Re-estimation of the activity median aerodynamic diameters of plutonium compound particles - incorporating molecular weight into specific activity calculations and comparing with log-normal distribution -

Information on workplace particle sizes is useful for radiation protection during plutonium handling and decommissioning. The International Commission on Radiological Protection advises using the activity median aerodynamic diameter (AMAD) together with the geometric standard deviation (GSD) to characterize the particle size distribution of inhaled aerosols. In this study, we re-estimated the AMADs and GSDs of plutonium compound particles released during the 2017 accidental contamination event at the Oarai plutonium-handling facility of the Japan Atomic Energy Agency. We explicitly incorporated molecular weight into specific activity calculations, whereas previous approaches did not account for molecular weight. We also examined the log-normality assumption, which is commonly adopted in AMAD assessments. The AMADs ranged from 4.33 to 8.9 μm (GSD 1.5–1.6) for plutonium nitrate, from 4.41 to 8.9 μm (GSD 1.4–1.6) for plutonium dioxide, and from 6.66 to 13.5 μm (GSD 1.4–1.6) for MOX. If provisional AMADs, excluded from the analysis because of measurement limitations, are taken into account, the corresponding AMADs may be larger. The re-estimated AMADs were approximately 1.5 times higher for plutonium nitrate and approximately 10% higher for plutonium dioxide than previously reported. The statistical tests indicated reasonable agreement between the relative radioactivity distributions and the log-normal model in the central region, while systematic deviations were observed in the tails. These deviations are unlikely to significantly affect dose assessments based on AMAD, but their quantitative impact should be evaluated if the fitted parameters are applied directly in dose models.