| 2. |
- Hansson, Edvin, 1987-
(författare)
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Internal Dosimetry in Nuclear Fuel Fabrication : Occupational Exposure to Uranium Aerosols
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The production of nuclear fuel can be associated with occupational exposure to ionizing radiation from radioactive decay of uranium. Such exposure must be sufficiently low and radiation doses adequately determined. Radiation doses from internal exposure, i.e., following intake (usually inhalation), cannot be estimated using dosimeters, but must be calculated based on indirect measurements in combination with biokinetic models.Such biokinetic models have been developed and refined for decades. Good knowledge of the material characteristics is crucial. However, the physicochemical properties of chemical compounds can vary between different production facilities. Aerosol size distributions and dissolution characteristics in lung fluid are of particular importance. The latter is important since dissolved material is absorbed to blood, whereupon a large fraction reaches the urine after filtering by the kidneys. This enables urine sampling as a method to monitor occupational exposure.The aim of this thesis was to investigate the physicochemical properties of uranium aerosols and their implication on internal dose assessments at a nuclear fuel fabrication plant in Sweden. Uranium aerosols were sampled and size fractionated using personal cascade impactors carried by workers at the factory’s different main workshops. Aerosols were studied using scanning electron microscopy in Paper I. In Paper II the activity size distributions were determined and in Paper III dissolution rates in simulated lung fluid were investigated. Paper IV is an internal dose assessment based on records of urine sample analyses from about 10 years of routine occupational exposure monitoring of uranium pelletizing workers at the site.For a median worker, the urinary daily excretion rate of uranium increased due to chronic exposure for about 1000 days, after which the excretion rate stabilized. This suggests that inhaled material dissolves in the respiratory tract rapidly enough to prevent a net buildup in the lung after several years of exposure. This could be modelled using the default recommendations for uranium oxide materials provided by the International Commission on Radiological Protection. However, the best model fit to measurement data was obtained using a different set of parameters, that showed some discrepancies with results from Papers II-III. For individual cases, excretion rates could vary between sampling occasions to a greater extent than predicted using the default recommendations, which could indicate a more rapid body clearance than expected. Whether this is an effect of experimental methods or simplifications in the biokinetic models should be further investigated in future work.
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| 3. |
- Hansson, Edvin, 1987-, et al.
(författare)
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Particle Size-dependent Dissolution of Uranium Aerosols in Simulated Lung Fluid : A Case Study in a Nuclear Fuel Fabrication Plant
- 2022
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Ingår i: Health Physics. - : Lippincott, Williams & Wilkins. - 0017-9078 .- 1538-5159. ; 123:1, s. 11-27
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Tidskriftsartikel (refereegranskat)abstract
- Inhalation exposure to uranium aerosols can be a concern in nuclear fuel fabrication. The ICRP provides default absorption parameters for various uranium compounds but also recommends determination of material-specific absorption parameters to improve dose calculations for individuals exposed to airborne radioactivity. Aerosol particle size influences internal dosimetry calculations in two potentially significant ways: the efficiency of particle deposition in the various regions of the respiratory tract is dependent on aerodynamic particle size, and the dissolution rate of deposited materials can vary according to particle size, shape, and porosity because smaller particles tend to have higher surface-to-volume ratios than larger particles. However, the ICRP model assumes that deposited particles of a given material dissolve at the same rate regardless of size and that uptake to blood of dissolved material normally occurs instantaneously in all parts of the lung (except the anterior portion of the nasal region, where zero absorption is assumed). In the present work, the effect of particle size on dissolution in simulated lung fluid was studied for uranium aerosols collected at the plant, and its influence on internal dosimetry calculations was evaluated. Size fractionated uranium aerosols were sampled at a nuclear fuel fabrication plant using portable cascade impactors. Absorption parameters, describing dissolution of material according to the ICRP Human Respiratory Tract Model, were determined in vitro for different size fractions using simulated lung fluid. Samples were collected at 16 time-points over a 100-d period. Uranium content of samples was determined using inductively coupled plasma mass spectrometry and alpha spectrometry. In addition, supplementary experiments to study the effect of pH drift and uranium adsorption on filter holders were conducted as they could potentially influence the derived absorption parameters. The undissolved fraction over time was observed to vary with impaction stage cut-point at the four main workshops at the plant. A larger fraction of the particle activity tended to dissolve for small cut-points, but exceptions were noted. Absorption parameters (rapid fraction, rapid rate, and slow rate), derived from the undissolved fraction over time, were generally in fair agreement with the ICRP default recommendations for uranium compounds. Differences in absorption parameters were noted across the four main workshops at the plant (i.e., where the aerosol characteristics are expected to vary). The pelletizing workshop was associated with the most insoluble material and the conversion workshop with the most soluble material. The correlation between derived lung absorption parameters and aerodynamic particle size (impactor stage cut-point) was weak. For example, the mean absorption parameters derived from impaction stages with low (taken to be <5 mu m) and large (>= 5 mu m) cut-points did not differ significantly. Drift of pH and adsorption on filter holders appeared to be of secondary importance, but it was found that particle leakage can occur. Undissolved fractions and to some degree derived lung absorption parameters were observed to vary depending on the aerodynamic size fraction studied, suggesting that size fractionation (e.g., using cascade impactors) is appropriate prior to conducting in vitro dissolution rate experiments. The 0.01-0.02 mu m and 1-2 mu m size ranges are of particular interest as they correspond to alveolar deposition maxima in the Human Respiratory Tract Model (HRTM). In the present work, however, the dependency on aerodynamic size appeared to be of minor importance, but it cannot be ruled out that particle bounce obscured the results for late impaction stages. In addition, it was noted that the time over which simulated lung fluid samples are collected (100 d in our case) influences the curve-fitting procedure used to determine the lung absorption parameters, in particular the slow rate that increased if fewer samples were considered.
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