On June 6, 2017, an accident occurred at the JAEA’s Oarai Research and Development Center, where 5 workers inhaled plutonium. This study aims to evaluate the validity of the internal dose assessments conducted by the QST and to share the findings with the dosimetry community.

In vivo measurements (lungs and liver) and bioassays were performed. The particle size of the inhaled material was estimated based on the ratio of early fecal excretion to residual lung content. The absorption types of Pu and 241Am were evaluated using urinary excretion levels from bioassay results of early fecal and pre-Ca- DTPA urine samples.

The estimated particle sizes of the inhaled materials were over 10 μm and several micrometers for 2 workers, respectively. The absorption types were evaluated as intermediate between M (Moderate) and S (Slow). A change in urinary excretion levels between before and after Ca-DTPA administration was observed, with the most significant reduction occurring after the initial treatment, followed by smaller changes.

The internal dose assessments for the 5 workers were confirmed to be conservatively conducted within a reasonable range. Comprehensive individual monitoring in cases of internal contamination by actinide is crucial to reducing uncertainties in dose assessment.