Abstract
A large amount of radioactive materials has been discharged to the atmosphere after the accident of the TEPCO's Fukushima Dai-ichi nuclear power stations (abbreviated to 1F). The total amount discharged from the reactors is estimated to be approximately 1.3 x 1017Bq of I-131 and 6.1x1015Bq of Cs-137. These radionuclides dispersed widely and some of them deposited on to the ground surface with rain water. The geographic pattern of contamination levels was much different from the shape of concentric circle; the most contaminated area is located along the line to the northwest from the 1F and the relatively high concentration is observed along the line from the east part of Fukushima city to a southwest region.
The Ministry of Education, Culture, Sports, Science and Technology (MEXT) has measured ambient dose rates in the area beyond 20km from 1F from March 15, 2011 and observed relatively high dose rates in the northwest are such as 0.33 mSv/h on that day at about 20km from 1F and also 0.17 mSv/h on March 17 at about 30km from 1F. Regarding radiation exposure of residents who lived in the vicinity of 1F, MEXT has estimated the dose of the residents at about 30km in the northwest region from the noon on March 13 to May 30 to be 36mSv. In the south-by-sourthwest region, the integral dose is maintained at relatively low level though it is known that a highly radioactive plume passed there during the initial period of the accident.
As to internal exposure, the local nuclear emergency response headquarter investigated thyroid exposure of 1,080 children in late March and found the radiation levels of all children who lived near the evacuation area were below the screening level. In addition, from the end of June to July, Fukushima prefecture performed whole-body-counter measurements for 122 residents in cooperation with the National Institute of Radiological Sciences and reported that the committed effective dose of radiocesium in all of them were below 1mSv.
