MODELING VARIABILITY AND UNCERTAINTY ASSOCIATED WITH INHALED WEAPONS-GRADE PuO₂

Abstract

The work presented relates to developing a stochastic version of the ICRP 66 respiratory tract deposition model and applying the stochastic model to characterize the variability/uncertainty associated with inhaled PuO₂ for a hypothetical population of nuclear workers engaged in light work-related exercise. The parameter uncertainty/variability distributions used are essentially the same as the FORTRAN-based stochastic deposition model of Bolch et al. known as LUDUC (LUng Dose Uncertainty Code). Based on Crystal Ball software, this stochastic deposition model includes particle polydispersity, which Bolch et al. did not discuss. This paper first compares model-simulated regional deposition probability distributions to deterministic results based on LUDEP (LUng Dose Evaluation Program) software, which implements the ICRP 66 deterministic deposition model. For these comparisons, a particle density of 3 g cm⁻³ (for hypothetical radioactive particles) was used. The range of possible depositions generated by LUDUC and the Crystal Ball program results revealed LUDEP’s limitations. Even though LUDEP tends to use parameters that represent average parameter values for adult males, it overestimates deposition in the lower regions of the lung for most of the population. The Crystal Ball program was then used to generate radioactivity intake distributions for single and multiple PuO₂ particle intakes by a hypothetical population of nuclear workers for the stochastic intake (STI) paradigm. These distributions of radioactivity intake are evaluated for the five primary regions of the respiratory tract as defined in the ICRP Publication 66. The results reveal that when a particle has been deposited, the radioactivity is likely to be low if it is in the lower regions (< 10 Bq for the bb and AI regions), but it may be quite large in the upper regions (as much as 600 Bq for the ET1, and ET2 regions), and the distributions for radioactivity become less and less skewed to the right, as particles penetrate deeper within the respiratory tract.