A problem concerning radiation exposure management of resident living in outside of evacuation area. The case of Moriya city in Ibaraki prefecture

The author points out the problem in the title as exemplified by the City 160 km afar from the Fukushima Daiichi Nuclear Power Station (FDNPS). The evacuation area is defined to be the zone within 20 km distance from FDNPS, where 60-80 thousands Bq of Cs (Cs-134 + Cs-137)/kg soil in average have been found. The author has found small spots of as high dose (hot) as 460 thousands Bq/kg in gutter materials probably due to the dry (Mar. 15, 2011) and wet (Mar. 21) deposit in the City, and the hot spots have been thought to disperse to form hot areas by measurement (June) with Ge-semiconductor detector of Cs levels at 12 spots including 10 waterway bottom precipitates, as high as 9-71 thousands Bq/kg (actual 0.60-2.74 micro-Sv/h, with NaI scintillation survey meter). The entrance is partly or thoroughly forbidden in some of these areas; however, children playing at other such hot areas can be often seen. This is the problem from the aspect of radiation protection out of the evacuation area and, further, it may be more problematic that resident does not know well about the fact. Although the rapid concentration of Cs like this is conceivably derived from the City's geographical features (many valleys and well-arranged waterway), the prediction of future distribution of Cs is difficult at present, but results may help to predict the extent of contamination in catchment areas of the evacuation zone in future. (T.T.)