Environmental radiation status in Bunkyo-ku, Tokyo, after the TEPCO Fukushima Dai-ichi Nuclear Power Plant disaster

Bunkyo-ku is located in the eastern part of the metropolitan area of Tokyo, Japan, and is roughly 220 km south of the TEPCO Fukushima Dai-ichi Nuclear Power Plant. The population of the city was 201,079 as of August 1, 2012, and its area is 11.31 km². The local government has officially been surveying the environmental radiation status after the disaster in response to numerous requests from its citizens. The radiation surveillance in this area has been technically guided by radiation protection specialists. The two main targets for surveillance are (1) the ambient radiation dose (microsieverts per hour) at all the school yards, public parks, and representative measurement points selected by the local government, and (2) the specific radioactivity (becquerels per kilogram) present in school lunch. These data have been reported to the citizens through the city website as well as in a bi-monthly report in the public relations magazine of the local government. This report presents the background status and technical information of the related activities, as well as the measured environmental radiation data. The ambient radiation dose in the city has been surveyed since July 2011. In the 1st period of surveillance (from July to August, 2012), over a total of 304 measurement points, the highest recorded value of the ambient radiation dose was 0.22 μSv h^-1 at the height of 1 m from the ground, the lowest was 0.05 μSv h^-1, and the average was around 0.09 μSv h^-1. These values include the natural background dose rate detected by the energy compensation type survey-meters. In the most recent surveillance records, the maximum value recorded was 0.10, the minimum was 0.05, and 0.07 μSv h^-1 was the average value. The specific radioactivity of drinking water has been monitored at local purification plants since the accident occurred. No water sample supplied to the city has exceeded the national limits for intake dose. The specific radioactivity of school lunch was also surveyed four times (in December, 2011; and May, July, and September, 2012) by the local government. Thus far, 162 sets of school lunch in the city have been checked using a Ge spectroscopy system. The highest specific radioactivity of ^134+137Cs detected was 2.5 Bq kg^-1 in school lunch and 15.4 Bq kg^-1 in milk, which were recorded in December, 2011. (author)