Isotopic compositions of 236U, 239Pu, and 240Pu in soil contaminated by the Fukushima Daiichi Nuclear Power Plant accident

Abstract

Six years after the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident, data for 236U and 236U/238U have remained limited to a few heavily contaminated samples. In the present study, activities of 236U, 239Pu, and 240Pu, along with other U isotopes in 46 soil samples both heavily and lightly contaminated by this accident were measured by inductively coupled plasma–mass spectrometry (ICP-MS) and triple-quadrupole ICP-MS. The 236U activities and 236U/238U atom ratios in these soil samples were in the range of (0.469–24.4) × 10−5 Bq kg−1 and ((0.099–1.35) × 10−7), respectively. Higher 240Pu/239Pu atom ratios (0.245–0.312) and 238Pu/239+240Pu activity ratios (0.859–1.62) indicated Pu contamination originated from this accident and global fallout in some samples. For those soil samples along with black substances collected along roads in Fukushima Prefecture, high linear correlations were presented between 236U activities and 239+240Pu activities (Pearson’s r = 0.755, p < 0.01), and between 236U activities and 238Pu activities (Pearson’s r = 0.844, p < 0.01). The analysis of these soil samples confirmed the release of 236U, although in trace amounts, during the FDNPP accident.

First determination of Pu isotopes (239Pu, 240Pu and 241Pu) in radioactive particles derived from Fukushima Daiichi Nuclear Power Plant accident

“…(0.103 ± 0.013) 21 as well as the 240 Pu/ 239 Pu atom ratio. As pointed out by previous studies 21,22 , these environmental samples were strongly influenced by global fallout, the value of which was 0.180 ± 0.007 for 240 Pu/ 239 Pu atom ratio 14–16 and 0.00194 ± 00014 for 241 Pu/ 239 Pu atom ratio 14 . The Pu contribution from the FDNPP accident can be estimated using the two end-member model 48 , and the extent of Pu contribution in soil samples was estimated as follows: 87% by Zheng et al .…”

“…21 and Yang et al . 22 were lower than that of DPs. The 241 Pu/ 239 Pu atom ratios in DPs agreed better with litters obtained by Zheng et al .…”

“…However, due to the relatively large error in the measured values, we cannot distinguish the source unit of the radioactive particles only from these atom ratios. The obtained 240 Pu/ 239 Pu and 241 Pu/ 239 Pu atom ratios in the radioactive particle were also compared with those of various environmental samples, such as soil 21,22 , litter 21,23 , black substance 23 , vegetation 24,25 , river water 26 , and air dust 27 , as shown in Figs 4 and 5. The 240 Pu/ 239 Pu atom ratio detected in environmental samples ranged from 0.14 to 0.381.…”

“…Thus, radioactive particle samples are completely free from the influence of the global fallout and the 240 Pu/ 239 Pu and 241 Pu/ 239 Pu atom ratio reflect the ratio in the core of the FDNPP directly. Therefore, both atom ratio of radioactive particles could provide us more accurate information on the ratio in the core of the FDNPP than other environmental samples and has great potential for source identification of the released radioactive particles in the environment, which should be further explored in a future study.

Figure 4Comparison of atom ratios of 240 Pu/ 239 Pu in various environmental samples, radioactive particles, and source terms: global fallout 15 , soils during 1966–1977 in Japan 16 , soil after the FDNPP accident 21,22 , litter 21,23 , black substance 23 , vegetation 24,25 , river water 26 , average of air dusts 27 , and calculated result of reactor core 28 .

Figure 5Comparison of atom ratios of 241 Pu/ 239 Pu in various environmental samples, radioactive particles, and source terms: global fallout 15 , soil and litter after the FDNPP accident 21 , and calculated result of reactor core 28 .

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Spatial pattern of plutonium and radiocaesium contamination released during the Fukushima Daiichi nuclear power plant disaster

“…It is estimated that only a fraction (~0.002%) of Pu from the total reactor core inventory was introduced to the environment 3 . For the data available, only trace quantities of FDNPP-derived Pu have been measured in environmental materials and mixing of the FDNPP Pu with trace amounts of Pu derived from atmospheric nuclear weapons testing complicates interpretation of the observed isotopic signature 5 , 8 . Heterogeneity in both isotopic ratio and specific activity of Pu in the vicinity of FDNPP has raised questions about the source and composition of contamination 4 , 9 .…”