Uranium dioxide leaching

Since the water movement will be very slow compared with the rate at which the wastes dissolve, we are concerned first and foremost with equilibrium solubility. Also, if only to relate behaviour on the geological time scale to that on the laboratory time scale, we will need to know about the mechanisms and kinetics of dissolution and leaching. The waste forms envisaged at present are glass blocks containing separated fission products and residual actinides fused into the glass and, alternatively, the uranium dioxide matrix of the used fuel containing unseparated fission products and plutonium. In the... 

Leach LJ, Maynard EA, Hodge CH, et al A five-year inhalation study with natural uranium dioxide (UO2) dust. I. Retention and biologic effect in the monkey, dog and rat. Health Phys 18 599-612, 1970... 

Exposure of 200 rats, 110 dogs, and 25 monkeys to 5 mg U/m as uranium dioxide dust for 1-5 years for 5.4 hours a day, 5 days a week did not result in histological damage in the lungs of the dogs or rats. Minimal patchy hyaline fibrosis was occasionally seen in the tracheobronchial lymph nodes of dogs and monkeys exposed for more than 3 years. No atypical epithelial changes were noted (Leach et al. 1970). 

No treatment-related renal effects were seen when Rhesus monkeys and dogs were exposed to uranium dioxide by inhalation at airborne concentrations as high as 5.1 mg U/m for 1-5 years (Leach et al. 1973). Blood NPN levels were consistently elevated in Rhesus monkeys although no renal histopathology was evident (Leach et al. 1973). 

In chronic-duration studies, exposure to inhalation concentrations of 3 mg U/m as uranium dioxide to monkeys for 5 years produced no significant body weight changes (Leach et al. 1970). 

One site of deposition for the soluble compounds (uranyl nitrate, uranium tetrachloride, uranium hexafluoride) in animals was the skeleton, but accumulation was not seen in bone at levels below 0.25 mg U/m over a period of 2 years in rats exposed to soluble compounds (uranyl nitrate, uranium tetrachloride, uranium hexafluoride) in one study. The insoluble compounds (uranium hexafluoride, uranium dioxide) were found to accumulate in the lungs and lymph nodes after the inhalation exposure. For uranyl nitrate exposure, no retention was found in the soft tissues. Accumulation of uranium was also found in the skeleton (Stokinger 1953). The amount distributed in the skeleton has been reported to be 23 5% of the intake in dogs (Morrow et al. 1972) 28-78% in rats (Leach et al. 1984) and 34-43% in guinea pigs (Leach et al. 1984). A biological half-time of 150-200 days (Ballou et al. 1986) or 70 days (Morrow et al. 1982) in the skeleton has been reported following inhalation exposure to soluble uranium compounds (e.g., uranium hexafluoride). 

Derivation Finely ground ore is leached under oxidizing conditions to give uranyl nitrate solution. The uranyl nitrate, purified by solvent extraction (ether, alkyl phosphate esters), is then reduced with hydrogen to uranium dioxide. This is treated with hydrogen fluoride to obtain uranium tetrafluoride, followed by either electrolysis in fused salts or by reduction with calcium or magnesium. Uranium can also be recovered from phosphate sand. 

Figure 9.2 presents a number of less commonly used stages, but all of them have commercial application in special cases. Pressure carbonate leaching of ores, which contain limestone can be cheaper than acid leaching and ion-exchange. This is followed by TBP extraction of the concentrate for final purification since any alternative would normally be based upon the same general chemical engineering principles and would only involve a different solvent. The older type of dryway process is then shown, with ammonia precipitation as the first step, since this still has applications for the production of special types of uranium dioxide. The final calcium reduction of oxide finds application on a relatively small scale, where the uranium metal product is required in powder form. 

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