Ten years have passed since the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident. In this special issue, we compiled review papers on the environmental behavior of the FDNPP-derived radionuclides from various research fields. Hopefully, it will be recognized that various research fields are closely related to each other as well as geochemistry.

Selective adsorption at the frayed edge site (FES) of micaceous minerals is the most important reactions controlling soil-to-plant transfer of radiocesium. In this article, radiocesium adsorption on soils and minerals are reviewed, especially focusing on the wide differences in the amount or effectiveness of the FES between soils. Micaceous mineral contents in parent materials primarily control the FES in soils. Whereas granitoids and shales typically contains biotite and illite, respectively, other rocks may have developed to FES-poor soils unless eolian dusts had been deposited from continental desert areas. Weathering of micaceous minerals into vermiculite increases the FES content. However, large portion of the FES could be inaccessible for radiocesium in soil environments, either through the mineral coverage with humic substances or hydroxy-Al polymers interlayering. Thus, soil formation processes are important to understand how soils obtained the ability to retain radiocesium.

Radiocesium was emitted into the environment due to the Fukushima Dai-ichi nuclear power plant accident, and it is strongly adsorbed by surface soil. The large-scale decontamination was succeeded. But a serious problem remains, i.e., long-term storage of a massive amount of the waste soil provided by the decontamination. It is known that micaceous clay minerals are the primary adsorbents of radiocesium in soil. Scientific knowledge of the states and mechanisms of cesium adsorption to micaceous clay minerals is required to develop volume reduction techniques and safe long-term storage. However, it is not easy to reveal them only by the experimental study because the adsorption is intrinsically a microscopic phenomenon that is difficult to be approached even by recent advanced experimental techniques. Numerical simulation is one of the best techniques to investigate microscopic phenomena. We developed the models of the five adsorption sites in the micaceous clay minerals: basal surfaces, edges, hydrated interlayers, frayed edges, and interlayers. The outline of the simulations of these adsorption sites is briefly reviewed. Particularly, the mechanism of the origin of the strong adsorption of cesium to frayed edge sites is discussed based on the results of our numerical simulations. A future direction of numerical simulations of clay minerals is shown.

Radiocesium-bearing microparticles (CsMPs),glassy water-resistant particles with highly concentrated radiocesium, were emitted by the Fukushima Daiichi Nuclear Power Plant accident. Since first discovery of CsMPs, a number of studies have analyzed the particles isolated from environmental samples and revealed their physical and chemical properties, distribution, and migration. This paper is intended to provide an overview focusing on the environmental transport and impact of CsMPs. First, we begin by reviewing the relationship between deposition areas and atmospheric plumes of CsMPs found on land. Next, search and separation methods for CsMPs will be described. Then, secondary transport via rivers and effect of CsMPs on K_d values of Cs in rivers will be discussed. Finally, CsMPs found in the ocean and their difference from terrestrial ones will be summarized.

Radiocesium (¹³⁷Cs),derived from Global Fallout, in the surface seawater showed a uniform value nationwide (range: 3.3–5.6 Bq/m³; mean: 4.1 Bq/m³) in 1984–1985, decreased with a half-life of 16–18 years, and then, converged into～1.5 Bq/m³ as of June 2010. After the Fukushima Dai-ichi Nuclear Power Plant accident in March 2011, there was a remarkable change in ¹³⁷Cs activities in surface waters during the first year period; the activity reached a maximum in the middle of April and thereafter decreased exponentially with time. Since then, ¹³⁷Cs of surface seawater showed different dynamics depending on the coastal waters of Japan. The activity of ¹³⁷Cs in the coastal waters around the Fukushima Daiichi Nuclear Power Plant has been maintained by (i) continuous direct releases from the facility, (ii) desorption/dissolution from marine sediments, and (iii) riverine transport of the facility fallout deposited on land including salinity-induced desorption of ¹³⁷Cs from fluvial particles. On the other hand, along the west coast of Japan remote from the facility (i.e., the Japan Sea),showed different behaviors. Immediately after the accident, a part of the ¹³⁷Cs subducted into the subsurface of the North Pacific, and then the entrainment of subsurface ¹³⁷Cs into the surface waters through vertical mixing. The ¹³⁷Cs activities started increasing by 2013 and its effect has been continuing.

Radiocesium, which was deposited in forests due to the Fukushima Dai-ichi nuclear power plant accident, is estimated to remain in the forest for a long period. Forest covers a large proportion of the land area contaminated by radiocesium but there are no decontamination plans for most of the forest area. Therefore, radiocesium is expected to be transferred throughout the material cycle systems in forests and to be taken up by trees. During the approximately 10 years since the accident, many studies on the dynamics of radiocesium in forests have been conducted in the contaminated area. In addition, radiocesium contamination in forests affects headwater stream ecosystems because organic matters such as dead leaves falling from forests are essential nutrient sources for food webs in the ecosystems. There have been several previous studies aimed at understanding the dynamics of radiocesium contamination in the ecosystems through organic matter derived from the contaminated forests. This review introduces previous studies into how radiocesium deposited in forests was transferred through forests and was absorbed by trees. In addition, the process by which radiocesium deposited in forests transfers into headwater streams, and the process of migration into benthic macroinvertebrates, are introduced.

This review is an exposition of published reports describing radioactive contamination of marine and freshwater fish, collectively revealing radiocesium contamination mechanisms. Radiocesium concentrations in marine fish caught in the waters off Fukushima have decreased drastically during the decade following the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident, mainly because of the area’s open and diffusive marine environments leading to lower radiocesium concentrations in seawater, sediments, and prey items, and because of the low physiological capability of retaining radiocesium in fish. Nevertheless, careful attention must be devoted to marine fish in the FDNPP port because some fish samples collected from the port have exceeded the Japanese regulatory limit of 100 Bq/kg-wet. Long-term monitoring of marine fish and surrounding environments is necessary because coastal waters around FDNPP have not reached an equilibrium state. Radiocesium contamination of freshwater fish has continued in rivers and lakes within areas with high degrees of deposition, although overall contamination levels have decreased considerably. Radiocesium contamination will be particularly persistent in lakes and ponds where dissolved radiocesium leaching from sediments can become a continuing source of contamination for biological magnification through the food web. In riverine environments, continuing supplies of contaminated prey items (e.g., insects) from riparian environments can be expected to engender persistent radiocesium contamination of freshwater fish (e.g., salmonid fish in forest rivers).

Iodine 129 (¹²⁹I) is a radionuclide that decays to ¹²⁹Xe with a half-life of 15.7 million years. The analysis of ¹²⁹I in the environment has played an important role not only in the fields of planetary science and earth science, but also in the nuclear field in recent years. Particularly, in the case of the Fukushima Dai-ichi Nuclear Power Plant accident in March, 2011, the scientists estimated the distribution and soil deposition of ¹³¹I over a wide area, and contributed to the assessment of internal exposure doses in the early stage of the accident. This is one of the most significant achievements in recent ¹²⁹I-related studies. Future studies are expected to be conducted to elucidate the transport mechanism of ¹²⁹I from land to river and sea, and to investigate the possible accumulation of ¹²⁹I in aquatic organisms. The mobility of ¹²⁹I in the soil studied by the author during the 10-years since the accident has been mainly reviewed in this manuscript.