Characterization of internal exposure to enriched uranium at a former gaseous diffusion plant

The National Institute for Occupational Safety and Health (NIOSH) is conducting a nested case-control study of mortality from multiple myeloma involving 581 subjects who worked at the Oak Ridge K-25 Gaseous Diffusion Plant. Internally-deposited uranium is the primary agent being considered in the exposure assessment. Routine operation and maintenance of the plant presented the potential for inhaling uranium of various enrichments. As part of the exposure assessment, records describing the various plant processes and procedures, documentation on the medical monitoring program, uranium urinalysis data, and procedures and analytical methods for monitoring uranium exposure were retrieved and reviewed. Uranium urinalysis data consisted of 161,055 uranium urinalysis results obtained by fluorometry and 171,914 results obtained by alpha particle counting. Approximately 20% of the workers were monitored for internal exposure using urine sampling. Mean and median uranium concentrations in urine for the monitored study subjects were slightly lower than for the entire population of monitored K-25 workers. The specific activity of uranium excreted in urine was determined by comparing results obtained using fluorometric and alpha activity measurements and indicate that the majority of internal exposure involved uranium that was depleted or enriched to no more than 4% U-235.