Techniques for Yellowcake Dissolution Studies in Vitro and Their Use in Bioassay Interpretation

Abstract

The high variability in solubility of yellowcake produced by different mills complicates the interpretation of routine bioassay data. A simple in vitro dissolution test is needed for yellowcake to improve this interpretation. A series of experiments was designed to evaluate the relative importance of solvent composition, method, pH and temperature in determining yellowcake dissolution according to a model developed from the known composition of a test yellowcake and data from inhalation exposures of humans to UO3 or U3O8. Useful in vitro dissolution results can be obtained using either simulated serum ultrafiltrate or simulated lung fluid as the solvent if dissolved and undissolved yellowcake are separated by a membrane filter. In vitro dissolution experiments estimated the soluble portion of the test yellowcake within +/- 6% (mean +/- 2 S.E.) and showed that the dissolution rates of the more soluble and less soluble portions corresponded to Class D and Class Y compounds, respectively. It was not necessary to maintain physiological pH or temperature conditions to approximate the "human" model. The greatest utility of in vitro dissolution results was in the estimation of the more soluble fraction of the yellowcake and to indicate whether prior excretion of a Class D uranium compound and possible kidney damage could have occurred before detection of an exposure. Some guidelines for the use of in vitro dissolution data in biassay interpretation are suggested based on ICRP Publication 30 recommendations.