Dose estimations of individuals occupationally exposed to ionising radiation are currently carried out by the evaluation of personal dosimeters, e.g. film badges, or by activity monitoring of the surroundings and subsequent calculations of the exposure. In the case of uncontrolled exposures, e.g. accidental whole- or partial-body exposures or incorporation of radionuclides, doses can occur one order of magnitude larger than the annual dose limit. In such situations personal dosimeter readings are often very imprecise or not available at all and a retrospective estimation of the dose will be necessary. Detailed knowledge of the doses received by individuals forms the basis for follow-up checks and their further medical treatment and will substantially reduce the possibility of further damage. At present, biological dosimetry, by the determination of the rate of unstable chromosome aberrations (dicentric chromosomes) in peripheral blood lymphocytes, is commonly used for retrospective dose estimations. In general, however, this method cannot be used to assess exposures which occurred more than a few years ago because these chromosome aberrations disappear with a half-life of about 2 years. For this reason, doses such as those probable in cases of overexposure cannot be detected by means of the determination of unstable chromosome aberrations after a longer period of time.

This project will study a group of employees of the Kurchatov Institute who took part in the clearing work at Chernobyl in 1986, all of whom have been exposed to ionising radiation. For all of these individuals dosimeter readings exist as well as a detailed description of their tasks during the post-accident work (duration, place etc.). The applicability of retrospective dosimetry by analysis of stable chromosome aberrations (fluorescene in situ hybridisation) in the low dose-range will be studied in the project. The possibility of using this method in the high dose-range has recently been under investigation. Furthermore, the possibility of using the method of electron spin resonance analysis as a standard method of retrospective dosimetry will be investigated in this project. In the first phase of the project each method will be established and improved at the institutes in the NIS. In the second phase, a comparison of the different methods of retrospective dose assessment will be carried out using the aforementioned described cohort with individuals exposed to different external dose levels. In parallel to these investigations, the individual doses of members of this cohort will be determined by analyses of personal dosimeter readings and will further be calculated by computer models which will be developed. A database will be established which will collect the dosimetric, medical, occupational and personal information of the employees of the Kurchatov Institute concerned. This will also contain information concerning the exposure conditions, radiation fields and dose-rates inside the destroyed reactor building. Both data sets will be linked and will be used for verification of the computer-based models.