NUCLEAR REACTOR SAFETY AEROSOLS

The deposition rate constants of the different unattached decay products of radon (²²²Rn) are evaluated from the measured radon and decay product concentrations in a 1-m³ chamber as a function of the degree of turbulence. The turbulence is induced by ventilation and/or generating heat. The coefficient of eddy diffusivity, k_e, in the theoretical formula of Crump and Seinfeld for calculating the deposition rate is examined by fitting the Crump and Seinfeld formula to our experimental results. The expression for k_e thus obtained is proportional to λ³ _v (ventilation) and W ^3/2 (generated heat). The deposition rate constant of ²¹⁸Po is found to be about three times that of ²¹⁴Pb, which means that the associated diameter of the ²¹⁴Pb particle is about twice as large as the diameter of the ²¹⁸Po particle. This difference could be due to the physical and chemical properties of the two elements     

Counting Efficiencies for Alpha Particles Emitted from Wire Screens

Counting efficiencies for alpha particles emitted from the front and the back of 30-, 105-, 200-, and 400-mesh wire screens were measured for ultrafine radon daughter aerosols deposited at face velocities in the range 5.1 to 30.8 cm s^?1. Mean activity median diameters for the ultrafine ²¹⁸Po, ²¹⁴Pb, and ²¹⁴Bi particles were 0.70 ± 0.16, 1.1 ± 0.3, and 1.0 ± 0.2 nm (0.062, 0.033, and 0.038 cm² s^?1), respectively, as determined from graded wire screen array analysis of the test atmosphere. For wire screen collection efficiencies < 0.8, the “front-to-total” (FT) ratio, denned as the ratio of measured alpha activity from the front of the screen to the total alpha activity (front and back), was found to be insensitive to the screen and sampling parameters, with a mean value of 0.67 ± 0.02. With increasing collection efficiency, the FT ratio was found to increase, up to a maximum value of 0.86 ± 0.03 for collection efficiencies > 0.999. Alpha-particle losses within the screens (screen loss factors) were determined by comparison with counting efficiencies for radon daughters deposited onto membrane filters. For the four screen types studied, the mean screen loss factor at a face velocity of 21.2 cm s^?1 was 1.04 ± 0.01. A Monte Carlo simulation of alpha-particle losses within a simple woven wire screen showed that the FT ratios were sensitive to the functional form of the deposition of the radioactive aerosol around the wire cylinders of each screen. Screen loss factors derived from the Monte Carlo analysis were found to be insensitive to the deposition on the wire, but dependent upon the counting geometry, in particular the distance between the wire screen and the detector.     

An annular diffusion channel equipped with a track detector film for long-term measurements of activity concentration and size distribution of nanometer 218Po particles

A new method is presented to estimate mean activity concentration and mean size distribution of nanometer ²¹⁸Po over long time, i.e. several weeks in indoor atmospheres. It uses an annular channel, equipped with an LR 115 solid-state alpha-track detector, as diffusion sampler. Design, experimental characterisation and analytical methods are described. The calibration was carried out in a 0.35 m³ test radon chamber without aerosol particles by comparing the number of tracks registered on the film detector with the nanometer ²¹⁸Po particle concentrations measured in parallel with a classical single screen-filter method. A calibration factor of 0.107 tracks cm^-2 h^-1 was established. The size distribution of these nanometer radioactive particles in the range of 0.5–5 nm was reconstructed from the track density recorded on the detector film with non-linear inversion techniques.     

A Method to Measure Static Charge on a Filter Used for Gravimetric Analysis

Static charge present on a filter contributes substantial error to low-level mass measurements. The measurement and sources of static charge are not well understood and this article presents a fundamental method of static charge measurement. As predicted by other researchers, triboelectric charging was found to generate significant static charge on a filter when using 2007 Diesel particulate matter (PM) measurement protocols. Measurements made using our method indicate that PM filters are rapidly and effectively neutralized by ²¹⁰ Polonuim ( ²¹⁰ Po) sources. The neutralization of charge occurred exponentially with characteristic time constants (the time it takes for the charge on the filter to decay by 63%) of 0.4 and 0.7 s, depending on the strength of the source. The experimental neutralization time constants were consistent with theory. The charge remaining on highly charged 47 mm Teflon and TX40 filters after 5 s of neutralization with year old ²¹⁰ Po sources biased the true filter mass by <1 μg when the filters were weighed with a 1 μg resolution Cahn microbalance.     

Deposition of Ultrafine Particles in Human Tracheobronchial Airways of Adults and Children

Inhalation exposure to ultrafine particles, including radon progeny and other combustion aerosols, has been implicated in potential health risks of ambient and indoor environments. These particles deposit in the respiratory tract mainly by diffusion. The purpose of this study was to determine the deposition pattern of nanometer-sized particles in the human tracheobronchial (TB) airways of children and young adults. The deposition was determined for 1.75, 10, and 40 nm ²¹²Pb particles at flow rates corresponding to respiratory minute volumes at rest and during moderate exercise. The 1.75 nm particles were unattached clusters, whereas the 10 and 40 nm particles were silver particles with attached ²¹²Pb clusters. Replicate casts of the upper TB airways of 3, 16, and 23 year old humans were used, including the larynx, trachea, and bronchial airways down to generations 5-8. Deposition in each generation and total deposition were measured by counting the ²¹²Pb gamma photopeak in a NaI (Tl) detector. The effects of airway geometry, particle size, and flow rate on deposition efficiency were studied. The deposition of the 1.75 nm particle, corresponding to unattached indoor radon progeny, was substantially higher than that of the 10 and 40 nm particles. The dependence of particle deposition on the flow rate was relatively weak, and deposition efficiencies were only slightly higher at the lower flow rates. The deposition models for diffusion from parabolic flow underestimated aerosol deposition, whereas the diffusion deposition predicted for plug flow overestimated the TB deposition. The deposition models resulting from this study can be used for developing lung deposition models and in the risk assessment of radon progeny and ultrafine ambient particles.     

Stable lead, 210Pb and 210Po in the liver and kidneys of cattle. II. Animals from an area near an abandoned lead mine

The concentrations of ²¹⁰Pb, ²¹⁰Po and stable lead in the livers and kidneys of 35 animals from an area close to an abandoned lead mine were determined. The median liver concentrations were 12·9 pCi ²¹⁰Pb, 35·8 pCi ²¹⁰Po and 0·58 mg stable lead/kg wet tissue, and the median kidney concentrations were 19·5 pCi ²¹⁰Pb, 152·4 pCi ²¹⁰Po and 1·20 mg stable lead/kg wet tissue. The specific activity of ²¹⁰Pb was 20·0 nCi/g Pb in both the livers and the kidneys. The median values for the ratios of the concentrations in the kidneys to those in the livers were 1·47 for ²¹⁰Pb, 4·15 for ²¹⁰Po and 2·04 for stable lead. The median ratio of ²¹⁰Po to ²¹⁰Pb was 2·50 in the livers and 8·18 in the kidneys. Significant correlations were found between the liver and the kidney levels of ²¹⁰Pb, ²¹⁰Po and stable lead and between ²¹⁰Po and ²¹⁰Pb in the kidneys, ²¹⁰Po and ²¹⁰Pb in the livers, and ²¹⁰Pb and stable lead in the livers. All the data and their relationships were compared with corresponding data from a control group of cattle from an area which was apparently unaffected by pollution from traffic or industry.     

The Deposition of Unattached Radon Progeny in a Tracheobronchial Cast as Measured with Iodine Vapor

ABSTRACT

The deposition of the unattached radon progeny in hollow cast models of the human tracheobronchial region was studied using iodine vapor. The experiments were conducted in a replicate cast whose inner surface was coated with NaOH impregnated charcoal powder. This coating can trap iodine molecules by converting iodine into iodide and iodate, so that the iodine gas molecules behave like particles and stick to the surface upon contact. The iodine vapor is selected as a surrogate of radon progeny because the effective diffusion coefficient of iodine vapor, 0.08 cm² s^?1, is close to the diffusivities of unattached radon progeny (0.03–0.07 cm² s^?1). Deposition experiments have been conducted under constant and cyclic inspiratory flow between 5 and 30 LPM. It was found that the deposition of iodine vapor under constant flow can be described by diffusion in laminar flow. The cyclic inspiratory flow pattern does not significantly change the total deposition in the tracheobronchial cast. This observation, combined with the enhanced particle deposition due to charge (Cohen et al., 1996) suggest that particle charge plays an important role in the deposition of submicron particles in human airways.     

Semi-continuous measurement of the unattached radon decay products size distributions from 0.5 to 5 nm by an array of annular diffusion channels

A new diffusion battery was designed to make precise measurements of the indoor activity size distribution of short-lived ²²²Rn decay products in the size range of 0.5–5 nm. It relies on the selection of particles by five annular diffusion channels (ADCs) and on the use of a non-linear inversion method, EVE, for the reconstruction of the particle size distributions. The ADC geometry proved to allow a better selection of the particles size compared to the wire screen and the implementation of an alpha detector set in front of the sampling filter for semi-continuous activity measurements. EVE algorithm was chosen in order to take into account the uncertainties of the measurement in the deconvolution process. The diffusion battery was tested in a radon chamber, and results showed a good reproducibility of the measurements. The degree of confidence on the obtained size distributions is then discussed. Results show that the relative uncertainty on the penetration data has to be less than 10% in order to obtain a reliable size distribution.     

Long-Term Measurements of Equilibrium Factor and Unattached Fraction of Short-Lived Radon Decay Products in a Dwelling - Comparison with Praddo Model

According to the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR 1993), the dose due to the inhalation of radon decay products represents almost 50% of the total natural radiation dose to the general population. The scientific community is interested in the assessment of the risk induced by domestic radon exposure. The dosimetric models used to estimate the dose are very sensitive to unattached fraction and size distributions, which makes the characterization of the indoor radon decay products aerosol necessary. For this purpose, longterm measurements of unattached fraction (f_p ) and equilibrium factor (F) were taken in a dwelling under typical indoor domestic aerosol conditions. An original device consisting of an annular diffusion channel set in parallel with an open filter was developed and calibrated to continuously measure the unattached fraction. Moreover, radon activity concentration and particle concentration were simultaneously monitored. With aged aerosol, particle concentration was found to be very low (between 500 and 5000 cm^-3), radon activity concentration ranged from 240 to 2800 Bq m^-3, and the mean values of f_p and F were, respectively, 0.31 (0.08-0.67) and 0.16 (0.04-0.45). With aerosol sources, the high increase in particle concentration led to a negligible unattached fraction and raised the equilibrium factor. A correlation relationship was determined between these two parameters under different aerosol conditions. Finally,our experimental results were compared to results obtained with the PRADDO model; this comparison showed a good agreement between these two different approaches.     

Measurement of 220Rn/222Rn progeny deposition velocities on surfaces and their comparison with theoretical models

The deposition velocities of ²²²Rn (radon) and ²²⁰Rn (thoron) progeny species have been measured in a chamber, in a test house, and in dwellings by relating the atom deposition fluxes of these species to their atom concentrations in air. These measurements were carried out using absorber-mounted nuclear track detectors (LR-115) which selectively register the tracks due to alpha emissions from ²¹²Po and ²¹⁴Po from the deposited atoms of ²²⁰Rn and ²²²Rn progeny species, respectively. These are termed as DRPS (direct radon progeny sensor) and DTPS (direct thoron progeny sensor). Measurement of parameters such as ventilation rate, particle size distribution and unattached fractions were also carried out along with deposition velocity. The experimental data on deposition velocity in test house and chamber were compared with the predictions based on the indoor progeny dynamics model and particle deposition models. These showed excellent agreement with experimental values although the data on radon progeny showed slightly higher dispersion. The progeny deposition velocities were also measured in living rooms of dwellings in Mumbai and were found to be close to the model results which in turn imply that in the long term, the average environmental conditions are similar to that in the test house. These results point at a plausible constancy of long time averaged indoor deposition velocities. From these studies, we are inclined to assign summary values of deposition velocities of 0.075 m h^?1 for ²²⁰Rn progeny and 0.132 m h^?1 for ²²²Rn progeny, for indoor conditions.     

Determination of Mass Transport Mechanism of Rutile by Sinusoidal Profile Decay Method

Amplitude decay of sinusoidal surface profiles on the c surface of TiO₂ single crystals was measured in air over the range 1500 to 1700 K. The rate of decay was proportional to –2.75 powers of the sinusoidal wave length (4.3 to 17.6 μrn), indicating that volume diffusion is the dominant mass-transport mechanism. The effective volume diffusion coefficient, Deff , given agreed with tracer diffusion data for the Ti ion. Hence Deff was assumed to be equal to the self-diffusion coefficient of the Ti ion as oxygen ion transport along the surface was enhanced. No anisotropy was observed for transport on this surface.     

An Analytical Model for Moisture Diffusion in Solid Objects During Drying

ABSTRACT

An analytical model is proposed to determine the moisture diffusivities in geometrical solid objects (namely, infinite slab, infinite cylinder, sphere) subjected to drying applications. The analysis for moisture diffusion is carried out on the basis of two imponant criteria: 0.l?Bi?100 and Bi>100. New correlations for the mots of the characteristic equations corresponding these objects are developed. For a drying application, using the similarity between cooling and drying profiles, drying coefficient and lag factor are obtained. The model is then verified using available data from literature.Results show that the presented model can be used to determine the moisture diffusion coefficients in such solid objects in a simple and accurate manner in a variety of drying applications.     

Ascorbic Acid Thermal Degradation during Hot Air Drying of Camu-Camu (Myrciaria dubia [H.B.K.] McVaugh) Slices at Different Air Temperatures

Abstract

Air drying of camu-camu slices was performed in order to estimate the effect of air temperature on the kinetics of ascorbic acid thermal degradation. Moisture variation during the air drying process was monitored gravimetrically by weighing the trays at predetermined time intervals. The experimental points were adjusted by Fick's diffusion model and by the Page empirical model. The effective diffusion coefficient (D_eff) ranged from 8.48 × 10^?10 to 1.34 × 10^?9 m²/s.The ascorbic acid content was evaluated in samples taken during the drying process using Iodine titration, and the results modeled by the Weibull equation. Concerning ascorbic acid retention the best drying condition required air at 50°C. The ascorbic acid retention was 78%, when the moisture content of the product reached 10% (wet basis).     

Characterization of Radioactive Aerosols in Florida Phosphate Processing Facilities

The health risks to workers in the Florida phosphate industry resulting from chronic inhalation of radionuclide-containing aerosols have not been adequately addressed. The present study establishes a database of information on the particle size distribution, density, shape, chemical composition, and radioactivity concentration for six phosphate facilities in the northern and central regions of the state. A seven-stage cascade impactor was employed to sample aerosols at various processing areas in these plants. Aerosol mass loadings are lowest mainly at shipping areas where they are approximately one order of magnitude less than at granulator areas within equivalent particle size intervals. Aerosol mass concentrations increase as the aerosol size increases for the majority of the plants and operational areas. The aerosol loading at each area varies widely depending on plant, and the variance is largest at the storage areas due to the variability of mechanical operations and patterns of building ventilation. The density of bulk dry product, settled dust, and airborne particles are between 1.6 to 1.7 g/cm ³ . Under electron microscopy, the particles appear as spheroids or rough spherical fragments across all plants, work areas, and sampled size intervals. The main elemental components of large-sized and medium-sized aerosols are similar to those found in the bulk dry product. For small-sized aerosols (0.2–0.4 μ m), the fraction of phosphorus is very small in comparison to elemental impurities such as silicon and sulfur. The ²³⁸ U decay series was found in both bulk dry product, settled dust, and airborne particles collected via high-volume samplers. The ²³⁸ U, ²²⁶ Ra, and ²¹⁰ Pb radioactivity concentrations in bulk dry product and settled dust from central Florida range from 63–112 pCi/g, 0.8–1.5 pCi/g, and 7–9 pCi/g, respectively. No significant differences in the radioactivity concentrations of ²³⁸ U and ²²⁶ Ra were found between dry product, settled dust, and sampled aerosols. However, ²¹⁰ Pb is highly concentrated in aerosols up to 25–87 pCi/g, most likely due to the deposition of ambient airborne radon decay products on workplace aerosols. The database of worker aerosol physicochemical characteristics established in this study for the Florida phosphate industry can be directly used for more realistic assessments of worker inhalation dose, thus providing a more firm basis for assessing the adequacy of existing respiratory and other radiological protection policies.     

Particle Charging and Transmission Efficiencies of Aerosol Charge Neutralizes

ABSTRACT

Diffusion losses and charging efficiency were measured for three types of charge neutralizers commonly used in aerosol research: two with ⁸⁵Kr and one with ²¹⁰Po as radiation sources. The diffusion losses were characterized at flows of 0.5 -6 1 min^?1 typically used in atmospheric aerosol physics measurements. All of the neutralizers tested exhibited high transmission efficiencies, with losses up to 25% at the smallest tested size of 3 nm, varying with size and flow in general agreement with diffusion loss theory. Charging efficiency was measured for a singly charged, monodisperse aerosol at the same flows and at concentrations of 10³-10⁴ particles cm^?3. Neither of the ⁸⁵Kr chargers brought the charge distribution close to equilibrium at 2 1 min^?1, except at concentrations ≤ 10³ cm^?3. The ²¹⁰Po charger produced the theoretically expected fraction of singly charged particles within the uncertainty of the experiment.     

BiScO3-PbTiO3 piezoelectric ceramics with Bi excess for energy harvesting applications under high temperature

0.36BiScO₃-0.64PbTiO₃ ceramic is a competitive piezoelectric material even though it contains lead and volatile Bi contents. It contains a relatively decreased lead content compared to that of the Pb(Zr,Ti)O₃ system but it has similar piezoelectric properties with high Curie temperature. However, due to the very volatile component of Bi the 0.36BiScO₃-0.64PbTiO₃ system has Bi-deficient composition. Therefore, in order to compensate for deficient Bi contents in the 0.36BiScO₃-0.64Pb,TiO₃ system, excess 0.005, 0.01, 0.015 and 0.02 mol of Bi were added to the ceramics to enhance the piezoelectric properties for the first time. By employing excess Bi addition, the piezoelectric charge coefficient, electromechanical coupling factor, output open circuit voltage, and generated output power density were improved from 417 pC/N, 51.48%, 19.69 V and 0.28 mJ/cm³ to 452 pC/N, 52.25%, 26.93 V and 0.48 mJ/cm³. We expect that piezoelectric properties of 0.36BiScO₃-0.64PbTiO₃ ceramics were improved by adding Bi excess.     

Size evolution of ion beam synthesized Pb nanoparticles in Al

The size evolution of Pb nanoparticles (NPs) synthesized by ion implantation in an epitaxial Al film has been experimentally investigated. The average radius R of Pb NPs was determined as a function of implantation fluence f. The R(f) data were analyzed using various growth models. Our observations suggest that the size evolution of Pb NPs is controlled by the diffusion-limited growth kinetics (R²∝f). With increasing implantation current density, the diffusion coefficient of Pb atoms in Al is evident to be enhanced. By a comparative analysis of the R(f) data, values of the diffusion coefficient of Pb in Al were obtained.     

THEORETICAL ANALYSIS OF THE DIFFUSION PROCESS INSIDE PROLATE SPHEROIDAL SOLIDS

ABSTRACT

A numerical solution of the diffusion equation to describe solute transport inside ellipsoids, assuming a constant difiusion coefficient and a corrective boundary condition, is presented. The difiusion equation in a prolate spheroidal coordinate system was used for a hidimensional case. The finite volume method was employed to discretize the basic equation, utilizing a uniform grid size. The equation was solved irteratively using the Gauss-Seidel method. The effect of Biot number and the aspect ratio of the body on diffusion rate and concentration during the process is presented. Plots are presented for Biot numbers from 0.05 to infinity and aspect ratios of 1.1, 2.0 and 5.0. To investigate the effect of the aspect ratio, different results changing the aspect ratio for Bi= 1.0 are shown. The results show that the model is consistent and it may be used to solve other cases such as those that include cylinder and sphere geometry, and/ or those with variable diffusion coefficients with small modifications.     

Superconducting Bi(2223) thick film on Ba2HoNbO6: a new perovskite ceramic substrate

Abstract

Barium holmium niobate Ba₂HoNbO₆ (BHNO) has been developed as a new substrate for (Bi,Pb)₂Sr₂Ca₂Cu₃O_x (Bi(2223)) superconductor film. Ba₂HoNbO₆ has a cubic perovskite structure with lattice constant a = 8·26 Å. The dielectric constant and loss factor of this material are in a range suitable for its use as a substrate for microwave applications. The Bi(2223) superconductor shows no detectable chemical reaction with BHNO, even under extreme processing conditions. Dip coated Bi(2223) thick film on Ba₂HoNbO₆ substrate had T _c(0) of 109 K and current density of around 4 × 10³ A cm ^{- 2} at 77 K and in zero magnetic field.     

Analytical Solution of Mass Transfer Equation with Interfacial Resistance in Food Drying

ABSTRACT

A simplification of boundary conditions of the mass transfer equation in food drying was obtained. An average distribution constant between food and air humidity was introduced. With this simplification and analytical solution for the mass transter equation with interfacial resistance was; found. and a medihcd Bi number is proposed: Thc analytical solution for infinite flat slab was comparcd to the numcrical solution without the simplification. The analytical solution showed a good approximation to thc numerical solution. The equation oblained from thc analytical solution predicted ha1 for modified Bi numbers greater han 30. the drying curve bccamcs indcpcndcnt o f thc boundary conditions, which means that drying is diffusion controlled. Experimental drying curves of carrots were obtained at modified Bi numbers between 10 and 60. The crperimenwl drying cumes did not show statistical diffcrencc for modified Bi numbcrs grcater than 40, which approaches to the same limit predicted by the analytical solution.