28th Seismic Research Review: Ground-Based Nuclear Explosion Monitoring Technologies

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Gerald Hicks
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1 DESIGN OF AEROSOL SAMPLER TO REMOVE RADON AND THORON PROGENY INTERFERENCE FROM AEROSOL SAMPLES FOR NUCLEAR EXPLOSION MONITORING Scot K. Waye, Steven R. Biegalski, and Ofodike A. Ezekoye The University of Texas at Austin Sponsored by Army Space and Missile Defense Command Contract No. W9113M ABSTRACT An aerosol sampler is being developed to physically separate naturally occurring radionuclides from anthropogenic radionuclides produced in nuclear weapons explosions. Studies show that aerosols with natural activity have an aerodynamics diameter in the range of 0.1 to 1.0 µm. In contrast, atmospheric nuclear explosions produce radioactive aerosols with aerodynamic diameters less than 0.1 µm. Surface nuclear explosions produce a bimodal distribution of radioactive aerosol particles with aerodynamic particles greater than 1.0 µm and less than 0.1 µm. A high volume (70 m 3 hr -1 ), low pressure aerosol impactor has been designed that will separate the particles into the three size distributions: aerosols with aerodynamic diameters greater than 1.0 µm, between 0.1 and 1.0 µm, and smaller than 0.1 µm. Computational fluid dynamics (CFD) modeling was implemented in order to examine sampler inlet velocities and cutoff efficiencies. An interactive design tool was developed to obtain the impactor geometry that yields the desired aerodynamic particle diameters. Filters were chosen that had good collection efficiencies for small particles and had very low natural radioactivity. The components of the aerosol impactor were manufactured or obtained and then assembled. Sample aerosols of known aerodynamic diameters were introduced to verify the aerosol cut-off diameters achieved by the sampler. 851

2 OBJECTIVES This work seeks to develop an aerosol sampler that physically separates aerosols containing radioactivity from natural origin from aerosols containing radioactivity produced in a nuclear weapons explosion. Aerosols with natural radioactivity have been shown to have a size distribution predominantly between 0.1 and 1.0 µm. Grundela and Porstend (2004) showed this was the case for data collected in Göttingen, Germany for thoron and radon progenies. Similar studies, including Bondietti, et al. (1987) also found this size range for thoron progeny. Aerosols with radioactivity from nuclear explosions were found to have a bimodal size distribution depending on the type of detonation. Aerosols were collected after a 20 kt nuclear explosion. For air explosions, the aerosol particles had a diameter less than 0.1 µm and for surface explosions, the particle size was larger than 1.0 µm (Storebø, 1974). This data was consistent with aerosols collected in Sweden from a September 26, 1976 Chinese kt nuclear explosion conducted above-ground (De Geer et al., 1978). It is therefore advantageous to separate aerosol particles into at least three size groups by particle diameter: greater than 1.0 µm, between 0.1 and 1.0 µm, and less than 0.1 µm. There are many designs of aerosol samplers that separate the particles by size. Inertial impactors, especially cascade impactors, are ideal for this task. Particle inertia in a curved flow makes the particles deviate from the streamline, impacting a collection surface (Willeke and Baron, 1993). By controlling the fluid flow through the geometry of the sampler, particular sizes can be separated. When multiple single-stage impactors are placed in series, particle sizes of decreasing diameter may be collected. One difficulty with the current study is the small particle size of radioactive aerosols from above-ground nuclear explosions (particle size is less than 0.1 µm). Conventional impactors normally only have a particle diameter limit of µm (Vanderpool et al., 1990). Two methods that have been used previously to obtain lower particle diameters are using a Micro-Orifice Uniform Deposit Impactor (MOUDI) or using a low-pressure impactor. The nozzles in a MOUDI are difficult to manufacture, especially for the case of a high volume flow since thousands of nozzles are needed for the lower stages. Lowering the pressure in a stage changes the mean free path between fluid particles and allows for smaller diameter particles to be collected. A critical stage is needed to create a pressure drop and a vacuum pump is needed in order to pull the desired pressures. Once the aerosols containing radioactivity from natural sources are separated from those with radioactivity from nuclear explosions, the samples are analyzed. With the reduced background, gamma-ray spectroscopy of the aerosol filters will result in detection limit improvements. The filters will not need significant decay times between collection and gamma-ray spectrum acquisition. These improvements will be most notable where radon and thoron levels are high. The size distribution will also enable identification of a surface or atmospheric explosion. The current focus of the study is the design and validation of the before mentioned aerosol sampler. A generic impactor was modeled using a CFD program, FLUENT 6.1 (2002). Both circular and rectangular nozzles were modeled and benchmarked with previous studies. This provided insight into the physics of the fluid flow and particle behavior. By calculating the necessary geometries and pressures needed to obtain the desired aerosol particle sizes, the cascade impactor was manufactured and assembled. In order to verify the size distribution for each stage, fluorescent test aerosol particles of known size were produced and collected by the sampler onto the filters of each stage. The filters were then analyzed for size distribution for design validation. RESEARCH ACCOMPLISHED Various geometries of impactor stages were modeled in FLUENT and benchmarked with previous literature. Both Lagrangian and Eulerian modeling approaches were attempted. For the Lagrangian approach, the fluid flow was solved and then discrete particles were introduced into the flow and tracked. This approach yielded particle paths, and by varying the particle size, collection efficiencies were obtained. The collection efficiency of a stage is defined as the percentage of particularly sized particles introduced into the flow that was collected. Each stage is designed to have a particle efficiency of 50%, although the curve is steep around this design point. In the Eulerian approach, user defined scalar transport equations were introduced into FLUENT and the scalars (particle density) were solved simultaneously with the fluid flow. Due to numerical diffusion and a poor resolution of particle fronts, this approach did not produce results that were as accurate, although it provided qualitative results. Many of the computational studies in the literature use the Lagrangian approach in order to determine the collection efficiency of an impactor stage. 852

3 Two stages of a round nozzle cascade impactor were modeled in FLUENT and benchmarked with the experimental and numerical results found in Huang and Tsai (2001). Grid independence was carefully monitored in order to ensure that the grid was properly discretized to obtain an accurate fluid flow, as seen in Figure 1. The resulting cutoff diameters changed very little between the grids with cells and cells. Each of the two stages was modeled independently with boundary conditions matching Huang and Tsai (2001). The results compared well, as shown in Table 1. Stages 1 and 2 of the cascade impactor yielded cutoff diameters (50% efficiency) of 8.9 and 6.0 µm for the current study, respectively, as compared to 9.0 and 5.5 µm for Huang and Tsai s (2001) experimental results and 9.1 and 6.1 µm for their numerical results, respectively. Figure 2 shows the particle paths and density using the Lagrangian and Eulerian approaches for stage 1. A second benchmark of FLUENT for aerosol sampling applications was completed. An impactor with rectangular nozzles was more desirable for this study due to available commercial products and high volume flow. A computational study that simulated rectangular nozzles (Hari et al., 2005) was benchmarked in FLUENT using the Lagrangian approach with equal success to the round nozzle impactor, as shown in Figure 3. Hari et al., (2005) used CFX, a CFD program supported by ANSYS. The cutoff diameter was 0.84 µm, while the current study, using FLUENT, found a cutoff diameter of 0.80 µm. The efficiency curve was steeper in the current study, which provides a more accurate analysis and a larger certainty of the particle size. The benchmarking proved useful in order to show that FLUENT could be successfully implemented for aerosol impactor applications. Collection Efficiency (percent) STAGE X 200 grids 400 X 400 grids 100 X 100 grids STAGE X 100 grids Aerodynamic Diameter (microns) Figure 1. Grid independence study to find particle cutoff diameters for two stages of a round nozzle cascade impactor modeled with the same geometry as Huang and Tsai (2001). Table 1. Comparison of particle aerodynamic cutoff diameters from current numerical study with Huang and Tsai (2001). Stage 1 Stage 2 Particle aerodynamic diameter (µm) Experimental [Huang and Tsai (2001)] Numerical [Huang and Tsai (2001)] FLUENT CFD [Current Study]

4 (a) (b) ( a ) ( b ) Figure 2. Particle paths and density for Stage 1 using (a) Lagrangian approach and (b) Eulerian approach. The particle diameter used for these figures was 9.0 µm. 100% 90% Collection Efficiency (%) 80% 70% 60% 50% 40% 30% 20% 10% 0% Particle Diameter ( µm AD) FLUENT (Current Study) CFX (Hari et. al 2004) Figure 3. Collection efficiency curve for rectangular slot impactor for current study compared to Hari et al. (2004). The geometry for the current study was modeled after Hari et al., (2004). The Stokes number characterizes the particle motion in the flow field and the Reynolds number characterizes the flow field (Hinds, 1999). The Stokes number is defined as the ratio of the relaxation time of the particle to the fluid (Crowe et al., 1998). Using the Stokes number (St) definition and the concept of a slip correction factor (C), which is a function of the mean free path of the air molecules (which conversely is a function of pressure only) (Marple and Willeke, 1976), some of the parameters in these definitions may be manipulated in order to yield the desired values for the other variables. St =! p = " CD 2 p p! f 18µ W 2 V 0 = " V CD 2 p 0 p 9µW C = D p P D p P exp(#6.66d p P) Smaller cut-off diameters may be attainable while maintaining a constant Stokes number by increasing the jet velocity in an impactor stage (V 0 ), or by increasing the Cunningham slip factor (C). The Cunningham slip factor is Eq. [1] 854

5 related to inverse pressure, so by lowering the pressure, raising the Cunningham slip factor can be achieved. Low-pressure impactors implement one or more critical stages that lower the pressure across the stage, usually by choking the flow (making the jet approach the sonic speed). This can be achieved by limiting the nozzle area while using holes or slits that are more easily manufactured with traditional tooling. An interactive electronic design tool to determine the stage parameters has been developed, based on the design methodology of Marple and Willeke (1976). If the cut-off diameter, Reynolds number, and flow rate are known, the geometry of the round or rectangular nozzles is calculated. If the geometry of the nozzles and the flow rate (or Reynolds number) is known, the cut-off diameter is calculated. This cascade impactor design tool was benchmarked with various pieces of literature and the documentation of the commercial cascade impactor that we own (Tisch Environmental, Inc. TE-230 High Volume Cascade Impactor). These comparisons and validation are found in Tables 2 5. The agreement is generally good with slight deviations in some geometry due to limited tooling dimensions as opposed to a theoretical dimensions calculated with the design tool. Table 2. Validation of aerosol impactor calculator geometry predictions with specifications for Tisch Environmental, Inc. Series 230 High Volume Cascade Impactor for Q = 20 cfm. Stage 1 Stage 2 Stage 3 Stage 4 Stage 5 Stage 6 Given Calculated Tisch Waye Tisch Waye Tisch Waye Tisch Waye Tisch Waye Tisch Waye D p (µm) Re Q (cfm) W (mm) L (cm) V (m/s) Table 3. Validation of aerosol impactor calculator geometry predictions with specifications for Tisch Environmental, Inc. Series 230 High Volume Cascade Impactor for Q = 40 cfm. Stage 1 Stage 2 Stage 3 Stage 4 Stage 5 Given Calculated Tisch Waye Tisch Waye Tisch Waye Tisch Waye Tisch Waye D p (µm) Re Q (cfm) W (mm) L (cm) V (m/s) Table 4. Validation of aerosol impactor calculator geometry predictions with specifications for round nozzle impactor stages used in Huang and Tsai (2001). Stage 1 Stage 2 Given Calculated Huang Waye Huang Waye D p (µm) Re Q (cfm) D j (cm) # jets V (m/s)

6 Table 5. Validation of aerosol impactor calculator geometry predictions with specifications for rectangular nozzle impactor stage used in Hari et al. (2004). Stage 1 Given Calculated Hari Waye D p (µm) Re Q (cfm) W (mm) L (cm) V (m/s) With this design tool, stages were designed for the specific purpose of separating particles with diameters greater than 1.0 µm, between 0.1 and 1.0 µm, and less than 0.1 µm. The specifications of the cascade are found in Table 6. The majority of radioactive aerosol particles from a surface or below ground explosion (particle diameter greater than 1.0 µm) is separated by Stage 1 and collected on a filter before Stage 2. The type of filter is critical in order to control the amount of background gamma-ray spectra. A glass fiber filter will be used since it has a lower level of background radiation when compared to the cellulose filters. Stage 5 separates the aerosol particles with radioactivity occurring from natural origins (particle diameter between 0.1 and 1.0 µm). Stage 7 is a filter that would collect all remaining particles that include aerosol particles with radioactivity from atmospheric explosions (smaller than 0.1 µm). Two critical stages, stages 3 and 4, drawn in Figures 4 and 5 create a pressure drop to 0.29 atm. Table 6. Cascade impactor specifications. Stage P inlet (atm) Q (scfm) L (cm) Re W (mm) V 0 (m/s) Cut-off diameter (µm) < 0.10 P exit (atm)

7 Figure 1. Critical Stage 1. Figure 5. Critical Stage 2. A six-stage cascade impactor was purchased that used parallel rectangular nozzles (TE-230 high volume cascade impactor with TE-5000 high volume air sampling system made by Environmental Tisch). The TE-230 impactor has a variable motor that can vary the volumetric flow rate. At 20 cfm, the cutoff diameters are 10.2, 4.2, 2.1, 1.4, 0.73, 0.41, and 0 µm. At 40 cfm, the cutoff diameters are 7.2, 3.0, 1.5, 0.95, 0.49, and 0 µm (the 6th stage is not used because the pump cannot handle the pressure drop). For the current study, this impactor setup was extensively modified. New plates were machined as mentioned above and a new pump was obtained that would accommodate the high flow rate and large pressure drop. In order to attain the relatively high flow rate of 40 scfm (1133 L/min) 857

with the low pressure of approximately 0.2 atm (absolute) after the last stage, a Rietschle Thomas Zephyr DLR60 dry compression claw compressor with a 5 HP motor was acquired.

Figure 6 shows a schematic of the new set up that regulates the flow.

8 with the low pressure of approximately 0.2 atm (absolute) after the last stage, a Rietschle Thomas Zephyr DLR60 dry compression claw compressor with a 5 HP motor was acquired. This vacuum pump has a capacity of up to 2 atm gage pressure at 42.4 cfm. With the addition of a pressure regulating kit, the pressure can be adjusted. Figure 6 shows a schematic of the new set up that regulates the flow. The cascade impactor assembly is shown in Figure 7, showing the particle cutoff diameters and the pressures at critical points through the impactor. Figure 6. Schematic of modified experimental setup of cascade impactor and aerosol generator. P = 1.0 atm D ae = 1.0 µm P = 0.55 atm P = 0.30 atm D ae = 0.1 µm D ae < 0.1 µm P = 0.19 atm Figure 7. Cascade impactor specifications. Cutoff diameters given for separation stages and absolute pressure given where pertinent. A submicrometer monodisperse aerosol generation system was procured, as shown in Figure 8 (TSI, 2005). A TSI Model 2076 constant output atomizer creates submicrometer aerosols from solutions or suspension. Compressed air expands through an orifice, forming a high-velocity jet, as shown in Figure 9a (TSI, 2005). As the liquid is drawn through the atomizing section, the large droplets impact on the wall opposite the jet and drain back. A fine spray leaves the atomizer and is dried by silica gel in an annular tube (TSI Model 3063 diffusion dryer), shown in Figure 9b (TSI, 2003). Fluorescent particles with particle sizes of 0.051, 0.10, 0.20, 0.92, 1.0, and 2.1 µm were obtained, which are suspended in solution and bracket the designed particle sizes. By releasing these particles for collection, the designed cutoff diameters are verified. 858

9 Figure 8. Diagram of submicrometer monodisperse aerosol generation system. (a) (b) ( a ) ( b ) Figure 9. (a) TSI Model 2076 constant output atomizer, (b) TSI Model 3063 diffusion dryer. CONCLUSIONS AND RECOMMENDATIONS A high-volume, low-pressure impactor was designed to physically separate radioactive aerosol particles by size in order to separate the radionuclides from natural origins from those from nuclear explosions. In order to separate ultra fine particles, down to 0.1 µm, custom impactor stages and a low-pressure flow field were used. A stronger vacuum pump and impactor stages were assembled. Fluorescent test particles of known size that bracket the desired particle cutoff diameters (0.1 and 1.0 µm) will be released near the impactor to validate the cutoff diameters of each stage. This will further demonstrate that the design tool correctly calculated the geometry for a particular particle size. In the next year The University of Texas at Austin will run the sampler in an outdoor environment and measure the activity on each of the three filters from the three size distributions. A Geiger Muller detector will be used 859

10 immediately after sample collection. This will provide a quantitative evaluation of the activity on each of the filters. This data will be compared to previous samples and correlated to the environmental conditions. System performance and reliability will also be monitored. Gamma-ray spectroscopy will be used using hyperpure germanium detectors to determine the fraction of natural radioactivity collected on the second filter in comparison with the first and third filters. Thoron and radon progeny daughter products from aerosols with radioactivity from natural origins will also be identified. Detection limits will be calculated and the sampling methodology will be optimized. Variations of collection and run times will be implemented to minimize the background interference from naturally occurring radionuclides. The flow rates and geometry of the impactor stages may also be adjusted according to the results of the environmental testing to best match the particle size distribution of the collected radionuclide aerosol particles. Testing will also be conducted in parallel with an Anderson 5-stage impactor. The samples for the two impactors will be compared using gravimetric analysis, gamma-ray spectroscopy, Geiger Muller measurements, and neutron activation analysis to determine elemental content on the filters. REFERENCES Bondietti, E. A., C. Papastefanou, and C. Rangarajan (1987). Aerodynamic size associations of natural radioactivity with ambient aerosols, in Radon and Its Decay Products Occurrence, Properties and Health Effects, P. K. Hopke (Ed.), ASC-Symposium Series 3131, pp Crowe, C., M. Sommerfield, and Y. Tsuji (1998). Multiphase Flows with Droplets and Particles. Boca Raton, Florida: CRC Press, pp De Geer, L.-E., R. Arntsing, I. Vintersved, J. Sisefsky, S. Jakobsson, and J.-A. Engstrom (1978). Particulate radioactivity, mainly from nuclear explosions, in air and precipitation in Sweden mid-year 1975 to mid-year 1977, FOA Report, FOA-C FLUENT 6.1, Fluent USA Inc., Lebanon, NH, USA. Hari, S., Y. A. Hassan, and A. R. McFarland (2005). Computational fluid dynamics simulation of a rectangular slit real impactor s performance, Nucl. Eng. Des. 235: Hinds, W. C. (1999). Aerosol Technology: Properties, Behavior, and Measurement of Airborne Particles. New York: Wiley, pp Huang, C. H. and C. J. Tsai (2001). Effect of gravity on particle collection efficiency of inertial impactors, J. Aerosol Sci. 32: Grundela, M., J. Porstend (2004). Differences between the activity size distributions of the different natural radionuclide aerosols in outdoor air, Atmos. Environ. 38: Marple, V. A. and K. Willeke (1976). Impactor design, Atmos. Environ. 10: Storebø, P. B. (1974). Formation of radioactivity size distributions in nuclear bomb debris, Aerosol Sci. 5: TSI Model 3062 Diffusion Dryer Instruction Manual, P/N , Revision G (2003). TSI Model 3076 Constant Output Atomizer Instruction Manual, P/N , Revision J (2005). Vanderpool, R. W., D. A. Lundgren, and P.E. Kerch (1990). Design and Calibration of an in-stack low-pressure impactor, Aerosol Sci. Technol. 12: Willeke, K. and P. A. Baron (1993). Aerosol Measurement: Principles Techniques and Applications. New York: Van Nostrand Reinhold. 860

29th Monitoring Research Review: Ground-Based Nuclear Explosion Monitoring Technologies

29th Monitoring Research Review: Ground-Based Nuclear Explosion Monitoring Technologies TESTING OF AEROSOL SAMPLER TO REMOVE RADON AND THORON PROGENY INTERFERENCE FROM AEROSOL SAMPLES FOR NUCLEAR EXPLOSION MONITORING Steven R. Biegalski 1, Christopher J. Weaver 1, Scot Waye 1, Ofodike A.