Estimation of lung absorption parameters for oxides of 238Pu

Following inhalation of an aerosol of relatively insoluble particles, it is usually found that the fractional dissolution rate of material retained in the lungs decreases with time, and the amount remaining undissolved can be represented simply by a decreasing exponential function with two or more components. A few exceptions are known, in which the dissolution rate increases with time. The most important in the context of radiological protection is probably that of Pu-238 dioxide. Several published comprehensive data sets, from animal studies and accidental human exposures, have been analysed using the Human Respiratory Tract Model (HRTM) of the International Commission on Radiological Protection. The HRTM contains a simplified representation of particle dissolution in the respiratory tract, suitable for routine radiological protection purposes. Still, it was found to have sufficient flexibility to represent the measurement data in most of these cases. Although the Pu-238 dioxide showed a wide range of behaviour in the different studies, there was good agreement between the absorption behaviour modelled for two studies involving 'ceramic' Pu-238 dioxide as used in spacecraft radioisotope thermoelectric generators: a long-term experimental study in dogs and an accidental exposure involving a group of workers.