Plutonium concentration and isotopic ratio in soil samples from central-eastern Japan collected around the 1970s

Obtaining Pu background data in the environment is essential for contamination source identification and assessment of environmental impact of Pu released from the Fukushima Daiichi nuclear power plant (FDNPP) accident. However, no baseline information on Pu isotopes in Fukushima Prefecture has been reported. Here we analyzed 80 surface soil samples collected from the central-eastern Japan during 1969–1977 for 239+240Pu activity concentration and 240Pu/239Pu atom ratio to establish the baseline before the FDNPP accident. We found that 239+240Pu activity concentrations ranged from 0.004 –1.46 mBq g−1, and 240Pu/239Pu atom ratios varied narrowly from 0.148 to 0.229 with a mean of 0.186 ± 0.015. We also reconstructed the surface deposition density of 241Pu using the 241Pu/239Pu atom ratio in the Japanese fallout reference material. The obtained results indicated that, for the FDNPP-accident released 241Pu, a similar radiation impact can be estimated as was seen for the global fallout deposited 241Pu in the last decades.

Obtaining Pu background data in the environment is essential for contamination source identification and assessment of environmental impact of Pu released from the Fukushima Daiichi nuclear power plant (FDNPP) accident. However, no baseline information on Pu isotopes in Fukushima Prefecture has been reported. Here we analyzed 80 surface soil samples collected from the central-eastern Japan during 1969-1977 for 2391240 Pu activity concentration and 240 Pu/ 239 Pu atom ratio to establish the baseline before the FDNPP accident. We found that 2391240 Pu activity concentrations ranged from 0.004 -1.46 mBq g 21 , and 240 Pu/ 239 Pu atom ratios varied narrowly from 0.148 to 0.229 with a mean of 0.186 6 0.015. We also reconstructed the surface deposition density of 241 Pu using the 241 Pu/ 239 Pu atom ratio in the Japanese fallout reference material. The obtained results indicated that, for the FDNPP-accident released 241 Pu, a similar radiation impact can be estimated as was seen for the global fallout deposited 241 Pu in the last decades.
T he Fukushima Daiichi Nuclear Power Plant (FDNPP) accident in 2011 resulted in trace release of the reactor core Pu into the atmosphere after intentional venting operation and reactor hydrogen explosions 1-7 . Atmospheric Pu was subsequently deposited on the ground by the wet and dry deposition processes 8 . To estimate the environmental impact of the FDNPP source Pu, background data on Pu distributions in the environment before the FDNPP accident are critical.
Due to the difficulty of Pu analysis, baseline information on Pu activity in Japanese soils is very limited. Yamamoto et al. 9 examined the concentrations of 2391240 Pu for 30 rice-field surface soil samples collected from 15 locations in Japan mostly in 1963 and 1976, and for 15 soil samples periodically collected from 2 locations during 1957-1980. Their study found the integrated deposits of Pu isotopes on the Japan Sea coast of Honshu were 2.5 to 3 times higher than those on the Pacific coast, and the concentrations of 2391240 Pu in rice-field soils ranged from 0.078 to 1.43 mBq g 21 . A similar 2391240 Pu concentration range of 0.07-0.7 mBq g 21 was observed in agricultural upland fields soils in Rokkasho, Aomori Prefecture 10 . In addition, 2391240 Pu concentration in surface layer (0-5 cm) soil samples collected in 1995 in Kyushu was reported to range from 0.50-0.65 mBq g 21 11 . A more recent study conducted by Muramatsu et al. 12 analyzed 2391240 Pu concentrations in 20 soil samples collected from agricultural fields (vegetable/wheat fields and rice paddy fields) and forests in several places in Japan. They found the range of the 2391240 Pu concentrations to be 0.15-4.31 mBq g 21 ; the highest concentration of 4.31 mBq g 21 was found in a surface soil (0-2 cm) sample collected from a forest in Aomori Prefecture. In addition to the Pu activity, the Pu isotopic ratio is an important fingerprint for contamination source identification. The 240 Pu/ 239 Pu atom ratio is of special interest; for nuclear tests this ratio changes with the weapon type and yield, while for nuclear reactors it changes with the reactor type and nuclear fuel burn up. Therefore, the 240 Pu/ 239 Pu atom ratio can provide valuable information on the nature of the Pu emitting source 13,14 . In previous investigations to obtain the background data for Pu in Japanese soil, the 240 Pu/ 239 Pu atom ratio was found to be in the range of 0.14-0.24, which revealed that the major source of Pu in the environment was global fallout from the atmospheric nuclear explosions conducted in the last century [9][10][11][12][13][14][15] .
However, to our knowledge, no baseline information on Pu activity distribution and atom ratio in Fukushima Prefecture are available for times prior to the 2011 nuclear accident. From 1967 to1994, the National Institute of Radiological Sciences (NIRS) collected surface soils mainly from school grounds in Japan to establish a surface soil database for understanding the natural radiation level. Therefore, in this work, we used the NIRS archived soil samples collected in during 1969 to 1977 from Fukushima and its adjacent Prefectures in central-eastern Japan ( Fig. 1) to establish background data on the 2391240 Pu activity concentrations and 240 Pu/ 239 Pu atom ratio. In addition, since a high 241 Pu/ 2391240 Pu activity ratio (higher than 100), was observed in the FDNPP-source Pu 1 , we also reconstructed 241 Pu activities in the analyzed soil samples based on the 241 Pu/ 239 Pu atom ratio in the Japanese fallout reference material. The obtained background data are important to estimate the radiation dose due to the deposition of global fallout Pu and the FDNPP-source Pu.

Results
The results of activity concentrations of 2391240 Pu and the atom ratios of 240 Pu/ 239 Pu in all soil samples are summarized in Table S1 Table 1. The 2391240 Pu activities in school ground soils are quite low, ranging from 0.004 to 0.412 mBq g 21 . In the two samples collected from residential area in Okuma and Futaba, Fukushima Prefecture, 2391240 Pu activities were 0.294 and 0.695 mBq g 21 , respectively. The highest 2391240 Pu activity of 1.46 mBq g 21 in this study was found in the soil collected from the grounds of one Park in Sendai, Miyagi Prefecture. Fig. 2a shows the frequency distribution of  Pu activity concentrations in Fig. 3. It can be seen that the 240 Pu/ 239 Pu atom ratios ranged from 0.148 to 0.229 in the investigated school grounds soil. For the two samples from the residential areas and the one sample from the park grounds, the 240 Pu/ 239 Pu atom ratios ranged from 0.182 to 0.188. The frequency distribution of 240 Pu/ 239 Pu atom ratios in all samples is plotted in Fig. 2b. A typical Gaussian distribution was obtained. Among the 80 soil samples, 30 samples had 240 Pu/ 239 Pu atom ratios ranging from 0.18-0.19, and 58 samples had 240 Pu/ 239 Pu atom ratios of 0.17-0.20. The mean 240 Pu/ 239 Pu atom ratio was 0.186 6 0.015, which is similar to that of global fallout (0.180 6 0.007) 13 , indicating that global fallout Pu deposition was the major source, although a small contribution of the Chinese Nuclear Tests at Lop Nor has been observed in fallout samples collected in the 1970s in Japan 16 .

Discussion
Eighty soil samples in this study were collected from central-eastern Japan in the period from 1969 to 1977. From 1945 to 1980, 543 atmospheric nuclear tests were conducted worldwide, and during 1964-1980, 22 atmospheric nuclear weapons tests were conducted by China at Lop Nor 17 . The Nagasaki atomic bomb detonation on August 9, 1945 released Pu into the environment; a study on the geographic distribution of Nagasaki atomic bomb-derived Pu indicated that plutonium from the atomic bomb was deposited in the eastern area from the hypocenter reaching up to 100 km eastwards 18 . In addition, there was no detectable deposition of Chernobyl accident-sourced Pu in 1986, so Pu in the soil samples of the present  study was assumed to have come mainly from the global fallout due to the atmospheric nuclear tests. The 2391240 Pu activity concentrations in our study were in the range of 0.004-1.46 mBq g 21 , which were comparable with previous studies on the activity levels of Pu in soils in Japan before the FDNPP accident (0.07-4.31 mBq g 21 ) as shown in Table 2. Xu et al. 19 demonstrated that the combined effects of many environmental factors were responsible for the variation of Pu concentrations in the surface soils. We compared the activity concentrations of 2391240 Pu with organic contents in the investigated surface soil samples; however, no significant correlation between Pu activities and organic matter contents was found. Therefore, more detailed information should be obtained in the future to reveal the variation of concentrations in the surface soils.
As shown in Table 1 20 . It was found that the surface deposition density (Bq m 22 ) ranged from 0.35 to 40; these values are comparable to those we detected in the soils collected in the 1970s, indicating that the FDNPP accident did not cause significant increase of Pu deposition in central-eastern Japan.
However, alteration of the 240 Pu/ 239 Pu atom ratio in various environmental samples was significant after the FDNPP accident. Pu, especially in the period of the peak of global fallout is critical. Due to the low activities of 241 Pu in the school ground soils after five decades of decay, 241 Pu was below the detection limit of the analytical method (1 mBq/g), and thus not detected in this study. In order to establish background data of the 241 Pu activity in the analyzed soil samples, we used the 241 Pu/ 239 Pu atom ratio of 0.00261 6 0.00026 ( 241 Pu decay reference to January 1, 2000) in fallout reference material reported by Zhang et al. 16 . This reference   After  fallout material was prepared from fallout deposition samples collected monthly at 14 stations throughout Japan in 1963-1979 22 . Since the fallout reference material and the soils samples we investigated in this study were sampled in the period after large scale atmospheric nuclear weapons tests and before the widespread operation of nuclear power plants in Japan, we consider that they have the same source of Pu isotopes, i.e. mainly from the global fallout with a small contribution from the Chinese nuclear tests, therefore, we can use the 241 Pu/ 239 Pu atom ratio detected in the fallout reference material to reconstruct the 241 Pu activities in the soil samples we analyzed. To understand the highest background level of 241 Pu activity in the soil samples, decay of 241 Pu was corrected to January 1, 1964, the year of peak deposition of global fallout Pu 17 . As shown in Fig. 6, the reconstructed 241 Pu activities were highly correlated with the 2391240 Pu activities, thus the 241 Pu/ 2391240 Pu activity ratio of 14.8 was obtained. Since the half-lives of 239 Pu and 240 Pu are very long (2.411 3 10 4 y and 6.563 3 10 3 y, respectively), in the timescale of several decades, the 2391240 Pu activity can be considered unchanged, the 241 Pu/ 2391240 Pu activity ratio of 14.8 obtained in this study can be used to estimate 241 Pu background in soils collected in other areas in Japan, once 2391240 Pu activity is measured. The estimated 241 Pu/ 2391240 Pu activity ratio in the present study is slightly higher than that of global fallout (ca. 12.1) obtained from lake sediments 23 .
The estimated 241 Pu activities are summarized in Table S1. It was found that the 241 Pu activities ranged from 0.06 to 6.07 mBq g 21 in 77   13 ; data for atmospheric fallout in Japan (1963-1979) are cited from Zhang et al. 16 ; data in surface soil and litter are cited from Zheng et al. 1 and Yamamoto et al. 5 ; data in black substances are cited from Yamamoto et al. 5 ; data in aerosol are cited from Shinonaga et al. 6 ; and data in reactor cores and spent fuel pools are cited from Nishihara et al. 21 ).
www.nature.com/scientificreports SCIENTIFIC REPORTS | 5 : 9636 | DOI: 10.1038/srep09636 school grounds soil samples. Higher 241 Pu activities were found in the soil samples collected from residential areas and the park grounds, ranging from 4.28 mBq g 21 to 21.25 mBq g 21 . It was noted that the distribution patterns of measured 2391240 Pu activities and the estimated 241 Pu activities were quite similar. For the 241 Pu activities, among the investigated soil samples, 89.6% were less than 0.3 mBq g 21 . The northern prefectures of Ibaraki, Fukushima, Miyagi and Iwate presented relatively higher 241 Pu activities than those in the four southern prefectures (Tochigi, Gunma, Saitama, and Chiba) and Tokyo. Fig. 7 showed the re-constructed surface deposition density of

Methods
Reagents and materials. High-purity water (18 MV cm 21 ) was prepared with a Millipore Milli-Q-Plus water purification system. All chemicals (HCl, HNO 3 , NaNO 2 , NH 4 I, H 2 O 2 , HBr) were of analytical grade, except for the final solution preparation for the ICP-MS measurement, in which ultrapure grade 68% HNO 3 (Tama Chemicals, Japan) was used. The two anion-exchange resins, AG 1X8 (100-200 mesh, Cl-form) and AG MP-1M (100-200 mesh, Cl-form) were obtained from Bio-Rad, which were packed in a Muromac mini-column (M type, 6.5-8.5 mm 3 58 mm i.d.) for Pu separation and purification. 242 Pu (CRM 130, plutonium spike assay and isotopic standard, New Brunswick Laboratory, USA) was used to spike the soil samples as a yield tracer. The mixed Pu isotope standard solution (NBS-947) with certified 240 Pu/ 239 Pu atom ratio of 0.242 was employed for mass bias correction. Two soil standard reference materials (IAEA-soil-6 and IAEA-375) were used to validate our analytical method.
As shown in Fig. 1, eighty soil samples were collected from central-eastern Japan (Fukushima, Ibaraki, Miyagi, Iwata, Tochigi, Gunma, Saitama, and Chiba Prefectures, and Tokyo) in the period from 1969 to 1977. Among them, two were from residential areas, one from a park, and others from school grounds. All samples were analyzed for 2391240 Pu concentrations and their isotope ratios of 240 Pu/ 239 Pu.

Instrumentation.
A high efficiency sample introduction system (APEX-Q) equipped with a conical concentric nebulizer was combined with SF-ICP-MS (Thermo Fisher Scientific, Element 2, Bremen, Germany) for Pu isotope analysis 24 . This system consisted of a heated cyclonic spray chamber, a Peltier cooled condenser and an ACM Nafion fluoropolymer membrane desolvation module. A small flow of nitrogen was used to increase transport efficiency and signal stability. The low resolution mode was used to utilize the maximal instrument sensitivity. All the measurements were made in the self-aspiration mode with an uptake rate of , 0.2 mL min 21 to reduce the risk of contamination from the peristaltic pump tubing. The SF-ICP-MS was optimized on a daily basis using 0.1 ng mL 21 U standard solution to provide optimum intensities and peak shapes.
Analytical procedure. The full experimental procedure was described elsewhere 25 . In brief, the soil samples were dried at 105uC for 24 h, and pulverized to about 80 mesh. About 1-3 g of a dried soil sample was weighed out. After ashing at 450uC for 5 h to destroy the organic matter, ca. 1 pg 242 Pu was added for each sample as a yield monitor. HNO 3 (20-40 mL, conc.) leaching at 180uC for at least 4 h was done in a tightened lidded Teflon vessel (120 mL, Savillex Corporation, Minnesota, USA) to avoid the loss of acid and improve the acid leaching efficiency. After cooling, the supernatant was filtered through an Advantec filter into a beaker (100 mL), and the Teflon vessel and filter paper were washed with 10-20 mL concentrated HNO 3 . High-purity water was added to adjust the sample solution to the concentration of 8 M HNO 3 . Then, NaNO 2 was added to a concentration of 0.2 M and heated at 40uC for 30 min to adjust Pu to the tetravalent state prior to loading onto the first AG 1X8 resin column.
The AG 1X8 resin column (2.5 mL) was preconditioned with 20 mL 8 M HNO 3 -0.2 M NaNO 2 . After sample loading, 50 mL 8 M HNO 3 was used to wash U, Pb and Fe from the column 26 . Then 30 mL 10 M HCl was used for washing Th and converting the resin back into the chloride form. Finally, Pu was eluted with 40 mL 0.1 M NH 4 I-8.5 M HCl, collected in a 100 mL Teflon beaker and evaporated to near dryness. Aqua regia (1 mL) was added and then the solution was heated to dryness again. This procedure was repeated twice to destroy the organic matter and remove the residual iodine. Subsequently, 2 mL concentrated HCl was added and the mixture was evaporated to dryness. After adding ca. 4 mL freshly prepared HCl-H 2 O 2 solution (10 mL conc. HCl with 0.01 mL 30% H 2 O 2 ) and heating at 40uC for about 30 min, the sample solution was ready for loading onto the second AG MP-1M resin column.
The AG MP-1M resin column (2.5 mL) was pre-conditioned with 8 mL HCl-H 2 O 2 solution. After sample loading, 20 mL 8 M HNO 3 was used for washing U. Then, 8 mL 10 M HCl was added to wash the residual HNO 3 in the column and for further Th washing. Pu was eluted from the column with 16 mL HBr into a 30 mL Teflon beaker. After evaporating to near dryness, 1 mL concentrated HNO 3 was added to the Teflon beaker and heated to remove any trace of HBr. When nearly dry, the final residual was dissolved in 0.8 mL 4% HNO 3 in preparation for the SF-ICP-MS analysis.