. Continuous air monitoring is a faster method for identifying the presence of (trans)uranium elements when their activity concentration is considerably higher than the typical detection limit. Separate sampling and counting in a vacuum chamber is a more sensitive method when concentrations are close to the detection limit and when the duration of the sampling-counting cycle is greater than approximately 2 h. The method may serve as a tool for rapid field measurements.
A continuous air monitor has been developed that includes provisions for improving the detection of alpha-emitting aerosol particles in the presence of radon/thoron progeny that are unattached to ambient aerosol particles. Wind tunnel tests show that 80% of 10-microns aerodynamic equivalent diameter particles penetrate the flow system from the ambient air to the collection filter when the flow rate is 57 L min-1 (2 cfm) and the wind speed is 1 m s-1. Uniformity of aerosol collection on the filter, as characterized by the coefficient of variation of the areal density deposits, is less than 15% for 10-microns aerodynamic-equivalent-diameter aerosol particles. Tests with unattached radon daughters in a flow-through chamber showed that approximately 99% of the 218Po was removed by an inlet screen that is designed to collect radon daughters that are in the size range of molecular clusters. The inlet screen offers the opportunity to improve the signal-to-noise ratio of energy spectra in the regions of interest (subranges of the energy spectrum) of transuranic elements and thereby enhance the performance of background compensation algorithms.
Aerosol separation characteristics of two prototype single-stage virtual impactors are described. Data indicate that the efficiency curve and the 50% cut-off Stokes number (Stk50) (or cut-off size) are not sensitive to the nozzle Reynolds number (Re), and can be predicted as long as the corresponding Re is between 1000 and 8000, the ratio of nozzle to probe distance (S) to the nozzle diameter (W) is about 1, and the ratio of the hole diameter (D1) to W is between 1.2 and 1.5. The results, however, show that the cut-off sizes are strongly affected by the ratio of minor flow rate (Qm) to the total flow rate (QT). Also, the efficiency data reveal that Stk50 of the liquid dioctyl phthalate aerosol is smaller than that of solid polystyrene Latex aerosol, suggesting internal circulation in the liquid particles at the acceleration nozzle exists. In addition, we found that, near the cut-off particle size, there were substantial losses (20 ∼ 40%) on the surface of the collection probe. The results also show that only parameters D1/W and Qm/QT have a slight effect on the shape of the efficiency curve.
A survey of published values of Activity Median Aerodynamic Diameter (AMAD) measured in working environments was conducted to assist in the selection of a realistic default AMAD for occupational exposures. Results were compiled from 52 publications covering a wide variety of industries and workplaces. Reported values of AMAD from all studies ranged from 0.12 μm to 25 μm, and most were well fitted by a log-normal distribution with a median value of 4.4 μm. This supports the choice of a 5 μm default AMAD, as a realistic rounded value for occupational exposures, by the ICRP Task Group on Human Respiratory Tract Models for Radiological Protection and its acceptance by ICRP. Both the nuclear power and nuclear fuel handling industries gave median values of approximately 4 μm. Uranium mills gave a median value of 6.8 μm with AMADs frequently greater than 10 μm. High temperature and arc saw cutting operations generated submicron particles and occasionally, bimodal log-normal particle size distributions. It is concluded that in view of the wide range of AMADs found in the surveyed literature, greater emphasis should be placed on air sampling to characterise aerosol particle size distributions for individual work practices, especially as doses estimated with the new 5 μm default AMAD will not always be conservative.
The TSI small-scale powder disperser was evaluated to study the effects of total flow rate, capillary flow rate, and particle shape, size, and density on dispersion efy and internal losses. Dry powders, including carbon fibers, talc, alumina, and polystyrene microspheres, were used to represent materials with different physical properties. Polystyrene microspheres in the size range of 3–97 μm were used to study the effects of particle size on performance. Results indicated that, for the same operating conditions, dispersion efficiencies for carbon fibers and polystyrene spheres were higher than talc and alumina. Dispersion efficiency increased as the total flow rate increased and reached a constant value when the low rate was >8 l min−1. Also the dispersion efficiency decreased as particle sizes increased at a given operating conditions. As for the internal losses in the disperser, particles were deposited primarily at the expansion zone of the venturi tube. Adjustment of the capillary tube location to increase the capillary flow rate did not result in any noticeable change in the efficiency, while poor alignment of the tube resulted in a significant loss as compared with a proper alignment. Along with these data, computer simulations of the flow field and theoretical predictions of the lift force as a result of high flow shear at the venturi throat were successfully used to interpret particle deposition in the TSI small-scale powder disperser.
An aerosol test chamber has been developed for the purpose of providing an environment for the evaluation of several aerosol measuring instruments and samplers at one time. Evaluations of the chamber have shown that the aerosol concentration is uniform throughout the area of the test section, which is 1.1 m2 in size, with good temporal stability. To ensure further that all instruments obtain the same aerosol exposure, the floor of the chamber's test section has been designed as a turntable. The turntable can be rotated for the duration of a test, continuously moving the instruments in the test section. The design of the chamber, the controls, and operating procedures are described, as well as the results of spatial uniformity and temporal stability evaluations using monodisperse liquid aerosols from 2.5-µm to 15-µm diameter and polydisperse dust particles.
Prompt detection of airborne contamination is essential in plutonium-handling facilities because the materials normally involved are extremely toxic. Impaction type continuous monitors for airborne alpha contamination, as described by Collins, have been in use at the Savannah River Plant (SRP) and at the Savannah River Laboratory (SRL) since 1956. The monitors originally used had definite limitations. They utilized moving tape as a collection medium and were limited by a delay in response and by low collection efficiency. A monitor which promptly detects low concentrations of airborne alpha contamination has now been developed. This article presents monitor design and SRP operating data. (C)1966Health Physics Society
Update on Selection and Use of Filter Media in Continuous Air Monitors for Alph a-Emitting Radionuelides , ITRI Annu al Report
- M D Hoover
Hoover, M. D., and Newton, G. J. (1992). Update
on Selection and Use of Filter Media in Continuous Air Monitors for Alph a-Emitting Radionuelides, ITRI Annu al Report 1991-1992, National Techn ical Information Service, Springfield, VA, pp. 5-7.
A Continuous Monitor for Airborne Plutonium
- D C Collins
Collins, D. C. (1956). A Continuous Monitor for
Airborne Plutonium, U.S.AE C Report DP-188,
Savann ah River Plant, E. I. du Pont de
Nemours and Company, Aiken, Sc.
A nn ular Kinetic Impactor, DPSP U 56-11-30, Savannah River Plant, E. I. du Pont de Nemours and Company Hum an Respiratory Tract Model for Radiological Protection: A Report of a Task Group of the International Commission on Radiological Protection
- J E Hoy
Hoy, J. E. (1956). A nn ular Kinetic Impactor,
DPSP U 56-11-30, Savannah River Plant, E. I.
du Pont de Nemours and Company, Aiken,
Sc.
International Council on Radiation Protection
(1994). Hum an Respiratory Tract Model for
Radiological Protection: A Report of a Task
Group of the International Commission on Radiological Protection, ICRP Publication 66,
Ann als of the ICRP, Vol 24, No. 1-3, Pergamon, Oxford.