11:00 AM - 11:15 AM
[MAG34-08] Radioactivity in soil from near the Fukushima Dai-ichi Nuclear Power Plant at five years after the accident
Keywords:Fukushima Dai-ichi Nuclear Power Plant, radiocesium, Cs-134, Cs-137, soil deposition density
Between June and September 2016, 176 researcher of total were work to measure the air dose rate and collect soil samples at 105 locations near the Fukushima Dai-ichi nuclear power plant, mainly on the difficult-to-return zone, for 9 days in total. The outline of the project will be reported in another presentation. In this report, we will present the results of the inventory of 134Cs and 137Cs in the soil, the degree of destribution, the ratio of 134Cs/137Cs concentration, and the correlation with the air dose rate.
[Sample and Measurement] Surface soil of 5 samples per one location was collected from 36 public facilities in the difficult-to-return area of Futaba-machi or Okuma-machi in Fukushima Prefecture in June and July 2016. The sample from depths of 5 cm was divided into two parts of 0 - 2.5 cm depth and 2.5 - 5 cm depth, filled in a U-8 container after drying. Radiocesium was quantified using gamma ray spectrometry with Ge detectors.
[Results and Discussion] The results are shown in Fig.1 The maximum 137Cs concentration was 68400 kBq/m2 in inventory and 1180 kBq/kg(dry) in specific activity. The inventory clearly correlated with the air dose rate. There was large difference in soil distribution degree (the ratio of radioactivity of 2.5 - 5.0 cm and 0 - 2.5 cm in depth) for each sample, and the values were almost less than 1. Moreover, it was found that there are points where the concentration ratio of 134Cs /137Cs clearly has a low value of 0.87 - 0.93 in the north-north-west direction from the nuclear power plant.