Changes of absorbed dose rate in air in metropolitan Tokyo relating to radiocesium released from the Fukushima Daiichi Nuclear Power Plant accident: Results of a five-year study

Abstract

Car-borne surveys were carried out in metropolitan Tokyo, Japan, in 2015, 2016, 2017 and 2018 to estimate the transition of absorbed dose rate in air from the Fukushima Daiichi Nuclear Power Plant accident. Additionally, the future transition of absorbed dose rates in air based on this five-year study and including previously reported measurements done in 2014 by the authors was analyzed because central Tokyo has large areas covered with asphalt and concrete. The average absorbed dose rate in air (range) in the whole area of Tokyo measured in 2018 was 59 ± 9 nGy h-1 (28-105 nGy h-1), and it was slightly decreased compared to the previously reported value measured in 2011 (61 nGy h-1; 30-200 nGy h-1). In the detailed dose rate distribution map, while areas of higher dose rates exceeding 70 nGy h-1 had been observed on the eastern and western ends of Tokyo after 2014, the dose rates in these areas have decreased yearly. Especially, the decreasing dose rate from radiocesium (Cs-134 + Cs-137) in the eastern end of Tokyo which is mainly covered by asphalt was higher than that measured in the western end which is mainly covered by forest. The percent reductions for the eastern end in the years 2014-2015, 2015-2016, 2016-2017 and 2017-2018 were 49%, 21%, 18% and 16%, and those percent reductions for western end were 26%, 18%, 6% and 3%, respectively. Additionally, the decrease for dose rate from radiocesium depended on the types of asphalt, and that on porous asphalt was larger than the decrease on standard asphalt.

Fig 1. Location of Tokyo municipalities consisting of 23 wards (A1) and 30 cities, towns and villages (A2).

The number for each administrative district (#1 - #53) is an ID number that is given in this paper by reference to the Japanese Industrial Standards. The color scale gives the altitudes within the districts. This map was drawn using the GMT [15] and GSI maps of the Geospatial Information Authority of Japan [16].

Fig 2. The survey routes for measuring the count rates in metropolitan Tokyo.

Car-borne surveys were carried out using a 3-in × 3-in NaI(Tl) scintillation spectrometer in November of the four years, 2015, 2016, 2017 and 2018. Total distances traveled were 725 km for each year. The circles represent the locations where fixed-point measurements were made outside the car (n = 61).

Fig 3. Calculated absorbed dose rates in air from natural and artificial radionuclides measured in 2014 [5]– 2018 in metropolitan Tokyo based on the measurements by the car-borne survey technique.

The measurement was done on the same route (red line in Fig 2) using the same 3-in × 3-in NaI(Tl) scintillation spectrometer.

Fig 4

Changes of absorbed dose rate in air from natural radionuclides in the eastern (A1) and western (A2) ends of Tokyo in 2014 [5]– 2018. The gamma-ray pulse height distributions were measured outside the car for 10 min, at 61 locations (Fig 2). The gamma-ray pulse height distributions were then unfolded using the 22 × 22 response matrix method, and separated as natural radionuclides (K-40, U-238 series and Th-232 series).

Fig 5

The distribution maps of absorbed dose rates in air in metropolitan Tokyo measured in 2015 (A), 2016 (B), 2017 (C) and 2018 (D). A minimum curvature algorithm was used for the data interpolation using the GMT [15]. Those maps were drawn using 4,018 data for 2015, 4,346 data for 2016, 4,717 data for 2017 and 5,138 data for 2018.

Fig 6

The distribution maps of absorbed dose rates in air from artificial radionuclides in 2015 (A), 2016 (B), 2017 (C) and 2018 (D). The gamma-ray pulse height distributions were measured for 10 min, at 61 locations (Fig 2). The gamma-ray pulse height distributions were then unfolded using the 22 × 22 response matrix method, and separated as artificial radionuclides (Cs-134 and Cs-137).

Fig 7

Changes of absorbed dose rate in air from artificial radionuclides in the eastern (A1) and western (A2) ends of Tokyo in 2014 [5]– 2018.

Fig 8. Changes of absorbed dose rate in air from artificial radionuclides measured at 1 m above the surface of porous and standard asphalt surfaces.

The gamma-ray pulse height distributions were measured 1 m above the surface of porous (n = 3) and standard asphalt (n = 5) materials for 10 min. The gamma-ray pulse height distributions were then unfolded using the 22 × 22 response matrix method, and dose rates were calculated for the artificial radionuclides.

Fig 9. The structures of porous and standard asphalt materials.