Different activity ratios of radioactive Cs were detected for DPs and SP (Table 1). Using the ORIGEN code, Nishihara et al.28 calculated the 134Cs/137Cs activity ratio as 0.95, 1.08, 1.05 for the units 1, 2, and 3, respectively. The 134Cs/137Cs activity ratios of DPs and SP were compared with the calculated values, and we estimated that DPs were derived from unit 1 and SP was derived from unit 2 or 3. DPs were identified as ‘Type B’ due to the similarity in particle size, Cs concentration, and 134Cs/137Cs activity ratio, according to the classification by Satou et al.38. SP was similar to ‘Type B’ with respect to the particle size and Cs concentration. However, with respect to the 134Cs/137Cs activity ratio, SP was similar to ‘Type A’ rather than ‘Type B’. We conclude that SP had differences with the features of both ‘Type A’ and ‘Type B’ and was classified as a new type of radioactive particle41.

The 240Pu/239Pu and 241Pu/239Pu atom ratios in the radioactive particles in this study were compared with the value calculated using ORIGEN code. The 240Pu/239Pu ratios in units 1, 2, and 3 reactor cores were 0.344, 0.320, and 0.350 calculated from fuel inventories by Nishihara et al.28. These ratios of DPs (DP1: 0.330 ± 0.077, DP2: 0.415 ± 0.069, DP3: 0.373 ± 0.045) generally agreed well with the calculated values of each reactor. The 241Pu/239Pu ratios in units 1, 2, and 3 reactor cores were 0.192, 0.192, and 0.183 also calculated by Nishihara et al.28. These ratios of DPs (DP2: 0.178 ± 0.016, DP3: 0.162 ± 0.028) are also consistent with these calculated values of each reactor. 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 240Pu/239Pu and 241Pu/239Pu atom ratios in the radioactive particle were also compared with those of various environmental samples, such as soil21,22, litter21,23, black substance23, vegetation24,25, river water26, and air dust27, as shown in Figs 4 and 5. The 240Pu/239Pu atom ratio detected in environmental samples ranged from 0.14 to 0.381. In particular, the ratios (the numbers in brackets) in litters obtained by Yamamoto et al. (0.335 ± 0.007)23 and Zheng et al. (0.323 ± 0.017–0.330 ± 0.032)21, in black substances reported by Yamamoto et al. (0.335 ± 0.004–0.365 ± 0.015)23, and in vegetable reported by Dunne et al. (0.324 ± 0.015 – 0.359 ± 0.07)25 showed relatively high values among them. It was inferred that these samples were less contaminated by the global fallout source of Pu due to the extremely low transfer factor of global fallout Pu from soil to plant47. The ratios in DPs were consistent with the ratios in these environmental samples. In contrast, the values of 240Pu/239Pu atom ratio in soils reported by Zheng et al.21 and Yang et al.22 were lower than that of DPs. The 241Pu/239Pu atom ratios in DPs agreed better with litters obtained by Zheng et al. (0.128 ± 0.034–0.135 ± 0.012)21 than soil obtained by Zheng et al. (0.103 ± 0.013)21 as well as the 240Pu/239Pu atom ratio. As pointed out by previous studies21,22, these environmental samples were strongly influenced by global fallout, the value of which was 0.180 ± 0.007 for 240Pu/239Pu atom ratio14,15,16 and 0.00194 ± 00014 for 241Pu/239Pu atom ratio14. The Pu contribution from the FDNPP accident can be estimated using the two end-member model48, and the extent of Pu contribution in soil samples was estimated as follows: 87% by Zheng et al.21 and 31–59% by Shibahara et al.49. In the radioactive particle, the Pu injection occurred only in the reactor unlike other environmental samples. Thus, radioactive particle samples are completely free from the influence of the global fallout and the 240Pu/239Pu and 241Pu/239Pu 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 4 Comparison of atom ratios of 240Pu/239Pu in various environmental samples, radioactive particles, and source terms: global fallout15, soils during 1966–1977 in Japan16, soil after the FDNPP accident21,22, litter21,23, black substance23, vegetation24,25, river water26, average of air dusts27, and calculated result of reactor core28. Full size image

Figure 5 Comparison of atom ratios of 241Pu/239Pu in various environmental samples, radioactive particles, and source terms: global fallout15, soil and litter after the FDNPP accident21, and calculated result of reactor core28. Full size image

Figure 6 shows the relationship between the activity ratio of 239+240Pu/137Cs and the distance to the sampling site from the FDNPP. The 239+240Pu/137Cs activity ratio of DPs was (7.26 ± 0.86) × 10−8, (3.92 ± 0.51) × 10−8 and (9.58 ± 0.78) × 10−8 for DP1, DP2 and DP3, respectively. These ratios were two orders of magnitude smaller than those in soils reported by Yamamoto et al.23. In contrast, these values agreed well with those in black substances and litters reported by Yamamoto et al.23 and Zheng et al.21, respectively. This result is consistent with the discussion on 240Pu/239Pu and 241Pu/239Pu atom ratio. The typical value of 239+240Pu/137Cs activity ratio of the global fallout was of the order of 10−3 50. Therefore, we conclude that the soil samples were strongly influenced by the global fallout compared with the black substances and litters as mentioned in the discussion on 240Pu/239Pu and 241Pu/239Pu atom ratio. The relationship between the 239+240Pu/137Cs activity ratio and the distance from the FDNPP is important for understanding the dispersion and deposition behavior of the volatile element such as Cs and non-volatile elements such as Pu in the environment. The 239+240Pu/137Cs activity ratios in black substances and litters agreed with those in the radioactive particles, although the distance from the FDNPP were completely different. The ratios of radioactive particles directly reflected the released ratio of Pu to Cs from the reactor. The agreement indicates that the behavior of dispersion and deposition of Pu in the environment after being released from the reactor was similar to that of Cs within a distance of 25 km, as pointed out by Yamamoto et al.23.

Figure 6 Relationship between the activity ratios of 239+240Pu/137Cs and the distance to sampling site from the FDNPP within 25 km around the FDNPP. (Data of soils, black substances are quoted from Yamamoto et al.23, and data of litters are quoted from Yamamoto et al.23 and Zheng et al.21). Full size image

The determination of Pu isotopes contained in the radioactive particles can reveal not only the accurate 240Pu/239Pu and 241Pu/239Pu atom ratio derived from the FDNPP accident but also provide insight into the formation process of the radioactive particles. In case of the Chernobyl accident, ‘hot particles’ containing U and other actinides were released. Lancsaris et al.51 reported that the 239+240Pu activities of the hot particles were in the range (4.2–92) × 10−3 Bq. On the other hand, the 239+240Pu activities of the radioactive particles released from the FDNPP accident were very small ((1.70–7.06) × 10−5 Bq) compared to that of the hot particles released from the accident in Chernobyl. Such a difference in Pu activity among the particles could be attributed to the formation process of these particles in the reactor between the radioactive and hot particle. The hot particles were considered to be originating from the fragments of the fuel core, and the particles were formed during the explosive break-up of the reactors52. In the case of radioactive particles released from the FDNPP accident, however, a previous study related to the formation process of the ‘Type B’ (which is of the same type as DPs) indicated that fission product gases were incorporated into the silicate-rich materials such as glass fiber insulator materials surrounding the reactor pressure vessel53. Our first identification of Pu in the radioactive particles indicated that fuel elements such as Pu were introduced to the silicate materials at the time of the formation of ‘Type B’ radioactive particles. Aberecht et al.54 estimated that refractory species having the least oxygen, such as Pu 2 O 3 , were evaporated by the reductive reaction in the reductive atmosphere of the reactor due to the large amounts of hydrogen gas during the meltdown. During the formation process of ‘Type B’ radioactive particle, we estimated that the mist, including the evaporated Pu 2 O 3 and other fission products such as Cs, were incorporated into the insulator materials in the reductive atmosphere. The contents of the mist should change with rising temperature of the mist according to the volatile pressure of each element. Therefore, investigating the amounts of various elements contained in the radioactive particles enabled us to understand the detailed formation process of the particles. The 239+240Pu/137Cs activity ratio was different for DPs ((3.92–9.58) × 10−8) and SP (<1.98 × 10−8), which implies that the formation processes of both were different. We estimated that the temperature in unit 1 was higher than that in unit 2 or 3 during the formation of the radioactive particles.

Similar to Pu, Sr is one of the trace elements contained in the radioactive particles. Zhang et al.41 reported that the activity ratio of 90Sr/137Cs in radioactive particles was of the order of 10−4. The activities of 90Sr were 0.171 ± 0.010 Bq, 0.589 ± 0.034 Bq and 0.0455 ± 0.0040 Bq for DP1, DP2 and SP, respectively. The relationship between DPs and SP for the 90Sr/137Cs activity ratio and 90Sr activity was similar, as summarised in Table 1. Pontillon et al.55 and Lewis et al.56 observed that the release fraction from the core inventory of Sr was much lower than that of Cs and that of Pu was even lower. Therefore, we considered that the amount of Pu in the radioactive particles, which is less volatile than Sr, would be clearly different between DPs and SP due to the rising fuel temperature in the core. Pontillon et al.55 also categorised Ag as a volatile element along with Cs and showed that the emission of Ag increased with the rise in temperature, while Cs was released from the fuel pellet until 2300 K. Satou57 considered that the release rate of 110mAg reflected the fuel temperature in the core and investigated the activity ratio of 110mAg/137Cs in soils. From the result of his investigation, he estimated that the temperature of unit 1 was higher than that of unit 2 or 3 when the radionuclides were released into the environment, which is characterized by the high 110mAg/137Cs activity ratio. Therefore, we estimated that DPs derived from unit 1 were formed under a high temperature atmosphere, where non-volatile radionuclides such as Pu were more volatile compared to SP derived from unit 2 or 3.

In this study, we first investigated the amounts of Pu in DP (Type B) radioactive particles. On the other hand, Pu was not detected from radioactive particles that were not categorized as ‘Type A’ or ‘Type B’. The amount of Pu in ‘Type A’ particles still remains to be investigated. The formation process of radioactive particles can be investigated by measuring Pu amount and comparing between different particle types. In this study, the error of obtained Pu isotopic ratio was relatively large because we quantified trace amounts of Pu in the radioactive particles. To obtain more accurate ratio, it is necessary to quantify Pu from the Pu-bearing particles that contain more concentrates Pu than the radioactive particles. For this purpose, we consider investigating the Pu-bearing particles and quantifying Pu in these particles by identification methods such as the alpha particle imaging detector developed by Morishita et al.58 in the future.