Internal dosimetry of tritium in the mouse given HTO or OBT as a model of human exposure to tritium sources

Nuclear fusion energy is expected as a possible choice of future energy source as a substitute of fossil energy. In fusion reactors a large amount of tritium is expectedly used as fuel. The fuel tritium might be released to the environment from fusion facilities accidentally and chronically. Tritium released to the environment will be taken up by plants and animals including man. Tritium taken up by human body exists mainly in two chemical forms, i.e. as tritiated water (HTO) and as organically bound tritium (OBT). The dose coefficient of OBT is several times larger than that of HTO. The exact estimation of OBT level in the human body must be quickly and conveniently estimated especially in accidental cases. However, the dosimetry of OBT for the humans has some difficulty because direct sampling of human tissues for the dosimetry purpose is limited. In our laboratory, a series of experiments on the dosimetry of OBT in the mice has been performed. The OBT localization to cell nuclei increases the microscopic dose to cell nuclei by a factor of 3 - 6 in the case of DNA-bound tritium in comparison with the tissue-averaged tritium dose. The tritium localization is of less importance in the case of protein-bound tritium. The blood level tritium is useful and convenient for OBT dosimetry in a practical case of radiation protection of humans after either acute and chronic intake of tritium. A new technique enabled us to estimate the dose to active red bone marrow. Conservative estimate of HTO and OBT level in the human body can be successfully made by blood tritium assay