Introduction of Isotope Ratio Analyses of 235U and 238U Nuclide in the Soil Using ICP-MS and Microwave Digestion Wide-Area Soil Survey Related to the Fukushima Daiichi Nuclear Power Plant Accident

This paper provides an overview of a wide-area soil study[1] related to the Fukushima Daiichi Nuclear Power Plant based on isotope ratio analysis of 235U and 238U in the soil using ICP-MS (Y. Takagai, Bunseki Kagaku, 60, pp. 947−958 (2011)). In order to perform precise isotope ratio analysis using ICP-MS, we
successfully established isotope ratio of uranium as index of the isotope ratio obtained from the actual test samples. This was done by using uranium in a geochemical reference material instead of a uranium reference sample. Furthermore, we used the ICP-MS cell pass voltage to calibrate and/or correct mass
bias when performing mass spectrometry. The result was that we were able to measure the isotope ratio of uranium with an accuracy of 0.37% without using a radioactive standard source. We were also able to determine both 235U and 238U, with a lower detection limit of 0.010 μg/kg for each. During nuclear disasters and other emergencies, this technique should be more effective than traditional ICPMS and α-particle spectroscopy for rapidly ascertaining conditions of wider area. We also monitored locations ranging between 5 and 80 km in distance from Tokyo Electric Power’s Fukushima Daiichi Nuclear Power Plant (115 locations within Fukushima Prefecture). Our results showed that although there were differences in the amount of uranium found at different sampling sites, the isotope ratio was nearly the same as uranium’s natural abundance.