Dissolution fractions and half-times of single source yellowcake in simulated lung fluids

Thirty-five samples of yellowcake, the primary product of uranium milling, were obtained from industry. Samples were selected from the archives of 6 yr of yellowcake production from a single uranium milling operation. Ten representative samples were selected on the basis of color variation and chemical composition. Dissolution fractions and half-times were obtained using both simulated interstitial and simulated surfactant lung fluids maintained in vials submerged in a shaker water bath at 37 degrees C for 26 days. Samples were collected at 12 different time intervals and fresh simulant replaced. Uranium concentrations were analyzed by fluorometric methods using ASTM techniques. The fraction of the uranium undissolved, F, can be well modelled by the sum of three exponential terms: (See formula in text). Dissolution fractions and half-times in the two simulated lung fluids were found to be not significantly different. The point estimates for the six parameters (three fractions of the amount of uranium and the three associated half-times for dissolution) based on the analyses are: f1 equals 16.0% and t1/2(1) equals 3.5 hr, f2 equals 16.5% and t1/2(2) equals 4.4 days, and f3 equals 67.5% and t1/2(3) equals 224 days. The final characterization of yellowcake resulting from this investigation strongly suggests that dissolution rates of yellowcake from a single source may be individually process specific or site specific.