Uranium Aerosol Activity Size Distributions at a Nuclear Fuel Fabrication Plant

Abstract

Inhalation of uranium aerosols is a concern in nuclear fuel fabrication. Determination of committed effective doses and lung equivalent doses following inhalation intake requires knowledge about aerosol characteristics; e.g., the activity median aerodynamic diameter (AMAD). Cascade impactor sampling of uranium aerosols in the breathing zone of nuclear operators was carried out at a nuclear fuel fabrication plant producing uranium dioxide via ammonium uranyl carbonate. Complementary static sampling was carried out at key process steps. Uranium on impaction substrates was measured using gross alpha counting and alpha spectrometry. Activity size distributions were evaluated for both unimodal and bimodal distributions. When a unimodal distribution was assumed, the average AMAD in the operator breathing zone at the workshops was 12.9–19.3 μm, which is larger than found in previous studies. Certain sampling occasions showed variable isotope ratios ( 234 U/ 238 U) at different impactor stages, indicating more than one population of particles; i.e., a multimodal activity size distribution. When a bimodal distribution (coarse and fine fraction) was assumed, 75–88% of the activity was associated with an AMAD of 15.2–18.9 μm (coarse fraction). Quantification of the AMAD of the fine fraction was associated with large uncertainties. Values of 1.7–7.1 μm were obtained. Static sampling at key process steps in the workshops showed AMADs of 4.9–17.2 μm, generally lower than obtained by breathing zone sampling, when a unimodal distribution was assumed. When a bimodal distribution was assumed, a smaller fraction of the activity was associated with the coarse fraction compared to breathing zone sampling. This might be due to impactor positioning during sampling and sedimentation of large particles. The average committed effective dose coefficient for breathing zone sampling and a bimodal distribution was 1.6–2.6 μSv Bq −1 for 234 U when Type M/S absorption parameters were assumed (5.0 μSv Bq −1 for an AMAD of 5 μm). The corresponding lung equivalent dose coefficient was 3.6–10.7 μSv Bq −1 (29.9 μSv Bq −1 for an AMAD of 5 μm). The predicted urinary excretion level 100 d after inhalation intake was found to be 13-34% of that corresponding to an AMAD of 5 μm. Uranium aerosols generated at a nuclear fuel fabrication plant using ammonium uranyl carbonate route of conversion were associated with larger AMADs compared to previous work, especially when sampling of aerosols was carried out in the operator breathing zone. A bimodal activity size distribution can be used in calculations of committed effective doses and lung equivalent doses, but parameters associated with the fine fraction must be interpreted with care due to large uncertainties.

Particle Size-dependent Dissolution of Uranium Aerosols in Simulated Lung Fluid: A Case Study in a Nuclear Fuel Fabrication Plant

“…Each sampling campaign was designed to collect sufficient amounts of uranium for the dissolution rate studies but avoid particle overload on the impactions stages (which can lead to increased particle bounce-off). Time periods for sampling were determined based on previous knowledge of typical levels of airborne uranium (Hansson et al 2020). Sampling was carried out during work in the main production lines during normal production conditions.…”

“…Considering that some materials (e.g., Type S) correspond to very long half-times (e.g., a s s of 1 Â 10 −4 d −1 corresponds to almost 7,000 d half-time), a long experimental U. To allow for easier comparison, the same AMADs (15 and 5 mm) were used for all workshops, although some variations exist (Hansson et al 2020). Absorption from the alimentary tract (f A ) was assumed to be 0.02…”

“…It has been demonstrated that activity size distributions at the plant can be described with a bimodal size distribution (fine and coarse fraction), with some variations across the different workshops (Hansson et al 2020). For the dosimetric evaluation, it was thus assumed that impactor stages with cut-points greater than 5 mm (i.e., the first four stages) represent the coarse fraction, and remaining impactor stages + final collection stage represent the fine fraction.…”

Particle Size Dependent Dissolution of Uranium Aerosols in Simulated Gastrointestinal Fluids

“…In the present study, the effective dose coefficient, CED per ingested, and inhaled Bq 234 U with an AMAD of 10 μm were evaluated. The AMAD chosen represent a simplification of the bimodal distributions found in previous work ( Hansson et al 2020 ), to evaluate the effect of site-specific f A on CED per Bq in comparison to using default f A for stated material type.…”

“…For the nuclear fuel fabrication plant in this study, previous work has been carried out to improve dose estimates, including characterization of uranium aerosols at various workshops ( Hansson et al 2017 ) and characterization of the bimodal uranium particle activity size distribution for several workshops ( Hansson et al 2020 ). In a recently concluded study, site-specific lung absorption parameters for different uranium aerosol sizes were investigated ( Hansson et al 2022 ).…”