TM1999-114.pdf

Following the recent restart of operations at the Y-12 Plant, the Radiological Control Organization (RCO) observed that the enriched uranium exposures appeared to involve insoluble rather than soluble uranium that presumably characterized most earlier Y-12 operations. These observations necessitated changes in the bioassay program, particularly the need for routine fecal sampling. In addition, it was not reasonable to interpret the bioassay data using metabolic parameter values established during earlier Y-12 operations. Thus, the recent urinary and fecal bioassay data were interpreted using the default guidance in Publication 54 of the International Commission on Radiological Protection (ICRP); that is, inhalation of Class Y uranium with an activity median aerodynamic diameter (AMAD) of 1 µm. Faced with apparently new workplace conditions, these actions were appropriate and ensured a cautionary approach to worker protection. As additional bioassay data were accumulated, it became apparent that the data were not consistent with Publication 54. Therefore, this study was undertaken to examine the situation.

This study uses state-of-the-art information on the behavior of uranium in the respiratory tract and in the body to assess the enriched uranium intakes in a small subset of Y-12 workers during late 1998 and early 1999. Specifically, the mathematical models applied are those that form the bases for ICRP Publication 78 which superseded Publication 54 noted above. The study confirmed that the enriched uranium exposures involve insoluble uranium and found that the Y-12 bioassay was consistent with the methods of ICRP Publication 78. The methods of Publication 78 for interpretation of the bioassay data also were found to reduce the estimated Y-12 worker doses from insoluble uranium by a factor of about five. Thus, the following recommendations are made:

• The methods of ICRP Publication 78 should be used in the interpretation of uranium bioassay data. The new models of the respiratory tract and of the behavior of uranium in the body in that publication are broadly consistent with the observed urinary and fecal bioassay data for the Y-12 workers.
• The routine bioassay program must continue to include both fecal and urine sampling to enable clarification of the workplace conditions. Monthly fecal sampling, while desirable from a statistical viewpoint, can probably be reduced to bimonthly or quarterly sampling once more experience is gained in the application of the new models to uranium exposures at the Y-12 Plant.
• Use of the methods of Publication 78 requires that the models underlying these methods be used in assigning the worker’s radiation dose. Components of these models have no counterpart in the models upon which 10 CFR 835 is based; thus, efforts should be promptly initiated to obtain the appropriate regulatory exemptions.
• Further assistance should be provided to Y-12 RCO staff in implementing the methods of ICRP Publication 78. Considering the demands on their time by routine tasks, it is unreasonable to expect that the staff can implement and evaluate these methods while performing their other duties. 

Although beyond the scope of this study, it is suggested that the cursory review of the Y-12 personal air monitoring program in this study be expanded to include more workers who are currently wearing personal air monitoring devices. While workers’ intakes can best be determined by bioassay methods, air sampling is a primary indicator of the potential exposure of a worker to airborne materials and can provide early information on changes in workplace conditions.