Biokinetic analyses of the latest actinide model for internal dose assessment after an accidental inhalation at a nuclear facility in Oarai, Japan

An accidental inhalation of actinide nuclides, e.g. plutonium and americium, by 5 workers occurred due to an accident at Oarai Research and Development Institute, Japan Atomic Energy Agency in June 2017. Although wearing half-face masks, overalls and gloves, the workers possibly inhaled the radionuclides through tiny gaps between their faces and masks. In vivo counting by using a lung monitor and bioassay measurements for urinal and faecal samples have been performed at the National Institute of Radiological Sciences (NIRS) to assess their intake amounts and internal doses. Decorporation agents, i.e. Caand/or Zn-DTPA, were intravenously administered to the all workers for the purpose of enhancing the urinal excretion of actinide nuclides in blood, leading to difficulty in internal dose assessment because the biokinetics may deviate from the general model prediction. The effective dose of the worker who received the highest exposure is currently estimated, still as an initial assessment, to be less than 200 mSv based on previous biokinetic models recommended by International Commission on Radiological Protection (ICRP). The modified biokinetic models for plutonium, however, had been proposed to reflect data become newly available since development of the previous models. Recently, ICRP also has published some relevant recommendations describing revised biokinetic models for several elements as the Occupational Intakes of Radionuclides (OIR) series. In this study, as part of our effort on improvement of the internal dose assessment for the workers, biokinetic characteristics after inhalation of Pu-239 and Am-241 based on the latest models are compared with those based on the previous models