Precise measurement 236U/238U isotope ratio measurement in 10-8 range as well as 234U/238U and 235U/238U using thermal ionization mass spectrometry: Application to nuclear accident soil samples

The principal isotopes of uranium e.g. 235U and 238U are of primordial origin and 234U present in radioactive equilibrium with 238U. Anthropogenic uranium also contains small amount of 236U, formed by neutron capture of 235U in nuclear industrial processes. 236U has been noticed in the environment as a result of nuclear activities including nuclear explosions, nuclear plant accidents or dumping of nuclear waste. Therefore, it can be a good fingerprint for investigating nuclear power plant accident soil samples. A multicollector Isotopx Ltd. Phoenix 62X thermal ionization mass spectrometer (TIMS) equipped with wide aperture retardation potential (WARP) energy filter was optimized for the determination of 236U/238U isotope ratios in the isotopic range of 10-8 in a reference material supplied by University of Vienna. We have selected soil samples from Chernobyl (Ukraine) and Fukushima (Japan) affected by nuclear power plant accident, Kosovo (Serbia) due to depleted uranium (DU) bombardment and Hiroshima (Japan) due to atom bomb. We validated our method with reference standard materials e.g. NIST 4350b (River sediment in USA). Soils were digested by microwave system with acid mixture of HNO3, HF and HClO4. A modified method was used for isolation of U using Anion Exchange resin-UTEVA to remove high concentration of Fe from Japanese soil sample for high precision analysis by TIMS. Detection limit of 236U/238U ratio by TIMS yielded 10-9~10-10. Uranium ore sample which has not been affected by nuclear accidents yielded to 10-10. Results of 236U/238U ratio for Chernobyl and Kosovo soil were in range of 10-4~10-5, whereas NIST 4350b, Fukushima and Hiroshima soil sample were under 10-7. The Radioactive dis-equilibrium of 234U/238U, enrichment and depletion of 235U/238U were noticed in some soil samples in addition to significant evidence of 236U/238U.URANIUM BIOGE