Retrospective Dosimetry of an Accidental Intake Case of Radioruthenium-106 at the Tokai Reprocessing Plant

摘要

On November 30, 1978, two workers in the acid recovery cell of the nuclear spent fuel reprocessing plant of JAEA-NFCEL were involved in an accident in which they became unconscious due to lack of oxygen. They were rescued immediately by other workers and were given artificial respiration to restore their normal breathing. Subsequent measurements by the whole-body counter showed that they were contaminated internally with ~(106)Ru. Prolonged lung monitoring was carried out for one of them. A significantly high activity of ~(106)Ru was obtained in the lung monitoring on the day of the accident. The physicochemical characteristics of the incorporated radioactive materials were not observed. In order to perform more reasonable internal dose assessment, the interpretation of the bioassay datasets of the worker was made based on the guideline demonstrated in the EU project IDEAS. The effective half-life of the materials in the lungs was determined to be 140 days which leads to the default Type S absorption type in the HRTM and the f_1 value was estimated to be less than 0.005 which is one-tenth of the default value. Simultaneous intakes via inhalation and ingestion were also suggested from several pieces of evidence although pure inhalation was assumed for internal dose assessment at the time of the accident. The aerosol size of the materials was not determined due to a lack of information if assuming simultaneous intakes; however, the resulting committed effective dose was about 1 mSv and its variation was small against the aerosol size ranging from 1 μm to 20 μm.