Absorbed dose rate in air in metropolitan Tokyo before the Fukushima Daiichi Nuclear Power Plant accident

Japanese population dose from natural radiation

The radiation doses from natural radiation sources in Japan are reviewed using the latest knowledge. The United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) and the Nuclear Safety Research Association report the annual effective doses from cosmic rays, terrestrial radiation, inhalation, and ingestion as natural sources. In this paper, the total annual effective dose from cosmic-ray exposure is evaluated as 0.29 mSv. The arithmetic mean of the annual effective dose from external exposure to terrestrial radiation is 0.33 mSv for the Japanese population using the data of nationwide surveys by the National Institute of Radiological Sciences. Previously in Japan, although three different groups have conducted nationwide indoor radon surveys using passive-type radon monitors, to date only the Japan Chemical Analysis Center (JCAC) has performed a nationwide radon survey using a unified method for radon measurements conducted indoor, outdoor, and in the workplace. Consequently, the JCAC results are used for the annual effective dose from radon and that for radon inhalation is estimated as 0.50 mSv using a current dose conversion factor. In this paper, UNSCEAR values are used for the mean indoor and outdoor thoron-progeny concentrations, and the annual effective dose from thoron is reported as 0.09 mSv. Thus, the annual effective dose from radon and thoron inhalation is 0.59 mSv. From a JCAC large-scale survey of foodstuffs, the committed effective dose from the main radionuclides in dietary intake is 0.99 mSv. Finally, the Japanese population dose from natural radiation is given as 2.2 mSv, which is similar to the reported global average of 2.4 mSv.

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

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.

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

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).

Dispersion of radiocesium-contaminated bottom sediment caused by heavy rainfall in Joso City, Japan

A large-scale heavy rainfall disaster occurred in Joso City, Japan, in September 2015, and one third of the city area (40 km²) was flooded by the Kinu River. Artificial radionuclides such as ¹³⁴Cs and ¹³⁷Cs were known to have accumulated in the river bottom sediment after their release in the 2011 Fukushima Dai-ichi Nuclear Power Plant accident. It was thought that these radionuclides might have been dispersed by the rainfall disaster. A car-borne survey of absorbed dose rate in air had been made by the authors in Joso City in August 2015. Then, the present study made a second car-borne survey in October 2015, to evaluate changes in the rate after the rainfall disaster. The absorbed dose rate in air and the standard deviation (range) measured in the flooded areas of Joso City after the disaster were 68 ± 9 nGy h^-1 (39–98 nGy h^-1), which was 10% higher than the rate before it. Additionally, higher dose rates (> 60 nGy h^-1) were observed for the flooded areas after the disaster; furthermore, up to 886 Bq kg^-1 of activity concentration from ¹³⁴Cs and ¹³⁷Cs was observed in these flooded areas, and this was 11 times higher than the activity concentration before the disaster. These results suggested the dispersion of artificial radionuclides accumulated in the bottom sediment of the Kinu River after the Fukushima Daiichi Nuclear Power Plant accident occurred by the heavy rainfall disaster.

Detailed Distribution Map of Absorbed Dose Rate in Air in Tokatsu Area of Chiba Prefecture, Japan, Constructed by Car-Borne Survey 4 Years after the Fukushima Daiichi Nuclear Power Plant Accident

A car-borne survey was carried out in the northwestern, or Tokatsu, area of Chiba Prefecture, Japan, to make a detailed distribution map of absorbed dose rate in air four years after the Fukushima Daiichi Nuclear Power Plant accident. This area was chosen because it was the most heavily radionuclide contaminated part of Chiba Prefecture and it neighbors metropolitan Tokyo. Measurements were performed using a 3-in × 3-in NaI(Tl) scintillation spectrometer in June 2015. The survey route covered the whole Tokatsu area which includes six cities. A heterogeneous distribution of absorbed dose rate in air was observed on the dose distribution map. Especially, higher absorbed dose rates in air exceeding 80 nGy h-1 were observed along national roads constructed using high porosity asphalt, whereas lower absorbed dose rates in air were observed along local roads constructed using low porosity asphalt. The difference between these asphalt types resulted in a heterogeneous dose distribution in the Tokatsu area. The mean of the contribution ratio of artificial radionuclides to absorbed dose rate in air measured 4 years after the accident was 29% (9-50%) in the Tokatsu area. The maximum absorbed dose rate in air, 201 nGy h-1 was observed at Kashiwa City. Radiocesium was deposited in the upper 1 cm surface layer of the high porosity asphalt which was collected in Kashiwa City and the environmental half-life of the absorbed dose rate in air was estimated to be 1.7 years.

Impact on ambient dose rate in metropolitan Tokyo from the Fukushima Daiichi Nuclear Power Plant accident

A car-borne survey was made in metropolitan Tokyo, Japan, in December 2014 to estimate external dose. This survey was conducted for all municipalities of Tokyo and the results were compared with measurements done in 2003. The ambient dose rate measured in the whole area of Tokyo in December 2014 was 60 nGy h(-1) (23-142 nGy h(-1)), which was 24% higher than the rate in 2003. Higher dose rates (>70 nGy h(-1)) were observed on the eastern and western ends of Tokyo; furthermore, the contribution ratio from artificial radionuclides ((134)Cs and (137)Cs) to ambient dose rate in eastern Tokyo was twice as high as that of western Tokyo. Based on the measured ambient dose rate, the effective dose rate after the accident was estimated to be 0.45 μSv h(-1) in Tokyo. This value was 22% higher than the value before the accident as of December 2014.

A comparison of the dose from natural radionuclides and artificial radionuclides after the Fukushima nuclear accident

Due to the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident, the evacuees from Namie Town still cannot reside in the town, and some continue to live in temporary housing units. In this study, the radon activity concentrations were measured at temporary housing facilities, apartments and detached houses in Fukushima Prefecture in order to estimate the annual internal exposure dose of residents. A passive radon-thoron monitor (using a CR-39) and a pulse-type ionization chamber were used to evaluate the radon activity concentration. The average radon activity concentrations at temporary housing units, including a medical clinic, apartments and detached houses, were 5, 7 and 9 Bq m(-3), respectively. Assuming the residents lived in these facilities for one year, the average annual effective doses due to indoor radon in each housing type were evaluated as 0.18, 0.22 and 0.29 mSv, respectively. The average effective doses to all residents in Fukushima Prefecture due to natural and artificial sources were estimated using the results of the indoor radon measurements and published data. The average effective dose due to natural sources for the evacuees from Namie Town was estimated to be 1.9 mSv. In comparison, for the first year after the FDNPP accident, the average effective dose for the evacuees due to artificial sources from the accident was 5.0 mSv. Although residents' internal and external exposures due to natural radionuclides cannot be avoided, it might be possible to lower external exposure due to the artificial radionuclides by changing some behaviors of residents.