Journal of Radioanalytical and Nuclear Chemistry, Articles, Vol. 185, No. 1 (1994) 157-165 CONCENTRATIONS OF THORIUM AND URANIUM IN FRESHWATER SAMPLES COLLECTED IN THE FORMER USSR K. SHIRAISHI,* Y. IGARASHI,** M. YAMAMOTO,*** T. NAKAJIMA,* I. P. LOS,**** A. V. ZELENSKY****, M. Z. BUZINNY**** *Division of Radioecology, National Institute of Radiological Sciences, 3609 lsozaki, Nakaminato, lbaraki 311-12 (Japan) **Geochemical Research Department, Meteorological Research Institute, 1-1 Nagamine, Tsukuba 305 (Japan) ***Low Level Radioactivity Laboratory, Kanazawa University, Tatsunokuchi, lshikawa 023-12, (Japan) ****Laboratory of Radioecology, Ukrainian Scientific Center of Radiation Medicine, 53 Melnikova, Kiev 254050 (Ukraine) (Received June 17, 1994) Approximately 100 freshwater samples were collected in Ukraine, Russia, and Belomssia with regard to the Chemobyl accident. Thorium and uranium were determined by both quantitative and semiquantitative analysis modes of inductively coupled plasma mass spectrometry (ICP-MS). Thorium-232 was detected in only a few samples. Uranium concentrations ranged from non-detectable to 1,000ng/ml. Mean and median concentrations of 2:~SU were found to be 30.7 + 139 and 0.7 ng/ml, respectively. The isotope ratio of 234U/238U ranged from 4.6 .-10 -5 to 4.4.10 -4. Mean ratio of 235U/238U was 0.00721 + 0.00006 (n=27). Since the Chemobyl accident much information in many countries has been accumulated concerning radiation protection) -3 Knowing concentrations of radioactive nuclides in drinking water sources in the former USSR is important for dose estimation of its inhabitants. Concentrations of nineteen elements in the same freshwater samples were previously reported. 4 Another study also established intakes of radioactive and non-radioactive nuclides in environmental samples (total diet, milk, ftsh, etc.) collected in the vicinity of Chemobyl: Thorium and uranium usually have been analyzed by a-spectrometry after chemical separation. 6-8 Isotope ratios of uranium were measured with good precision (better than 0.05%) by thermal ionization mass spectrometry and grow discharge mass spectrometry. However, sub-micrograms of uranium are required for those determinations.9,1~ Recently, ICP-MS has been recognized as a powerful and versatile analytical method at the sub-ppb level) 1-14 Approximately one hundred freshwater samples had been collected in the former USSR with regard to the Chemobyl accident. In this report, their concentrations of thorium apd uranium were determined by both quantitative and semiquantitative analysis modes of ICP-MS. Isotope ratios of uranium are discussed from the viewpoints of contamination due to the Chemobyl accident. Elsevier Science S. A., Lausanne Akad6miai Kiad6, Budapest