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Abstract
We investigated the distribution in seawater of anthropogenic radionuclides from the Fukushima Daiichi Nuclear Power Plant (FNPP1) as preliminary attempt using a rapid aerial radiological survey performed by the U.S. Department of Energy National Nuclear Security Administration on 18 April 2011. We found strong correlations between in-situ activities of 131I, 134Cs, and 137Cs measured in surface seawater samples and gamma-ray peak count rates determined by the aerial survey (correlation coefficients were 0.89 for 131I, 0.96 for134Cs, and 0.92 for137Cs). The offshore area of high radionuclide activity extended south and southeast from the FNPP1. The maximum activities of 131I, 134Cs, and 137Cs were 329, 650, and 599 Bq L−1, respectively. The 131I/137Cs ratio in surface water of the high-activity area ranged from 0.6 to 0.7. Considering the radioactive decay of 131I (half-life 8.02 d), we determined that the radionuclides in this area were directly released from FNPP1 to the ocean. We confirm that aerial radiological surveys can be effective for investigating the surface distribution of anthropogenic radionuclides in seawater. Our model reproduced the distribution pattern of radionuclides derived from the FNPP1, although results simulated by a regional ocean model were underestimated.
1. Introduction
The giant Tohoku earthquake and the resulting huge tsunami of 11 March 2011 caused the release of radionuclides from the Fukushima Daiichi Nuclear Power Plant (FNPP1) to the ocean and atmosphere. The Consequence Management Response Team of the U.S. Department of Energy National Nuclear Security Administration (DOE/NNSA) conducted aerial radiological surveys using its Aerial Measuring System (AMS) to determine the distribution of ambient gamma-ray count rates on land [1,2]. The survey results provided a complete map of the distribution of the radionuclides 131I, 134Cs, and 137Cs on the ground surface in Japan [3,4]. However, the map did not depict radionuclide levels offshore. There are no established analytical methods to estimate activities in seawater in the absence of accurate attenuation coefficients. Methods of determining activities of FNPP1-derived radionuclides in seawater have been limited to chemical analysis of in-situ samples [5]. Shipborne measurements of radionuclides in seawater are necessarily limited in spatial coverage. It is, therefore, important to establish aerial surveys as a viable means of measuring the distribution of radionuclides over wide regions of the ocean. This study was a preliminary attempt to investigate the distribution of radionuclides in seawater near the FNPP1 by an aerial survey.
2. Materials and method
On 18 April 2011 (at 10:21–12:48 and 16:27–18:40 local time), the U.S. DOE/NNSA conducted aerial radiological surveys by using fixed-wing aircraft (U.S. Air Force C12 Huron airplanes) and the agency's AMS in an offshore survey area less than 30 km from the FNPP1 (141.055–141.265°E and 37.14–37.68°N) [1–4]. The AMS uses an array of three thallium-activated sodium iodide [NaI(Tl)] scintillator crystals, 5 cm×10 cm×40 cm (2.1 L) in size, each mounted with a photomultiplier tube and digital multichannel analyzer (MCA). The AMS measures gamma rays of energies up to 3066 keV at an interval of 1 s. The signals are recorded in channels 0–1022 by a digital channel analyzer. For this study, we used channels 102−134 (304−404 keV) for 131I, channels 169−230 (507−690 keV) for 134Cs and 137Cs, and channels 235−290 (704−869 keV) for 134Cs, based on energy peaks identified by Torii et al. [4]. Gamma-ray count rates for 137Cs were estimated by subtraction of 134Cs count rates from the composite count rates for 137Cs and 134Cs based on the gamma-ray abundance ratio between 137Cs and 134Cs. Location information (latitude, longitude, and altitude) was obtained from the global positioning system. The measurement altitude was 237 ± 38 m above sea level.
Surface seawater samples were collected and analyzed by Tokyo Electric Power Company (TEPCO). On 18 April 2011 (6:55–14:25 local time), 12 in-situ samples were collected within the area measured by AMS. At a flight altitude of 300 m, aerial radiological surveys can detect gamma-ray count rates from the area within 300–600 m of the survey aircraft. For comparison of in-situ and aerial data, we used aerial measurements made within 500 m of the sampling location. Background gamma-ray count rates were measured in the ocean south of the FNPP1 (139–141°E and 35.8–37.1°N) at altitudes from 650 to 1500 m.
Estimated activities of radionuclides were interpolated by the optimum interpolation method with a resolution of 0.015° in longitude and 0.01° in latitude [6]. This method is effective for statistically investigating the horizontal distribution of irregularly distributed measurement data. It has the advantage of using statistical estimates to determine appropriate relative weighting between observations and a first guess to minimize the resulting error of all the data in the analysis. The optimal value of the influence radius (R) parameter was determined by comparing the optimum interpolation results with those of another interpolation and gridding algorithm, the Akima spline method [7]. By comparing the correlation coefficients of calculated radionuclide activity for each grid cell by the optimum interpolation and Akima spline methods, we selected 1 km as the optimal value of R.
The calculated activities of FNPP1-derived radionuclides in seawater were compared with the corresponding activities simulated by the Regional Ocean Modeling System (ROMS) [8–10]. The model domain covered the oceanic area off FNPP1 (35.33−38.67°N and 140.33−142.67°E) and the horizontal resolution was 0.008°×0.008°. The simulated data were interpolated to 0.015°×0.01° resolution for comparison with the aerial measurement data from the optimum interpolation analysis.
3. Results and discussion
3.1. Relation between gamma-ray count rates and radionuclide activity in seawater samples
There were strong relationships between the gamma-ray count rates in aerial surveys and the FNPP1-derived radionuclide activities in seawater samples (correlation coefficients, 0.89 for 131I, 0.96 for 134Cs, and 0.92 for 137Cs) (Figure 1). The peak counts for each radionuclide were calculated as total counts in the radionuclide's respective peak range by the Covell method [11]. Although the correlation between 131I activities and the net count rate by the Covell method was not significant, the correlation between 131I activities and the total count rate was strong. The non-significant correlation with the net count rate might reflect the high background energy in the peak range and the small peak count rates of 131I. For 134Cs, we also compared peak counts by the Covell method and the Gaussian peak extraction method and found no significant difference. Therefore, we used the total count rates to estimate the radionuclide activities in seawater. From the linear relation between peak count rates and in-situ measured activity, we estimated the lower detection limits of 131I, 134Cs, and 137Cs to be 307, 218, and 354 counts per second (cps), respectively, in this aerial survey. These detection limits include the effects of attenuation in seawater and the atmosphere (using observed altitudes and water vapor concentrations in the atmosphere).
Fukushima NPP Accident Related
Published online:
25 June 2014Figure 1. Relationship between gamma-ray count rates by aerial radiological survey and in-situ measurement of (a) 131I, (b) 134Cs, and (c) 137Cs activities in surface seawater. White circle is the peak count rate in the background region.
![]({"type":"image","src":"/na101/home/literatum/publisher/tandf/journals/content/tnst20/2014/tnst20.v051.i09/00223131.2014.914451/20140625/images/medium/tnst_a_914451_f0001_b.gif"})
Figure 1. Relationship between gamma-ray count rates by aerial radiological survey and in-situ measurement of (a) 131I, (b) 134Cs, and (c) 137Cs activities in surface seawater. White circle is the peak count rate in the background region.
We considered the measurement conditions for the peak counts to be uniform because the AMS measurement was conducted at an almost constant altitude. Furthermore, we considered the variation of the peak channel to be negligible because the two aerial radiological surveys were conducted on the same day using the same AMS. Although the seawater sampling and aerial survey were not conducted at the same time, these close relationships imply that the aerial survey reliably depicted the distribution of radionuclides in surface seawater within a time scale of half a day.
3.2. Horizontal distribution of radionuclides in surface seawater near the FNPP1
Figure 2 displays the horizontal distribution of gamma-ray count rates from the aerial survey as well as the estimated and simulated 137Cs activities in surface seawater near the FNPP1 on 18 April 2011. The area of high gamma-ray count rates and 137Cs activities extended south and southeast at least 20 km from the FNPP1. The distributions of 131I and 134Cs (not shown) were similar to that of 137Cs. The maximum activities of 131I, 134Cs, and 137Cs were 329, 650, and 599 Bq L−1, respectively. The simulated distribution of 137Cs activity was similar to the distribution of gamma-ray counts from the aerial survey. On the other hand, the simulated 137Cs activities were less than the estimated activities based on the aerial survey (Figure 2(c)). However, the simulated temporal variation of 137Cs activity near the FNPP1 agreed well with observations [9]. Because the gamma-ray count rates near the FNPP1 include the effect of artificial as well as natural (soil) radionuclides, the aerial survey had high uncertainty near coastal monitoring sites. We, then, did not directly compare the measured in-situ activities and gamma-ray count rates at coastal sites. One possible interpretation of the underestimation by the simulation is that the effects of advection and vertical mixing strength were not reproduced well in the model simulation. However, there are no data on the vertical distribution of radionuclide activity in this area for this period, although the surface 137Cs activity strongly depends on vertical as well as horizontal transport. Therefore, radionuclide activities estimated from an aerial radiological survey should be used for model validation.
Fukushima NPP Accident Related
Published online:
25 June 2014Figure 2. Distribution of (a) gamma-ray count rates (cps) from aerial radiological survey, (b) calculated 137Cs activities (Bq L−1), and (c) simulated 137Cs activities (Bq L−1) over surface seawater near the FNPP1 on 18 April 2011. The radius of the circle centered at the FNPP1 (triangle) is 30 km; the black rectangle shows the aerial radiological survey region and small circles are in-situ sampling points.
![]({"type":"image","src":"/na101/home/literatum/publisher/tandf/journals/content/tnst20/2014/tnst20.v051.i09/00223131.2014.914451/20140625/images/medium/tnst_a_914451_f0002_c.jpg"})
Figure 2. Distribution of (a) gamma-ray count rates (cps) from aerial radiological survey, (b) calculated 137Cs activities (Bq L−1), and (c) simulated 137Cs activities (Bq L−1) over surface seawater near the FNPP1 on 18 April 2011. The radius of the circle centered at the FNPP1 (triangle) is 30 km; the black rectangle shows the aerial radiological survey region and small circles are in-situ sampling points.
The distribution of 131I/137Cs ratios in surface seawater is shown in Figure 3. The 131I/137Cs ratios in the high-activity area shown in Figure 2 were 0.6−0.7. It has been reported that the 131I/137Cs ratio in the puddled cooling water from the FNPP1 on 26 March 2011 was 5.7 [9]. Given the 8.02-day half-life of 131I, the 131I/137Cs activity ratio of directly released water should be 0.72 as of the survey date. It is reasonable to conclude that the radionuclides in this region were dominated by direct releases from the FNPP1 to the ocean rather than atmospheric input.
Fukushima NPP Accident Related
Published online:
25 June 2014Figure 3. Distribution of 131I/137Cs ratio over surface seawater near the FNPP1 on 18 April 2011. Triangle is the location of FNPP1. Contour line is 131I/137Cs ratio of 0.72, which incorporates radiological decay after 26 March 2011. The radius of the circle centered at the FNPP1 (triangle) is 30 km.
![]({"type":"image","src":"/na101/home/literatum/publisher/tandf/journals/content/tnst20/2014/tnst20.v051.i09/00223131.2014.914451/20140625/images/medium/tnst_a_914451_f0003_c.jpg"})
Figure 3. Distribution of 131I/137Cs ratio over surface seawater near the FNPP1 on 18 April 2011. Triangle is the location of FNPP1. Contour line is 131I/137Cs ratio of 0.72, which incorporates radiological decay after 26 March 2011. The radius of the circle centered at the FNPP1 (triangle) is 30 km.
3.3. Estimated total amount and inventory of radionuclides near the FNPP1
The estimated inventories of 131I, 134Cs, and 137Cs in the area affected by direct release were 868, 1307, and 1208 kBq m−2, respectively, at the time of the aerial survey (Table 1). Assuming that the mixed layer depth in this area is 10 m [12], these inventories mean that the average activities derived from direct released seawater were 682, 918, and 1061 Bq L−1 for 131I, 134Cs, and 137Cs, respectively. The total amounts of 131I, 134Cs, and 137Cs in the aerial survey area were 0.83, 1.22, and 1.13 PBq, respectively (Table 1). Given that the total amount of directly released 137Cs from the FNPP1 as of 18 April 2011 was 3.4 PBq [10], more than one-third of that total remained in this area 12 days after the release ended on 6 April 2011.
Table 1. Estimated total amounts of radioactivity and inventories of three radionuclides in the survey region.
4. Conclusions
We investigated the distribution in seawater of the radionuclides 131I, 134Cs, and 137Cs derived from FNPP1 with an aerial radiological survey. We found that gamma-ray count rates observed during this survey showed a linear relationship with the in-situ measured activities of these nuclides. We subsequently estimated the distribution of radioactivity near the FNPP1 on the basis of this empirical analysis.
The analytical results show that an area with high radionuclide activities extended south and southeast of the FNPP1. In that area, the maximum activities of 131I, 134Cs, and 137Cs reached 329, 650, and 599 Bq L−1, respectively, in the surface seawater. The 131I/137Cs activity ratio (0.6–0.7) suggests that high concentrations of radionuclides in this region were dominated by direct releases from the FNPP1 to the ocean. Estimated inventory indicated that more than one-third of the total 137Cs directly released from the FNPP1 remained in this area 12 days after large-scale releases ended. We confirm that aerial radiological survey is a useful way to rapidly monitor the distribution of radionuclides in surface seawater, particularly on an urgent basis.
| Inventory | |||
|---|---|---|---|
| Total amount | (kBq m−2)* | ||
| (PBq) Calculated | Calculated | ||
| Mixed layer | Simulated | Mixed layer | |
| 10 m (20 m)# | (PBq) | 10 m (20 m) | |
| 131I | 0.83 (1.7) | 868 (1736) | |
| 134Cs | 1.2 (2.4) | 1307 (2614) | |
| 137Cs | 1.1 (2.2) | 0.93 | 1208 (2416) |
Acknowledgements
The authors are grateful to the staff of U.S. DOE/NNSA for conducting the aerial radiological survey.