Abstract
Following the Fukushima Daiichi nuclear power plant accident on March 11th 2011, the Village of Iitate located 25 - 45 km northwest from the plant is found with radioactive contamination and suspected as a Hot Spot, where the contamination is greater than other parts of Fukushima Prefecture. Given the urgency of the situation of residents living in the region and in order to grasp the extent and spread of Hot Spot, radiation survey activities have been carried out with the cooperation of Iitate Village and related authorities on March 28th and 29th.
With radiation detectors brought into a van provided by Iitate Village office, the radiation dose rate inside the vehicle was measured as it travelled within the village. The detectors used were ALOKA Co. pocket survey meter PDR-101(CsI detector) and ALOKA Co. Ionization Chamber Survey Meter ICS-313. The air dose rates at 130 locations have been measured. The survey was conducted in and outside the vehicle at several locations, and this confirmed the shielding effect of the vehicle. In addition, soil samples were obtained at 5 locations for the propose of the radionuclide analysis, and later with Ge-detector, gamma ray radionuclide analysis was carried out at Hiroshima University. Radionuclides of 132Te, 131I, 129Te, 129mTe, 137Cs, 134Cs, 136Cs and 140La were identified. The time variation of air dose rate was calculated by using the contamination density. The estimated accumulative exposure to radiation over 90 days beginning from the March 15 low is 74mSv for Nagadoro-Magata and 22mSv for the Iitate Village Hal. A cumulative exposure map in Iitate village are has been evaluated from the above mentioned data. The cumulative exposure map is compared with the cumulative exposure map by MEXT. In conclusion, the both cumulative exposure maps show a similar trend. Our map shows the detailed distribution cumulative exposure.
