Introduction

As a result of the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident, a large amount of 131I, 134Cs, 137Cs and other radionuclides have been released into the environment. Especially for the waters, Tsumune et al. [1] estimated the total amount of 137Cs activity directly released into the sea was estimated to be 3.5 ± 0.7 PBq by the end of May 2011. A comprehensive survey project was initiated immediately after the accident to clarify the distribution of highly volatile radionuclides [2, 3]. However, scientific information on the Pu isotopes and 241Am is limited both in the number of samples and on the extent of the survey, if any [4–6] in comparison with that obtained by the intensive surveys on the volatile fission and activation products. Thus, efforts toward actinide elements derived from the accident in the marine environment have not yet been sufficient to evaluate their impacts on a nationwide scale.

We have been engaged in a monitoring survey since 2008 to determine the levels of Pu isotopes and 241Am in the sediments of the coastal sea areas near nuclear power plants all over Japan. This paper represents the results of this nationwide survey, which provides information on the variations of Pu isotopes and 241Am in the sediments during the period from 2008 to 2011. Emphasis was placed on clarifying what was the extent of the impacts of the FDNPP accident on the levels of actinides on a nationwide scale.

Experimental

Bottom sediment samples were collected at 15 sites in the waters off the nuclear power stations (Fig. 1) once a year in May–July from 2008 to 2011 using a box-type sampler, which took sediments from the ocean floor with a conventional method [2, 7]. For the annual monitoring survey, the upper 3 cm layer samples of the sediments in the sampler were taken for analysis that the same as usual [2, 7]. The sediments were dried at 105 °C, passed through a sieve (<2 mm mesh), and then pulverized again to a homogeneous powder in a tabletop grinder. Particle size distribution of the sediments was observed by a laser diffraction particle size analyzer when necessary.

Fig. 1
figure 1

Sampling sea areas (totally 15 areas) set up off the nuclear power stations in Japan

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Plutonium in the sediments were extracted in HNO3 and several drops of H2O2 with a known amount of 242Pu tracer. The Pu in the leachate was separated by co-precipitation with Fe(OH)3. The Pu was further purified by passing through an anion exchange column and then electrodeposited on a stainless steel disk. Alpha spectrometry was used to measure 239+240Pu radioactivity on the disk. After measuring 239+240Pu radioactivity, the Pu on the disk was leached by immersion in mixture of HF/HNO3 for a few minutes. The isotopes of Pu were purified by repeating the anion exchange resin procedure. A sector field ICP-MS (ELEMENT 2) was used to measure the 240Pu/239Pu atom ratio. Details of the operating conditions have been described elsewhere [7].

Another aliquot of sediments was used to measure 241Pu and 241Am through the same procedure as mentioned above after an addition of a known amount of 242Pu and 243Am tracers. The leachate was concentrated by evaporation. After adding 8 M HNO3/NaNO2 solution to the condensate, the solution was heated to adjust the Pu valence to the IV state. Pu was then separated from Am by using an anion exchange column. The 241Pu activity was measured by a liquid scintillation counter for 500 min and 241Pu activity was decay-corrected to the sampling date. The Am was further purified using co-precipitation and an anion exchange step. The purified Am was electrodeposited on a stainless steel disk. Alpha spectrometry was used to measure 241Am radioactivity.

Results and discussion

Levels of Pu isotopes and 241Am before the FDNPP accident

Analytical results of the sediments prior to the accident are summarized in Table 1 with information relevant to the sampling sites. The concentration of 239+240Pu widely varied from one site to another within the range from 0.37 to 3.5 Bq/kg-dry, which was comparable to those reported previously [6]. Although surveying there only for a short period, the temporal variations of 239+240Pu concentration at each sampling site were not so great as the spatial variation from the 3 years variation of 239+240Pu concentration at each site. The 240Pu/239Pu atom ratio ranged from 0.212 to 0.270, which was higher than the global fallout Pu(0.178) as shown in land-source Pu [8] and comparable to the reported values [6, 7, 9]. It was likely that the 240Pu/239Pu atom ratio was nearly constant at each site during the period of the survey, that it remained at a level specific to the site, and that it clearly depended on the depth (or apparent density and particle size distribution); namely there were higher 240Pu/239Pu atom ratios at shallower sites (e.g. Shizuoka, Saga and Kagoshima) and lower ratios at deeper sites (e.g. Hokkaido, Aomori and Niigata), and vice versa for 239+240Pu concentrations. The depth dependence of the 240Pu/239Pu atom ratio had been also observed in previous survey carried out in the coastal sea off Aomori and Miyagi Prefectures, although what caused the depth dependence and the convergence on a given value still remained unknown [7].

Table 1 Summary of 239+240Pu, 241Pu and 241Am concentration and 240Pu/239Pu atom ratio in the surface sediments (0–3 cm) of the coast of the Japanese islands. (Before the FDNPP accident)
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The concentrations of both 241Pu and 241Am widely varied from one sampling site to another within the respective ranges from 0.87 to 3.9 Bq/kg-dry and from 0.22 to 2.1 Bq/kg-dry. The lowest values were observed in sandy (the percentage contribution of coarse, medium and fine sands was larger than 70 % to the total weight of a sediment sample) sediments from the shallower sites in Ishikawa and Saga Prefectures for 241Pu and 241Am, whereas the highest values were observed in clayey (the percentage contribution of silt and clay was larger than 70 %) sediments from the deeper site in Aomori. Generally, the levels of 241Pu and 241Am concentrations were similar to those of the 239+240Pu concentrations in sediments.

After the FDNPP accident

Analytical results of the sediments collected after the accident are summarized in Table 2. The locations of sampling sites in Table 2 were the same as those in Table 1 except for the site in Shizuoka, which was moved 20 km west of the site before the accident.

Table 2 Analytical results of 239+240Pu, 241Pu and 241Am concentration and 240Pu/239Pu atom ratio in the surface sediments (0–3 cm) of the coast of the Japanese islands. (Immediately after the FDNPP accident)
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The 239+240Pu concentrations in April–June 2011 after the accident ranged from 0.42 to 3.7 Bq/kg-dry, which was corresponding to the typical background levels shown above. The highest concentration of 3.7 Bq/kg-dry was found in the clayey sediments from Aomori and the lowest of 0.42 Bq/kg-dry in the sandy sediments from Ehime. The 240Pu/239Pu atom ratio was nearly constant (ca. 0.24), ranging from 0.218 to 0.267 with the average value of 0.240 ± 0.014, which comparable to the pre-accident values shown above. The 240Pu/239Pu atom ratio decreased with increasing depth of the site and seemed to converge on the value of 0.22. The Pu found in the sediments of the coastal sea area of the Japanese islands is a mixture of Pu from global fallout deposition and Pu from the nuclear weapons testing conducted at the Pacific Proving Ground (PPG) in the Marshall Islands; the latter has a higher 240Pu/239Pu atom ratio of 0.33–0.36 and this Pu was transported mainly by the Kuroshio Current [6]. It is well known that heavier Pu isotopes become more abundant with increased burn-up of the nuclear fuels, and the 240Pu/239Pu atom ratio can be regarded as a good indicator to identify the source of Pu contamination. The 240Pu/239Pu atom ratio would have been altered if an appreciable amounts of Pu isotopes from the accident were added to pre-existing ones. The 240Pu/239Pu atom ratio found in the sediments in the present study, however, had no significant differences between the pre-accident and the post-accident ones.

The Pu isotope, 241Pu, which had been released from the atmospheric nuclear weapons tests mainly conducted in the 1950s–1960s was detected in some samples having relatively high 239+240Pu concentration (ca. > 0.7 Bq/kg-dry). The 241Pu concentrations ranging from 0.63 to 3.9 Bq/kg-dry after the accident would correspond to the background level in the sediment samples in the coast of the Japanese islands before the FDNPP accident. The 241Pu/239+240Pu activity ratio in the sediments varied among the sites and ranged from 0.75 to 1.2 although the sediment samples were collected in the same time period. Zheng et al. [5, 10] estimated that the 241Pu/239+240Pu activity ratio derived from the FDNPP accident was 107.8 on the 15th of March 2011 and that was higher than in the Chernobyl accident (83 on the 1st of May 1986). They also mentioned that the relatively higher 241Pu/239+240Pu activity ratio resulted from damage to the Unit 3 reactor which had a mixed oxide fuel containing both enriched U and Pu (about 6 % Pu). By comparing the FDNPP accident and the Chernobyl accident values, it could be concluded that there was no additional 241Pu input to the sediments on the occasion of the accident judging from the 241Pu/239+240Pu activity ratio found in the global fallout (1.2 in March 2011) [5, 10].

The 241Am concentration ranged from 0.22 to 2.1 Bq/kg-dry and there were also regional differences as seen in the 239+240Pu concentration. The 241Am/239+240Pu activity ratio was nearly constant (ca. 0.56 ± 0.07) except at Shizuoka. The half-life of 241Am is much longer than its ancestor nuclide 241Pu and therefore its activity increases as time passes, namely the 241Pu/239+240Pu activity ratio decreases and as a result, the 241Am/239+240Pu activity ratio increases exponentially [5]. In 1976, Krey et al. [8] estimated that the 241Am/239+240Pu activity ratio will increase and reach a peak value of 0.42 in the year 2037. In 1990, Yamamoto et al. [11] concluded that the 241Am/239+240Pu activity ratio of global fallout had increases and will reach a maximum value of 0.41 in the year 2030. By using the FDNPP-derived 241Pu/239+240Pu activity ratio of 107.8, Zheng et al. [5, 10] estimated that the 241Am/239+240Pu activity ratio would increase quickly, surpass the value of 1 in the year 2018, and reach the maximum value of 3.18 in 2081 followed by a gradual decrease. As can be seen in Table 2 the 241Am/239+240Pu activity ratio (ca. 0.62 ± 0.04 and 0.59 ± 0.04) in the sediments off Fukushima Prefecture was higher than expected value for the global fallout that was expected to reach the maximum value of 0.36 in 2042 [5, 11, 12], and the PPG (27 in 1952–1954 [10, 13]), presumably resulting from the composition of Pu isotopes of different origins. It is difficult to quantify the contribution of each origin to the isotopic composition of Pu based on the concentrations of Pu isotopes even if it is taken into account that the ingrowth of 241Am as a result of the decay of 241Pu from the PPG could raise the 241Am/239+240Pu activity ratio.

Conclusions

A radioactivity monitoring survey particularly for actinides in sediments in the coastal seas around the Japanese islands has been done since 2008. The concentrations of 239+240Pu, 241Pu and 241Am were in the surface sediments were within the range of 0.42–3.7, 0.63–3.9 and 0.22–2.1 Bq/kg-dry, respectively, soon after the FDNPP accident, which corresponded to the background levels. The highest concentration was found in clayey sediments at deeper sites, while the lowest concentration was in sandy sediments at shallower sites. The 240Pu/239Pu atom ratios ranged from 0.212 to 0.267 and were relatively higher than the global fallout Pu(0.178). The 241Pu/239+240Pu and 241Am/239+240Pu activity ratios were nearly identical among the sites around the Japanese islands. In addition, there were significant differences neither in the concentrations of Pu isotopes and 241Am nor in the activity ratios, 241Pu/239+240Pu and 241Am/239+240Pu between the situations before and soon after the Fukushima accident.