Research on inhaled industrial uranium compounds has shown that solubility influences the target organ, the toxic response, and the mode of uranium excretion. Consideration of physical chemical properties indicates that the dissolution of industrial uranium oxides is expected to be strongly dependent on process history, and that dissolved uranium exists in vivo in the hexavalent state regardless of the oxidation state of the inhaled compound. The overall clearance rate of uranium compounds from the lung reflects both mechanical and dissolution processes. Mechanical clearance rates are highly variable among individual workers studied, but dissolution rates of inhaled compounds are similar among the mammalian species studied. Results from experiments in vivo and accidental worker exposures indicate that the uptake of dissolved uranium from the lung is more rapid than the dissolution rate of most industrial uranium compounds. These results indicate that the absorption rate of inhaled uranium can be approximated by the dissolution rate of most industrial compounds. Dissolution rates of UF6 and UO2(NO3)2 are more rapid than the mechanical clearance rates and dominate the overall lung clearance rate. UF4, UO3, and ammonium diuranate have intermediate dissolution rates that are similar to mechanical clearance rates and exhibit high variability among uranium specimens. U3O8 and UO2 have slow dissolution rates such that pulmonary clearance rates are dominated by mechanical processes. Industrial uranium ores, oxides, and fluorides are often variable mixtures of relatively soluble and insoluble fractions. Dissolution rates measured in vitro can be used with biokinetics models to reduce the uncertainties in dosimetry associated with inhalation exposures to mixtures.
Dissolution characteristics of mixed-oxide nuclear fuels are important considerations for prediction of biological behavior of inhaled particles. Four representative industrial mixed-oxide powders were obtained from fuel fabrication enclosures. Studies of the dissolution of Pu, Am and U from aerosol particles of these materials in a serum simulant solution and in 0.1M HCl showed: (1) dissolution occurred at a rapid rate initially and slowed at longer times, (2) greater percentages of U dissolved than Pu or Am: with the dissolution rates of U and Pu generally reflecting the physical nature of the UO2-PuO2 matrix, (3) the temperature history of industrial mixed-oxides could not be reliably related to Pu dissolution except for a 3-5% increase when incorporated into a solid solution by sintering at 1750 degrees C, and (4) dissolution in the serum simulant agreed with the in vivo UO2 dissolution rate and suggested the dominant role of mechanical processes in PuO2 clearance from the lung. The rapid initial dissolution rate was shown to be related, in part, to an altered surface layer. The advantages and uses of in vitro solubility data for estimation of biological behavior of inhaled industrial mixed oxides, such as assessing the use of chelation therapy and interpretation of urinary excretion data, are discussed. It was concluded that in vitro solubility tests were useful, simple and easily applied to individual materials potentially inhaled by humans.
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