Graduate Thesis Or Dissertation

Development and Application of Voxelized Dosimetric Models for Biota : Characterization of the Uncertainty in the International Commission on Radiological Protection's Wildlife Dosimetry System

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  • A system for radiological dosimetry for nonhuman biota developed by International Commission on Radiological Protection (ICRP) relies on calculations that utilize the Monte Carlo simulations of simple, ellipsoidal geometries with internal radioactivity distributed homogeneously throughout. In this manner it is quick and easy to estimate whole-body dose rates to biota. This system relies on the validity of three major assumptions. First, that any organism can be reasonably represented by a simplified dosimetric phantom; second, that for dosimetric purposes four-component human tissue (composed of hydrogen, carbon, nitrogen, and oxygen) adequately mimics real tissue, and third, that assuming a homogeneous distribution of radionuclides within an organism's body is not a large source of uncertainty. This work characterizes the uncertainty each of these assumptions adds to wildlife dosimetry calculations by comparing ellipsoidal and voxel calculated dose rates for a rabbit to determine whether or not ellipsoidal models are fit for regulatory purposes. The voxel model is then used to compare homogeneous versus particulate lung dose rates resulting from exposures to small, highly radioactive fragments of material incorporated into metallic matrices (i.e. hot particles). Voxel models are detailed anatomical phantoms that were first used for calculating radiation dose to humans, which are now being extended to nonhuman biota dose calculations. These more complex phantoms can be used to test the validity of simple ellipsoidal models by comparing dose rate estimates from each. Here we show that the ellipsoidal method provides conservative estimates of organ dose rates to small mammals. Organ dose rates were calculated for environmental source terms from Maralinga, the Nevada Test Site, Hanford and Fukushima using both the ellipsoidal and voxel techniques, and in all cases the ellipsoidal method yielded more conservative dose rates by factors of 1.2-1.4 for photons and 5.3 for beta particles. Dose rates for alpha-emitting radionuclides are identical for each method as full energy absorption in source tissue is assumed. The voxel procedure includes contributions to dose from organ-to-organ irradiation (shown here to comprise 2-50% of total dose from photons and 0-93% of total dose from beta particles) that is not specifically quantified in the ellipsoidal approach. The maximum potential uncertainty added to the wildlife dosimetry calculation from geometry is a factor of 5.3, and the assumption is conservative (i.e. ellipsoidal model over predicts dose rates as compared to the voxel model). In most voxel models created to date, human tissue composition and density values have been used in lieu of biologically accurate values for nonhuman biota. This has raised questions regarding variable tissue composition and density effects on the fraction of radioactive emission energy absorbed within tissues (e.g. the absorbed fraction - AF). The results of this study on rabbits indicates that the variation in composition between two mammalian tissue types (e.g. human vs rabbit bones) made little difference in self-AF (SAF) values (within 5% over most energy ranges). However, variable tissue density (e.g. bone vs liver) can significantly impact SAF values. AFs for electron energies of 0.1, 0.2, 0.4, 0.5, 0.7, 1.0, 1.5, 2.0, and 4.0 MeV and photon energies of 0.01, 0.015, 0.02, 0.03, 0.05, 0.1, 0.2, 0.5, 1.0, 1.5, 2.0, and 4.0 MeV are provided for eleven rabbit tissues. The maximum potential uncertainty added to the wildlife dosimetry calculation from tissue composition and density is a factor of 1.5, and the assumption is not conservative (i.e. ellipsoidal model under predicts dose rates as compared to the voxel model). Hot particles are commonly found at nuclear weapons test and accident sites, and can be inhaled by wildlife. Inhaled particles often partition heterogeneously in the lungs, with aggregation occurring in the periphery of the lung, and are tenaciously retained. However, dose rates are typically calculated as if the material were homogeneously distributed throughout the entire organ. Here we quantify the variation in dose rates for alpha, beta, and gamma emitting radionuclides with particles sizes from 1-150 µm and considering three averaging volumes- the entire lung, a 10 cm³ and a 1 cm³ volume of tissue. Dose rates from beta-emitting particles (e.g. ⁹⁰Sr) were approximately one order of magnitude higher than those from gamma-emitting radionuclides (e.g. ¹³⁷Cs). Self-shielding within the particle was negligible for gammas and minor for betas. For alpha-emitting particles (e.g. ²³⁹Pu) it was found that particles in the respirable size range of less than 5 µm are not greatly self-shielded, but rather deposit a significant amount of energy into the surrounding tissue. As such particles may remain lodged deep in the lung, they represent a considerable contribution to long term lung dose rates. This study demonstrates one possible approach to dose assessments for biota in environments contaminated by radioactive particles, which may prove useful for those engaged in environmental radioprotection. Overall, the voxel models provide robust dosimetry for the nonhuman mammals considered in this study, and though the level of detail is likely extraneous to demonstrating regulatory compliance today, voxel models may nevertheless be advantageous in resolving ongoing questions regarding the effects of ionizing radiation on wildlife.
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