Radioactive contamination in the Tokyo metropolitan area in the early stage of the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident and its fluctuation over five years.

Masanobu Ishida,Hideo Yamazaki,Tim A Mousseau

doi:10.1371/journal.pone.0187687

Abstract

Radioactive contamination in the Tokyo metropolitan area in the immediate aftermath of the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident was analyzed via surface soil sampled during a two-month period after the accident. 131I, 134Cs, and 137Cs were detected in these soil samples. The activity and inventory of radioactive material in the eastern part of Tokyo tended to be high. The 134Cs/137Cs activity ratio in soil was 0.978 ± 0.053. The 131I/137Cs ratio fluctuated widely, and was 19.7 ± 9.0 (weighted average 18.71 ± 0.13, n = 14) in the Tokyo metropolitan area. The radioactive plume with high 131I activity spread into the Tokyo metropolitan area and was higher than the weighted average of 6.07 ± 0.04 (n = 26) in other areas. The radiocesium activity and inventory surveyed in soil from a garden in Chiyoda Ward in the center of Tokyo, fell approximately 85% in the four months after the accident, and subsequently tended to rise slightly while fluctuating widely. It is possible that migration and redistribution of radiocesium occurred. The behavior of radiocesium in Tokyo was analyzed via monitoring of radiocesium in sludge incineration ash. The radiocesium activity in the incineration ash was high at wastewater treatment centers that had catchment areas in eastern Tokyo and low at those with catchment areas in western Tokyo. Similar to the case of the garden soil, even in incineration ash, the radiocesium activity dropped rapidly immediately after the accident. The radiocesium activity in the incineration ash fell steadily from the tenth month after the accident until December 2016, and its half-life was about 500 days. According to frequency analysis, in central Tokyo, the cycles of fluctuation of radiocesium activity in incineration ash and rainfall conformed, clearly showing that radiocesium deposited in urban areas was resuspended and transported by rainfall run-off.

Highlights

The tsunami triggered by the East Japan Great Earthquake Disaster of March 11, 2011 cut off all electric power to Fukushima Daiichi Nuclear Power Plant (FDNPP)
Regarding the radioactive contamination of soil caused by the FDNPP accident, the geographical heterogeneity of the radioactive materials deposited on the ground surface was uneven and narrow ranging
Tokyo metropolitan pollution immediate aftermath of the accident, 131I, 134Cs, and 137Cs were detected in the soil of the Tokyo metropolitan area

Summary

Introduction

The tsunami triggered by the East Japan Great Earthquake Disaster of March 11, 2011 cut off all electric power to Fukushima Daiichi Nuclear Power Plant (FDNPP). Part of the radioactive material discharged into the atmosphere was deposited across eastern Japan [7,8,9,10,11]. Based on monitoring data from each location, it is assumed that radioactive material in the plume that spread into the Tokyo metropolitan area on March 16 and 22 was deposited on the ground via rainfall. The timing and route of the spread of the radioactive plume have been variously hypothesized, but have not been clarified in detail as of yet [12,13,14,15,16,17]. When we started our measurements, almost no public information about the radioactive contamination in the Tokyo metropolitan area and Kanto district had been shared

Methods

Results

Conclusion