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Distribution of Pu Isotopes in the Marine Sediments off Japan After the Accident

To assess the impact of the Fukushima nuclear accident on possible Pu contamination in the marine environment, sediment core samples were collected in the Western North Pacific 30 km off the FDNPP during several cruises from July 2011 to July 2012 after the nuclear accident. The sampling information for each location is shown in Table 9.1. Pu distribution in surface sediments and sediment cores has been investigated [8, 9]. Results for 239+240Pu activity in the surface sediments, inventory-weighted 240Pu/230Pu atom ratios, and 239+240Pu inventories for each sample station are presented in Fig. 9.3.

Table 9.1 Sampling information about the sediment cores collected in the western North Pacific off the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) site after the nuclear accident

Sample

Cruise

Sampling time

Water depth (m)

Location

MC1

MR 11-05

18 July 2011

1,327

36°28.97′N, 141°29.93′E

MC5

MR 11-05

19 July 2011

141

37°35.01′N, 141°30.95′E

ES2

KH 11-07

17 July 2011

2,200

37°4.00′N, 142°16.00′E

ES4

KH 11-07

18 July 2011

5,400

37°53.00′N, 143°35.00′E

ES5

KH 11-07

18 July 2011

7,300

37°47.00′N, 143°54.00′E

FS1

KH 11-07

2 August 2011

150

37°20.00′N, 141°25.00′E

F1

MR 12-02

7 July 2012

1,322

36°29.09′N, 141°30.01′E

Fig. 9.3 Plutonium (Pu) distribution in the marine sediments 30 km off the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) site after the nuclear accident. (Data from references [8, 9])

Distribution of Pu Activities and Pu Inventories

Zheng et al. [8] reported Pu isotopic concentration in the surface sediments; the surface 239+240Pu activities ranged from 0.48 to 3.53 mBq g−1. Bu et al. [9] investigated the vertical distribution of Pu isotopes in sediment cores. 239+240Pu activities for all the surface sediments collected after the nuclear accident were less than 4 mBq g−1, typically less than the background upper limit data (5.81 mBq g−1) before the accident. Activity of 241Pu in the investigated sediments was found to be quite low, most values being below the detection limit of 2 mBq g−1.

The inventories of Pu in the sediment cores ranged from 29 to 141 Bq m2. High Pu inventories have been commonly reported in the coastal sediments in the Western North Pacific and its marginal seas. For example, Pu inventories in the Southern Okinawa Trough were extremely high (201–693 Bq m2) [25]. For the Japan Sea and Okhotsk Sea, Pu inventories ranged from 5.7 to 241 Bq m2 and from 81 to 271 Bq m2, respectively [27]. Therefore, isotopic composition information should be combined with the inventory to reach a more reliable conclusion.

Pu Atom Ratios

The 240Pu/239Pu atom ratios in the surface sediments were between 0.188–0.255, and for all the sediment cores collected after the accident, 240Pu/239Pu atom ratios ranged from 0.188 to 0.293 [8, 9], typically higher than the global fallout value (0.18). 240Pu/239Pu atom ratios in the upper layers of the sediment cores showed nearly uniform distribution patterns, whereas for the sediment cores of MC1, FS1, and F1, the ratios increased from a definite depth. The higher 240Pu/239Pu atom ratios (>0.25) could be attributed to the presence of the PPG close-in fallout Pu.

As shown in Fig. 9.3, the inventory-weighted 240Pu/239Pu atom ratios for the sediment cores ranged from 0.189 to 0.243. The 240Pu/239Pu atom ratios for ES4 and ES5, which are located northeast of the FDNPP site, were lower than the values for the other stations. As the ES4 and ES5 stations are in the pathway of the Oyashio Current, they received more global fallout Pu than did the other stations investigated. The inventory-weighted 240Pu/239Pu atom ratio values were significantly lower than the reported values (>0.30) for the FDNPP accident-released Pu [6] but typically in the background range before the nuclear accident.

The 241Pu/239Pu atom ratio observed in the surface sediments ranged from 0.0012 to 0.0016, almost two orders of magnitude lower than the values (>0.1) derived from the FDNPP accident [6]. As mentioned in Sect. 9.2, global fallout of Pu is characterized by a 241Pu/239Pu atom ratio of 0.0011 and PPG close-in fallout has a 241Pu/239Pu atom ratio ~0.0020 [13, 16, 17]. The 241Pu/239Pu atom ratios in the Fukushima surface sediments were between the global fallout value and the PPG close-in fallout value. These results suggested that 241Pu in the Fukushima surface sediment originated from global fallout and PPG close-in fallout.

 
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