Uncertainty Analysis of the ICRP Systemic Model for Uranium as applied to Interpretation of Bioassay Data for Depleted Uranium

This report provides a review of the ICRP model for the systemic biokinetics of uranium absorbed to blood, identifies key model parameters that affect urinary excretion, considers uncertainties in values for these parameters, and determines the effect of these uncertainties on dose assessments based on urine measurements. A parallel study of the application of the ICRP respiratory tract model to inhaled DU is reported separately. Consistent with available human and animal data, the ICRP systemic model predicts rapid urinary excretion of uranium entering blood, about 75% after 1 week and 95% at 1 year, with long-term retention mainly in the skeleton (4% at 1 year, 1% at 20 years). Following a sensitivity analysis of the ICRP systemic model, uncertainty analysis was confined to values for key parameters: the urinary excretion rate and rates associated with skeletal retention. These uncertainties were considered together and in combination with respiratory tract model uncertainties obtained in the parallel study. Considering committed effective dose following inhalation of DU, assessed from a measurement of DU in urine, probabilistic analysis showed that uncertainties in dose estimates were dominated by respiratory tract model uncertainties at all times. For measurements at early times up to about 6 months (200 days) after intake, the 2.5% 97.5% range in dose estimates was a factor of about 50 – 70, independent of the inclusion of systemic model uncertainties. For measurements at later times, from 3 years to approaching 30 years after intake, the range in uncertainties in dose estimates was reduced to < 10, considering uncertainties in respiratory tract model parameter values only. Systemic model uncertainties made the greatest contribution to overall uncertainties in dose estimates at the latest time-point considered, approaching 30 years after intake (10,000 days), but only increase the 2.5% 97.5% range from 10 to 15. The maximum assessed doses for the example of measured urinary excretion of 1 ng DU d (97.5% values) were below 1 mSv up to about 14 years after intake but could exceed 3 mSv for measurements at times approaching 30 years after intake. This study was partly funded by the Defence Science and Technology Laboratory under contract number RD034 9781.