INFLUENCES OF PARAMETER UNCERTAINTIES WITHIN THE ICRP-66 RESPIRATORY TRACT MODEL: REGIONAL TISSUE DOSES FOR 239PuO2 AND 238UO2/238U3O8

Abstract

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 ∼50 and ∼70 for plutonium and uranium oxides, respectively, over the particle size range from 0.1 to 20 μm. For typical exposure scenarios involving both radionuclides, the ratio of the 95% dose fractile to the 5% dose fractile ranged from ∼8–10 (corresponding to a geometric standard deviation, or GSD, of about 1.7–2) for particle diameters of 0.1 to 1 μm. This ratio increased to about 370 for plutonium oxide (GSD ∼4.5) and to about 600 for uranium oxide (GSD ∼5) as the particle diameter approached 20 μm. 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 μm. However, median doses to the basal cell nuclei of the bronchial airways (BBbas) calculated by LUDUC were found to be approximately 6 times higher than LUDEP reference doses. The higher BBbas doses were directly attributed to discrepancies between the ICRP default thickness for the bronchial epithelium (55 μm) and the probability density function assumed within LUDUC (uniform distribution from 20 to 60 μm based upon detailed literature reviews).