Reduction of External Exposure for Residents from the Fukushima Nuclear Accident by Weathering and Decontamination

External exposure for residents resulting from the Fukushima nuclear accident was measured in the Marumori and Kosugo regions for the period from September 2011 to March 2013 after the occurrence of the Fukushima nuclear accident. Marumori is a rural settlement, and Kosugo is a suburban city along a freeway. The initial substantial reduction in personal dose equivalent [Hp(10)] for Marumori residents, which was in accordance with the reduction in the air dose rate, was observed. Both values of the Hp(10) results and the air dose rates dropped and remained low during the heavy snow season. The values returned to previous levels and then followed a relatively faster reduction than the radioactive decay rate of 134Cs and 137Cs after the snow had thawed. These faster reductions are considered to be caused by weathering from snow melting or migration of radionuclides down the soil column. However, neither a drop resulting from an accumulation of snow nor faster reduction was observed in Hp(10) for Kosugo residents, except the reduction from the radiocesium decay, although the same reduction tendency as that in Marumori was observed in the air dose rates. The discrepancy between the air dose rate and Hp(10) for Kosugo residents might be caused by dose contributions from the fixed contamination in houses in the suburban environment. The effects of schoolyard decontamination on the Hp(10) values for schoolchildren in Kosugo have been observed in the readings recorded since October 2012, and dose reduction was evaluated as approximately 10–20 μSv/month on average.