P-11-258 1 Assessments of effective dose to population groups in Sweden and Russia after the Chernobyl accident E Wallstrom, 1 (2) C Thornberg 3 , R Vesanen 2 , I Zvonova 4 , T Jesko 4 , V Golikov 4 , V Erkin 4 , T Wohni 5 , A Cederblad 2 , S Mattsson 3 , M Alpsten 2 , M Balonov 4 1 Dept. of Radiology, Trollhattan Hospital - NAL, SE-461 85 Trollhattan, Sweden 2 Dept. of Radiation Physics, Goteborg University, Sahlgren University Hospital, SE-413 45 Goteborg, Sweden 3 Dept. of Radiation Physics, Lund University, Malmoe University Hospital, 205 02 Malmoe, Sweden 4 Institute of Radiation Hygiene, Mira St 8, St Petersburg, Russia 5 Norwegian Radiation Protection Authority, Box 55, N-1345 Osteraas, Norway 1 INTRODUCTION The Chernobyl accident gave rise to a great number of questions of which many directly concerned the radiation exposure of affected persons. In this paper, mainly based on a thesis (Wallström, 1998), the results of different studies in Western Sweden and in the Brjansk region in Russia are presented. All studies deal with various components in the assessment of effective dose, both from external and internal irradiation. At 20.00 hrs on April 28, the first sample containing Chernobyl deposits was taken at the Department of Radiation Physics in Göteborg (Figure 1). Systematic sample collection was immediately initiated (Mattsson and Vesanen, 1988). The continuos Swedish studies were carried out in Göteborg in Western Sweden, and at three different farms around Göteborg. The work in Göteborg showed that, during the first week, the radionuclides were deposited via dry deposition. The deposition was low. However, on May 8, the deposition in Western Sweden increased rapidly during a single rain fall (Figure 2). After that day almost no further deposition occurred. Around 98% of the resulting total deposition in this area was deposited during the rainfall on May 8. The resulting total deposition of 137 Cs was, however, still low; 1-2 kBqm -2 . The investigations at the farms studied started on May 13, 1986, when the cows were let out for grazing for the first time that year and continued up to 1989. Figure 1 Pulse height distribution from the first wipe-test sample taken at 20.00 hrs, April 28, at the Sahlgrenska University Hospital (Information Department, Göteborg University, data from Mattsson and Vesanen, 1986). Figure 2 The deposition on the ground of 137 Cs, 134 Cs, 131 I, 106 Ru and 103 Ru in Göteborg after the Chernobyl accident (Mattsson and Vesanen, 1988) In 1990 a collaboration started between laboratories in the Nordic countries and the Leningrad Inst. of Radiation Hygiene in the former Soviet Union (now Inst. of Radiation Hygiene, St. Petersburg, Russia). The studies were performed in Brjansk 150-250 km NNE of the Chernobyl NPP. Parts of these studies still continues, e.g. this one, devoted to various dosimetry questions, as a joint work between the Depts of Radiation Physics in Göteborg and Malmö, and the Inst. of Radiation Hygiene, St. Petersburg. The results presented here pertain to the period 1990 - 1998. The deposition in Brjansk mainly originated from rainfall during April 28-29 (Balonov,1993). The 137 Cs deposition levels in the areas where the measurements were performed were 0.9 - 2.7 MBq m -2 ; about 1000 times higher than in Göteborg, Sweden. The aim of this paper is to present and discuss the various methods used for assessing effective dose from internal and external irradiation, mainly based on different field studies. The aim is also to give the results of effective dose in villages in Brjansk, Russia, and in Western Sweden, and to discuss the contribution from internal and external sources. The results and discussions refer to the two radioactive caesium isotopes 134 Cs and 137 Cs, as well as to 131 I. Most results of effective dose are summarised in Table 3.