Study on discrimination of environmental plutonium

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Plutonium in the environment is generally composed of isotopes of mass numbers ranging from 238 to 241. These isotopes, except Pu-241 (beta-emitter), are all long-lived alpha-ray emitters with high radiotoxicites. Of the characteristics for isotopic composition of Pu, the ratio of Pu-240/Pu-239 particularly gives strong evidence of the contamination due to Pu of non-fallout origin. This work was focused on the studies for developing the precise measurement of Pu-240/Pu-239 ratio by ICP-MS and TIMS and their application.As for ICP-MS measurmenet, satisfactory results were obtaind wit the following consideration : 1) U contents introduced into the ICP-MS should be less than 0.1 ppb, 2) Pu-240 content must be adjust to become more than 0.1 ppt, 3) mass area (238 amu) must be measured and 4) care must be taken for interference of polyatomic species with Pb and Ar. On the other hand, as for TIMS, high bacground peaks in the mass range of 235 to 240 amu were detected in the instrument transf … More erred from Tokyo Univ., and now reducing of background is conducted. Therefore, further studies are needed for precise measurement of the Pu-240/Pu-239 atomic ratio.Application of ICP-MS was done for soil samples in and around the former Soviet Union's Semipalatinsk nuclear test site. More than 300 soil samples were subjected to radiochemical analysis, and measured by usual alpha-ray spectrometer and ICP-MS.As a results, while the overall Cs-137 levels were as same as or slightly lower than Japanese global fallout, Pu levels in most of sites were several ten to several hundred times higher than the Japanese level. The ratios of Pu-240/Pu-239 were significantly lower than the value of 0.18 commonly accepted for global fallout Pu. These areas are clearly contaminated with weapon-grade Pu. Discrmination of Pu was performed using the Pu-240/Pu239 ratio in the samples from Ust'-kamenogorsk City (300 km east of test site) It was found thata fraction of 30-60% of total Pu were due to the close-in fallout Pu from the SNTS debris. Less