Temporal change in the 137Cs concentration ratio between coarse and fine particles in Fukushima

Many radionuclides were released into the atmosphere by the Fukushima Daiichi Nuclear Power Plant accident, which was caused by the tsunami following the Tohoku Region Pacific Coast Earthquake on March 11, 2011. A major radionuclide released into the environment was radiocesium. Radiocesium has a long lifetime, which allows it to remain in the environment for a prolonged period. Most radiocesium exists in soil particles, and a potential source of radiocesium in coarse particles was resuspended from soil particles. Therefore, the 137Cs concentration ratios between coarse and fine particles were important for the evaluation of the radiocesium source. In this study, the 137Cs atmospheric concentrations in coarse and fine particles were measured in Fukushima. Airborne particles were collected from September 2012 to July 2013 at two sites, Fukushima and Date, which are 62 and 55 km, respectively, from the Fukushima Daiichi Nuclear Power Plant. The coarse and fine particles were categorized by a 50% cutoff diameter of 1.1 μm. The sampling filters were exchanged once every half month. The radioactivity of 137Cs in the samples was measured for 10800-86400 s by the Ge detector. In Fukushima, the total 137Cs concentration in coarse and fine particles ranged from 20 to 370 μBq m-3: 7-270 μBq m-3 for coarse particles and 10-170 μBq m-3 for fine particles. In Date, the total concentration ranged from 100 to 310 μBq m-3: 60-220 μBq m-3 for coarse particles and 10-170 μBq m-3 for fine particles. The 137Cs concentrations at the two sites were found to be comparable. The average concentration at the two sites for the course and fine particles were 110 and 55 μBq m-3, respectively, and the total concentration was 166 μBq m-3. The 137Cs concentration in coarse particles accounted for 30%-90% of the total concentration, with an average of 67%. On comparing with the ratio before the study, the 137Cs concentration in coarse particles tended to increase after the accident. A potential cause for this result is the increase in the ratio of 137Cs concentration in the resuspended soil particles to the total concentration. The 137Cs concentration ratio between coarse and fine particles indicates that the 137Cs concentration in resuspended soil particles increased in atmospheric radiocesium. (authors)