IMPACT ON ABSORBED DOSE RATE IN AIR IN THE IZU ISLANDS FROM LONG HALF-LIFE RADIONUCLIDES RELEASED BY THE FUKUSHIMA DAIICHI NUCLEAR POWER PLANT ACCIDENT

IMPACT ON ABSORBED DOSE RATE IN AIR IN THE IZU ISLANDS FROM LONG HALF-LIFE RADIONUCLIDES RELEASED... Abstract Car-borne surveys were carried out on eight islands of the Izu Islands located 339–570 km southwest of the Fukushima Daiichi Nuclear Power Plant. The mean dose rates measured in 2015, 2016 or 2017 on each island were from 12 to 47 nGy h−1, meaning that the contribution ratios of artificial radionuclides were 5–31%. Based on the environmental half-life for long half-life radionuclides (134Cs + 137Cs) measured on Izu-Oshima (3.1 y), the mean dose rates in March 2011 were estimated to be 15–53 nGy h−1 and the contribution ratios of artificial radionuclides were 11–55%. The estimated annual external effective doses were 0.06–0.21 mSv which were 13–44% of the worldwide average (0.48 mSv). INTRODUCTION The total activities of artificial radionuclides released by the Fukushima Daiichi Nuclear Power Plant (F1-NPP) accident have been estimated by the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) to be in the ranges of 100–500 PBq for 131I and 6–20 PBq for 137Cs(1). They are ~10 and 20% of the respectively estimated amounts released to the atmosphere in the 1986 Chernobyl accident. According to the report from the Science Council of Japan, 93% of the total 137Cs assumed in the simulation model was wet-deposited over the globe in the F1-NPP accident(2). Atmospheric simulation models for the F1-NPP released artificial radionuclides have substantial role in estimations on health effects. The Science Council of Japan has compared the results obtained from nine regional atmospheric models and six global atmospheric models(2) and reported there were large variabilities in the horizontal distribution of the accumulated deposition caused by differences in deposition model treatments(3). Thus, accurate estimation of emissions to the air is important for estimation of the atmospheric behaviors of radionuclides and their subsequent behaviors in absorbed dose rate in air and soil. The Izu Islands are a chain of volcanic islands located 339–570 km southwest of the F1-NPP (Figure 1). On Izu-Oshima, located closest to the F1-NPP (339 km), a survey was carried out in August 2011 by a group including some of the present authors(4), but no measurements on other islands (381–570 km from the F1-NPP) have been made. Since Japan is an island country with the sea on all sides, it is difficult to estimate the amounts of artificial radionuclides from a seawater sampling survey because the released artificial radionuclides have been diluted by the surrounding sea water. Thus, an accumulation of data measured on islands that are scattered over the Pacific Ocean would be important to enhance the precision of regional atmospheric models and allow accurate estimation of health effects. In this study, car-borne surveys were carried out in 2015, 2016 or 2017 for eight islands of the Izu Islands; these islands are an administrative part of the Tokyo Metropolitan Government. Additionally, the absorbed dose rates in air in the Izu Islands just after the accident were estimated based on the environmental half-life for long half-life radionuclides (134Cs + 137Cs) reported previously. This study will contribute to an improved understanding of the release and subsequent dispersion of artificial radionuclides on the Pacific Ocean side of Japan. Figure 1. View largeDownload slide Location of the Izu Islands and the Fukushima Daiichi Nuclear Power Plant. Figure 1. View largeDownload slide Location of the Izu Islands and the Fukushima Daiichi Nuclear Power Plant. MATERIALS AND METHODS Survey area The measurements of the count rates were carried out during May 2015, August 2016 and February 2017 for the Izu Islands (Figure 1 and Table 1) and then converted to absorbed dose rate in air (nGy h−1) by using conversion factors. The survey routes were selected as main roads on these islands (Figure 2). The weather conditions were sunny or cloudy throughout the measurement days, and there was no rainfall on them. These routes and altitude maps were drawn using Generic Mapping Tools (GMT)(5) and the geographic information published by the Geospatial Information Authority of Japan(6). Table 1. Statistical and geological data for the Izu Islands and the measurement period. Island  Populationa  Area (km2)  Distance from Fukushima Daiichi Nuclear Power Plant (km)  Measurement period  Basement geology(32)  Izu-Oshima  7 611  91  339  August 2016  Basalt/Pyroclastic rocks  Niijima  2 224  28  381  May 2015  Dacite/Rhyolite  Shikinejima  532  4  388  May 2015  Dacite/Rhyolite  Kouzushima  1 847  19  402  May 2015  Dacite/Rhyolite  Miyakejima  2 551  55  404  August 2016  Basalt/Pyroclastic rocks  Mikurashima  339  20  423  August 2015  Andesite/Basalt  Hachijojima  7 337  72  500  February 2015  Basalt/Pyroclastic rocks  Aogashima  175  6  570  February 2017  Basalt/Pyroclastic rocks  Island  Populationa  Area (km2)  Distance from Fukushima Daiichi Nuclear Power Plant (km)  Measurement period  Basement geology(32)  Izu-Oshima  7 611  91  339  August 2016  Basalt/Pyroclastic rocks  Niijima  2 224  28  381  May 2015  Dacite/Rhyolite  Shikinejima  532  4  388  May 2015  Dacite/Rhyolite  Kouzushima  1 847  19  402  May 2015  Dacite/Rhyolite  Miyakejima  2 551  55  404  August 2016  Basalt/Pyroclastic rocks  Mikurashima  339  20  423  August 2015  Andesite/Basalt  Hachijojima  7 337  72  500  February 2015  Basalt/Pyroclastic rocks  Aogashima  175  6  570  February 2017  Basalt/Pyroclastic rocks  aAs of October 2017(31). Table 1. Statistical and geological data for the Izu Islands and the measurement period. Island  Populationa  Area (km2)  Distance from Fukushima Daiichi Nuclear Power Plant (km)  Measurement period  Basement geology(32)  Izu-Oshima  7 611  91  339  August 2016  Basalt/Pyroclastic rocks  Niijima  2 224  28  381  May 2015  Dacite/Rhyolite  Shikinejima  532  4  388  May 2015  Dacite/Rhyolite  Kouzushima  1 847  19  402  May 2015  Dacite/Rhyolite  Miyakejima  2 551  55  404  August 2016  Basalt/Pyroclastic rocks  Mikurashima  339  20  423  August 2015  Andesite/Basalt  Hachijojima  7 337  72  500  February 2015  Basalt/Pyroclastic rocks  Aogashima  175  6  570  February 2017  Basalt/Pyroclastic rocks  Island  Populationa  Area (km2)  Distance from Fukushima Daiichi Nuclear Power Plant (km)  Measurement period  Basement geology(32)  Izu-Oshima  7 611  91  339  August 2016  Basalt/Pyroclastic rocks  Niijima  2 224  28  381  May 2015  Dacite/Rhyolite  Shikinejima  532  4  388  May 2015  Dacite/Rhyolite  Kouzushima  1 847  19  402  May 2015  Dacite/Rhyolite  Miyakejima  2 551  55  404  August 2016  Basalt/Pyroclastic rocks  Mikurashima  339  20  423  August 2015  Andesite/Basalt  Hachijojima  7 337  72  500  February 2015  Basalt/Pyroclastic rocks  Aogashima  175  6  570  February 2017  Basalt/Pyroclastic rocks  aAs of October 2017(31). Figure 2. View largeDownload slide The survey routes for measuring the count rate in air in the Izu-Islands (a, Izu-Oshima; b, Niijima; c, Shikinejima; d, Kouzushima; e, Miyakejima; f, Mikurashima; g, Hachijojima; h, Aogashima). Car-borne surveys were carried out using a 3-in × 3-in NaI(Tl) scintillation spectrometer during May 2015, August 2016 or February 2017. The fixed-point measurements outside the car were also carried out for 10 min at 145 locations (circle). Figure 2. View largeDownload slide The survey routes for measuring the count rate in air in the Izu-Islands (a, Izu-Oshima; b, Niijima; c, Shikinejima; d, Kouzushima; e, Miyakejima; f, Mikurashima; g, Hachijojima; h, Aogashima). Car-borne surveys were carried out using a 3-in × 3-in NaI(Tl) scintillation spectrometer during May 2015, August 2016 or February 2017. The fixed-point measurements outside the car were also carried out for 10 min at 145 locations (circle). Car-borne surveys Car-borne surveys were carried out over the asphalt pavement (width: 4–10 m) using a 3-in × 3-in NaI(Tl) scintillation spectrometer (EMF-211, EMF Japan Co., Osaka, Japan) with a global positioning system (GPS). The NaI(Tl) scintillation spectrometer was positioned 1 m above the ground surface at the center of the car. Measurements of the count rates inside the car were performed every 30 s along the route, and consecutive gamma-ray energies of 50 keV–3.2 MeV were recorded. Simultaneously, latitude and longitude at each measurement point were recorded at the same time with the GPS. Car speeds were kept around 30 km h−1. The car windows were kept closed during those measurements. Walking surveys were also carried out over unpaved surfaces for mountain areas of Shikinejima, Kouzushima and Mikurashima where the car was unable to be driven using the same scintillation spectrometer which was carried by a researcher. The photon peak of 40K (Eγ = 1.464 MeV) was used for calibration from the channel number and gamma-ray energy before the measurements. All measurements were carried out with the permission of the town office branches. Additionally, the measurements on Mikurashima were carried out with the permission of the Tokyo Metropolitan Government because most of this island is a nature reserve. The shielding effect of the car body was estimated by measuring the count rates inside and outside the car for each island because count rate was measured inside the car. There were 145 measurement locations for the eight islands (circles in Figure 2). Measurements were recorded for consecutive 30-s intervals during a total recording period of 2 min inside and outside the car. Those measurements were done above bare soil areas which were not cultivated soil areas (i.e. areas unaffected from natural radionuclides contained in fertilizers). The shielding factors (SFs) were calculated as regression coefficients from the correlation between count rates inside and outside the car for each island. The count rates inside the car were then multiplied by this SF. The gamma-ray pulse height distributions were also measured outside the car for 10 min at the same 145 locations (Figure 2). The NaI(Tl) scintillation spectrometer was positioned 1 m above the bare soil surface. The measured gamma-ray pulse height distribution measured with the NaI(Tl) scintillation spectrometer was then unfolded using a 22 × 22 response matrix method(7) and absorbed dose rates in air were calculated. The dose conversion factors (nGy h−1/cps) for each island were then estimated from a correlation between calculated dose rates and measured count rates. The absorbed dose rate in air outside the car (Dout) was calculated using the following equation:   Dout=Cin×SF×DCF (1)where Cin is count rate (cps) inside the car, obtained by the car-borne survey. SF is shielding factor and DCF is dose conversion factor (nGy h−1/cps). For the count rate obtained from the walking survey, only DCF was utilized to calculate the absorbed dose rate in air. All data obtained from the car-borne and walking surveys were plotted on a distribution map of absorbed dose rate in air using GMT(5). A minimum curvature algorithm was used for the data interpolation by GMT. This is the method for interpolating data by presuming a smooth curved surface from the data of individual points. For a more detailed analysis, clear peaks from 134Cs (energy ranges: 0.55–0.65 MeV and 0.75–0.85 MeV), 137Cs (0.65–0.75 MeV), 40K (1.39–1.54 MeV), 214Bi (1.69–1.84 MeV and 2.10–2.31 MeV) and 208Tl (2.51–2.72 MeV) were observed in the energy spectrum after unfolding the gamma-ray pulse height distribution obtained at 145 locations. The absorbed dose rates in air from natural radionuclides (40K, 238U series and 232Th series) and artificial radionuclides (134Cs and 137Cs) were then calculated to estimate the impact from the F1-NNP accident. The detailed method has been reported by Minato(7). RESULTS AND DISCUSSION Shielding and dose conversion factors The SFs for each island survey were obtained from correlations between count rates inside and outside the car and applied to calculation of absorbed dose rates in air. Table 2 shows obtained SFs and the standard uncertainties (Type A evaluation)(8). The standard uncertainties were calculated using the following equation:   u=sN (2)where u is standard uncertainty, s is standard deviation of SF and DCF, and N is the number of measurement locations. The coefficient of determinations (R2) from measurement correlations ranged from 0.807 to 0.928. The SFs and the standard uncertainties were also calculated and ranged from 1.23 to 1.55 and from 0.02 to 0.06, respectively. The SF is influenced by the type of car used in the survey, the number of passengers and the scintillation spectrometer position inside the car. The obtained SF for Izu-Oshima was lower than that obtained for other islands because a smaller car was utilized. SFs have been reported previously as ranging from 1.1 to 1.9(4, 9–18) and the presently obtained SFs were in this range. Table 2. Absorbed dose rates in air in the Izu Islands assessed from the measurements by the car-borne survey technique. Island  na  Shielding factor  Standard uncertainty of shielding factor  Dose conversion factor (nGy h−1/cps)  Standard uncertainty of dose conversion factor  Calculated absorbed dose rate in air (nGy h−1)  Mean  SD  Range  Izu-Oshima  1011  1.23  0.02  0.144  1.89 × 10−3  22  6  6–54  Niijima  418  1.55  0.05  0.137  2.58 × 10−3  33  5  22–52  Shikinejima  338  1.50  0.06  0.148  3.27 × 10−3  42  6  27–62  Kouzushima  805  1.50  0.03  0.144  3.37 × 10−3  49  7  17–83  Miyakejima  1022  1.47  0.03  0.146  2.40 × 10−3  22  7  7–43  Mikurashima  846  1.40  0.04  0.162  3.36 × 10−3  27  7  12–45  Hachijojima  832  1.49  0.03  0.162  3.27 × 10−3  20  7  6–65  Aogashima  179  1.43  0.04  0.144  1.44 × 10−3  15  8  5–42  Island  na  Shielding factor  Standard uncertainty of shielding factor  Dose conversion factor (nGy h−1/cps)  Standard uncertainty of dose conversion factor  Calculated absorbed dose rate in air (nGy h−1)  Mean  SD  Range  Izu-Oshima  1011  1.23  0.02  0.144  1.89 × 10−3  22  6  6–54  Niijima  418  1.55  0.05  0.137  2.58 × 10−3  33  5  22–52  Shikinejima  338  1.50  0.06  0.148  3.27 × 10−3  42  6  27–62  Kouzushima  805  1.50  0.03  0.144  3.37 × 10−3  49  7  17–83  Miyakejima  1022  1.47  0.03  0.146  2.40 × 10−3  22  7  7–43  Mikurashima  846  1.40  0.04  0.162  3.36 × 10−3  27  7  12–45  Hachijojima  832  1.49  0.03  0.162  3.27 × 10−3  20  7  6–65  Aogashima  179  1.43  0.04  0.144  1.44 × 10−3  15  8  5–42  aNumber of measurements. Table 2. Absorbed dose rates in air in the Izu Islands assessed from the measurements by the car-borne survey technique. Island  na  Shielding factor  Standard uncertainty of shielding factor  Dose conversion factor (nGy h−1/cps)  Standard uncertainty of dose conversion factor  Calculated absorbed dose rate in air (nGy h−1)  Mean  SD  Range  Izu-Oshima  1011  1.23  0.02  0.144  1.89 × 10−3  22  6  6–54  Niijima  418  1.55  0.05  0.137  2.58 × 10−3  33  5  22–52  Shikinejima  338  1.50  0.06  0.148  3.27 × 10−3  42  6  27–62  Kouzushima  805  1.50  0.03  0.144  3.37 × 10−3  49  7  17–83  Miyakejima  1022  1.47  0.03  0.146  2.40 × 10−3  22  7  7–43  Mikurashima  846  1.40  0.04  0.162  3.36 × 10−3  27  7  12–45  Hachijojima  832  1.49  0.03  0.162  3.27 × 10−3  20  7  6–65  Aogashima  179  1.43  0.04  0.144  1.44 × 10−3  15  8  5–42  Island  na  Shielding factor  Standard uncertainty of shielding factor  Dose conversion factor (nGy h−1/cps)  Standard uncertainty of dose conversion factor  Calculated absorbed dose rate in air (nGy h−1)  Mean  SD  Range  Izu-Oshima  1011  1.23  0.02  0.144  1.89 × 10−3  22  6  6–54  Niijima  418  1.55  0.05  0.137  2.58 × 10−3  33  5  22–52  Shikinejima  338  1.50  0.06  0.148  3.27 × 10−3  42  6  27–62  Kouzushima  805  1.50  0.03  0.144  3.37 × 10−3  49  7  17–83  Miyakejima  1022  1.47  0.03  0.146  2.40 × 10−3  22  7  7–43  Mikurashima  846  1.40  0.04  0.162  3.36 × 10−3  27  7  12–45  Hachijojima  832  1.49  0.03  0.162  3.27 × 10−3  20  7  6–65  Aogashima  179  1.43  0.04  0.144  1.44 × 10−3  15  8  5–42  aNumber of measurements. The DCFs were then obtained from the correlation between dose rate (nGy h−1) calculated from the software using the 22 × 22 response matrix method(7) and count rate outside the car (Table 2). The decision coefficients (R2) obtained were correlations from measurements on the eight islands and ranged from 0.870 to 0.945. The absorbed dose rates in air were calculated using the SFs and DCFs. Absorbed dose rates in air The mean absorbed dose rates (ranges) in the whole area of the eight islands measured by car-borne and walking surveys are shown in Table 2. The highest dose rate was observed on Kouzushima (49 nGy h−1) located 402 km southwest of the F1-NPP, whereas the lowest dose rate was observed on Aogashima (570 km southwest; 15 nGy h−1). Although large scale surveys for measuring the absorbed dose rates in air from terrestrial radiation in Japan had been done in the 1960 to 1970s by the National Institute of Radiological Sciences (NIRS) using an integrated methodology with an ionization chamber system and NaI(Tl) scintillation detectors based on in situ measurements(19, 20), no surveys for isolated islands such as the Izu Islands were included. Some members of the present authors’ group had done measurements of absorbed dose rate in air by the car-borne survey technique for Izu-Oshima (n = 137) and Miyakejima (n = 425) in 2005 (i.e. before the F1-NPP accident) and the mean absorbed dose rates were 13(4) and 19 nGy h−1(21), respectively. The dose rates measured in 2016 (i.e. 5 years after the F1-NPP accident) were still 1.7 and 1.2 times higher values compared to those measured in 2005; and the 2016 dose rates were lower than rates measured in metropolitan Tokyo excluding the Pacific Ocean islands that are within the Tokyo Metropolitan Government’s jurisdiction (49 nGy h−1)(11) and in all of Japan (50 nGy h−1)(20). To allow a more detailed discussion, absorbed dose rates in air obtained from all the radionuclides, the natural radionuclides (40K, 238U series and 232Th series) and the artificial radionuclides (134Cs and 137Cs) in the Izu Islands are shown in Table 3. In this analysis, the effects from nuclear tests performed in the 1950 to 1980s and the Chernobyl NPP accident in 1986 were not considered. According to reporting from the Meteorological Research Institute(22), the integrated deposited activity of 137Cs with decay from 1954 to just before the F1-NPP accident was estimated to be 2 kBq m−2, from which it was estimated that the measured dose rate in this study included contribution of a few nano-gray per hour from the earlier events. The absorbed dose rates in air from the natural radionuclides were calculated in the range of 11–43 nGy h−1 depending on the basement geology that is given in Table 1. Additionally, the calculated dose rates from the natural radionuclides for Izu-Oshima (13 nGy h−1) and Miyakejima (18 nGy h−1) were similar to those of previous reports (Izu-Oshima, 13 nGy h−1; Miyakejima, 19 nGy h−1)(4, 21). In the present study, Niijima, Shikinejima and Kouzushima which are mainly formed by the same dacite/rhyolite basement geology had higher dose rates because higher activity concentrations of natural radionuclides such as 40K, 238U series and 232Th series are present. According to Minato(23), the averaged dose rates for rhyolite/dacite and dacite/rhyolite basements were 43 nGy h−1 (n = 175) and 64 nGy h−1 (n = 96), respectively, whereas that for basalt basement was 20 nGy h−1 (n = 49). Thus, it could be assumed that impact on absorbed dose rates in air from the F1-NPP accident was low for Niijima, Shikinejima and Kouzushima. Table 3. Calculated absorbed dose rates in air (nGy h−1) from natural and artificial radionuclides in the Izu Islands. Island  na  All radionuclides  Natural radionuclides  Artificial radionuclides  Contribution ratio of artificial radionuclides (%)  Mean  SD  Range  Mean  SD  Range  Mean  SD  Range  Mean  SD  Range  Izu-Oshima  25  19  10  7–41  13  10  5–41  6  4  0−14  31  19  0–66  Niijima  15  32  6  23–47  29  7  17–45  3  1  1–5  9  5  4–23  Shikinejima  17  41  7  23–53  39  8  19–51  2  1  0–4  5  5  0−17  Kouzushima  25  47  6  28–59  43  6  25–54  4  2  1–9  7  4  2−17  Miyakejima  15  19  10  7–39  18  11  7–39  1  1  0–4  8  8  0–26  Mikurashima  20  23  10  8–38  20  12  6–38  3  3  0–9  18  20  0–53  Hachijojima  16  18  8  7–32  17  9  5–32  1  1  0–3  8  11  0–33  Aogashima  12  12  10  5–44  11  10  5–44  1  1  0–3  11  9  0–25  Island  na  All radionuclides  Natural radionuclides  Artificial radionuclides  Contribution ratio of artificial radionuclides (%)  Mean  SD  Range  Mean  SD  Range  Mean  SD  Range  Mean  SD  Range  Izu-Oshima  25  19  10  7–41  13  10  5–41  6  4  0−14  31  19  0–66  Niijima  15  32  6  23–47  29  7  17–45  3  1  1–5  9  5  4–23  Shikinejima  17  41  7  23–53  39  8  19–51  2  1  0–4  5  5  0−17  Kouzushima  25  47  6  28–59  43  6  25–54  4  2  1–9  7  4  2−17  Miyakejima  15  19  10  7–39  18  11  7–39  1  1  0–4  8  8  0–26  Mikurashima  20  23  10  8–38  20  12  6–38  3  3  0–9  18  20  0–53  Hachijojima  16  18  8  7–32  17  9  5–32  1  1  0–3  8  11  0–33  Aogashima  12  12  10  5–44  11  10  5–44  1  1  0–3  11  9  0–25  aNumber of measurements. Table 3. Calculated absorbed dose rates in air (nGy h−1) from natural and artificial radionuclides in the Izu Islands. Island  na  All radionuclides  Natural radionuclides  Artificial radionuclides  Contribution ratio of artificial radionuclides (%)  Mean  SD  Range  Mean  SD  Range  Mean  SD  Range  Mean  SD  Range  Izu-Oshima  25  19  10  7–41  13  10  5–41  6  4  0−14  31  19  0–66  Niijima  15  32  6  23–47  29  7  17–45  3  1  1–5  9  5  4–23  Shikinejima  17  41  7  23–53  39  8  19–51  2  1  0–4  5  5  0−17  Kouzushima  25  47  6  28–59  43  6  25–54  4  2  1–9  7  4  2−17  Miyakejima  15  19  10  7–39  18  11  7–39  1  1  0–4  8  8  0–26  Mikurashima  20  23  10  8–38  20  12  6–38  3  3  0–9  18  20  0–53  Hachijojima  16  18  8  7–32  17  9  5–32  1  1  0–3  8  11  0–33  Aogashima  12  12  10  5–44  11  10  5–44  1  1  0–3  11  9  0–25  Island  na  All radionuclides  Natural radionuclides  Artificial radionuclides  Contribution ratio of artificial radionuclides (%)  Mean  SD  Range  Mean  SD  Range  Mean  SD  Range  Mean  SD  Range  Izu-Oshima  25  19  10  7–41  13  10  5–41  6  4  0−14  31  19  0–66  Niijima  15  32  6  23–47  29  7  17–45  3  1  1–5  9  5  4–23  Shikinejima  17  41  7  23–53  39  8  19–51  2  1  0–4  5  5  0−17  Kouzushima  25  47  6  28–59  43  6  25–54  4  2  1–9  7  4  2−17  Miyakejima  15  19  10  7–39  18  11  7–39  1  1  0–4  8  8  0–26  Mikurashima  20  23  10  8–38  20  12  6–38  3  3  0–9  18  20  0–53  Hachijojima  16  18  8  7–32  17  9  5–32  1  1  0–3  8  11  0–33  Aogashima  12  12  10  5–44  11  10  5–44  1  1  0–3  11  9  0–25  aNumber of measurements. Information related to the contribution from artificial radioactivity relies on data from measurement points. In principle, the nuclide-specific information could be derived from data obtained by the car-borne survey technique with a NaI(Tl) scintillation spectrometer. However, this method is commonly used to rapidly assess dose rates over an extended area(4). Thus, measurement of the count rate inside the car is generally set within a short time (e.g. 30 s–1 min), resulting in it being difficult to use this information from the car-borne survey technique for evaluating the contribution of artificial radionuclides. The highest absorbed dose rate in air from artificial radionuclides (6 nGy h−1) and the highest contribution ratio of artificial radionuclides (31%; range, 0–66%) were observed on Izu-Oshima (Table 3). Dose rate from all artificial radionuclides (including short half-lived radionuclides) previously measured with a CsI(Tl) scintillation survey meter in August 2011 was 21 nGy h−1 (after subtracting 13 nGy h−1 as background)(4); hence, dose rate measured in the present study has decreased by 71% in the 5 years since August 2011 in Izu-Oshima. According to the previous report for Izu-Oshima, the environmental half-life for long half-life radionuclides (134Cs and 137Cs) was estimated to be 3.1 y based on changes of the absorbed dose rate in air at 1 m above the ground surface(24). When the environmental half-life in the other seven islands was assumed to be the same as that of Izu-Oshima, the calculated absorbed dose rates in air from long half-life radionuclides (134Cs and 137Cs) in March 2011 were as shown in Table 4. The estimated total absorbed dose rate in air without short-lived radionuclides could be calculated as dose rate from natural radionuclides (Table 3) plus estimated dose rate from artificial radionuclides based on the environmental half-life. The mean dose rate from all natural and artificial radionuclides (n = 23) measured at 1 m above the ground surface on Izu-Oshima in August 2011 was reported to be 34 nGy h−1(4), and the estimated total absorbed dose rate in air there (n = 25) in August 2011 based on the environmental half-life was 31 nGy h−1. This small difference might be due to the difference in the number of measurement locations (n = 23 vs. n = 25). Table 4. Estimated absorbed dose rates in air (nGy h−1) in March 2011 based on the environmental half-live of artificial radionuclides (134Cs + 137Cs). Island  na  Estimated absorbed dose rates in air of artificial radionuclides (134Cs + 137Cs) (nGy h−1)  Estimated total absorbed dose rate in air (nGy h−1)  Estimated contribution ratio of artificial radionuclides (%)  Estimated annual external effective doseb (mSv)  Mean  SD  Range  Mean  SD  Range  Mean  SD  Range  Izu-Oshima  25  19  14  0–48  32  15  11–58  55  25  0–87  0.12  Niijima  15  7  3  3–13  36  6  28–51  19  8  9–43  0.14  Shikinejima  17  5  3  0–10  43  6  29–56  11  9  0–34  0.17  Kouzushima  25  11  6  2–24  53  8  32–67  19  10  4–37  0.21  Miyakejima  15  4  4  0–12  22  10  9–39  20  19  0–54  0.09  Mikurashima  20  7  7  0–24  27  8  12–38  30  29  0–76  0.11  Hachijojima  16  2  2  0–7  19  7  8–32  14  19  0–54  0.07  Aogashima  12  4  3  0–11  15  12  7–52  29  19  0–56  0.06  Island  na  Estimated absorbed dose rates in air of artificial radionuclides (134Cs + 137Cs) (nGy h−1)  Estimated total absorbed dose rate in air (nGy h−1)  Estimated contribution ratio of artificial radionuclides (%)  Estimated annual external effective doseb (mSv)  Mean  SD  Range  Mean  SD  Range  Mean  SD  Range  Izu-Oshima  25  19  14  0–48  32  15  11–58  55  25  0–87  0.12  Niijima  15  7  3  3–13  36  6  28–51  19  8  9–43  0.14  Shikinejima  17  5  3  0–10  43  6  29–56  11  9  0–34  0.17  Kouzushima  25  11  6  2–24  53  8  32–67  19  10  4–37  0.21  Miyakejima  15  4  4  0–12  22  10  9–39  20  19  0–54  0.09  Mikurashima  20  7  7  0–24  27  8  12–38  30  29  0–76  0.11  Hachijojima  16  2  2  0–7  19  7  8–32  14  19  0–54  0.07  Aogashima  12  4  3  0–11  15  12  7–52  29  19  0–56  0.06  aNumber of measurements. bEstimated period: March 2011–February 2012. Table 4. Estimated absorbed dose rates in air (nGy h−1) in March 2011 based on the environmental half-live of artificial radionuclides (134Cs + 137Cs). Island  na  Estimated absorbed dose rates in air of artificial radionuclides (134Cs + 137Cs) (nGy h−1)  Estimated total absorbed dose rate in air (nGy h−1)  Estimated contribution ratio of artificial radionuclides (%)  Estimated annual external effective doseb (mSv)  Mean  SD  Range  Mean  SD  Range  Mean  SD  Range  Izu-Oshima  25  19  14  0–48  32  15  11–58  55  25  0–87  0.12  Niijima  15  7  3  3–13  36  6  28–51  19  8  9–43  0.14  Shikinejima  17  5  3  0–10  43  6  29–56  11  9  0–34  0.17  Kouzushima  25  11  6  2–24  53  8  32–67  19  10  4–37  0.21  Miyakejima  15  4  4  0–12  22  10  9–39  20  19  0–54  0.09  Mikurashima  20  7  7  0–24  27  8  12–38  30  29  0–76  0.11  Hachijojima  16  2  2  0–7  19  7  8–32  14  19  0–54  0.07  Aogashima  12  4  3  0–11  15  12  7–52  29  19  0–56  0.06  Island  na  Estimated absorbed dose rates in air of artificial radionuclides (134Cs + 137Cs) (nGy h−1)  Estimated total absorbed dose rate in air (nGy h−1)  Estimated contribution ratio of artificial radionuclides (%)  Estimated annual external effective doseb (mSv)  Mean  SD  Range  Mean  SD  Range  Mean  SD  Range  Izu-Oshima  25  19  14  0–48  32  15  11–58  55  25  0–87  0.12  Niijima  15  7  3  3–13  36  6  28–51  19  8  9–43  0.14  Shikinejima  17  5  3  0–10  43  6  29–56  11  9  0–34  0.17  Kouzushima  25  11  6  2–24  53  8  32–67  19  10  4–37  0.21  Miyakejima  15  4  4  0–12  22  10  9–39  20  19  0–54  0.09  Mikurashima  20  7  7  0–24  27  8  12–38  30  29  0–76  0.11  Hachijojima  16  2  2  0–7  19  7  8–32  14  19  0–54  0.07  Aogashima  12  4  3  0–11  15  12  7–52  29  19  0–56  0.06  aNumber of measurements. bEstimated period: March 2011–February 2012. The estimation of absorbed dose rate in air from artificial radionuclides with only long half-lives radionuclides (134Cs and 137Cs) for other Izu Islands in March 2011 can be possibly based on the environmental half-life. Izu-Oshima has an oceanic climate and the other seven islands do also. Additionally, the eight islands are the same volcanic island chain belonging to the Fuji volcanic zone. The effect from the sedimentation of artificial radionuclides in the soil cannot be completely ignored in the estimation of dose rate based on the environmental half-life. However, the speed of the sedimentation of the radioactive fallout in the soil is very slow (1−2 mm y−1)(25). Thus, the mean absorbed dose rate in air from the artificial radionuclides on each island and the range of contribution ratios of long half-life radionuclides in March 2011 could be estimated to be 2–19 nGy h−1 and 11–55%, respectively, and the error range of this calculation could be estimated as a few nGy h−1. In these dose rates assessments, the contributions from short-lived radionuclides (131I and others) could not be discussed. It is likely that these contributions had dominated dose rate for a few weeks after the accident. The data (i.e. continuous measured dose rate or nuclide-specific information from soil samples or from in-situ spectrometry) measured just after the accident are needed to assess contributions from short-lived radionuclides. The estimated annual external effective doses (mSv) from natural and long half-life artificial radionuclides (134Cs and 137Cs) for the adult population of each island are shown in Table 4. In this estimation, the external effective dose was calculated based on the environmental half-life using the following equation:   e(t)=(Dnat+Dart(0)exp(−λt))×DE×(Qin×R+Qout)×10−6,λ=ln2Tenviron (3)where e(t) is effective dose rate (mSv h−1) per adult at time t, Dnat is the absorbed dose rate in air (nGy h−1) from natural radionuclides, Dart(0) is the calculated absorbed dose rate in air (nGy h−1) from artificial radionuclides (134Cs and 137Cs) in March 2011, λ is decay constant, Tenviron is environmental half-life (i.e. 3.1 y), DE is dose conversion factor from the dose in air to the external effective dose for adults (0.748 ± 0.007 Sv Gy−1)(26), Qin and Qout are the indoor (0.9) and outdoor (0.1) occupancy factors(27) and R is the ratio of indoor dose rate to outdoor dose rate (0.55)(28). The annual external effective dose (mSv) was then calculated using the following equation:   E(T)=∫0Te(t)dt (4)where E(T) is annual external effective dose (mSv) from time 0 (March 2011) to time T (February 2012). The estimated annual external effective doses (mSv) for the Izu-Islands are shown in Table 4. The values were in the range of 0.06–0.21 mSv. This range was 18–64% of the average for Japan before the F1-NPP accident (0.33 mSv)(20) and 13–44% of the worldwide average (0.48 mSv)(29). However, the external effective dose from short half-life radionuclides (131I and others) is to be added to the above values. This is a limitation to estimate external effective dose including the effect from short half-life radionuclides in this study. The standard uncertainties of the one-time count rate measurement (30 s) can be calculated from the measurement values using the following equation and they are shown in Table 5.   un=nt (5) Table 5. Maximum combined relative standard uncertainties on car-borne survey in the Izu Islands. Island  na  Relative standard uncertainty of shielding factor (%)  Relative standard uncertainty of dose conversion factor (%)  Relative standard uncertainty of minimum count rate (%)  Combined relative standard uncertainty (%)  Izu-Oshima  1011  1.7  1.3  5.3  5.7  Niijima  418  3.2  1.9  3.5  5.1  Shikinejima  338  4.0  2.2  3.3  5.6  Kouzushima  805  2.0  2.3  4.0  5.0  Miyakejima  854  1.7  1.6  6.6  7.0  Mikurashima  846  2.7  2.1  5.0  6.0  Hachijojima  832  1.7  2.0  7.7  8.1  Aogashima  179  2.5  1.0  7.5  8.0  Island  na  Relative standard uncertainty of shielding factor (%)  Relative standard uncertainty of dose conversion factor (%)  Relative standard uncertainty of minimum count rate (%)  Combined relative standard uncertainty (%)  Izu-Oshima  1011  1.7  1.3  5.3  5.7  Niijima  418  3.2  1.9  3.5  5.1  Shikinejima  338  4.0  2.2  3.3  5.6  Kouzushima  805  2.0  2.3  4.0  5.0  Miyakejima  854  1.7  1.6  6.6  7.0  Mikurashima  846  2.7  2.1  5.0  6.0  Hachijojima  832  1.7  2.0  7.7  8.1  Aogashima  179  2.5  1.0  7.5  8.0  aNumber of measurements. Table 5. Maximum combined relative standard uncertainties on car-borne survey in the Izu Islands. Island  na  Relative standard uncertainty of shielding factor (%)  Relative standard uncertainty of dose conversion factor (%)  Relative standard uncertainty of minimum count rate (%)  Combined relative standard uncertainty (%)  Izu-Oshima  1011  1.7  1.3  5.3  5.7  Niijima  418  3.2  1.9  3.5  5.1  Shikinejima  338  4.0  2.2  3.3  5.6  Kouzushima  805  2.0  2.3  4.0  5.0  Miyakejima  854  1.7  1.6  6.6  7.0  Mikurashima  846  2.7  2.1  5.0  6.0  Hachijojima  832  1.7  2.0  7.7  8.1  Aogashima  179  2.5  1.0  7.5  8.0  Island  na  Relative standard uncertainty of shielding factor (%)  Relative standard uncertainty of dose conversion factor (%)  Relative standard uncertainty of minimum count rate (%)  Combined relative standard uncertainty (%)  Izu-Oshima  1011  1.7  1.3  5.3  5.7  Niijima  418  3.2  1.9  3.5  5.1  Shikinejima  338  4.0  2.2  3.3  5.6  Kouzushima  805  2.0  2.3  4.0  5.0  Miyakejima  854  1.7  1.6  6.6  7.0  Mikurashima  846  2.7  2.1  5.0  6.0  Hachijojima  832  1.7  2.0  7.7  8.1  Aogashima  179  2.5  1.0  7.5  8.0  aNumber of measurements.Here n is measured minimum or maximum count rate (cpm) and t is measurement time (min). The ranges of relative standard uncertainties of shielding factor, dose conversion factor and minimum count rate were 1.7–4.0%, 1.0–2.3% and 3.3–7.7%, respectively. The maximum combined relative standard uncertainties of the estimated absorbed dose rate in air in this study were then calculated using the following equation:   uc(D)D=(unn)2+(uDCFDCF)2+(uSFSF)2 (6)where uc(D)D is maximum combined relative standard uncertainty, un is standard uncertainty for the measured value, n is minimum counts per 30 s, and uDCF and uSF are standard uncertainties for dose conversion factor and shielding factor. In this study, the maximum combined relative standard uncertainties were calculated to be in the range of 5.1–8.1%. The maximum combined relative standard uncertainty calculated from measurements in 2011(4) was 6.0% which was similar to the above values. Those obtained uncertainties were higher compared to the previous study (4.9%)(14). This might be due to the difference in the number of measurement locations (n = 12–25 vs. n = 35). Additionally, the uncertainty for estimating annual external effective dose is associated with the uncertainty of equation (3) and its parameters and the extrapolation process of dose rate from later measurement periods back to March 2011 based on the environmental half-life. Distributions of absorbed dose rates in air of the Izu Islands Figure 3 shows the distribution map of absorbed dose rate in air measured in May 2015, August 2016 or February 2017. Those maps were drawn with the same magnification and gradation scale using the GMT. Higher dose rates exceeding 40 nGy h−1 were observed for Shikinejima and Kouzushima among the results shown in Table 3, but the heterogeneous distributions were shown to be due to the presence of artificial radionuclides. For a more detailed analysis, Figure 4 shows absorbed dose rates in air from the artificial radionuclides (134Cs + 137Cs). For the Izu Islands, it was estimated by previous simulation studies that the artificial radionuclides were wet-deposited on 21–23 March 2011(3, 30). According to past weather information collected by the Japan Meteorological Agency, north-northeasterly winds were blowing for Izu-Oshima, east-northeasterly winds were blowing for Niijima and Kouzushima, north winds were blowing for Miyakejima and northeast winds were blowing for Hachijojima on 21–23 March (arrows in Figure 4). Weather observations were not made by the Japan Meteorological Agency for the other three islands. Along the direction of those winds and influenced by the topography of each island, elevated dose rates from artificial radionuclides were observed on each island. Additionally, while rainfall was observed over the whole area of the five observed island, on mountains with an altitude of 428–854 m high dose rates were not observed on the opposite mountainside to the direction of the winds because it was thought that the released artificial radionuclides were diffused within an altitude of 0–1000 m from ground level(3, 30). Figure 3. View largeDownload slide The distribution maps of absorbed dose rate in air in the Izu Islands measured during May 2015, August 2016 or February 2017 by the car-borne survey technique (a, Izu-Oshima; b, Niijima; c, Shikinejima; d, Kouzushima; e, Miyakejima; f, Mikurashima; g, Hachijojima; h, Aogashima). Figure 3. View largeDownload slide The distribution maps of absorbed dose rate in air in the Izu Islands measured during May 2015, August 2016 or February 2017 by the car-borne survey technique (a, Izu-Oshima; b, Niijima; c, Shikinejima; d, Kouzushima; e, Miyakejima; f, Mikurashima; g, Hachijojima; h, Aogashima). Figure 4. View largeDownload slide Absorbed dose rates in air from the artificial radionuclides (134Cs + 137Cs) at 145 locations in the Izu Islands (a, Izu-Oshima; b, Niijima; c, Shikinejima; d, Kouzushima; e, Miyakejima; f, Mikurashima; g, Hachijojima; h, Aogashima) in 2015-2017. The obtained absorbed dose rates in air were separated into artificial radionuclides and natural radionuclides (40K, 238U series and 232Th series) using the response matrix method. Figure 4. View largeDownload slide Absorbed dose rates in air from the artificial radionuclides (134Cs + 137Cs) at 145 locations in the Izu Islands (a, Izu-Oshima; b, Niijima; c, Shikinejima; d, Kouzushima; e, Miyakejima; f, Mikurashima; g, Hachijojima; h, Aogashima) in 2015-2017. The obtained absorbed dose rates in air were separated into artificial radionuclides and natural radionuclides (40K, 238U series and 232Th series) using the response matrix method. CONCLUSION The car-borne and walking surveys using a NaI(Tl) scintillation spectrometer were carried out in the Izu Islands during May 2015, August 2016 or February 2017 and absorbed doses in air and annual effective external doses from long half-lived cesium radionuclides to the island residents in 2011 were calculated. The absorbed dose rates in air and the contribution ratios from artificial radionuclides were observed to be in the range of 1–6 nGy h−1 and 5–31%, respectively. After calculating dose rates based on the environmental half-life measured on Izu-Oshima, absorbed dose rates in air from artificial radionuclides (134Cs + 137Cs) in March 2011 were estimated to be 2−19 nGy h−1 and the contribution ratios from those artificial radionuclides were 11–55%. The estimated annual external effective doses to the residents on the Izu Islands in 2011 were then calculated to be 0.06–0.21 mSv and these values were 18–64% of the average in Japan before the F1-NPP accident (0.33 mSv). Heterogeneous absorbed dose rates in air were observed for the Izu Islands and high absorbed dose rates in air from artificial radionuclides were observed depending on the wind directions in 21–23 March 2011. Additionally, for mountains with an altitude of 428–854 m, high dose rates were not observed for the opposite mountainside to the direction of the winds. Thus, it was estimated that the released artificial radionuclides which reached the Izu Islands were diffused within an altitude of 0–1000 m from ground level and that was consistent with previous reports. FUNDING This work was funded by the strategic research fund of Tokyo Metropolitan University. REFERENCES 1 United Nations Scientific Committee on the Effects of Atomic Radiation. UNSCEAR 2013 report annex A: levels and effects of radiation exposure due to the nuclear accident after the 2011 great east-Japan earthquake and tsunami ( 2013). 2 Committee on Comprehensive Synthetic Engineering., Science Council of Japan. 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Google Scholar CrossRef Search ADS   31 Statistics Division., Bureau of General Affairs, Tokyo Metropolitan Government. Statistics of Tokyo ( 2017). Available on http://www.toukei.metro.tokyo.jp/jsuikei/js-index.htm (accessed Nov. 15, 2017) (in Japanese). 32 National Institute of Advanced Industrial Science and Technology. Geological Survey of Japan ( 2015). Available on https://gbank.gsj.jp/geonavi/?lang=en (accessed Nov. 15, 2017). © The Author(s) 2018. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oup.com This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/about_us/legal/notices) http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Radiation Protection Dosimetry Oxford University Press

IMPACT ON ABSORBED DOSE RATE IN AIR IN THE IZU ISLANDS FROM LONG HALF-LIFE RADIONUCLIDES RELEASED BY THE FUKUSHIMA DAIICHI NUCLEAR POWER PLANT ACCIDENT

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Abstract

Abstract Car-borne surveys were carried out on eight islands of the Izu Islands located 339–570 km southwest of the Fukushima Daiichi Nuclear Power Plant. The mean dose rates measured in 2015, 2016 or 2017 on each island were from 12 to 47 nGy h−1, meaning that the contribution ratios of artificial radionuclides were 5–31%. Based on the environmental half-life for long half-life radionuclides (134Cs + 137Cs) measured on Izu-Oshima (3.1 y), the mean dose rates in March 2011 were estimated to be 15–53 nGy h−1 and the contribution ratios of artificial radionuclides were 11–55%. The estimated annual external effective doses were 0.06–0.21 mSv which were 13–44% of the worldwide average (0.48 mSv). INTRODUCTION The total activities of artificial radionuclides released by the Fukushima Daiichi Nuclear Power Plant (F1-NPP) accident have been estimated by the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) to be in the ranges of 100–500 PBq for 131I and 6–20 PBq for 137Cs(1). They are ~10 and 20% of the respectively estimated amounts released to the atmosphere in the 1986 Chernobyl accident. According to the report from the Science Council of Japan, 93% of the total 137Cs assumed in the simulation model was wet-deposited over the globe in the F1-NPP accident(2). Atmospheric simulation models for the F1-NPP released artificial radionuclides have substantial role in estimations on health effects. The Science Council of Japan has compared the results obtained from nine regional atmospheric models and six global atmospheric models(2) and reported there were large variabilities in the horizontal distribution of the accumulated deposition caused by differences in deposition model treatments(3). Thus, accurate estimation of emissions to the air is important for estimation of the atmospheric behaviors of radionuclides and their subsequent behaviors in absorbed dose rate in air and soil. The Izu Islands are a chain of volcanic islands located 339–570 km southwest of the F1-NPP (Figure 1). On Izu-Oshima, located closest to the F1-NPP (339 km), a survey was carried out in August 2011 by a group including some of the present authors(4), but no measurements on other islands (381–570 km from the F1-NPP) have been made. Since Japan is an island country with the sea on all sides, it is difficult to estimate the amounts of artificial radionuclides from a seawater sampling survey because the released artificial radionuclides have been diluted by the surrounding sea water. Thus, an accumulation of data measured on islands that are scattered over the Pacific Ocean would be important to enhance the precision of regional atmospheric models and allow accurate estimation of health effects. In this study, car-borne surveys were carried out in 2015, 2016 or 2017 for eight islands of the Izu Islands; these islands are an administrative part of the Tokyo Metropolitan Government. Additionally, the absorbed dose rates in air in the Izu Islands just after the accident were estimated based on the environmental half-life for long half-life radionuclides (134Cs + 137Cs) reported previously. This study will contribute to an improved understanding of the release and subsequent dispersion of artificial radionuclides on the Pacific Ocean side of Japan. Figure 1. View largeDownload slide Location of the Izu Islands and the Fukushima Daiichi Nuclear Power Plant. Figure 1. View largeDownload slide Location of the Izu Islands and the Fukushima Daiichi Nuclear Power Plant. MATERIALS AND METHODS Survey area The measurements of the count rates were carried out during May 2015, August 2016 and February 2017 for the Izu Islands (Figure 1 and Table 1) and then converted to absorbed dose rate in air (nGy h−1) by using conversion factors. The survey routes were selected as main roads on these islands (Figure 2). The weather conditions were sunny or cloudy throughout the measurement days, and there was no rainfall on them. These routes and altitude maps were drawn using Generic Mapping Tools (GMT)(5) and the geographic information published by the Geospatial Information Authority of Japan(6). Table 1. Statistical and geological data for the Izu Islands and the measurement period. Island  Populationa  Area (km2)  Distance from Fukushima Daiichi Nuclear Power Plant (km)  Measurement period  Basement geology(32)  Izu-Oshima  7 611  91  339  August 2016  Basalt/Pyroclastic rocks  Niijima  2 224  28  381  May 2015  Dacite/Rhyolite  Shikinejima  532  4  388  May 2015  Dacite/Rhyolite  Kouzushima  1 847  19  402  May 2015  Dacite/Rhyolite  Miyakejima  2 551  55  404  August 2016  Basalt/Pyroclastic rocks  Mikurashima  339  20  423  August 2015  Andesite/Basalt  Hachijojima  7 337  72  500  February 2015  Basalt/Pyroclastic rocks  Aogashima  175  6  570  February 2017  Basalt/Pyroclastic rocks  Island  Populationa  Area (km2)  Distance from Fukushima Daiichi Nuclear Power Plant (km)  Measurement period  Basement geology(32)  Izu-Oshima  7 611  91  339  August 2016  Basalt/Pyroclastic rocks  Niijima  2 224  28  381  May 2015  Dacite/Rhyolite  Shikinejima  532  4  388  May 2015  Dacite/Rhyolite  Kouzushima  1 847  19  402  May 2015  Dacite/Rhyolite  Miyakejima  2 551  55  404  August 2016  Basalt/Pyroclastic rocks  Mikurashima  339  20  423  August 2015  Andesite/Basalt  Hachijojima  7 337  72  500  February 2015  Basalt/Pyroclastic rocks  Aogashima  175  6  570  February 2017  Basalt/Pyroclastic rocks  aAs of October 2017(31). Table 1. Statistical and geological data for the Izu Islands and the measurement period. Island  Populationa  Area (km2)  Distance from Fukushima Daiichi Nuclear Power Plant (km)  Measurement period  Basement geology(32)  Izu-Oshima  7 611  91  339  August 2016  Basalt/Pyroclastic rocks  Niijima  2 224  28  381  May 2015  Dacite/Rhyolite  Shikinejima  532  4  388  May 2015  Dacite/Rhyolite  Kouzushima  1 847  19  402  May 2015  Dacite/Rhyolite  Miyakejima  2 551  55  404  August 2016  Basalt/Pyroclastic rocks  Mikurashima  339  20  423  August 2015  Andesite/Basalt  Hachijojima  7 337  72  500  February 2015  Basalt/Pyroclastic rocks  Aogashima  175  6  570  February 2017  Basalt/Pyroclastic rocks  Island  Populationa  Area (km2)  Distance from Fukushima Daiichi Nuclear Power Plant (km)  Measurement period  Basement geology(32)  Izu-Oshima  7 611  91  339  August 2016  Basalt/Pyroclastic rocks  Niijima  2 224  28  381  May 2015  Dacite/Rhyolite  Shikinejima  532  4  388  May 2015  Dacite/Rhyolite  Kouzushima  1 847  19  402  May 2015  Dacite/Rhyolite  Miyakejima  2 551  55  404  August 2016  Basalt/Pyroclastic rocks  Mikurashima  339  20  423  August 2015  Andesite/Basalt  Hachijojima  7 337  72  500  February 2015  Basalt/Pyroclastic rocks  Aogashima  175  6  570  February 2017  Basalt/Pyroclastic rocks  aAs of October 2017(31). Figure 2. View largeDownload slide The survey routes for measuring the count rate in air in the Izu-Islands (a, Izu-Oshima; b, Niijima; c, Shikinejima; d, Kouzushima; e, Miyakejima; f, Mikurashima; g, Hachijojima; h, Aogashima). Car-borne surveys were carried out using a 3-in × 3-in NaI(Tl) scintillation spectrometer during May 2015, August 2016 or February 2017. The fixed-point measurements outside the car were also carried out for 10 min at 145 locations (circle). Figure 2. View largeDownload slide The survey routes for measuring the count rate in air in the Izu-Islands (a, Izu-Oshima; b, Niijima; c, Shikinejima; d, Kouzushima; e, Miyakejima; f, Mikurashima; g, Hachijojima; h, Aogashima). Car-borne surveys were carried out using a 3-in × 3-in NaI(Tl) scintillation spectrometer during May 2015, August 2016 or February 2017. The fixed-point measurements outside the car were also carried out for 10 min at 145 locations (circle). Car-borne surveys Car-borne surveys were carried out over the asphalt pavement (width: 4–10 m) using a 3-in × 3-in NaI(Tl) scintillation spectrometer (EMF-211, EMF Japan Co., Osaka, Japan) with a global positioning system (GPS). The NaI(Tl) scintillation spectrometer was positioned 1 m above the ground surface at the center of the car. Measurements of the count rates inside the car were performed every 30 s along the route, and consecutive gamma-ray energies of 50 keV–3.2 MeV were recorded. Simultaneously, latitude and longitude at each measurement point were recorded at the same time with the GPS. Car speeds were kept around 30 km h−1. The car windows were kept closed during those measurements. Walking surveys were also carried out over unpaved surfaces for mountain areas of Shikinejima, Kouzushima and Mikurashima where the car was unable to be driven using the same scintillation spectrometer which was carried by a researcher. The photon peak of 40K (Eγ = 1.464 MeV) was used for calibration from the channel number and gamma-ray energy before the measurements. All measurements were carried out with the permission of the town office branches. Additionally, the measurements on Mikurashima were carried out with the permission of the Tokyo Metropolitan Government because most of this island is a nature reserve. The shielding effect of the car body was estimated by measuring the count rates inside and outside the car for each island because count rate was measured inside the car. There were 145 measurement locations for the eight islands (circles in Figure 2). Measurements were recorded for consecutive 30-s intervals during a total recording period of 2 min inside and outside the car. Those measurements were done above bare soil areas which were not cultivated soil areas (i.e. areas unaffected from natural radionuclides contained in fertilizers). The shielding factors (SFs) were calculated as regression coefficients from the correlation between count rates inside and outside the car for each island. The count rates inside the car were then multiplied by this SF. The gamma-ray pulse height distributions were also measured outside the car for 10 min at the same 145 locations (Figure 2). The NaI(Tl) scintillation spectrometer was positioned 1 m above the bare soil surface. The measured gamma-ray pulse height distribution measured with the NaI(Tl) scintillation spectrometer was then unfolded using a 22 × 22 response matrix method(7) and absorbed dose rates in air were calculated. The dose conversion factors (nGy h−1/cps) for each island were then estimated from a correlation between calculated dose rates and measured count rates. The absorbed dose rate in air outside the car (Dout) was calculated using the following equation:   Dout=Cin×SF×DCF (1)where Cin is count rate (cps) inside the car, obtained by the car-borne survey. SF is shielding factor and DCF is dose conversion factor (nGy h−1/cps). For the count rate obtained from the walking survey, only DCF was utilized to calculate the absorbed dose rate in air. All data obtained from the car-borne and walking surveys were plotted on a distribution map of absorbed dose rate in air using GMT(5). A minimum curvature algorithm was used for the data interpolation by GMT. This is the method for interpolating data by presuming a smooth curved surface from the data of individual points. For a more detailed analysis, clear peaks from 134Cs (energy ranges: 0.55–0.65 MeV and 0.75–0.85 MeV), 137Cs (0.65–0.75 MeV), 40K (1.39–1.54 MeV), 214Bi (1.69–1.84 MeV and 2.10–2.31 MeV) and 208Tl (2.51–2.72 MeV) were observed in the energy spectrum after unfolding the gamma-ray pulse height distribution obtained at 145 locations. The absorbed dose rates in air from natural radionuclides (40K, 238U series and 232Th series) and artificial radionuclides (134Cs and 137Cs) were then calculated to estimate the impact from the F1-NNP accident. The detailed method has been reported by Minato(7). RESULTS AND DISCUSSION Shielding and dose conversion factors The SFs for each island survey were obtained from correlations between count rates inside and outside the car and applied to calculation of absorbed dose rates in air. Table 2 shows obtained SFs and the standard uncertainties (Type A evaluation)(8). The standard uncertainties were calculated using the following equation:   u=sN (2)where u is standard uncertainty, s is standard deviation of SF and DCF, and N is the number of measurement locations. The coefficient of determinations (R2) from measurement correlations ranged from 0.807 to 0.928. The SFs and the standard uncertainties were also calculated and ranged from 1.23 to 1.55 and from 0.02 to 0.06, respectively. The SF is influenced by the type of car used in the survey, the number of passengers and the scintillation spectrometer position inside the car. The obtained SF for Izu-Oshima was lower than that obtained for other islands because a smaller car was utilized. SFs have been reported previously as ranging from 1.1 to 1.9(4, 9–18) and the presently obtained SFs were in this range. Table 2. Absorbed dose rates in air in the Izu Islands assessed from the measurements by the car-borne survey technique. Island  na  Shielding factor  Standard uncertainty of shielding factor  Dose conversion factor (nGy h−1/cps)  Standard uncertainty of dose conversion factor  Calculated absorbed dose rate in air (nGy h−1)  Mean  SD  Range  Izu-Oshima  1011  1.23  0.02  0.144  1.89 × 10−3  22  6  6–54  Niijima  418  1.55  0.05  0.137  2.58 × 10−3  33  5  22–52  Shikinejima  338  1.50  0.06  0.148  3.27 × 10−3  42  6  27–62  Kouzushima  805  1.50  0.03  0.144  3.37 × 10−3  49  7  17–83  Miyakejima  1022  1.47  0.03  0.146  2.40 × 10−3  22  7  7–43  Mikurashima  846  1.40  0.04  0.162  3.36 × 10−3  27  7  12–45  Hachijojima  832  1.49  0.03  0.162  3.27 × 10−3  20  7  6–65  Aogashima  179  1.43  0.04  0.144  1.44 × 10−3  15  8  5–42  Island  na  Shielding factor  Standard uncertainty of shielding factor  Dose conversion factor (nGy h−1/cps)  Standard uncertainty of dose conversion factor  Calculated absorbed dose rate in air (nGy h−1)  Mean  SD  Range  Izu-Oshima  1011  1.23  0.02  0.144  1.89 × 10−3  22  6  6–54  Niijima  418  1.55  0.05  0.137  2.58 × 10−3  33  5  22–52  Shikinejima  338  1.50  0.06  0.148  3.27 × 10−3  42  6  27–62  Kouzushima  805  1.50  0.03  0.144  3.37 × 10−3  49  7  17–83  Miyakejima  1022  1.47  0.03  0.146  2.40 × 10−3  22  7  7–43  Mikurashima  846  1.40  0.04  0.162  3.36 × 10−3  27  7  12–45  Hachijojima  832  1.49  0.03  0.162  3.27 × 10−3  20  7  6–65  Aogashima  179  1.43  0.04  0.144  1.44 × 10−3  15  8  5–42  aNumber of measurements. Table 2. Absorbed dose rates in air in the Izu Islands assessed from the measurements by the car-borne survey technique. Island  na  Shielding factor  Standard uncertainty of shielding factor  Dose conversion factor (nGy h−1/cps)  Standard uncertainty of dose conversion factor  Calculated absorbed dose rate in air (nGy h−1)  Mean  SD  Range  Izu-Oshima  1011  1.23  0.02  0.144  1.89 × 10−3  22  6  6–54  Niijima  418  1.55  0.05  0.137  2.58 × 10−3  33  5  22–52  Shikinejima  338  1.50  0.06  0.148  3.27 × 10−3  42  6  27–62  Kouzushima  805  1.50  0.03  0.144  3.37 × 10−3  49  7  17–83  Miyakejima  1022  1.47  0.03  0.146  2.40 × 10−3  22  7  7–43  Mikurashima  846  1.40  0.04  0.162  3.36 × 10−3  27  7  12–45  Hachijojima  832  1.49  0.03  0.162  3.27 × 10−3  20  7  6–65  Aogashima  179  1.43  0.04  0.144  1.44 × 10−3  15  8  5–42  Island  na  Shielding factor  Standard uncertainty of shielding factor  Dose conversion factor (nGy h−1/cps)  Standard uncertainty of dose conversion factor  Calculated absorbed dose rate in air (nGy h−1)  Mean  SD  Range  Izu-Oshima  1011  1.23  0.02  0.144  1.89 × 10−3  22  6  6–54  Niijima  418  1.55  0.05  0.137  2.58 × 10−3  33  5  22–52  Shikinejima  338  1.50  0.06  0.148  3.27 × 10−3  42  6  27–62  Kouzushima  805  1.50  0.03  0.144  3.37 × 10−3  49  7  17–83  Miyakejima  1022  1.47  0.03  0.146  2.40 × 10−3  22  7  7–43  Mikurashima  846  1.40  0.04  0.162  3.36 × 10−3  27  7  12–45  Hachijojima  832  1.49  0.03  0.162  3.27 × 10−3  20  7  6–65  Aogashima  179  1.43  0.04  0.144  1.44 × 10−3  15  8  5–42  aNumber of measurements. The DCFs were then obtained from the correlation between dose rate (nGy h−1) calculated from the software using the 22 × 22 response matrix method(7) and count rate outside the car (Table 2). The decision coefficients (R2) obtained were correlations from measurements on the eight islands and ranged from 0.870 to 0.945. The absorbed dose rates in air were calculated using the SFs and DCFs. Absorbed dose rates in air The mean absorbed dose rates (ranges) in the whole area of the eight islands measured by car-borne and walking surveys are shown in Table 2. The highest dose rate was observed on Kouzushima (49 nGy h−1) located 402 km southwest of the F1-NPP, whereas the lowest dose rate was observed on Aogashima (570 km southwest; 15 nGy h−1). Although large scale surveys for measuring the absorbed dose rates in air from terrestrial radiation in Japan had been done in the 1960 to 1970s by the National Institute of Radiological Sciences (NIRS) using an integrated methodology with an ionization chamber system and NaI(Tl) scintillation detectors based on in situ measurements(19, 20), no surveys for isolated islands such as the Izu Islands were included. Some members of the present authors’ group had done measurements of absorbed dose rate in air by the car-borne survey technique for Izu-Oshima (n = 137) and Miyakejima (n = 425) in 2005 (i.e. before the F1-NPP accident) and the mean absorbed dose rates were 13(4) and 19 nGy h−1(21), respectively. The dose rates measured in 2016 (i.e. 5 years after the F1-NPP accident) were still 1.7 and 1.2 times higher values compared to those measured in 2005; and the 2016 dose rates were lower than rates measured in metropolitan Tokyo excluding the Pacific Ocean islands that are within the Tokyo Metropolitan Government’s jurisdiction (49 nGy h−1)(11) and in all of Japan (50 nGy h−1)(20). To allow a more detailed discussion, absorbed dose rates in air obtained from all the radionuclides, the natural radionuclides (40K, 238U series and 232Th series) and the artificial radionuclides (134Cs and 137Cs) in the Izu Islands are shown in Table 3. In this analysis, the effects from nuclear tests performed in the 1950 to 1980s and the Chernobyl NPP accident in 1986 were not considered. According to reporting from the Meteorological Research Institute(22), the integrated deposited activity of 137Cs with decay from 1954 to just before the F1-NPP accident was estimated to be 2 kBq m−2, from which it was estimated that the measured dose rate in this study included contribution of a few nano-gray per hour from the earlier events. The absorbed dose rates in air from the natural radionuclides were calculated in the range of 11–43 nGy h−1 depending on the basement geology that is given in Table 1. Additionally, the calculated dose rates from the natural radionuclides for Izu-Oshima (13 nGy h−1) and Miyakejima (18 nGy h−1) were similar to those of previous reports (Izu-Oshima, 13 nGy h−1; Miyakejima, 19 nGy h−1)(4, 21). In the present study, Niijima, Shikinejima and Kouzushima which are mainly formed by the same dacite/rhyolite basement geology had higher dose rates because higher activity concentrations of natural radionuclides such as 40K, 238U series and 232Th series are present. According to Minato(23), the averaged dose rates for rhyolite/dacite and dacite/rhyolite basements were 43 nGy h−1 (n = 175) and 64 nGy h−1 (n = 96), respectively, whereas that for basalt basement was 20 nGy h−1 (n = 49). Thus, it could be assumed that impact on absorbed dose rates in air from the F1-NPP accident was low for Niijima, Shikinejima and Kouzushima. Table 3. Calculated absorbed dose rates in air (nGy h−1) from natural and artificial radionuclides in the Izu Islands. Island  na  All radionuclides  Natural radionuclides  Artificial radionuclides  Contribution ratio of artificial radionuclides (%)  Mean  SD  Range  Mean  SD  Range  Mean  SD  Range  Mean  SD  Range  Izu-Oshima  25  19  10  7–41  13  10  5–41  6  4  0−14  31  19  0–66  Niijima  15  32  6  23–47  29  7  17–45  3  1  1–5  9  5  4–23  Shikinejima  17  41  7  23–53  39  8  19–51  2  1  0–4  5  5  0−17  Kouzushima  25  47  6  28–59  43  6  25–54  4  2  1–9  7  4  2−17  Miyakejima  15  19  10  7–39  18  11  7–39  1  1  0–4  8  8  0–26  Mikurashima  20  23  10  8–38  20  12  6–38  3  3  0–9  18  20  0–53  Hachijojima  16  18  8  7–32  17  9  5–32  1  1  0–3  8  11  0–33  Aogashima  12  12  10  5–44  11  10  5–44  1  1  0–3  11  9  0–25  Island  na  All radionuclides  Natural radionuclides  Artificial radionuclides  Contribution ratio of artificial radionuclides (%)  Mean  SD  Range  Mean  SD  Range  Mean  SD  Range  Mean  SD  Range  Izu-Oshima  25  19  10  7–41  13  10  5–41  6  4  0−14  31  19  0–66  Niijima  15  32  6  23–47  29  7  17–45  3  1  1–5  9  5  4–23  Shikinejima  17  41  7  23–53  39  8  19–51  2  1  0–4  5  5  0−17  Kouzushima  25  47  6  28–59  43  6  25–54  4  2  1–9  7  4  2−17  Miyakejima  15  19  10  7–39  18  11  7–39  1  1  0–4  8  8  0–26  Mikurashima  20  23  10  8–38  20  12  6–38  3  3  0–9  18  20  0–53  Hachijojima  16  18  8  7–32  17  9  5–32  1  1  0–3  8  11  0–33  Aogashima  12  12  10  5–44  11  10  5–44  1  1  0–3  11  9  0–25  aNumber of measurements. Table 3. Calculated absorbed dose rates in air (nGy h−1) from natural and artificial radionuclides in the Izu Islands. Island  na  All radionuclides  Natural radionuclides  Artificial radionuclides  Contribution ratio of artificial radionuclides (%)  Mean  SD  Range  Mean  SD  Range  Mean  SD  Range  Mean  SD  Range  Izu-Oshima  25  19  10  7–41  13  10  5–41  6  4  0−14  31  19  0–66  Niijima  15  32  6  23–47  29  7  17–45  3  1  1–5  9  5  4–23  Shikinejima  17  41  7  23–53  39  8  19–51  2  1  0–4  5  5  0−17  Kouzushima  25  47  6  28–59  43  6  25–54  4  2  1–9  7  4  2−17  Miyakejima  15  19  10  7–39  18  11  7–39  1  1  0–4  8  8  0–26  Mikurashima  20  23  10  8–38  20  12  6–38  3  3  0–9  18  20  0–53  Hachijojima  16  18  8  7–32  17  9  5–32  1  1  0–3  8  11  0–33  Aogashima  12  12  10  5–44  11  10  5–44  1  1  0–3  11  9  0–25  Island  na  All radionuclides  Natural radionuclides  Artificial radionuclides  Contribution ratio of artificial radionuclides (%)  Mean  SD  Range  Mean  SD  Range  Mean  SD  Range  Mean  SD  Range  Izu-Oshima  25  19  10  7–41  13  10  5–41  6  4  0−14  31  19  0–66  Niijima  15  32  6  23–47  29  7  17–45  3  1  1–5  9  5  4–23  Shikinejima  17  41  7  23–53  39  8  19–51  2  1  0–4  5  5  0−17  Kouzushima  25  47  6  28–59  43  6  25–54  4  2  1–9  7  4  2−17  Miyakejima  15  19  10  7–39  18  11  7–39  1  1  0–4  8  8  0–26  Mikurashima  20  23  10  8–38  20  12  6–38  3  3  0–9  18  20  0–53  Hachijojima  16  18  8  7–32  17  9  5–32  1  1  0–3  8  11  0–33  Aogashima  12  12  10  5–44  11  10  5–44  1  1  0–3  11  9  0–25  aNumber of measurements. Information related to the contribution from artificial radioactivity relies on data from measurement points. In principle, the nuclide-specific information could be derived from data obtained by the car-borne survey technique with a NaI(Tl) scintillation spectrometer. However, this method is commonly used to rapidly assess dose rates over an extended area(4). Thus, measurement of the count rate inside the car is generally set within a short time (e.g. 30 s–1 min), resulting in it being difficult to use this information from the car-borne survey technique for evaluating the contribution of artificial radionuclides. The highest absorbed dose rate in air from artificial radionuclides (6 nGy h−1) and the highest contribution ratio of artificial radionuclides (31%; range, 0–66%) were observed on Izu-Oshima (Table 3). Dose rate from all artificial radionuclides (including short half-lived radionuclides) previously measured with a CsI(Tl) scintillation survey meter in August 2011 was 21 nGy h−1 (after subtracting 13 nGy h−1 as background)(4); hence, dose rate measured in the present study has decreased by 71% in the 5 years since August 2011 in Izu-Oshima. According to the previous report for Izu-Oshima, the environmental half-life for long half-life radionuclides (134Cs and 137Cs) was estimated to be 3.1 y based on changes of the absorbed dose rate in air at 1 m above the ground surface(24). When the environmental half-life in the other seven islands was assumed to be the same as that of Izu-Oshima, the calculated absorbed dose rates in air from long half-life radionuclides (134Cs and 137Cs) in March 2011 were as shown in Table 4. The estimated total absorbed dose rate in air without short-lived radionuclides could be calculated as dose rate from natural radionuclides (Table 3) plus estimated dose rate from artificial radionuclides based on the environmental half-life. The mean dose rate from all natural and artificial radionuclides (n = 23) measured at 1 m above the ground surface on Izu-Oshima in August 2011 was reported to be 34 nGy h−1(4), and the estimated total absorbed dose rate in air there (n = 25) in August 2011 based on the environmental half-life was 31 nGy h−1. This small difference might be due to the difference in the number of measurement locations (n = 23 vs. n = 25). Table 4. Estimated absorbed dose rates in air (nGy h−1) in March 2011 based on the environmental half-live of artificial radionuclides (134Cs + 137Cs). Island  na  Estimated absorbed dose rates in air of artificial radionuclides (134Cs + 137Cs) (nGy h−1)  Estimated total absorbed dose rate in air (nGy h−1)  Estimated contribution ratio of artificial radionuclides (%)  Estimated annual external effective doseb (mSv)  Mean  SD  Range  Mean  SD  Range  Mean  SD  Range  Izu-Oshima  25  19  14  0–48  32  15  11–58  55  25  0–87  0.12  Niijima  15  7  3  3–13  36  6  28–51  19  8  9–43  0.14  Shikinejima  17  5  3  0–10  43  6  29–56  11  9  0–34  0.17  Kouzushima  25  11  6  2–24  53  8  32–67  19  10  4–37  0.21  Miyakejima  15  4  4  0–12  22  10  9–39  20  19  0–54  0.09  Mikurashima  20  7  7  0–24  27  8  12–38  30  29  0–76  0.11  Hachijojima  16  2  2  0–7  19  7  8–32  14  19  0–54  0.07  Aogashima  12  4  3  0–11  15  12  7–52  29  19  0–56  0.06  Island  na  Estimated absorbed dose rates in air of artificial radionuclides (134Cs + 137Cs) (nGy h−1)  Estimated total absorbed dose rate in air (nGy h−1)  Estimated contribution ratio of artificial radionuclides (%)  Estimated annual external effective doseb (mSv)  Mean  SD  Range  Mean  SD  Range  Mean  SD  Range  Izu-Oshima  25  19  14  0–48  32  15  11–58  55  25  0–87  0.12  Niijima  15  7  3  3–13  36  6  28–51  19  8  9–43  0.14  Shikinejima  17  5  3  0–10  43  6  29–56  11  9  0–34  0.17  Kouzushima  25  11  6  2–24  53  8  32–67  19  10  4–37  0.21  Miyakejima  15  4  4  0–12  22  10  9–39  20  19  0–54  0.09  Mikurashima  20  7  7  0–24  27  8  12–38  30  29  0–76  0.11  Hachijojima  16  2  2  0–7  19  7  8–32  14  19  0–54  0.07  Aogashima  12  4  3  0–11  15  12  7–52  29  19  0–56  0.06  aNumber of measurements. bEstimated period: March 2011–February 2012. Table 4. Estimated absorbed dose rates in air (nGy h−1) in March 2011 based on the environmental half-live of artificial radionuclides (134Cs + 137Cs). Island  na  Estimated absorbed dose rates in air of artificial radionuclides (134Cs + 137Cs) (nGy h−1)  Estimated total absorbed dose rate in air (nGy h−1)  Estimated contribution ratio of artificial radionuclides (%)  Estimated annual external effective doseb (mSv)  Mean  SD  Range  Mean  SD  Range  Mean  SD  Range  Izu-Oshima  25  19  14  0–48  32  15  11–58  55  25  0–87  0.12  Niijima  15  7  3  3–13  36  6  28–51  19  8  9–43  0.14  Shikinejima  17  5  3  0–10  43  6  29–56  11  9  0–34  0.17  Kouzushima  25  11  6  2–24  53  8  32–67  19  10  4–37  0.21  Miyakejima  15  4  4  0–12  22  10  9–39  20  19  0–54  0.09  Mikurashima  20  7  7  0–24  27  8  12–38  30  29  0–76  0.11  Hachijojima  16  2  2  0–7  19  7  8–32  14  19  0–54  0.07  Aogashima  12  4  3  0–11  15  12  7–52  29  19  0–56  0.06  Island  na  Estimated absorbed dose rates in air of artificial radionuclides (134Cs + 137Cs) (nGy h−1)  Estimated total absorbed dose rate in air (nGy h−1)  Estimated contribution ratio of artificial radionuclides (%)  Estimated annual external effective doseb (mSv)  Mean  SD  Range  Mean  SD  Range  Mean  SD  Range  Izu-Oshima  25  19  14  0–48  32  15  11–58  55  25  0–87  0.12  Niijima  15  7  3  3–13  36  6  28–51  19  8  9–43  0.14  Shikinejima  17  5  3  0–10  43  6  29–56  11  9  0–34  0.17  Kouzushima  25  11  6  2–24  53  8  32–67  19  10  4–37  0.21  Miyakejima  15  4  4  0–12  22  10  9–39  20  19  0–54  0.09  Mikurashima  20  7  7  0–24  27  8  12–38  30  29  0–76  0.11  Hachijojima  16  2  2  0–7  19  7  8–32  14  19  0–54  0.07  Aogashima  12  4  3  0–11  15  12  7–52  29  19  0–56  0.06  aNumber of measurements. bEstimated period: March 2011–February 2012. The estimation of absorbed dose rate in air from artificial radionuclides with only long half-lives radionuclides (134Cs and 137Cs) for other Izu Islands in March 2011 can be possibly based on the environmental half-life. Izu-Oshima has an oceanic climate and the other seven islands do also. Additionally, the eight islands are the same volcanic island chain belonging to the Fuji volcanic zone. The effect from the sedimentation of artificial radionuclides in the soil cannot be completely ignored in the estimation of dose rate based on the environmental half-life. However, the speed of the sedimentation of the radioactive fallout in the soil is very slow (1−2 mm y−1)(25). Thus, the mean absorbed dose rate in air from the artificial radionuclides on each island and the range of contribution ratios of long half-life radionuclides in March 2011 could be estimated to be 2–19 nGy h−1 and 11–55%, respectively, and the error range of this calculation could be estimated as a few nGy h−1. In these dose rates assessments, the contributions from short-lived radionuclides (131I and others) could not be discussed. It is likely that these contributions had dominated dose rate for a few weeks after the accident. The data (i.e. continuous measured dose rate or nuclide-specific information from soil samples or from in-situ spectrometry) measured just after the accident are needed to assess contributions from short-lived radionuclides. The estimated annual external effective doses (mSv) from natural and long half-life artificial radionuclides (134Cs and 137Cs) for the adult population of each island are shown in Table 4. In this estimation, the external effective dose was calculated based on the environmental half-life using the following equation:   e(t)=(Dnat+Dart(0)exp(−λt))×DE×(Qin×R+Qout)×10−6,λ=ln2Tenviron (3)where e(t) is effective dose rate (mSv h−1) per adult at time t, Dnat is the absorbed dose rate in air (nGy h−1) from natural radionuclides, Dart(0) is the calculated absorbed dose rate in air (nGy h−1) from artificial radionuclides (134Cs and 137Cs) in March 2011, λ is decay constant, Tenviron is environmental half-life (i.e. 3.1 y), DE is dose conversion factor from the dose in air to the external effective dose for adults (0.748 ± 0.007 Sv Gy−1)(26), Qin and Qout are the indoor (0.9) and outdoor (0.1) occupancy factors(27) and R is the ratio of indoor dose rate to outdoor dose rate (0.55)(28). The annual external effective dose (mSv) was then calculated using the following equation:   E(T)=∫0Te(t)dt (4)where E(T) is annual external effective dose (mSv) from time 0 (March 2011) to time T (February 2012). The estimated annual external effective doses (mSv) for the Izu-Islands are shown in Table 4. The values were in the range of 0.06–0.21 mSv. This range was 18–64% of the average for Japan before the F1-NPP accident (0.33 mSv)(20) and 13–44% of the worldwide average (0.48 mSv)(29). However, the external effective dose from short half-life radionuclides (131I and others) is to be added to the above values. This is a limitation to estimate external effective dose including the effect from short half-life radionuclides in this study. The standard uncertainties of the one-time count rate measurement (30 s) can be calculated from the measurement values using the following equation and they are shown in Table 5.   un=nt (5) Table 5. Maximum combined relative standard uncertainties on car-borne survey in the Izu Islands. Island  na  Relative standard uncertainty of shielding factor (%)  Relative standard uncertainty of dose conversion factor (%)  Relative standard uncertainty of minimum count rate (%)  Combined relative standard uncertainty (%)  Izu-Oshima  1011  1.7  1.3  5.3  5.7  Niijima  418  3.2  1.9  3.5  5.1  Shikinejima  338  4.0  2.2  3.3  5.6  Kouzushima  805  2.0  2.3  4.0  5.0  Miyakejima  854  1.7  1.6  6.6  7.0  Mikurashima  846  2.7  2.1  5.0  6.0  Hachijojima  832  1.7  2.0  7.7  8.1  Aogashima  179  2.5  1.0  7.5  8.0  Island  na  Relative standard uncertainty of shielding factor (%)  Relative standard uncertainty of dose conversion factor (%)  Relative standard uncertainty of minimum count rate (%)  Combined relative standard uncertainty (%)  Izu-Oshima  1011  1.7  1.3  5.3  5.7  Niijima  418  3.2  1.9  3.5  5.1  Shikinejima  338  4.0  2.2  3.3  5.6  Kouzushima  805  2.0  2.3  4.0  5.0  Miyakejima  854  1.7  1.6  6.6  7.0  Mikurashima  846  2.7  2.1  5.0  6.0  Hachijojima  832  1.7  2.0  7.7  8.1  Aogashima  179  2.5  1.0  7.5  8.0  aNumber of measurements. Table 5. Maximum combined relative standard uncertainties on car-borne survey in the Izu Islands. Island  na  Relative standard uncertainty of shielding factor (%)  Relative standard uncertainty of dose conversion factor (%)  Relative standard uncertainty of minimum count rate (%)  Combined relative standard uncertainty (%)  Izu-Oshima  1011  1.7  1.3  5.3  5.7  Niijima  418  3.2  1.9  3.5  5.1  Shikinejima  338  4.0  2.2  3.3  5.6  Kouzushima  805  2.0  2.3  4.0  5.0  Miyakejima  854  1.7  1.6  6.6  7.0  Mikurashima  846  2.7  2.1  5.0  6.0  Hachijojima  832  1.7  2.0  7.7  8.1  Aogashima  179  2.5  1.0  7.5  8.0  Island  na  Relative standard uncertainty of shielding factor (%)  Relative standard uncertainty of dose conversion factor (%)  Relative standard uncertainty of minimum count rate (%)  Combined relative standard uncertainty (%)  Izu-Oshima  1011  1.7  1.3  5.3  5.7  Niijima  418  3.2  1.9  3.5  5.1  Shikinejima  338  4.0  2.2  3.3  5.6  Kouzushima  805  2.0  2.3  4.0  5.0  Miyakejima  854  1.7  1.6  6.6  7.0  Mikurashima  846  2.7  2.1  5.0  6.0  Hachijojima  832  1.7  2.0  7.7  8.1  Aogashima  179  2.5  1.0  7.5  8.0  aNumber of measurements.Here n is measured minimum or maximum count rate (cpm) and t is measurement time (min). The ranges of relative standard uncertainties of shielding factor, dose conversion factor and minimum count rate were 1.7–4.0%, 1.0–2.3% and 3.3–7.7%, respectively. The maximum combined relative standard uncertainties of the estimated absorbed dose rate in air in this study were then calculated using the following equation:   uc(D)D=(unn)2+(uDCFDCF)2+(uSFSF)2 (6)where uc(D)D is maximum combined relative standard uncertainty, un is standard uncertainty for the measured value, n is minimum counts per 30 s, and uDCF and uSF are standard uncertainties for dose conversion factor and shielding factor. In this study, the maximum combined relative standard uncertainties were calculated to be in the range of 5.1–8.1%. The maximum combined relative standard uncertainty calculated from measurements in 2011(4) was 6.0% which was similar to the above values. Those obtained uncertainties were higher compared to the previous study (4.9%)(14). This might be due to the difference in the number of measurement locations (n = 12–25 vs. n = 35). Additionally, the uncertainty for estimating annual external effective dose is associated with the uncertainty of equation (3) and its parameters and the extrapolation process of dose rate from later measurement periods back to March 2011 based on the environmental half-life. Distributions of absorbed dose rates in air of the Izu Islands Figure 3 shows the distribution map of absorbed dose rate in air measured in May 2015, August 2016 or February 2017. Those maps were drawn with the same magnification and gradation scale using the GMT. Higher dose rates exceeding 40 nGy h−1 were observed for Shikinejima and Kouzushima among the results shown in Table 3, but the heterogeneous distributions were shown to be due to the presence of artificial radionuclides. For a more detailed analysis, Figure 4 shows absorbed dose rates in air from the artificial radionuclides (134Cs + 137Cs). For the Izu Islands, it was estimated by previous simulation studies that the artificial radionuclides were wet-deposited on 21–23 March 2011(3, 30). According to past weather information collected by the Japan Meteorological Agency, north-northeasterly winds were blowing for Izu-Oshima, east-northeasterly winds were blowing for Niijima and Kouzushima, north winds were blowing for Miyakejima and northeast winds were blowing for Hachijojima on 21–23 March (arrows in Figure 4). Weather observations were not made by the Japan Meteorological Agency for the other three islands. Along the direction of those winds and influenced by the topography of each island, elevated dose rates from artificial radionuclides were observed on each island. Additionally, while rainfall was observed over the whole area of the five observed island, on mountains with an altitude of 428–854 m high dose rates were not observed on the opposite mountainside to the direction of the winds because it was thought that the released artificial radionuclides were diffused within an altitude of 0–1000 m from ground level(3, 30). Figure 3. View largeDownload slide The distribution maps of absorbed dose rate in air in the Izu Islands measured during May 2015, August 2016 or February 2017 by the car-borne survey technique (a, Izu-Oshima; b, Niijima; c, Shikinejima; d, Kouzushima; e, Miyakejima; f, Mikurashima; g, Hachijojima; h, Aogashima). Figure 3. View largeDownload slide The distribution maps of absorbed dose rate in air in the Izu Islands measured during May 2015, August 2016 or February 2017 by the car-borne survey technique (a, Izu-Oshima; b, Niijima; c, Shikinejima; d, Kouzushima; e, Miyakejima; f, Mikurashima; g, Hachijojima; h, Aogashima). Figure 4. View largeDownload slide Absorbed dose rates in air from the artificial radionuclides (134Cs + 137Cs) at 145 locations in the Izu Islands (a, Izu-Oshima; b, Niijima; c, Shikinejima; d, Kouzushima; e, Miyakejima; f, Mikurashima; g, Hachijojima; h, Aogashima) in 2015-2017. The obtained absorbed dose rates in air were separated into artificial radionuclides and natural radionuclides (40K, 238U series and 232Th series) using the response matrix method. Figure 4. View largeDownload slide Absorbed dose rates in air from the artificial radionuclides (134Cs + 137Cs) at 145 locations in the Izu Islands (a, Izu-Oshima; b, Niijima; c, Shikinejima; d, Kouzushima; e, Miyakejima; f, Mikurashima; g, Hachijojima; h, Aogashima) in 2015-2017. The obtained absorbed dose rates in air were separated into artificial radionuclides and natural radionuclides (40K, 238U series and 232Th series) using the response matrix method. CONCLUSION The car-borne and walking surveys using a NaI(Tl) scintillation spectrometer were carried out in the Izu Islands during May 2015, August 2016 or February 2017 and absorbed doses in air and annual effective external doses from long half-lived cesium radionuclides to the island residents in 2011 were calculated. The absorbed dose rates in air and the contribution ratios from artificial radionuclides were observed to be in the range of 1–6 nGy h−1 and 5–31%, respectively. After calculating dose rates based on the environmental half-life measured on Izu-Oshima, absorbed dose rates in air from artificial radionuclides (134Cs + 137Cs) in March 2011 were estimated to be 2−19 nGy h−1 and the contribution ratios from those artificial radionuclides were 11–55%. The estimated annual external effective doses to the residents on the Izu Islands in 2011 were then calculated to be 0.06–0.21 mSv and these values were 18–64% of the average in Japan before the F1-NPP accident (0.33 mSv). Heterogeneous absorbed dose rates in air were observed for the Izu Islands and high absorbed dose rates in air from artificial radionuclides were observed depending on the wind directions in 21–23 March 2011. Additionally, for mountains with an altitude of 428–854 m, high dose rates were not observed for the opposite mountainside to the direction of the winds. Thus, it was estimated that the released artificial radionuclides which reached the Izu Islands were diffused within an altitude of 0–1000 m from ground level and that was consistent with previous reports. FUNDING This work was funded by the strategic research fund of Tokyo Metropolitan University. REFERENCES 1 United Nations Scientific Committee on the Effects of Atomic Radiation. UNSCEAR 2013 report annex A: levels and effects of radiation exposure due to the nuclear accident after the 2011 great east-Japan earthquake and tsunami ( 2013). 2 Committee on Comprehensive Synthetic Engineering., Science Council of Japan. 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Radiation Protection DosimetryOxford University Press

Published: May 4, 2018

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