Compared to the Pu isotopic composition seen after the Chernobyl accident14,15, the Fukushima accident Pu had a slightly higher 241Pu/239Pu atom ratio, but lower ratio of 240Pu/239Pu (Fig. 2). However, due to the large amount of 239+240Pu (about 8.7×1013 Bq) released from the Chernobyl accident17, the activity ratio of 241Pu/239+240Pu of the Chernobyl accident (83±5)15,18is much lower than that of the Fukushima DNPP accident (107.8, average of S2 and S3 litter, 241Pu decay corrected to March 15, 2011). The Pu atom ratios increase with the increase of the fuel burn-up time in the reactor. The relatively higher 241Pu/239+240Pu activity ratio of the Fukushima DNPP accident might be because of the damage to the Unit 3 reactor, which had a mixed core, containing both uranium fuel and mixed uranium and plutonium oxide (MOX) fuel; the latter was about 6% of the core fuel. The additional production of 241Pu from the 239Pu fuel may have enhanced the 241Pu/239+240Pu activity ratio and 241Pu/239Pu atom ratio inside the reactor during normal operation before the accident.

Figure 2 Mixing plot of 241Pu/239Pu atom ratio vs. 240Pu/239Pu atom ratio for litter and surface soil samples collected in the 20-30 km zones of Fukushima prefecture, Japan and a comparison of isotopic composition with those of the Chernobyl accident and the global fallout sources. Error bars are ± 1 standard deviation. Data on the Chernobyl accident are cited from Muramatsu et al.14 and Ketterer et al.15. Data on the global fallout are cited from Kelley et al.12. Data on atmospheric fallout in Japan are cited from Zhang et al.13; these data were obtained from atmospheric fallout reference material prepared from samples collected at 14 stations through Japan in 1963-1979 by the Meteorological Research Institute (MRI), Japan. Full size image

The atom ratios of 240Pu/239Pu and 241Pu/239Pu found in the surface soil of J-Village were slightly lower than those in litter samples in Namie Town (S2) and Iitate Village (S3) in the NW direction of the Fukushima DNPP. The plot of 241Pu/239Pu vs. 240Pu/239Pu for the data of Table 1 for the global fallout, the soil in J-Village and the litter at sites S2 and S3 could be described by a linear function (r2 = 0.9901): 241Pu/239Pu = 0.9024×(240Pu/239Pu)−0.1656 (Fig. 2). It indicated that the Pu in J-Village surface soil (0–2 cm) contained a small proportion of global fallout Pu.

Using a two end-member mixing model (see Methods Section) based upon the work of Krey19, we found the percentage of Fukushima-derived 239+240Pu in the J-Village soil was 87 %; and the other 13 % 239+240Pu was of global fallout origin. We noted that Pu activities in the J-Village surface soil were ca. one order of magnitude lower than those in northwest of Fukushima DNPP.

In the samples that showed Pu contamination from the Fukushima accident, we detected extremely high 137Cs activities. They ranged from 1.15×104 to 4.65×106 mBq/g (Table 1). The activity ratios of 137Cs/239+240Pu for these samples ranged from 1.95×105 to 2.53×107 and they were 1–3 orders of magnitude higher than that of the Chernobyl accident (770, 137Cs corrected for decay to June, 1997)14, indicating that the release of 239+240Pu from the Fukushima DNPP accident was very small. This was supported by the 239+240Pu activity data in Table 1; even in the samples with high 137Cs contamination, the detected 239+240Pu activities were still in the typical activity range of the global fallout. To understand the differences of Pu emissions between the Fukushima DNPP accident and the Chernobyl accident, we made a rough estimation on the amount of atmospheric release of Pu and the percentage of core inventory released. The estimation was made based on the average of 137Cs/239+240Pu activity ratios (1.48×107) observed in litter samples at site S2 and S3, relative to the total amount of 137Cs releases, 1.5×1016 Bq and 3.58×1016 Bq, estimated by METI (Ministry of Economy, Trade and Industry, Japan)20 and Stohl et al.3, respectively, assuming 137Cs and Pu isotopes followed same deposition mechanism and no significant variation of 137Cs/239+240Pu activity ratio during the release and deposition. It should be noted that there is no attempt to make an accurate estimation on the release of Pu from the Fukushima DNPP accident due to the limited data on the deposition of Pu, but rather a rough estimation to obtain the information on the order of magnitude of Pu release from the accident. As shown in Table 2, the amounts of released 239+240Pu and 241Pu were 1.0×109 − 2.4×109 Bq and 1.1×1011 − 2.6×1011 Bq, respectively. These values are very close to those estimated by METI20 and about 4 orders of magnitude lower than those of the Chernobyl accident.17,18,21 Kirchner et al.22 recently calculated the mean fuel inventory of Pu isotopes in the Fukushima DNPP reactors using ORIGEN-ARP module of the SCALE-5.1 code system, this made it possible to estimate the percentages of the amounts of released Pu isotopes to core inventory with the information of fuel load in Unit 1, Unit 2 and Unit 3 reactors (in total 250 t). It was found that although the inventories of Pu isotopes in the reactors in the Fukushima DNPP were ca. 3.5 times those in the Chernobyl No. 4 reactor23, the percentages of core inventory released for both 239+240Pu and 241Pu were about 5 orders of magnitude lower than those of the Chernobyl accident. These results suggested that for the Fukushima DNPP accident, the plutonium emitted into the environment was mainly due to the release of Pu associated with fuel fragments as a consequence of the hydrogen explosions, as suggested by Kirchner et al.22.

Table 2 Comparison of Pu releases during the Fukushima DNPP accident and the Chernobyl accident Full size table

MEXT has estimated the 239+240Pu dose of external exposure and inhalation from resuspension as 0.12 mSv for a person living for 50 years in the contaminated area8. On the other hand, the 241Pu/239+240Pu activity ratio of the Fukushima-derived Pu was found to be higher than 100. The additional dose contribution from 241Pu has to be estimated. As an example, assuming a similar contamination of 241Pu in the surface soil as that in the litter layer and using the method of IAEA-TECDOC-95524, we estimated the 241Pu dose for a person living for 50 years in the vicinity of S2 site to be 0.44 mSv, about 4 times the 239+240Pu dose.

241Pu is a beta-emitting isotope, as a result of241Pu decay the increase of 241Am may significantly enhance the alpha-activity level in the contaminated area for a certain period of time. Using the 241Pu/239+240Pu activity ratio of the Fukushima-derived Pu (107.8), we made a prognostic prediction on the ingrowth of 241Am (Fig. 3). Details of the theoretical calculation on the decay of 241Pu and ingrowth of 241Am are described in Methods Section. The result showed that the 241Am/239+240Pu activity ratio would increase quickly reaching a value of 1 in the year 2018 and it would reach a maximum value of 3.12 in the year 2081, followed by a gradual decrease. This calculated maximum value of 3.12 is almost one order of magnitude higher than that of the expected global fallout 241Am/239+240Pu in the year 204225. Furthermore, the increased amount of 241Am may remain in the surface soil for decades together with Pu isotopes. In our previous study on the migration of 241Am and Pu released from the atomic bomb detonation in Nagasaki26, we found that the 241Am/239+240Pu activity ratio (0.036±0.006) detected in a soil core (0–15 cm) in Nishiyama area, Nagasaki, Japan in 2008 approached the expected maximum value27, indicating that 241Am and Pu were still together in the soils after six decades and showing no significant difference regarding their downward migration behavior (Fig. 3). In addition, a more efficient transfer of 241Am into plants may be expected. A recent study showed that the coefficients of 241Am transfer from soil to wild plants28, particularly to legumes, are 3–5 times higher than those of 239,240Pu. Therefore, it is highly necessary to investigate the distribution and surface activity of 241Pu inside the 20 km zone, where much higher 241Pu could be expected. This is important for the long-term dose assessment of actinides and will have important implications in the strategy for decontamination procedures.

Figure 3 Curves of the calculated activity ratios of 241Pu/239+240Pu and 241Am/239+240Pu from the Nagasaki atomic bomb Pu, the global fallout Pu and the Fukushima DNPP accident Pu with elapsed time. The 241Am ingrowth from the Nagasaki atomic bomb detonation was based on the initial 241Pu/239+240Pu activity ratio estimated by Yamamoto et al.27. The 241Am/239+240Pu activity ratio (0.036 ±0.006) detected in a soil core collected in Nishiyama area, Nagasaki, Japan in 2008 approached the calculated maximum value, indicating that 241Am and Pu were still together in the soils after 6 decades. 241Am from the global fallout source was expected to reach the maximum 241Am/239+240Pu activity ratio of 0.36 in the year 204225. The theoretic calculation indicated that 241Am/239+240Pu activity ratio would quickly approach the value of 1 by 7 years after the Fukushima DNPP accident and it would reach a maximum value of 3.18 in the year 2081. Full size image

For soil samples collected in Mito, Chiba and Kamagaya Cities, although 137Cs activities were significantly higher than the activity level before the accident (Table 1), for example, the Kamagaya soil sample 2 (0–2 cm) collected on the grounds of a Japanese shrine, near the drain pipes from a building roof, had a 137Cs activity of 11429±88 mBq/g, the 239+240Pu activities and 240Pu/239Pu atom ratios were the typical values of the global fallout and no 241Pu could be detected. We concluded this Pu was consistent with global fallout origin. If any, the Fukushima source contribution to the total Pu activity was negligible.

In addition to the atmospheric releases, the cooling of the reactors with fresh water and seawater and the release of highly contaminated water from the damaged reactor buildings resulted in the direct discharges of radionuclides into the Pacific Ocean29,30. The water-soluble 137Cs released from the atmospheric fallout and the directly discharged radioactive waste water caused serious contamination in the marine environment31. However, information on the distribution of plutonium in the marine environment is very limited. It remains unknown if there was a Pu contamination derived from the release of radioactive waste water. Starting from May, 2011, MEXT monthly reported the monitoring results of 239+240Pu and 238Pu activities in seawater and sediments in the 15 km zone in the Pacific off Fukushima32. As the monitoring was conducted using the analytical methods for emergency monitoring, the reported activities of 239+240Pu and 238Pu in seawater were always lower than the detection limits (<0.55 mBq/L for 239+240Pu and <0.61 mBq/L for 238Pu). Although the 239+240Pu activities in sediments ranged from 0.015 to 0.97 mBq/g, were within the range of 239+240Pu activities observed in sediments in the Japanese coastal area before the Fukushima DNPP accident33, no information on the Pu isotopic composition is available for source identification. Obviously, the possible release of Pu isotopes and their impact on the marine environment need further studies.