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^ (23-142 nGy h^), which was 24% higher than the rate in 2003. Higher dose rates (>70 nGy h^) were observed on the eastern and western ends of Tokyo; furthermore, the contribution ratio from artificial radionuclides (^Cs and ^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 0.45μSv h^ in Tokyo. This value was 22% higher than the value before the accident as of December 2014

Dispersion of radiocesium-contaminated bottom sediment caused by heavy rainfall in Joso City, Japan - Fig 3

<p><b>Correlation between count rates inside and outside the car (<i>p</i> = 0.781) (a). Correlation between dose rates in air and count rates outside the car (<i>p</i> = 0.215) (b).</b> The absorbed dose rates in air were calculated using software that implemented the 22 × 22 response matrix method [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0171788#pone.0171788.ref009" target="_blank">9</a>]. The slopes of these regression lines were used as the shielding factor and the dose conversion factor, respectively.</p

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

Absorbed dose rate in air from natural and artificial radionuclides before and after the 2015 heavy rainfall disaster.

<p>Absorbed dose rate in air from natural and artificial radionuclides before and after the 2015 heavy rainfall disaster.</p

The distribution maps of absorbed dose rate in air in Joso City before (a) and after (b) the heavy rainfall disaster.

<p>The map for August 2015 (a) was drawn using 169 data and that for October 2015 (b) was drawn using 623 data.</p

Activity concentrations of ¹³⁴Cs and ¹³⁷Cs in soil samples after the heavy rainfall disaster.

<p>Activity concentrations of ¹³⁴Cs and ¹³⁷Cs in soil samples after the heavy rainfall disaster.</p

The survey routes for measuring the absorbed dose rates in Joso City before (a) and after (b) the heavy rainfall disaster.

<p>Car-borne surveys were carried out using a 3-in × 3-in NaI(Tl) scintillation spectrometer in August and October 2015. Total distances traveled were 59.3 km and 88.0 km, respectively. The fixed-point observations outside the car were also done for 10 min at 11 locations. These route maps were drawn using the Generic Mapping Tools (GMT) created by Wessel and Smith [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0171788#pone.0171788.ref003" target="_blank">3</a>].</p

Absorbed dose rate in air from natural and artificial radionuclides in the Tokatsu area of Chiba Prefecture.

<p>Absorbed dose rate in air from natural and artificial radionuclides in the Tokatsu area of Chiba Prefecture.</p

Absorbed dose rate in air in the Tokatsu area of Chiba Prefecture measured in 2015.

<p>Absorbed dose rate in air in the Tokatsu area of Chiba Prefecture measured in 2015.</p