The effects of protracted exposure on the biokinetics and health effects of radionuclides: the example of uranium

The current radioprotection system is based on the risks of radio-induced cancer occurrence and on the hypothesis of linear non-threshold dose/risk relationship. the risk coefficients used in this system come mainly from epidemiological studies performed on population irradiated in Hiroshima and Nagasaki, that was exposed to high doses resulting rom external irradiation with neutrons and gamma rays. Researches carried out in the field of internal contamination at lower doses have focused mainly on the occupational exposure of workers in the nuclear fuel cycle industry. The models available for this category of population correspond to limited situations of exposure (acute contamination by inhalation or injury) that are different from the pattern of exposure of pubic members (chronic contamination by ingestion). In that latter case, assumptions are made that chronic exposure can be treated, for the purpose of does calculation, as a sum of acute exposures. This hypothesis can lead to a miscalculation of doses since chronic accumulation of radionuclides can lead to bioaccumulation and/or biological effects that could influence both the deposition and the micro-localisation of the nuclides. A large-scale programme has been developed in IRSN in order to address these issues. the efforts have been focused on the study of the biokinetics (kinetics of absorption and retention, micro localisation) and biological effects of some radionuclides, in order to test the effects of protracted exposures in comparison with acute exposures. first experiments were realized with uranium, given to rodents for times ranging between 1 day to 2 years (life-span) via drinking water. Preliminary data indicate that kinetics of excretion of U from rodents contaminated every day for 1 to 12 months with 40 mg U.L-1 are changes when compared to those contaminated by a single ingestion. Similarly, biological effects on the central nervous system and on kidneys appear from 6 months of exposure, whereas existing models predict that, at these dosages, the internal concentration of uranium in these tissues should not be toxic. The overall results pointed out the fact that specific models should be set up for chronic exposure and that specific researches should be carried out on that point. (Author)