Abstract
135Cs/137Cs is a potential tracer for radiocesium source identification. However, due to the challenge to measure 135Cs, there were no 135Cs data available for Japanese environmental samples before the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident. It was only 3 years after the accident that limited 135Cs values could be measured in heavily contaminated environmental samples. In the present study, activities of 134Cs, 135Cs, and 137Cs, along with their ratios in 67 soil and plant samples heavily and lightly contaminated by the FDNPP accident were measured by combining γ spectrometry with ICP-MS/MS. The arithmetic means of the 134Cs/137Cs activity ratio (1.033 ± 0.006) and 135Cs/137Cs atom ratio (0.334 ± 0.005) (decay corrected to March 11, 2011), from old leaves of plants collected immediately after the FDNPP accident, were confirmed to represent the FDNPP derived radiocesium signature. Subsequently, for the first time, trace 135Cs amounts before the FDNPP accident were deduced according to the contribution of global and FDNPP accident-derived fallout. Apart from two soil samples with a tiny global fallout contribution, contributions of global fallout radiocesium in other soil samples were observed to be 0.338%–52.6%. The obtained 135Cs/137Cs database will be useful for its application as a geochemical tracer in the future.
Highlights
The Fukushima Daiichi Nuclear Power Plant (FDNPP) accident in 2011 released tremendous amounts of radionuclides into the terrestrial environment, and the volatile radionuclides, especially radiocesium, have contributed to most of the radioactivities[1,2,3,4,5]
Increasing the numbers of data for lightly contaminated samples is highly desired for the analysis of low 135Cs activities, especially for obtaining a database of 135Cs/137Cs atom ratios to illustrate the influence of the FDNPP accident
Trace amount of 135Cs with the highest activity of 0.0212 ± 0.0024 Bq kg−1 and 135Cs/137Cs atom ratios up to 4.02 before the FDNPP accident were deduced according to the contribution of the global fallout and the FDNPP accident-derived fallout
Summary
The Fukushima Daiichi Nuclear Power Plant (FDNPP) accident in 2011 released tremendous amounts of radionuclides into the terrestrial environment, and the volatile radionuclides, especially radiocesium, have contributed to most of the radioactivities[1,2,3,4,5]. Two recent studies reported relatively higher 135Cs/137Cs atom ratios for the standard reference materials released by the Japan Society for Analytical Chemistry (JSAC)[14,26] These values were somewhat higher than the data (0.32–0.41) from heavily contaminated samples[6,25,27,28]. This indicated a rising proportion of the contribution from global fallout and other background sources in lightly contaminated samples due to the FDNPP accident, since the 134Cs activities (< 360 Bq kg−1, decay corrected to March 11, 2011) were relatively low for these standard reference materials. The low detection limits of 2.59 × 10−5 and 3.28 Bq kg−1 for 135Cs and 137Cs make it possible to carry out precise 135Cs/137Cs ratio analysis in lightly contaminated samples
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