Precise measurement of 234U/238U, 235U/238U and 236U/238U isotope ratios in Fukushima soils using thermal ionization mass spectrometry

Abstract

Precise and accurate isotope ratio measurements of 234U/238U, 235U/238U and 236U/238U were performed simultaneously in Fukushima contaminated soil samples using thermal ionization mass spectrometry (TIMS). 234U, 235U and 236U isotopes were measured by Daly ion counting system whereas 238U was measured by Faraday collector using mass peak jumping method. Fukushima radio‑cesium contaminated soil samples were digested using microwave oven for complete decomposition. A combined AER-UTEVA separation scheme was applied for purification of U and its accurate isotope ratios measurement in Fukushima soils rich with Fe. Analytical validation of the modified U separation method was tested with certified reference material (CRM) JSd-2. The analytical precision (RSD%) of 234U/238U, 235U/238U and 236U/238U isotope ratios obtained by long-term repeatability for CRM NBL U010 was 0.288% (2σ), 0.017% (2σ) and 0.235% (2σ), respectively. Validation of accurate U isotope ratios measurement was checked with a CRM IRMM-184. This procedure was applied successfully to CRM JSd-2, SRM NIST 4354 and Fukushima soil samples. The concentration of 236U in Fukushima contaminated soils varied from 21.7–194.7 fg/g which showed the mixing effect of global fallout and Fukushima Dai-ichi Nuclear Power Station (FDNPS) accident fallout.