Ruthenium-106 is a fission product radionuclide present in nuclear fuel cycles and has potential for release as ruthenium tetroxide. Ruthenium tetroxide is a vapor, diffuses rapidly in air and through porous barriers and is rapidly reduced to the dioxide form when in contact with organic compounds. The current ICRP model for inhaled particles does not predict respiratory tract deposition and clearance for inhaled vapors. This study provides data needed to assess radiation doses to humans exposed to IMRu tetroxide. Rats were given a nose-only exposure to IMRuO4 vapor to determine the subsequent distribution and retention. Initial whole-body activity ranged from 4.1 to 15 pCi of IMRu. Fifty-four per cent was in the upper respiratory tract, less than 1% was in the pulmonary region, the remaining 45% was external contamination. Clearance of l ' Ru from the body was predominately through the gastrointestinal tract to feces. A simulation model was developed to describe retention and translocation of inhaled IMRu04 and to provide estimates of radiation doses to internal organs. Current ICRP recommendations consider lung and gastrointestinal tract as critical organs for inhaled particles of radioactive ruthenium. This study indicated that the nasopharynx should be considered a critical region for vapor forms of radioactive ruthenium in human inhalation exposures.

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