Determination of ¹³⁵Cs/¹³⁷Cs isotopic ratio in soil collected near Fukushima Daiichi Nuclear Power Station through mass spectrometry

Determining the completeness of nuclear reactor decommissioning is an important step in safely utilizing nuclear power. For example, ¹³⁷Cs from the Fukushima Daiichi Nuclear Power Station (FDNPS) accident can be treated as background radioactivity, so determining the origin of ¹³⁷Cs is essential. To accomplish this, measuring the ¹³⁵Cs/¹³⁷Cs isotope ratio can be useful, so this study optimized a solvent extraction method with calix[4]arene-bis(t-octylbenzo-crown-6) [BOBCalixC6] in 1-octanol to purify Cs. This optimized method was applied to Cs purification in soil samples collected near the FDNPS, and the ¹³⁵Cs/¹³⁷Cs isotope ratios were measured using both thermal ionization mass spectrometry (TIMS) and triple quadrupole induced coupled plasma mass spectrometry (ICP-QQQ). The results of each of these measurements were compared, and we found that Cs isotope ratios obtained by TIMS were more precise, by an order of magnitude, while the ICP-QQQ results possessed good abundance sensitivities. A slightly higher ¹³⁵Cs/¹³⁷Cs ratio in the northwest area of the FDNPS was observed, while other areas exhibited similar values, all within the measurement error range, which indicated different origins of radiocesium. These results agreed with previously reported ¹³⁴Cs/¹³⁷Cs activity distributions, suggesting that this ratio may be useful in identifying radiocesium origins for evaluating future nuclear reactor decommissions. (author)