Internal dosimetry of uranium workers exposed during the nuclear fuel fabrication process in Spain

Abstract

Workers in the Juzbado plant of ENUSA in Spain are exposed to intakes of low enriched uranium during the fabrication of nuclear fuel elements for Nuclear Power Plants. CIEMAT started in 2014 an individual monitoring program for ENUSA workers consisting in routine bioassay measurements of uranium in urine by alpha spectrometry and a methodology of dose assessment based on ICRP Publications 78/68, ISO 27048 standard and IDEAS Guidelines V2. The individual monitoring program is complemented with workplace monitoring using Static Air Samplers (SAS) at the Juzbado facility. Results of routine monitoring data of 24 h-urine samples of Juzbado workforce confirmed low level of chronic intakes of uranium oxides in the plant, combined with acute intakes associated to incidents. IMBA software allows the dose assessments in complex intake regimes of occupational inhalation of Type S uranium compounds. Uncertainties of monitoring data were evaluated using the Scattering Factor approach (ISO 27048) for the intake calculation in the defined internal exposure scenario. A total of 200 workers at risk of internal exposures in the Juzbado facility were included in routine, special, confirmatory or task-related monitoring programs established by CIEMAT in agreement with ENUSA. Annual chronic intakes were assessed for 106 workers with monitoring results above the Minimum Detectable Activity (MDA = 0.50 mBq.sample−1 of 234U, 238U and 235U). Committed Effective Doses E(50) > 1 mSv/year were initially detected in 50% of these cases. Maximum annual intakes correspond to few cases of long term exposed workers with E(50) around 5 mSv/year. The preliminary study of the impact on the dose assessment using the new uranium model according to ICRP Publication 137 is presented here, showing a reduction of E(50) around a factor of 4 for workers exposed to uranium oxides re-defined as Type M/S materials (more soluble compounds, higher urinary excretion, lower dose coefficients than Type S uranium materials defined by ICRP Publications 78/68).