Activity size distribution of radionuclei from the Fukushima nuclear accident and their carrier in the atmosphere

The Fukushima Dai-ichi nuclear power plant accident discharged an enormous amount of radionuclides. The microphysical properties of the particulate radionuclides have not yet been revealed. Here, we show that the activity size distributions of radionuclides Cs-134 and Cs-137 collected 47 days after the accident at Tsukuba suggest that sulphate is the main transport medium of these radionuclides. The bimodal distributions in the accumulation mode size range can be explained by the size of host particles in the different growth stages of sulphate. This contradicts the common perception that the major airborne radioactive substances in eastern Japan may be re-suspended soil particles that had adsorbed radionuclides. Our results explain the activity sizes of radiocaesium measured at the time of the Chernobyl accident, which were independent of the transport distances and showed inconsistent temporal changes.