In this paper, we summarized the state-of-art knowledge of source term, atmospheric dispersion, and deposition of radionuclides during the Fukushima Daiichi Nuclear Power Station accident in March 2011. Six years later from the accident, a large amount of internal and international research activities increased our understanding of total release amount and spatiotemporal radiological plume movement. On the other hand, the mechanism of deposition of radionuclides was still largely unknown particularly in mountainous areas of East Japan. Further investigation is required to obtain the detailed information of fog, drizzle, and weak rain events and the physicochemical information of radiological aerosols during the accident.
The spatio-temporal distributions of atmospheric 137Cs concentrations in and around the Fukushima prefecture just after the Fukushima Daiichi Nuclear Power Plant (FD1NPP) accident were retrieved, by measuring radionuclides in suspended particulate matter (SPM) hourly collected on used filter-tapes of operational air quality monitoring stations. Analyzing a published dataset of radiocesium (134Cs and 137Cs) at around 100 SPM monitoring sites, 10 radioactive plumes/polluted air masses in which the maximum 137Cs concentrations were higher than 10 Bq m-3 were found in the period of March 12–23, 2011. In these plumes, 5 plumes were transported to the eastern and/or central parts of the Fukushima prefecture, and another 5 plumes were transported to the Kantou area located more than 100 km south of the FD1NPP, respectively. In the period, the maximum 137Cs concentration of about 575 Bq m-3 was observed in the east coast of the Fukushima prefecture on the evening of March 12, 2011, after a vent process and the hydrogen explosion of Unit 1. Furthermore, high 137Cs concentrations of around 10 Bq m-3 were found in the northern part of the FD1NPP on the morning of March 21 when a strong northerly wind began to blow. These results indicate that further study is expected on the relationship between the release of radionuclides and the events happened in the reactors of the FD1NPP, and on the effects of the vertical structure of the atmosphere on the surface concentrations of radionuclides.
Insoluble radioactive particles emitted from the Fukushima nuclear accident have been found from the samples collected in various environments. This manuscript summarizes the studies that characterized such particles using electron microscopy and synchrotron radiation analyses. The shapes and compositions of 33 radioactive particles from aerosol particles, soil, plant tissues, non-woven fabric cloth, and masks have been identified. These radioactive particles contained radioactive Cs ranging from 0.5 to 109 becquerel per particle. Diameters of spherical particles ranged from 0.8 to 4 μm, and those of non-spherical particles were from 1.4 to 250 μm. They consisted of silicate glass with Cs, Zn, and Fe as well as many minor elements (in total, 33 elements were detected). These particles tended to have more Cs on their surface than the core and contained nano-crystals consisting of, for example, Cu-Zn-Mo, Ag-Te, CaSO4, FeCr2O4, Sn-Te, Ag-Sb, and iron oxides. Such crystalline structures will be important to understand the formation and degradation processes.
This paper summaries the reports on early internal exposure to the public due to the accident of TEPCO Fukushima Daiichi Nuclear Power Station in March 2011 and the issues on the internal dose assessments. The results by whole-body-counter (WBC) measurement for early internal exposure showed that the committed effective doses to most of the residents were less than 1 mSv. Thyroid equivalent doses were much smaller than the mean thyroid dose in the Chernobyl accident. Through the internal exposure measurements made so far, there are the issues such as lack of the data associated with the internal dose assessments, radioactivity measurement by the WBC, selection of intake scenarios, and internal dose assessments for insoluble and radiocesium-bearing radioactive particles with a high activity.
Electrospray is capable of generating functional substances as multiply charged nanoparticles and ions. It is widely used for the liquid sample introduction to mass spectrometers and air purification. In this study, particle size distribution and charge number distribution of nanoparticles of amino acids generated by electrostatic spraying were measured. The break-up process of multivalent charged droplets was estimated. The droplets immediately after spraying were break-up by the Rayleigh fission due to evaporation of the solvent. They were divided into the droplets with the particle size of about 140±80 nm and the daughter droplets of about 40±15 nm. Some of the droplets exists in an ion emission region of 10 nm or less, and single molecule ions of amino acids would be released from these microdroplets.