Metabolic models for tritium dosimetry

Tritium (3H or T) is the radioactive isotope of hydrogen, which is produced by both natural, and man made sources. Tritium has a small relative natural abundance compared to hydrogen and deuterium (D). As was assessed by the United Nations scientific committee the contribution of cosmogenic tritium to annual effective dose in human is very small, only about 0.01 μSv (UNSCEAR, 2000). In case of heavy water reactors annual tritium doses for critical groups are also very small but theoretically could reach values of several μSv. In this case professionally exposed personnel could be exposed to tritium doses of some mSv. According to these considerations environmental and dosimetric aspects of this radionuclide are of special concern for health physicists. Tritium dose assessment methodologies have special particularities because hydrogen is a chemical element with an important metabolic role in the human body. Operating experience to date of CANDU reactors has indicated that the major contributor to the internal dose of professionally exposed people is the tritiated heavy water (DTO). DTO, like the tritiated water HTO, is assumed to be uniformly mixed with body water pool and reaching equilibrium immediately after the intake. All the statements in this paper related to HTO dosimetry are also considered valid in case of DTO. The results of the computations performed with different retention functions corresponding to different compartment models are presented. The differences between the models due to OBT (Organically Bound Tritium) contribution are 5.6% for two- compartment model and 13% for three-compartment Dunford - Johnson model, respectively. In practice the contribution of OBT is considered to be about 10%. (authors)