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Home arrow Environment arrow Radiation Monitoring and Dose Estimation of the Fukushima Nuclear Accident

Validation of Source Term by the Regional-Scale Atmospheric Dispersion Simulation

The regional-scale atmospheric dispersion and surface deposition of 131I and 137Cs released from the FNPP1 from 05:00 JST on 12 March to 00:00 JST on 1 May 2011 were simulated over eastern Japan using WSPEEDI. In this simulation, the source term, which was estimated in previous studies primarily using observed air concentrations of radionuclides and air dose rates over land around FNPP1, was verified and refined on the basis of its agreement of calculated daily surface deposition with measurements [4]. For the period from 03:00 to 21:00 JST on 21 March, th13e1I/137Cs activity ratio was determined from dust sampling data at different points from those used for the release rate of 131I in the preliminary source term estimation [1] because of the lack of 137Cs measurements. This change caused the 137Cs deposition to be underestimated in the Kanto region on 21 and 22 March by setting a lower release rate of 137Cs, although good agreement was seen for 131I. Considering the range of observed 131I/137Cs activity ratios (from 1.5 to 19.1) in the 12 sampling datasets at FNPP2 [22], Tokai [23], and Setagaya [24] from 06:00 to 22:00 JST on 21 March, we modified the 131I/137Cs activity ratio from 131 in the preliminary source term to 10. We also added new release rates for the period from 21:00 JST on 29 March to 11:00 JST on 30 March, estimated using the dust sampling data at FNPP2 [22], which were not used in the preliminary source term estimation [1]. The release rate of 131I was estimated to be 1.5 × 1013 Bq h−1 and the 131I/137Cs activity ratio was 1.7. The value of the 131I/137Cs activity ratio of 4.9 obtained from the dust sampling data

Table 15.1 Release start time, release duration, release rates of131I and 137Cs, 131I/137Cs radioactivity ratio, and release height for the period from 05:00 JST on 12 March and 00:00 JST on 1 May 2011

No. Start time (JST) Duration (h) 131I (Bq h−1) 137Cs (Bq h−1) 131I/137Cs Height (m)

1

2011/3/12 05:00

4.5

3.7E + 13

3.7E + 12

10

20

2

2011/3/12 09:30

6.0

1.7E + 13

1.7E + 12

10

120

3

2011/3/12 15:30

0.5

3.0E + 15

3.0E + 14

10

100 (volume)

4

2011/3/12 16:00

31.0

8.4E + 13

8.4E + 12

10

120

5

2011/3/13 23:00

12.0

3.6E + 13

3.6E + 12

10

120

6

2011/3/14 11:00

0.5

3.0E + 15

3.0E + 14

10

300 (volume)

7

2011/3/14 11:30

10.0

2.3E + 13

2.3E + 12

10

20

8

2011/3/14 21:30

2.5

1.3E + 15

1.3E + 14

10

120

9

2011/3/15 00:00

7.0

3.5E + 14

4.0E + 13

8.8

120

10

2011/3/15 07:00

3.0

3.0E + 15

3.0E + 14

10

20

11

2011/3/15 10:00

3.0

8.0E + 13

8.0E + 12

10

20

12

2011/3/15 13:00

4.0

4.0E + 15

4.0E + 14

10

20

13

2011/3/15 17:00

37.0

2.1E + 14

3.0E + 12

70

20

14

2011/3/17 06:00

57.0

4.1E + 14

1.0E + 13

41

20

15

2011/3/19 15:00

36.0

3.8E + 14

3.5E + 13

11

20

16

2011/3/21 03:00

18.0

1.4E + 14

1.4E + 13

10

20

17

2011/3/21 21:00

26.0

4.1E + 14

4.7E + 12

87

20

18

2011/3/22 23:00

25.0

7.1E + 14

8.9E + 12

80

20

19

2011/3/24 00:00

24.0

1.9E + 14

2.9E + 12

66

20

20

2011/3/25 00:00

35.0

5.6E + 13

1.2E + 12

45

20

21

2011/3/26 11:00

47.0

4.0E + 12

1.7E + 11

23

20

22

2011/3/28 10:00

35.0

7.5E + 12

4.7E + 12

1.6

20

23

2011/3/29 21:00

14.0

1.5E + 13

8.8E + 12

1.7

20

24

2011/3/30 11:00

13.0

1.8E + 14

1.4E + 14

1.3

20

25

2011/3/31 00:00

22.0

2.4E + 13

4.5E + 12

5.3

20

26

2011/3/31 22:00

35.0

1.8E + 12

1.6E + 12

1.1

20

27

2011/4/02 09:00

48.0

1.8E + 12

5.8E + 11

3.1

20

28

2011/4/04 09:00

80.0

7.0E + 11

1.4E + 11

4.9

20

29

2011/4/07 17:00

150.0

7.0E + 11

3.5E + 11

2.0

20

30

2011/4/13 23:00

409.0

7.0E + 11

1.8E + 11

4.0

20

Volume sources with the sizes of (x, y, z) = (100, 100, 100 m) and (100, 100, 300 m) were assumed for hydrogen explosions at unit 1 (No. 3) and 3 (No. 6), respectively [4]. Japanese standard time (JST) is used

on 5 April was simply assumed to continue until 00:00 JST on 1 May. This assumption caused underestimation of the daily surface deposition of 137Cs after 6 April, although the calculated 131I deposition showed relatively good agreement. In the refined source term, the 131I/137Cs activity ratios after 6 April were modified to be 2.0 from 17:00 JST on 7 April to 23:00 JST on 13 April and to be 4.0 from 23:00 JST on 13 April to 00:00 JST on 1 May on the basis of the mean values from the measured air concentration data sampled at Tokai [23]. As a result, the release rates of 131I and 137Cs discharged into the atmosphere were refined as in Table 15.1, and those for the period from 12 March to 5 April are shown in Fig. 15.6.

Fig. 15.6 Temporal variation in the refined release rates of 131I and 137Cs for the period from 12 March to 5 April 2011. Solid and dashed lines show release rates of 131I and 137Cs, respectively. Japanese standard time (JST) is used

Fig. 15.7 Scatter diagrams of the daily surface deposition of 131I and 137Cs comparing measurements and calculations for the period from 18 to 31 March. Solid lines show 1:1 lines; the areas between two dashed lines indicate the bands within a factor of 10

Daily surface deposition of 131I and 137Cs calculated using the refined source term agreed better with the measurements than those using the preliminary source term. Scatter diagrams and statistics of the daily surface deposition of 131I and 137Cs for calculations and for measurements from March 18 to 31 are shown in Fig. 15.7 and Table 15.2, respectively. In Table 15.2, FA2, FA5, and FA10 denote the percentage of calculations that are within factors of 2, 5, and 10 of the measurements, respectively.

Table 15.2 Statistics of the daily surface deposition of 131I and 137Cs for calculations and for measurements from March 18 to 31, 2011

Using the refined source term, the calculation reproduced daily and monthly surface deposition distributions over land in eastern Japan, without apparent biases toward underor overestimation. Therefore, it is concluded that the source term over the period when the plume flowed over land in Japan is reasonable, although the source term during the period when the plume flowed and deposited over the ocean could not be verified in this study.

Formation Processes of 137Cs Deposition over Eastern Japan

The analysis of the regional-scale atmospheric dispersion was carried out using the estimated source term [4]. It suggested that the present surface deposition distribution of 137Cs over eastern Japan, observed by airborne monitoring [21], was produced mainly by the following events (Fig. 15.8):

• Dry deposition in the northeastern coastal area of Miyagi Prefecture on 12 March

• Wet and dry deposition in Fukushima Prefecture and the north of Tochigi and

Gunma Prefectures on 15 March

• Dry deposition in the north of Ibaraki Prefecture on 15 March and, possibly, dry and wet deposition on 16 March

• Wet deposition in the area from the south of Iwate Prefecture to the northwest of Miyagi Prefecture on 20 March

• Wet deposition in the Kanto region, especially in the area from the south of

Ibaraki Prefecture to the northwest of Chiba Prefecture, from 21 to 23 March

A general feature is that the areas corresponding to a large amount of wet deposition were distributed heterogeneously far from FNPP1, as well as around FNPP1, whereas the areas corresponding to a large amount of dry deposition were mainly located near FNPP1. However, the ratio of dry deposition to the total was relatively high not only near FNPP1, but also in the northeastern coastal area of Miyagi Prefecture, because of dry deposition on 12 March, and in the north of Ibaraki Prefecture from dry deposition on 15 and 16 March.

Fig. 15.8 Formation process of surface deposition distribution of 137Cs over eastern Japan. Shaded area show the surface deposition of 137Cs made from the airborne monitoring data by MEXT (MEXT 2011c). Arrows and enclosed areas show the movements of major plumes and dominant deposition processes and dates (JST), respectively, causing the measured 137Cs deposition

 
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