Characterisation of Enriched Uranium Dioxide Particles from a Uranium Handling Facility

Abstract

A case study was conducted on approaches for using a prospective measurement of the particle size distribution of a process powder in a uranium handling facility to assign an appropriate annual limit on intake (ALI) for potential inhalation exposures of workers to that powder. The aerodynamic size distribution was determined for particles to which workers might be exposed during dispersion of dusts in an enriched uranium casting operation. Bulk powders were obtained during cleaning of the casting crucibles. The material was 0.846 uranium by mass, which corresponds to the theoretical uranium mass fraction for U 3 O 8 . The material was aerosolised using a DeVilbis dry powder blower and separated by aerodynamic diameter using a five-stage aerosol cyclone train. Approximately 0.08 of the total mass processed in the casting operation was retained as residual oxide, and approximately 0.01 of that mass was less than 10 μm aerodynamic diameter. Assumptions from ICRP 30 and from ICRP 68 and 69 were used to calculate the appropriate dose coefficients, depending on the fraction of the bulk powder that might be suspended into the workplace. Using the assumptions of ICRP 30 for Class D material, the potential location of particle deposition in the respiratory tract has little influence on assignment of an appropriate dose; using the actual particle size distribution of the powder reduced the ALI to about 90% of the value based on assuming that all re-suspended material had an activity median aerodynamic diameter of I μm. Using the assumptions of ICRP 30 for Class W and Y materials, the ALI was increased by a factor of 2.4, 6.3, 9.2, or 10.9 depending on whether the suspended material was limited to particles of aerodynamic diameter less than 10 μm, 20 μm, 50 μm, or all measured particle sizes, respectively. Using the assumptions of ICRP 68 and 69 the ALI for powders of Type F is more dependent on particle size than the ALI for Class D material and would be increased by a factor of 1.1, 1.5, 1.6, and 1.6 over the ICRP 30 model values for the four assumptions of particle size. The ALI for Type M material was changed by factors of 0.8, 1.8, 2.4, and 2.2 (an overall factor of 3) for the four assumptions of available particle size. For Type S material, the ALI changes by factors of 5.0, 10.4, 13.5, and 12.2 (again, an overall factor of about 3) compared to the ICRP 30 values for the four assumptions of particle size. Thus, the potential radiation doses to workers can vary substantially for Type M and Type S materials depending on which size fractions of the bulk powder are suspended into the breathing zone. The approaches and results of this case study provide insight into when prospective studies of particle size and solubility should be used to improve worker protection.