. ORNLP 1313 FFEFEEEE 11.4 11.6 MICROCOPY RESOLUTION TEST CHART NATIONAL BUREAU OF STAMOARDS -1963 . . . . . . V * WWW HE . LEGAL NOTICE This report was prepared as an account of Government sponsored work. Neither the United States, nor the Commission, nor any person acting on behalf of the Commission: A. Makes any warranty or representa- tions, expressed or implied, with respect to the accuracy, completeness, or usefulness of the information contained in this report, or that the use of any information, appa- ratus, method, or process disclosed in this report may not infringe privately owned rights; or B. Assumes any liabilities with respect to the use of, or for damages resulting from the use of any information, apparatus, method, or process disclosed in this report. As used in the above, "person acting on behalf of the Commission" includes any em- ployee or contractor of the Commission, or ernployee of such contractor, to the extent that such employee or contractor of the Commission, or employee of such contractor prepares, disseminates, or provides access to, any information pursuant to his employ- ment or contract with the Commission, or his employment with such contractor. w . . - - - i . . . . oron .P-13/3 февьяг انا لے ۔ ملوک Paper presented at International Symposium on Fission Product Release and Transport under Accident Conditions, Oak Ridge, Tennessee, April 5-7, 1965 la sonnigen dan Gambary *g JJTINTO ABSTRACT Me w wow boorlater h wted the cosumington, or Alemployment with much contractor. stogne of contractor of the Contestom, or on o sborn, pero wa mployee or autructor of the Couston, or a ploy Gerechten, a prop de wood be, wag totormation purnus to We emplogut or contract traballar , of producted a po roporto player of much coat actor, to the action that al mah soalructor properes, coasteddon" dachudw ay • So, w metry printly own e thay, completeness, or formation, paratum, modern, or proov Coloston, nor w other para Kctt A. Makes may warranty or representation, aprued or lleolied, with repect to the accu. a bella detaloration contained ba the report, or that the ws declaced to date raport may not loforlage e Corandados: ng bong Sunroos selurup 20g ao 'yo on an au ssedans AEROSOL PHYSICS OF RADIOACTIVE PARTICLES* -LEGAL NOTICE рма а защих моод цоороо арваше B. R. Fish and R. L. Walker Health Physics Division Oak Ridge National Laboratory The extent to which the behavior of radioactive aerosols differs from that [ of nonradioactive aerosols has not been clearly established. Such information is . potentially of importance with respect to transport, deposition, resuspension, and filtration of fission product aerosols released in a reactor accident. Theoretically, electrostatic charging of the particles and of ungrounded surfaces, ionization of the gaseous environment, and surface effects, such as desorption of gases, can have a direct bearing on the properties of radioactive particles. Studies of the behavior of radioactive aerosols produced by exploding wires suggest that properties which depend on the presence of asymmetry in the distribution of electrostatic charge are modified by the enhanced conductivity of the air. Thus, in a low radiation environment, aerosols bearing a unipolar charge are deposited more rapidly in a container, because of space charge effects, than are radioactive aerosols of conductive particles which tend to lose an asymmetry of charge at a faster rate. In principle, nonconductive radioactive particles can accumulate a net unipolar charge which can increase their deposition rate; however, both high specific activity and extremely low conductivity are required to produce a significant effect. Concurrent studies on the adhesion of ORNL - AEC - OFFICIAL "Research sponsored by the U. S. Atomic Enerxtereggission under contract with Union Carbide Corporation. THE PUBLIC IS APPROVED. PROCEDURES ARE ON EILE IN THE RECEIVING SECTION, ORNI - AEC - OFFICIAL - 2 - radioactive particles to other particles or to surfaces indicate that, depending upon conductivity and upon the radiation properties of the materials involved, the effect of unipolar charge is to decrease adhesion between radioactive nonconductor particles but to increase deposition and adhesion on irradiated dielectric surfaces. ****** ORNI - AEC - OFFICIAL ORNL - AEC - OFFICIAL AEROSOL PHYSICS OF RADIOACTIVE PARTICLES* ORNL - AIC - OFFICIAL B. R. Fish and R. L. Walker Health Physics Division Oak Ridge National Laboratory Oak Ridge, Tennessee INTRODUCTION There is a considerable body of literature on the physical behavior of aerosols. The recent translation of Fuchs!!book is an excellent compilation of some of the best available information on aerosol mechanics. Unfortunately, however, there are very few articles in the literature that relate to possible differences in behavior between radioactive and nonradioactive aerosols. There are indications of enhanced penetration of filters by radioactive particles (Thomas and Yoder(2,3); Gillespie (4, and one study that yielded significant changes in the coagulation rate as a function of radioactivity of the particles (Rosinski, Werle, and Nagamoto"). - - In order to understand the transport, deposition, and resuspension of radioactive particles, we have initiated a series of studies on the effects of interaction of radiation with particles, with surfaces, and with their gaseous environment. RADIATION POLARIZATION OF DIELECTRICS Deposition of dust on electrically charged surfaces is a well-known phenomena (White); Lowel) which is applied in various sampling and air cleaning devices. Dust collection on environmental surfaces also has been ORNI - AEC - OFFICIAL Research sponsored by the U. S. Atomic Energy Commission under contract with Union Carbide Corporation. ORNL - AEC - OFFICIAL 1 - 2 - OANI - ALC - OFFICIAL observed as a result of adventitious electrical fields (Woodlandlo)). Thus, because an enhanced rate of surface deposition due to local accumulations of electrostatic charge can directly affect the concentration of airborne particles, it is of interest to explore possible charging mechanisms. The buildup of electrostatic charge in dielectrics has been observed after irradiation with monoenergetic electrons (Gross") with beta particles from an isotope (Murphy and Ribeiro (10), and with gamma rays (Murphy and Gross 11). These studies show that a definite polarization is induced in a dielectric as a result of charge separation by trapping forward scattered electrons and by the accumulation of a net excess of electrons following electron or beta-ray irradiation. The production of positive Lichtenberg figures in electron-irradiated materials by point discharge is a well-known phenomena and offers simple and direct evidence for the existence of a negative space charge trapped within the dielectric (Gross(9), An illustration of the influence of radiation polarized surfaces on the hierdinen an. local deposition of airborne particles is given in Fig. 1. Specimens of plexiglass and of carnauba wax (3 1/4" diameter x 3/8" thick) were irradiated with monoenergetic electrons and then exposed to an airborne suspension of small particles. The irradiations were performed with a Van de Graaf generator using 1 MeV (6. 1x 1011 electrons/cm²; range 3 mm) and 1.9 MeV electrons (2.6 x 102 electrons/cm”; range 7.5 mm). An aluminum plate having a square hole ORNI - Air-OFFICIAL all the way through and a circular hole part way through was used to confine ORNI - AEC - OFFICIAL A.'.: . ir...... - .... Mw.asso...?....no obis .:::::: :: : ... .. - odin 5' 1 . i . . . A nastatiniais nustatomas de PLEXIGLASS IMeV cchi itse du CARNAUBA WAX 1.9 MeV (FRONT) "",name: wap ***.groups soos DAVOS. Y mixedmés... . ... PLEXIGLASS - 1.9 MeV CARNAUBA WAX 1.9 Lime I: POWDER PATTERNS OF ELECTRON IRRADIATED DIELECTRICS ORNI - AEC - OFFICIAL ORNI - AEC - OFFICIAL - 4 - inte ORNI - AEC - OFFICIAL on the reclamation the electron beam to a known portion of the dielectric surface. After irradiation , the specimens were set on edge and exposed to an airborne cloud of particles in such a way that the irradiated surfaces were vertical. Thus the observed deposition patterns relate to diffusion of particles to a wall. There is good qualitative agreement between the deposition and the irradiation patterns. The effective surface charge density was measured with a capacitor divider netwo: K and an electrometer. In this apparatus the surface is treated as one plate of a fixed-geometry air capacitor which is in series with a known capacitor ---- connected to ground. The charge induced on the known capacitor is directly -- related to the effective charge density on the test surface. Observations of : .. effective surface charge, Fig. 2, indicate an approximately exponential decay . with a half-life on the order of 9 days. It is seen (Plexiglass, Fig. 2) that the apparent surface charge is lost somewhat more rapidly from the front (negative) than from the back (positive) of the test specimens. This suggests that the space charge cloud (positive) controlling the front surface charge may be nearer the surface, hence lost faster by migration to the surface, than the .. space charge (negative) affecting the back surface. The observations are in qual itative agreement with the proposed mechanisms diagrammed in Fig. 3. In addition to the trapping of a net excess of electrons, the penetration and absorption of monoenergetic electrons in a dielectric may be expected to produce a charge separation in the absorber, resulting in an electron ORNI - AEC - OFFICIAL ORNI - AEC - OFFICIAL 5. ORNL-DWG 65-5277 ORNI - AEC - OFFICIAL Digure 2 DECAY OF CHARGE IN ELECTRON IRRADIATED · DIELECTRICS Soos ELECTRON CHARGES PER SQUARE CENTIMETER PLEXIGLASS (BACK) PLEXIGLASS I MeV (FRONT) Q LPLEXIGLASS 1.9 Mev CARNAUBA WAX IMeV CARNAUBA WAX 1.9 MeV . d .lmän to ORNI - AEC - OFFICIAL 20 30 . TIME SINCE IRRADIATION (DAYS) IYO ORNL-DWG 65-5276 MONOENERGETIC ELECTRON B-CONTINUUM OR (EQUILIBRIUM y j* · OR (NON-EQUILIBRIUM y I^. OOOOOOOOOO MAX O0000000000000000 RANGE 20_0_0_000_0 фии 3: PROPOSED MECHANISM FOR RADIATION POLAR- IZATION OF DIELECTRICS *(SPACE CHARGE MORE DIFFUSE – NO MAX. RANGE) ORNI - AEC - OFFICIAL ORNI - AEC - OFFICIAL ------ . .. sonder en omgevuld me.. tikrinimo .initiation • 7. ORNI - ABC-OFFICIAL depleted region near the entrance face. The effect of these mechanisms is to produce a positive space charge region near the front surface and a deeper region of negative space charge. Under the influence of the relatively fixed space charge clouds, charged air ions are selectively attracted to the surface, forming a more mobile charge layer. Because of the high concentration of ions in normal air (50-500 ion pairs per cm3,114) and their high mobility (~2 cni/sec per volt/cm), the lifetime of a given surface charge carrier may be short; however, an equilibrium charge is maintained on the surface depending upon the polarity, the magnitude and the geometry of the nearest space charge in the dielectric In earlier work, radioactive glass beads were deposited on a quartz surface, and the resulting effective surface charge was positive. This case, the Be-continuum, also is prosented in Fig. 3. As a result of the attenuation of the low-energy components of the B-spectrum, there may be no net accumulation of a depleted layer; instead, there may be a residual negative charge extending from the entrance face to the max imum range of the highest energy electron. . Under reactor accident conditions, dielectric surfaces may be expected to become polarized in such a way that the effective surface charge is positive. When the total radiation dose to local materials is of the order of 104 RAD, significant polarization may be observed in dielectrics. 10131310- )?V - IN8O. - 8- . SELF-CHARGING OF RADIOACTIVE SOURCES There is a not loss of electrons from BⓇ-radioactive sourcos, and, depending upon the size and shape of the source and its conductivity, some of the excess positive charge may be retained for significant periods of time. Compton and photoelectric processes of y-rays can also yield a net loss of electrons from the source. However, it should be noted that the smaller the particle, the more rapidly the charge is leaked off, hence the smaller the fraction of emitted charge that is retained as an effective equilibrium charge. The simple apparatus shown in Fig. 4 is adequate to demonstrate the buildup of an equilibrium charge on a dielectric ß-source. For dielectric sources that are much smaller in size than the maximum range of the Bo-particles in the source material, the net positive charge within the source will result in the accumulation of an effective negative charge on the surface of the source. Thus, externally, the particles will behave as if they were negatively charged. The result, unipolar charging of a dielectric particle, may be expected to produce an increased rate of diffusion to environmental surfaces. If the surface on which deposition occurs is either a good conductor (image charge attraction) or a radiation polarized dielectric (Bay continuum, Fig. 3), the force of adhesion between the particle and the surface should be enhanced. ORNI - AEC - OFFICIAL ORNI - AEC - OFFICIAL ORNI - AC - OFFICIAL . e ri. a sosem ., - diriny - 3 figure 4. Apparatus Used to monitor charge Buildup ORNL-DWG 65-5275 in a Radioactive Source wo ELECTROMETER GOLD PLATED PLEXIGLASS CONTAINING P-32 SOURCE t + + VACUUM OR ATMOSPHERE SOURCE CHAMBER ORNI - AEC - OFFICIAL - 10 - AIRBORNE RADIOACTIVE PARTICLES In contrast with nonconductive particles, particles with relatively high conductivity cannot maintain a significant unipolar charge for an appreciable length of time. For an aerosol composed of highly charged conductive particles, the time to reach charge equilibrium (related to the Boltzmann distribution) is inversely proportional to the average number of charges per particle and to the number of ion pairs per unit volume of air." Clearly, there will be no buildup of a net excess charge on a radioactive, conductive particle; furthermore, if a charge is induced, in the process of forming the aerosol, the more radioactive . the particle, the more rapidly will the charge bor iost. The decrease with time of the airborno concentration of radioactive and nonradioactive particles is studied in the aerosol chamber shown schematically in Fig. 5. For the work reported here, the aerosol was produced by electrically exploding a silver wire to produce spherical, metallic particles in the size range 0.01 to 0.1 microns diameter."") Silver aerosol is diluted and stored in a polyethylene bag chamber and introduced into the test chamber through a diffusion ionizer of a design suggested by Whitby.""? In the test chamber the particles can be mixed with radioactive iodine, and the particle concentration can be followed by several different sampling techniques. ORNL - AEC - OFFICIAL ORNI - AEC - OFFICIAL ORNI - ACC - OFFICIAL - - - Us ORNL-DWG 65-5278 FILTER CLEAN FILTERED AIR CLEAN FILTERED AIR ONIZER-MIXING CHAMBER FAN - TUBE 1 SUPPLY 1 18 O Ing in. OD CU TUBE AG PLATED INSIDE EXPLODING WIRE AEROSOL GENERATOR COLLIMATED Y-DETECTOR FLOW RATE METER VACUUM PUMP - EXHAUST TO HOOO FILTER ASSEMBLY 21 SAMPLING PORTS, 5 OPENINGS EACH WEEDLE VALVE POLYETHYLENE BAG CIIini PUMP COMPRESSED AIR DEPOSITION SAMPLER You AEROSOL STORAGE FIBERBOARD-220 liters GROUNDED CABLE FILTERED EXHAUST TO HOOD AEROSOL CHAMBER ALUMINUM - 820 liters digure 5. Diagram of Aerosol chanter and sampling Device ORNI – AEC - OFFICIAL ORNI - AC - OFFICIAL - 12 - Theoretically, the number concentration of airborne particles in the chamber can decrease by any one of four major mechanisms. First, particles may agglomerate, thus reducing the number concentration but not affecting the mass concentration in suspension. Some of the agglomerates may grow to sufficient size to settle under the action of gravity. Individual particles can diffuse to interior surfaces under the action of convective currents and thermal diffusion. Finally, if the aerosol is made to bear an unipolar charge, the rate of diffusion to the interior surfaces will be enhanced by space charge repulsion. In the present study, the number concentration, which ranged from 3 x 103 to 1.8 x 105 particles/cm®, was measured with a Nolan-type condensation nuclei counter. The number concentration data expressed in percent of the initial concentration is presented in Fig. 6. Two runs were made in which the ionizer was operated so as to produce a local high concen- tration of positive and negative air ions, thus promoting the approach to charge equilibrium. The addition of 1 mc of iodine-130 made no obvious change in the loss of particle concentration from that observed for the particles without radioactivity. In contrast, when the particles were deliberately given a net excess of positive charge, the addition of radioactive iodine made a definite change in the rate of loss of particles compared with the nonradioactive runs. - . - - ORNL-OWG 65-5279 6041 50 404 EFFECT OF RADIOACTIVITY AND ELECTROSTATIC CHARGE ON NUMBER CONCENTRATION OF AIRBORNE PARTICLES* *( SILVER-EXPLODING WIRE ) 30 POS. CHARGE + Imc Boy POS. CHARGE + O.Imc "4 EQUIL. CHARGE + Imc "So I EQUIL. CHARGE (NON-RAD.) POS. CHARGE (NON – RAD) PERCENT OF INITIAL CONCENTRATION Lan POS. CHARGE (NON - RAD) brine Hours TIME (HOURS)* - 14 - The observations are consistent with the view that, for conductive particles and for radioactivity levels of 0. 1 to 1 mc/meters, the major difference in behavior is that radioactive particles cannot retain a significant asymmetry of charge for an appreciable length of time, whereas nonradioactive particles can. Thus, transport, deposition, and filtration testing with the use of nonradioactive tracers is an acceptable substitution for radioactive, conductive aerosols, but care should be taken to insure that the tracer is approximately in charge equilibrium. SUMMARY It is seen that Buy irradiation of a dielectric surface can result in an effective positive charge on the surface. This can lead to enhanced particle deposition and retention provided that either the particle be conductive and of a relatively low charge (by induction), or, if a nonconductor, the effective particle charge is negative. Both of these conditions are observed to occur in the case of radioactive particles. Thus the influence of an irradiated dielectric surface is to increase deposition and adhesion of Bony radioactive particles regardless of their electrical conductivity (qualitatively). On the other hand, the deposition rate of nonconductor particles may be expected to be enhanced by the accumulation of a unipolar charge, while conductive particles will not be affected by this mechanism. - 15 - REFERENCES 1. N. A. Fuchs, The Mechanics of Aerosols (Pergamon, 1964). 2. J. Thomas and R. Yoder, "Aerosol Size for Maximum Penetration Through Fiberglas and Sand Filters, “ AMA Arch. Indust. Health 13, 545 (1956). 3. J. Thomas and R. Yoder, "Aerosol Penetration Through a Lead-Shot Column," AMA Arch. Indust. Health 13, 550 (1956). 4. T. Gillespie, "The Role of Electric Forces in the Filtration of Aerosols by Fiber Filters, “ J. Coll. Sci. 10. 299 (1955). 5. J. Rosinski, D. Werle, and C. T. Nagamoto, "Coagulation and Scavenging of Radioactive Aerosols, " J. Coll. Sci. 17, 703 (1962). 6. H. J. White, Industrial Electrostatic Precipitation (Addison-Wesley Publishing Co., 1962). 7. H. J. Lowe and D. H. Lucas, "The Physics of Electrostatic Precipitation, Brit. J. Applied Phys. 4, 5-40 (1953). 8. P. C. Woodland and E. E. Ziegler, Static Dust Collection of Plastics, Dow Chemical Co., Plastics Technical Service, Midland, Michigan (3rd Ed., 1956). 9. B. Gross, "Irradiation Effects in Plexiglass, " J. Polymer Sci. 27, 135 (1958). 10. P. V. Murphy and S. C. Ribeiro, "Polarization of Dielectrics by Nuclear Radiation. I. Release of Space Charge in Electron Irradiated Dielectries," J. Appl. Phys. 34, 2061 (1963). - 16 - 11. P. V. Murphy and B. Gross, "Polarization of Dielectrics by Nuclear Radiation. II. Gamma-Ray-Induced Polarization, " J. Appl. Phys. 35, 171 (1964). 12. K. T. Whitby, Personal Communication (1965). 13. F. G. Karioris, B. R. Fish, and G. W. Royster, Jr., "Aerosols from Exploding Wires," p. 299 in Exploding Wires, Vol. 2 (Plonum Press, 1962). 14. K. T. Whitby, "Generator for producing High Concentrations of Small lons," Rev. Sci. Instr. 32, 1351 (1961). END DATE FILMED 18 / 130/65 1. .- -- -.. -..