uranium-octoxide and plutonium-dioxide

This paper extends an examination of the influence of parameter uncertainties on regional doses to respiratory tract tissues for short-ranged alpha particles using the ICRP-66 respiratory tract model. Previous papers examined uncertainties in the deposition and clearance aspects of the model. The critical parameters examined in this study included target tissue depths, thicknesses, and masses, particularly within the thoracic or lung regions of the respiratory tract. Probability density functions were assigned for the parameters based on published data. The probabilistic computer code LUDUC (Lung Dose Uncertainty Code) was used to assess regional and total lung doses from inhaled aerosols of 239PuO2 and 238UO2/238U3O8. Dose uncertainty was noted to depend on the particle aerodynamic diameter. Additionally, dose distributions were found to follow a lognormal distribution pattern. For 239PuO2 and 238UO2/238U3O8, this study showed that the uncertainty in lung dose increases by factors of approximately 50 and approximately 70 for plutonium and uranium oxides, respectively, over the particle size range from 0.1 to 20 microm. For typical exposure scenarios involving both radionuclides, the ratio of the 95% dose fractile to the 5% dose fractile ranged from approximately 8-10 (corresponding to a geometric standard deviation, or GSD, of about 1.7-2) for particle diameters of 0.1 to 1 microm. This ratio increased to about 370 for plutonium oxide (GSD approximately 4.5) and to about 600 for uranium oxide (GSD approximately 5) as the particle diameter approached 20 microm. However, thoracic tissue doses were quite low at larger particle sizes because most of the deposition occurred in the extrathoracic airways. For 239PuO2, median doses from LUDUC were found be in general agreement with those for Reference Man (via deterministic LUDEP 2.0 calculations) in the particle range of 0.1 to 5 microm. However, median doses to the basal cell nuclei of the bronchial airways (BB(bas)) calculated by LUDUC were found to be approximately 6 times higher than LUDEP reference doses. The higher BB(bas) doses were directly attributed to discrepancies between the ICRP default thickness for the bronchial epithelium (55 microm) and the probability density function assumed within LUDUC (uniform distribution from 20 to 60 microm based upon detailed literature reviews).

Topics: Adolescent; Adult; Aerosols; Air Pollutants, Radioactive; Body Height; Child; Child, Preschool; Computer Simulation; Humans; Inhalation Exposure; Lung; Lymph Nodes; Male; Models, Biological; Organ Specificity; Oxides; Plutonium; Radiation Dosage; Radiometry; Reproducibility of Results; Respiratory System; Sensitivity and Specificity; Uranium Compounds

Quantifying radiological risk following the inhalation of radioactive aerosols entails not only an assessment of particle deposition within respiratory tract regions but a full accounting of clearance mechanisms whereby particles may be translocated to adjacent respiratory tissue regions, absorbed to blood, or released to the gastrointestinal tract. The model outlined in ICRP Publication 66 represents to date one of the most complete overall descriptions of particle deposition and clearance, as well as localized radiation dosimetry, within the respiratory tract. In this study, a previous review of the ICRP-66 deposition model is extended to the study of the subsequent clearance model. A systematic review of the clearance component within the ICRP 66 respiratory tract model was conducted in which probability density functions were assigned to all input parameters for both 239PuO2 and 238UO2/238U3O8. These distributions were subsequently incorporated within a computer code LUDUC (Lung Dose Uncertainty Code) in which Latin hypercube sampling techniques are used to generate multiple (e.g., 1,000) sets of input vectors (i.e., trials) for all model parameters needed to assess mechanical clearance and particle dissolution/absorption. Integral numbers of nuclear disintegrations, U(s), in various lung regions were shown to be well-described by lognormal probability distributions. Of the four extrathoracic clearance compartments of the respiratory tract, uncertainties in U(s), expressed as the ratio of its 95% to 5% confidence levels, were highest within the LN(ET) tissues for 239PuO2 (ratio of 50 to 130) and within the ET(seq) tissues for 238UO2/238U3O8 (ratio of 12 to 50). Peak uncertainties in U(s) in these respiratory regions occurred at particle sizes of approximately 0.5-0.6 microm where uncertainties in ET2 particle deposition fractions accounted for only approximately 10% of the total U(s) uncertainty for 239PuO2, and only approximately 30% of the total U(s) uncertainty for 238UO2/238U3O8 (the remainder is attributed to the clearance model alone). Of the eight clearance compartments within the thoracic regions of the respiratory tract, and for particle sizes below approximately 5 microm, uncertainties in U(s) were highest within the LN(TH) tissues for 239PuO2 (ratio of 60 to 80) and within the BB(seq) tissues for 238UO2/238U3O8 (ratio of 20 and 60). At particle sizes exceeding approximately 5 microm in aerodynamic diameter, peak uncertainties in U(s) are no

Topics: Aerosols; Air Pollutants, Radioactive; Computer Simulation; Humans; Inhalation Exposure; Lung; Models, Biological; Oxides; Particle Size; Plutonium; Radiation Dosage; Radiometry; Reproducibility of Results; Respiratory System; Sensitivity and Specificity; Uranium Compounds