Dose assessment for inhaling hafnium particles based on laboratory rats study

Internal radiation from inhalation of hafnium tritide aerosols may be a significant radiation protection problem encountered by nuclear facility workers. Based on experimental results of the rat intratracheally instilled with hafnium tritide particles and on a self-absorption factor of beta particles determined by a numerical method, a biokinetic model was developed for inhaled particles of hafnium tritide. Results show that lung burdens of the tritide are well represented by a two-component exponential equation; biological half-lives derived for the retention of 3H in lung were 4.9 d and 1,257 d for the short- and long-term clearance, respectively. The tritium clearance rate via urine or feces was described by bi-phase exponential components. At the end of the experiment (180 d after instillation), only approximately 30% of the initial lung burden of 3H had been eliminated, of which approximately 98% was excreted via feces and 2% in urine, but none through exhaled air. Results also showed that a large percentage (70%) of the hafnium tritide initially present in lung still remained in the organ 6 mo after the exposure. The calculation of the radiation dose indicates that the cumulative dose to the lung directly from the tritide particles was approximately 10(6) times the lung dose from the dissolved tritium in the lung region. The committed effective dose to the lung was estimated to be 5.41 x 10(-10) Sv Bq(-1), which is over 99% of that to the whole body. The dose to the liver was 6.00 x 10(-15) Sv Bq(-1). This information will be useful in developing new guidelines for radiation protection purposes.