Comparative Dosimetry of Radon in Mines and Homes (1991) / Chapter Skim
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Aerosols in Homes and Mines
Aerosols in Homes and Mines
Pages 90-136
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From page 90... ...
Thus, to understand the experimental methods used for activity-weighted size distribution and/or unattached fraction measurements, it is necessary to present the theoretical considerations upon which the measurement instruments are based. In the next sections, the relationship between size and diffusivity will be presented along with the theory of diffusive deposition of particles on the inside surfaces of cylinders and on strands of wire mesh screen.
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From page 91... ...
. In order to use a single equation for the particle diffusion coefficient over the entire size range for dp ~ 0.5 nm, the Einstein-Cunningham Equations 6-1 and 6-2 may be fitted to the kinetic theory Equation 6-3 in the 0.5-1.75-nm size range in a manner that yields the original Einstein-Cunningham Equations 6-1 and 6-2 for dp > 1.75 nm.
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From page 92... ...
Of particular interest when sampling highly diffusive ultrafine cluster aerosols are tube penetration equations for the prediction of diffusional deposition or wall losses in the entrances of sampling tubes. Typically, the "wall loss" lengths L in these situations are smaller than the tube length required for the development of a laminar flow profile ("entrance lengthy.
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From page 93... ...
attempted to verify the laminar flow tube penetration theory equations using gas molecules and reported agreement to within 20-30%. The discrepancies were attributed to experimental difficulties and possible entrance effects.
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From page 94... ...
have further verified the fan model for particle sizes dp ~ 4 nm. Recent work with charged and uncharged Gyro clusters in the 0.5-1.5-nm size range has also indicated general agreement with the wire screen fan model penetration Equation 6-7 (Holub and Knutson, 1987; Ramamurthi et al., in press)
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From page 95... ...
The diffusion coefficients of the radon decay products are assumed to have a single, very much larger value than the diffusion coefficients of the condensation nuclei to which the radon progeny become attached. Thus, by comparing the amount of activity penetrating through a tube of given dimensions at a given flow rate with the total airborne decay product activity, the fraction of unattached activities could be estimated.
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From page 96... ...
Wire Screen Samplers The diffusion samplers were fairly cumbersome devices to use, and therefore a simpler and more portable system was developed based on the collection of the activity on wire mesh screens. Wire mesh screens have become the most commonly used method for estimating unattached radon daughter fractions.
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From page 97... ...
Wire screens with collection efficiencies that differ only slightly are plotted together for comparison purposes. The collection efficiencies of the screens are between 70 and 90% and 6 and 12%, respectively, for 1- and 10-nm-diameter particles and decrease rapidly in the 1- to 3-nary size range.
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From page 98... ...
In this case, devices with sharp cutoff points such as cyclones or impactors can be designed (Lodge and Chan, 19861. However, because of the stochastic nature of diffusional deposition, the collection curves for wire screens or tubes are much more gradual functions of particle size.
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From page 99... ...
It can be seen that the use of wire screens underestimates the unattached fraction if it indeed consists of an ultrafine cluster mode in the 0.5- to 3-nary size range (Reineking and Porstendorfer, 19861. The cumulative collection of activity as a function of size is presented in Figure 6-6.
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From page 100... ...
(1982) for the collection of attached activity in this size range by wire screens sampling the unattached mode.
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From page 101... ...
However, they defined their fraction as the ratio of unattached radon daughter alpha-energy concentration to the total radon decay product alpha-energy concentration. They concluded that there is so much variability from location to location and mine to mine that it is not possible to select any particular value of unattached activity as being representative of actual mine conditions.
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From page 102... ...
Wire Screen Measurements Raghavayya and Jones (1974) made measurements of radon, decay product concentrations, unattached fractions of each decay product, and condensation nuclei concentration in three mines in Colorado and New Mexico.
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From page 103... ...
1 E6 FIGURE 6-9 "Unattached" fraction of 218po as defined relative to total 218po measured in New Mexico uranium mines as a function of particle concentration. Data for this plot were taken from George and Hinchliffe (1972)
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From page 104... ...
They counted the wire screens and had difficulties getting the total activity measured with a separate filter to match the sum of the activity on the screen plus that on the backup filter. They developed an empirical correction factor to obtain the values they present as the fraction of activity for each decay product that is unattached and activity for a total unattached fraction relative to the activity of radon.
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From page 105... ...
Bigu and Kirk (1980) measured unattached fractions in two Canadian mines using both a diffusion sampler and a wire screen system.
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From page 106... ...
Subsequently, the same group made more extensive measurements in two mines using a wire screen sampler (Khan et al., 1987~. Although that report followed publication of the detailed theory of wire screen collection in the early 1980s, this group continued to use the collection efficiency curve of Thomas and Hinchliffe (1972~.
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From page 107... ...
Summary of Mining Exposure From the limited number of measurements of activity-size distributions made in active mines, it is necessary to estimate the typical exposure conditions in the mine. In this review, an attempt has been made to estimate the conditions both in the areas where active drilling, slushing, and other activities could produce substantial airborne particle concentrations, in the haulage drifts where the material was being transported from the active mine sites to the sites where it could be conveyed to the surface, and in areas where nonmining activities were being conducted (workshops, lunchrooms, etc.~.
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From page 108... ...
. 0 10 20 60 70 FIGURE 6-13 Distributions of the "unattached" fractions of the three radon decay products and the potential alpha-energy concentration (PAEC)
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From page 109... ...
INDOOR ATMOSPHERES Diffusion Sampler Measurements Duggan and Howell (1969) first attempted to use electrostatic collection of the decay products to determine the unattached fractions.
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From page 110... ...
of 4 rim for "unattached" fraction measurements in indoor air. In addition aerosol size distributions were made with an automated aerosol spectrometer system (Raes et al., 19841.
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(1988~. The results of the intercomparison of radon and radon decay product concentrations are presented in Figure 6-18.
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From page 112... ...
112 Ct be .= Cal au O Ct ED ~ ^ Ct = ~ _4 At.
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From page 113... ...
_ CJ U] FIGURE 6-17 Equilibrium factor and the "unattached" fraction of potential alpha-energy concentration as a function of the estimated attachment rate based on measured aerosol size distributions.
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12 16 29-JUL-87 30-JUL-87 TIME (h) FIGURE 6-18 Radon and radon decay product concentrations measured during the GhentGottingen intercomparison study.
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From page 115... ...
The instrument was then deployed in a detached, two-story concrete house used in the typical Japanese life-style. Figure 6-20 provides the diurnal variations of the unattached fractions of each of the three decay products and the aerosol concentration.
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From page 116... ...
the unattached fraction is, in reality, an ultrafine cluster mode in the 0.5- to 3-nary size range, (2) the collection efficiency versus particle diameter characteristics for wire screens do not allow a distinct separation of the unattached and attached fractions, and (3)
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From page 117... ...
Because unattached fraction measurements are inexact representations of the particle size information needed to utilize the dosimetric model developed and presented elsewhere in this report, the next section will review the more recent measurements that have been developed and employed to characterize the indoor radioactive aerosol. ACTIVITY-WEIGHTED SIZE DISTRIBUTIONS Recently, methods have been developed by which the entire radioactive aerosol size distribution can be deduced from data regarding the collection of activity on or its penetration through a series of screens.
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From page 118... ...
Tu and Knutson (1988a,b) used 25-liter/min screen diffusion batteries to measure the 2~8Po-weighted size distributions in the presence of several specific aerosol sources.
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From page 119... ...
, have also developed these graded screen diffusion batteries for activity-size distribution measurements. An intercomparison between these three groups (EML, NRPB, and ARL)
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From page 120... ...
developed a system with six sampling heads containing 0, 1, 3, 7, 18, and 45 stainless steel, 400-mesh wire screens. He measured the size distributions in several rooms in two houses at two times of the year.
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From page 121... ...
at 1 nm, a nuclei mode at 10 nm, and an accumulation mode at 100 to 130 nm. In Table 6-6, the unattached fractions presented are the integrated values from the size distributions.
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From page 122... ...
FIGURE 6-24 218 Po-weighted size distributions measured in house II after a kerosene heater was operated for: 1. 0 mini 2.
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From page 123... ...
123 FIGURE 6-25 Activity-size distributions measured in a rural house kitchen by Strong (1988)
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From page 124... ...
Radon concentrations varied between 5 and 500 psi/liter during the 7-day period with the time period for fluctuations being much longer than the 15-min sampling interval. A total of 15 measurements were made during the 7-day period, with a remarkable degree of consistency in the shape of the measured activity-size distributions.
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From page 125... ...
FIGURE 6-27 Typical Po-218, Pb-214, and Bi-214 activity-size distributions observed in a house basement in Princeton, N.J. smallest inferred size interval, with a midpoint diameter of ~0.9 nm (diffusion coefficient, D ~ 0.04 cm2/s)
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From page 126... ...
The differences in the size distributions obtained in the basement and in the kitchen area related primarily to the 1.6- to 5.0-nm size interval, with the attached activity modes remaining in the 160- to 500-nm size range. This result suggests the presence of condensable constituents leading to the formation of particles in the 1.6- to 5.0-nm size interval or a source of very fine primary particles.
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From page 127... ...
The size distributions obtained in the experiments were found to be stable and relatively insensitive to perturbations in the input data of the order of the associated measurement errors. The errors in the size interval fractions estimated from this procedure are too small to be seen in these figures and thus were not included.
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From page 128... ...
on the radon progeny size distributions in a closed bedroom are shown in Figure 6-30. The measurements were made 5 min after lighting the cigarette (5-20 min)
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From page 131... ...
of typo was observed in the attached mode and peaked at the 50- to 500-nm size range, with only small changes (from 40 to 50%) in 2~4Pb and 2~4Bi distributions.
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From page 132... ...
1981. Measurement of the total and radioactive aerosol size distributions in a Canadian uranium mine.
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From page 133... ...
1988. Tests of the graded wire screen technique for measuring the amount and size distribution of unattached radon progeny.
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From page 134... ...
1988. Intercomparison of three diffusion batteries for the measurement of radon decay product particle size distributions.
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From page 135... ...
1986. High-volume screen diffusion batteries and a-spectroscopy for measurement of the radon daughter activity size distributions in the environment.
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From page 136... ...
1982. Evaluation of various types of wire screens as diffusion battery cells.
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