Temporal variation in environmental radioactivity and radiation exposure doses in the restricted areas around the Fukushima Daiichi Nuclear Power Plant

doi:10.21203/rs.3.rs-3098169/v1

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Temporal variation in environmental radioactivity and radiation exposure doses in the restricted areas around the Fukushima Daiichi Nuclear Power Plant

https://doi.org/10.21203/rs.3.rs-3098169/v1

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published 18 Dec, 2023

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Temporal variation and fluctuation in environmental contamination in Futaba town and Okuma town, the location of the Fukushima Daiichi Nuclear Power Plant (FDNPP), were evaluated based on a car-borne survey conducted from October 2021 to November 2022. Although the environmental radioactivity levels in the interim storage facility area (ISF) were higher than those in open areas (i.e., the evacuation-order-lifted areas in Futaba town and the specific reconstruction and regeneration base area [SRRB] in Okuma town), only minor changes were seen in the ambient dose and radiocesium detection rates in the ISF and SRRB, respectively. These findings suggest that such differences may be the result of multiple factors such as physical decay (decreasing factor) and radiocesium resuspension (non-decreasing factor) accompanied by human activity such as traffic and decontamination construction in these areas, in addition to weather conditions such as wind direction. However, the higher radiation exposure doses in the ISF and SRRB were estimated to be at a limited level (lower than the public dose). Therefore, to help ensure the safety and future prosperity of residents and communities in the affected areas around the FDNPP, long-term follow-up monitoring of temporal dose levels during the recovery and reconstruction phases is extremely important.

Earth and environmental sciences/Environmental sciences

Earth and environmental sciences/Environmental social sciences

Health sciences/Risk factors

On March 11, 2011, the Great East Japan Earthquake (magnitude 9.0) occurred off the east coast of Honshu Island and triggered a massive tsunami that severely affected Iwate, Miyagi, and Fukushima Prefectures, causing a nuclear accident at the Fukushima Daiichi Nuclear Power Plant (FDNPP), located approximately 200 km northeast of Tokyo^1–4. Immediately after the FDNPP accident, the Japanese government implemented emergency protective measures for the public, such as planning for evacuation, sheltering, and relocation, distributing stable iodine to help prevent further radioactive iodine uptake by the thyroid gland, and restricting food and water consumption. Evacuation of the 20-km area around the FDNPP began immediately on March 11, 2011, and was completed 1 day later. In areas within a 20–30 km radius, residents were ordered to remain indoors and then advised to evacuate voluntarily⁴. Subsequently, a comprehensive monitoring program was initiated, including measurements of environmental dose rates and radionuclide activity concentrations in soil, crops, food, and animal feed^2,3,5. The nuclear accident resulted in the release of various artificial radionuclides, including cesium-134 (¹³⁴Cs), cesium-137 (¹³⁷Cs), and iodine-131 (¹³¹I), into the atmosphere and eventual deposition on land and sea in the areas around the FDNPP. Therefore, such survey and monitoring programs are extremely important for the revitalization of Fukushima Prefecture.

Although more than 12 years have passed since the FDNPP accident, radionuclides with long half-lives such as ¹³⁴Cs (half-life: 2.1 years) and ¹³⁷Cs (half-life: 30 years) remain in the environment^2,3. By March 19, 2018, the residential areas that had been under evacuation orders, including surrounding roads, residential areas, farmland, and forests, but excluding the difficult-to-return zones (DRZs), were nearly completely decontaminated⁵. At the same time, an Interim Storage Facility (ISF) was built to store and manage the contaminated soil and waste removed during off-site decontamination work, as well as specified waste (radioactive waste exceeding 8000 Bq/kg) in Fukushima Prefecture, safely until final disposal. Construction of the ISF began in November 2016, and storage of the removed soil and waste began in Okuma town in October 2017 and Futaba town in December 2017^6,8,9. Moreover, under the provisions of the Act on Special Measures for the Reconstruction and Revitalization of Fukushima, which was revised in May 2017, six municipalities in restricted areas, including the DRZs in Futaba town and Okuma town, developed revitalization plans^6,8,9. Under these plans, the Ministry of the Environment conducts decontamination and demolition work in these areas^6,8,9. Due to the efficiency of the decontamination work, on March 4, 2020, the evacuation order was lifted in part of Futaba (evacuation-order-lifted area)⁶. Subsequently, the evacuation orders for the Specific Reconstruction and Regeneration Base area (SRRB) in Okuma and Futaba were lifted on June 30 and August 30, 2022, respectively¹⁰.

Although environmental remediation in the SRRB might allow new residential areas to be constructed in Futaba town and Okuma town in the future, environmental radioactivity levels around the restricted areas, including the ISF, should be monitored and controlled because these areas are next to the SRRB. Therefore, in the present study, a car-borne survey was conducted to evaluate temporal variation in ambient dose and radiocesium detection rates in Futaba town and Okuma town (Fig. 1).

In the present study, 10 surveys were conducted in Futaba town and Okuma town from October 2021 to November 2022. The frequency distributions of the ambient dose rate and detection points of ¹³⁴Cs and ¹³⁷Cs within the ISF, SRRB, and evacuation-order-lifted areas of Futaba town and Okuma town are shown in Tables 1 and 2 and Figs. 2 and 3. The median ambient dose rates in the ISF and evacuation-order-lifted areas in Futaba town ranged from 0.15–0.21 µSv/h (measurement points ranged from 887–1800) and from 0.053–0.086 µSv/h (477–781), respectively, and those in the ISF and SRRB in Okuma town ranged from 0.68–0.90 µSv/h (2420–3688) and from 0.23–0.38 µSv/h (503–1317), respectively. During the survey period, the estimated effective doses measured at all routine monitoring sites in the ISF and evacuation-order-lifted areas in Futaba town ranged from 0.13–0.20 mSv/y in the monthly data and from 0.042–0.15 mSv/y, respectively, and those in the ISF and SRRB in Okuma town ranged from 0.74–1.0 mSv/y and from 0.61–1.1 mSv/y, respectively (Tables 1 and 2). The ¹³⁴Cs and ¹³⁷Cs detection points ranged from 0–2.6% and from 1.7–5.9% in Futaba town, respectively, and from 10.4–24.2% and from 13.2–29.5% in Okuma town, respectively (Figs. 2 and 3). The detection points in Okuma town were significantly higher than those in Futaba town (p < 0.01). Additionally, the radiocesium detection points in the evacuation-order-lifted area and SRRB were lower compared to those for the ISF area, with ¹³⁷Cs ranging from 0–1.2% in Futaba town (¹³⁴Cs not detected), and ¹³⁴Cs ranging from 0–8.2% and ¹³⁷Cs ranging from 0.5–7.8% in Okuma town. These radiocesium detection points in the ISF in Futaba town and Okuma town indicated that the physical decay and ambient dose rates were decreasing, respectively (Supplementary Table S1). The ¹³⁴Cs detection points were lower than those of ¹³⁷Cs because 12 years had passed since the FDNPP accident (approximately six times the physical half-life of ¹³⁴Cs).

Table 1

**Ambient dose rates in the interim storage facility (ISF) area and evacuation-order-lifted area in Futaba town from October 2021 to November 2022.**
Survey date	ISF			Evacuation-order-lifted area
Survey date	Measurement points	Median (min–max) (µSv/h)	Estimated effective doses in mSv/y (monthly)	Measuring points	Median (min–max) (µSv/h)	Estimated effective doses in mSv/y (monthly)
19/10/2021	887	0.21 (0.050–1.8)	0.19	477	0.059 (0.029–0.63)	0.060
20/11/2021	1800	0.20 (0.048–1.8)	0.18	746	0.062 (0.029–0.60)	0.067
18/12/2021	1701	0.21 (0.039–2.0)	0.20	781	0.064 (0.029–0.51)	0.077
28/5/2022	1450	0.16 (0.042–1.3)	0.14	778	0.086 (0.025–1.0)	0.15
25/6/2022	1266	0.16 (0.032–1.3)	0.14	697	0.060 (0.025–0.71)	0.062
23/7/2022	1189	0.15 (0.040–1.3)	0.13	727	0.053 (0.020–0.49)	0.042
27/8/2022	1178	0.21 (0.034–1.6)	0.19	600	0.070 (0.030–0.68)	0.096
17/9/2022	1185	0.19 (0.035–1.5)	0.17	567	0.067 (0.027–0.82)	0.084
15/10/2022	1109	0.19 (0.040–1.5)	0.17	579	0.083 (0.024–0.72)	0.14
19/11/2022	1147	0.21 (0.049–1.9)	0.19	585	0.083 (0.030–0.85)	0.13
Annual estimated effective doses of workers in mSv/y (average)			0.17	Annual estimated effective doses of residents in mSv/y (average)		0.091

Table 2

**Ambient dose rates in the interim storage facility (ISF) area and specific reconstruction and regeneration base (SRRB) area in Okuma town from October 2021 to November 2022.**
Survey date	ISF			SRRB
Survey date	Measurement points	Median (min–max) (µSv/h)	Estimated effective doses in mSv/y (monthly)	Measuring points	Median (min–max) (µSv/h)	Estimated effective doses in mSv/y (monthly)
18/10/2021	2420	0.83 (0.11–3.4)	0.91	535	0.38 (0.13–1.3)	1.1
21/11/2021	3211	0.86 (0.11–4.0)	0.94	836	0.35 (0.15–1.3)	0.97
19/12/2021	2839	0.90 (0.10–3.4)	1.0	1317	0.32 (0.081–1.3)	0.89
29/5/2022	3688	0.69 (0.10–3.3)	0.75	503	0.23 (0.12–2.0)	0.61
26/6/2022	3493	0.68 (0.10–4.1)	0.74	526	0.25 (0.12–2.1)	0.67
24/7/2022	2484	0.69 (0.10–2.9)	0.75	942	0.24 (0.11–1.9)	0.64
28/8/2022	3464	0.77 (0.11–3.7)	0.84	576	0.27 (0.15–2.2)	0.74
18/9/2022	2960	0.75 (0.10–4.2)	0.81	560	0.26 (0.11–2.2)	0.69
16/10/2022	2669	0.73 (0.094–5.1)	0.80	627	0.24 (0.12–2.0)	0.63
20/11/2022	2658	0.82 (0.094–4.9)	0.90	526	0.25 (0.11–2.4)	0.64
Annual estimated effective doses of workers in mSv/y (average)			0.84	Annual estimated effective doses of residents in mSv/y (average)		0.76

Moreover, the ambient dose rates in the ISF areas (> 0.95 µSv/h) were significantly higher than those in the evacuation-order-lifted areas in Futaba town and the SRRB in Okuma town (p < 0.01). The proportions of the ISF area exceeding this value were approximately 1–5% in Futaba town and 29–46% in Okuma town, and approximately 0 and 1–10% in the evacuation-order-lifted areas and SRRB, respectively. The ambient dose rates in the ISF area were < 0.38 µSv/h in Futaba town (79–89%) and 0.38–1.9 µSv/h in Okuma town (73–83%). By contrast, the ambient dose rates were < 0.19 µSv/h (79–90%) in the evacuation-order-lifted areas in Futaba town and 0.19–0.95 µSv/h (66–94%) in the SRRB in Okuma town. In other words, the ambient dose rates (> 0.38 µSv/h) in the ISF areas showed a significantly higher distribution (p < 0.01) in Okuma town (80–90%) than in Futaba town (10–23%). On the other hand, the ambient dose rate in the SRRB in Okuma town (0.38 µSv/h; 59–87%) was higher than that in the evacuation-order-lifted areas in Futaba town (< 0.19 µSv/h; 79–90%). The ambient dose rates were stable in the evacuation-order-lifted areas in Futaba town and SRRB in Okuma town. Although the median ambient dose rate in the SRRB in Okuma town was 0.38 µSv/h in October 2021, the ambient dose rates showed minor changes (0.25 ± 0.02 µSv/h). The radiocesium detection points were also low in the evacuation-order-lifted area and SRRB (Figs. 2 and 3 and Supplementary Figure S1, S2 and S3).

The annual external effective doses for decontamination workers in the ISF were estimated at 0.17 mSv/y in Futaba town and 0.84 mSv/y in Okuma town. On the other hand, the annual effective doses (corresponding to indoor and outdoor activities of daily living) for residents who might return to the evacuation-order-lifted areas and SRRB were estimated at 0.091 mSv/y in Futaba town and 0.76 mSv/y in Okuma town (Supplementary Tables S2 and S3, Figure S2 and S3).

Construction of the ISF in Futaba town and Okuma town began in 2017, with storage facility for removed soil set up in three areas in Futaba town and five areas in Okuma town^7,8,11. The total capacity for contaminated soil was 3.1 million m³ in Futaba town and 10 million m³ in Okuma town. As of March 2023, two storage facilities in Futaba town and one in Okuma town have been completed¹¹. From October 2021 to November 2022, the ambient dose rates and radiocesium detection points were significantly higher in the ISF than in the evacuation-order-lifted area and SRRB in Futaba town and Okuma town (p < 0.001). Additionally, the values at the ISF in Okuma town were significantly higher than those in Futaba town (p < 0.001). Immediately after the FDNPP accident, various radionuclides, including radiocesium, were released from around the FDNPP into the atmosphere, eventually being deposited on land and at sea in the surrounding areas⁵. The amount of artificial radionuclides released from nuclear reactors and diffusion scales absolutely differed between Futaba town and Okuma town^4,6. Therefore, the difference in the initial contamination in Futaba town and Okuma town are considered to provide a direct reflection of the ambient dose rates and radiocesium detection points, even after the decontamination process, because whole contamination materials such as deposits, soil, and trees were not completely removed and storaged^6,7,11. Moreover, the ISF remains a restricted area and decontamination work has not yet been carried out⁷. In our previous study in Tomioka town, Fukushima Prefecture, a significant difference in ambient dose rates since 2017 was observed between the decontaminated and non-decontaminated areas^12–14. According to another report, the average ambient dose rate in decontaminated locations was about 20% lower than that in non-decontaminated areas¹⁵. In Futaba town and Okuma town, the decontamination processes at the SRRB started on September 12, 2017 and March 9, 2018, respectively, with total areas of approximately 860 and 555 ha, respectively^6,7,9,10. The decontamination process in the evacuation-order-lifted area in Futaba town started much earlier than that at the SRRB and was completed in March 2016, well before the evacuation order was lifted on March 4, 2020⁶. According to reports from the Ministry of the Environment, decontamination reduced the ambient dose rates by 60% at 1 m above ground level in residential areas and by 42% on roads^6,16. Therefore, a more effective and thorough decontamination process could reduce the ambient dose rates as a result of the removal of radiocesium (environmental remediation) in the affected areas. Moreover, in our previous study, decontamination was carried out effectively in Tomioka, reducing the ambient dose rates in the decontaminated area by 71.9% (from 1.0 µSv/h to 0.32 µSv/h within 1 year)¹². Other studies have also reported the effectiveness of decontamination for reducing ambient dose rates to a low level^13,14,17. Therefore, our findings suggest that the environmental radioactivity in the SRRB and evacuation-order-lifted areas was continuously stable at extremely low levels as a result of decontamination.

On the other hand, the radiocesium (mainly ¹³⁷Cs) detection points in the SRRB remained at a consistently lower level (< 7.8%). In our previous study, accident-derived ¹³⁷Cs levels in the SRRB in Tomioka were observed in airborne dust samples, which suggested that the ¹³⁷Cs radioactivity in the airborne dust was primarily associated with particles that were resuspended by localized winds and the transfer of construction vehicles as opposed to the decontamination and demolition operations (Supplementary Table S4)¹⁸. Furthermore, human activities such as the transportation of contaminants (removal of soil and radioactive waste) and land restoration might have caused some fluctuations in the ambient dose rates^12,18. According to another report, weekly changes in vehicular traffic tends to affect the accumulation of airborne dust particles and radioactive materials resuspended in the air, thereby contributing to temporary variation in the concentration of radiocesium¹⁹. The findings of the present study suggest that the radiocesium detected was from not only materials derived from the FDNPP accident, but also the subsequent resuspension of radiocesium in the air. In addition, wind direction, wind speed, and other meteorological factors can also cause changes in radiocesium concentrations^20–23. According to previous studies in Fukushima, the wind can affect the deposition level of ¹³⁷Cs, with high concentrations in the air associated with areas of high ¹³⁷Cs deposition²⁰. Radiocesium resuspension and deposition can also be influenced by meteorological events such as rain out (washout), which can transfer radiocesium in the surface layer to the lower layer²¹. According to reports of the Chernobyl accident, a positive relationship was found between airborne radiocesium concentrations and wind speed^22,23.

However, the estimated annual effective doses for decontamination workers and residents of the decontamination area were lower than the recommended limit set by the Japanese government based on the recommendation of the International Commission on Radiological Protection^24–26 (Supplementary Tables S2 and S3, Figure S2 and S3). Nevertheless, to control artificial radioactivity and avoid unnecessary radiation exposure due to the FDNPP accident in these areas, environmental radioactivity monitoring and special education, including radiation safety for workers who engage in decontamination work and residents who will return to the SRRB, are necessary.

In the present study, changes in the ambient dose rate by season and weather were difficult to identify through a horizontal comparison based on car-borne surveys. However, the main artificial radionuclides derived from the FDNPP accident, such as ¹³⁷Cs, could be analyzed to sufficiently low levels. Moreover, the combination of radionuclide analysis of environmental samples such as soil and extensive monitoring via a car-borne survey could accurately evaluate the decontamination effects and external and internal exposure levels. These findings suggest that long-term follow-up monitoring is extremely important for the reconstruction of affected areas, including the SRRB, around the FDNPP.

Survey location. The FDNPP (37°25′ N, 141°02’ E) is located on the boundary between Futaba town (37°25′ N, 141°02’ E) and Okuma town (37°25′ N, 141°02’ E) in Fukushima Prefecture on the east coast of Honshu Island, Japan. Both towns include DRZ and SRRB areas (Fig. 1)⁴. Further decontamination efforts have been continuing in the SRRB of Futaba town and Okuma town.

Survey of ambient dose rates and radiocesium detection. The ISF within the DRZ and SRRB areas (including the evacuation-order-lifted area) of Futaba town and Okuma town was surveyed using the Radi-probe car-borne survey system (Chiyoda Technology Corp., Tokyo, Japan) with a handheld radiation detector (HDS-101GN; Mirion Technologies, Inc., Japan) from October 2021 to November 2022 (10 times in the ISF, SRRB, and evacuation-over-lifted area of Futaba town and Okuma town)²⁷ This Radi-probe system, including the radiation detector, a laptop computer, GPS, and a micro-camera, was installed in a vehicle, and the radiation detector was set on the front passenger seat about 1 m above the ground. The ambient dose rates were measured and the position coordinates and a photo were automatically taken every 5 s in addition to spectrum segments every 0.2 s. Gamma detection was performed using a large thallium-doped cesium iodide scintillator with high sensitivity (typical 1400 cps per µSv/h for the ¹³⁷Cs source). The measurable range of gamma-ray energy using a multichannel analyzer with 512 channels was 30 keV–6 MeV. The detected energy peaks of radiocesium (¹³⁴Cs and ¹³⁷Cs) were registered in the detected net count values and their associated confidence (with levels 1–10 used as reference values)^12,28. Software with a graphical interface installed on the laptop computer enabled automatic output of real-time maps with color-scaled environmental dose rates and gamma-ray energy spectra. Snapshots taken by a micro-camera set at the front of a vehicle could also confirm the geographical environment and weather^11,13. Generally, the chassis and walls of a vehicle provide a shielding effect against external radiation; numerous other factors, such as the type of vehicle and the number of occupants, also affect this shielding factor¹³. Therefore, the shielding effect was calculated by measuring the interior and exterior of the vehicle in an open and flat area at a height of 1 m above the ground before each monitoring session. The shielding factors ranged from 1.19 to 1.84. In all measurements, the vehicle was driven at a constant speed by the same person.

Effective dose. The effective dose through external exposure is calculated using the following formula¹²:

\({E}_{i}\) = (D_out – D_BG)·T·R	(a)
E_w =\({\sum }_{i=1}^{12}{E}_{i}\)	(b)
E = E_out + E_in	(c)
E_out/in = (D_out/in – D_BG)·T·F·R	(d)
D_in = r·D_out	(e)

where E_i is the estimated external effective dose (mSv/month by median), E_w the external effective dose for decontamination workers (mSv/y), E the external effective dose for residents who are going to return to the decontaminated area (mSv/y), E_out/in the external effective dose for outdoor and indoor workers, D_out/in the dose rate for a height of 1 m above ground outside and inside the house (µSv/h), D_BG 0.04 µSv/h, which was measured in the area of interest before the accident, T the work time (240 days × 8 h; the normal labor standard in Japan), F the occupancy factor (with 16 and 8 h [24 h/day] considered to represent the indoor and outdoor activities of daily living, respectively, based on the guideline of the Ministry of the Environment, Japan), R the age-dependent dose conversion coefficient for adults (0.6 corresponds to the effective dose for an adult), and r the deposited gamma location factor for a wooden house (0.4)²⁹.

Data analysis. All data followed a non-normal distribution. To compare differences among the measurement areas in the same period and the temporal variation within the same areas, the data were analyzed using the Mann–Whitney U and Kruskal–Wallis H tests.

Acknowledgments

We would like to thank all the study participants and the staffs of Futaba town, Okuma town, and Tomioka town for their cooperation.

Author contributions

Conceived and designed the observations: N.T. and Y.T.; performed the observations: M.L., Y.T., M.M., M.O., H.M., Y.K., and X.X.; analyzed the data: M.L. and Y.T.; wrote the paper: M.L. and Y.T. All authors approved the final version of the manuscript.

Supplementary information

Physical decay of radiocesium from October 2021 to November 2022. Effective doses for workers and residents in Futaba town, Okuma town, and Tomioka town. Real-time map of color-scaled ambient dose rates and ¹³⁷Cs detection points among the sampling locations in the ISF sites in the DRZs in Futaba town and Okuma town from October 2021 to November 2022.

Funding

This work was supported by Research on the Health Effects of Radiation organized by the Ministry of the Environment, Japan.

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No competing interests reported.

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Editor invited by journal
30 Jun, 2023
Submission checks completed at journal
30 Jun, 2023
First submitted to journal
22 Jun, 2023

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Temporal variation in environmental radioactivity and radiation exposure doses in the restricted areas around the Fukushima Daiichi Nuclear Power Plant

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Abstract

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Introduction

Results

Discussion

Materials and Methods

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Supplementary Files

Status:

Journal Publication

Version 1