The Fukushima Daiichi nuclear accident, initiated by the Great East Japan Earthquake, has resulted in environmental pollution across eastern Japan, especially in Fukushima Prefecture. The amount of radioactive substances released was far less than from the Chernobyl accident, and only radionuclide cesium has needed to be controlled. It was a daunting challenge, however, to implement the necessary countermeasures while obtaining a community consensus in the heavily populated country of Japan with its advanced information orientation. One of the main factors that made implementation of the countermeasures more challenging was a lack of necessary scientific knowledge for decision-making by policy makers. Nevertheless, Japan’s academic circles and industrial sectors have been making steady progress in research and development toward environmental restoration.

This special issue focuses on research and development toward restoration from environmentally radioactive pollution caused by the nuclear accident, and provides an integrated body of relevant scientific knowledge collected to date. The articles in this issue cover a wide range of topics, including a study on the environmental dynamics of radioactive substances and its impact on ecosystems, and the adoption and evaluation of decontamination and contaminated waste treatment technologies. Many articles have been provided by researchers of the National Institute for Environmental Studies and the Japan Atomic Energy Agency, Japan’s leading research institutes in the environmental and nuclear fields, respectively. We are profoundly grateful for their contribution of valuable scientific knowledge and technical expertise in the midst of their hard work toward environmental restoration, and strongly hope that this information will be utilized in Japan’s steady environmental restoration efforts in the years to come.

Toshimasa OHARA

Kaname MIYAHARA

Masahiro OSAKO

Seiji HAYASHI

Kuniaki SAITO

Minoru TAKEISHI

The Fukushima Daiichi Nuclear Power Station accident caused radioactive pollution of various environmental media. In the face of a perceived unprecedented environmental issue, many organizations in the public and private sectors and academia have carried out multifaceted research to help restore the environment of affected areas starting from immediately after the accident. The national and local governments have conducted off-site cleanup, treatment of contaminated waste and other endeavors. These challenging efforts need to be further augmented for the environmental regeneration of Fukushima Prefecture and other affected areas.

Large-scale environmental monitoring that has been repeatedly performed has revealed the characteristics of radiological conditions around the Fukushima NPP site. Several radionuclides other than radiocesium were detected three months after the accident, but radiocesium turned out to be far more important than the other radionuclides from the viewpoint of long-term exposures. Radiocesium deposition was found to be heterogeneous at different scales. On the one hand, air dose rates in environments related to human living have decreased much faster than the physical decay of radiocesium; on the other hand, the reduction has been close to physical decay in pure forests. Movement of deposited radiocesium has generally been very slow except in urban areas, while radiocesium deposited on the ground has gradually penetrated into the ground, resulting in decreased air dose rates because of the increased radiation shielding effect.

Atmospheric simulation models have played key roles in elucidating the atmospheric behaviors and deposition patterns of radioactive materials emitted from the Fukushima Daiichi Nuclear Power Plant after the nuclear accident in March 2011. In this review article, we summarize atmospheric modeling studies conducted after that accident. In particular, atmospheric models were used for source term estimates and analysis of atmospheric behaviors of radionuclides. In addition, atmospheric models provided results utilized in estimating health effects, forecasting, assessing long-term mitigation, and other tasks. Performance of atmospheric models for those purposes is evaluated in detail. Many problems remain, however, and future studies will be required for gaining a better understanding of radionuclides’ environmental impact.

To investigate detailed sorption behaviors of cesium (Cs) into clay minerals, thereby contributing to resolution of soil contamination issues due to Fukushima Daiichi Nuclear Power Station accident, we applied synchrotron-based X-ray analysis along with molecular orbital calculation (MO) using density functional theory and SEM/TEM-EDS combined with imaging plate (IP) methods. Conventional extended X-ray absorption fine structure (EXAFS) results suggest that Cs is preferentially incorporated into interlayers of vermiculite clay, which has water-swollen interlayers, but, by contrast, is dominantly sorbed onto the surface of clays such as illite in which most of the interlayers are closed off. The processes of incorporation of Cs into the swollen interlayers, followed by collapse of the interlayers, were first observed with time resolved dispersive X-ray fine structure (TR-DXAFS). Small angle X-ray diffraction (SAXS) results show that successional sorption of Cs into clay minerals follows if one Cs ion can be sorbed into the clay minerals. Molecular orbital calculation based on density functional theory (DFT) methods suggests, furthermore, that the Cs incorporated into clay minerals can stabilize in the interlayers of clay minerals by forming covalent bonds with the clay minerals after the incorporation process. From analysis of litter soil collected from a forest in Iitate, Fukushima Prefecture, using SEM/TEM-EDS and IP sensitive to radioactivity, weathered biotite clay was confirmed to be important as a super-selective absorbent for Cs.

Huge amounts of radioactive cesium (Cs) were released by the accident at the Fukushima Daiichi Nuclear Power Plant into the environment and widely contaminated the Tohoku and Kanto regions, especially mountainous area of Fukushima Prefecture. Much research on the migration and accumulation of radioactive Cs in the heavily contaminated upstream region mainly consisting of forested catchments and water bodies has been conducted to understand the current status and process of the contamination from the Fukushima accident. This paper organizes the scientific knowledge about the migration and accumulation of deposited radioactive Cs in contaminated upstream regions after the Fukushima accident, especially regarding the runoff characteristics from forest catchments, accumulation characteristics in bottom sediment of dam reservoirs and ponds, and the behavior of bioavailable radioactive Cs. It then highlights future research issues for providing local residents safe and secure lives based on the above knowledge.

This paper outlines circumstances regarding development of models to evaluate radioactive cesium behavior in river systems and discusses issues to be considered. The study used a relatively simple approach, parameterizing the transfer rate of radionuclides from one component to others, using, for example, soil-loss models and multi-compartment models. These models were applied to calculate the annually averaged transport behavior of radioactive cesium across entire river basins. On the other hand, detailed river models and water transport models were applied to evaluate the behavior during each high water event. A comparison of these models indicates that the amounts of discharged radioactive cesium brought mostly by wash loads are more or less successfully calculated by all models investigated. Liquid wash-off and/or desorption behavior is expected to be modeled in detail to calculate the future behavior of dissolved cesium, which is thought to migrate easily into ecosystems.

Since the Fukushima Daiichi Nuclear Power Plant accident, vast amounts of radionuclide monitoring data obtained from various governmental and institutional surveys have been released to the public. In this study, we analyzed these large amounts of data to evaluate the long-term, large-scale fate of Cesium-137 in eastern Japan. Analysis of in situ measurements showed that temporal changes in the virtual amount of Cs-137 deposited, which were estimated from measured air dose rate data, varied greatly among measurement points, depending on air dose level and land use. The decrease in virtual amounts deposited was comparable to the decay rate of radioactive cesium, but was large in areas where the air dose rate had been relatively high and human activities had occurred. An analysis of Cs-137 concentrations in river and lake sediments showed Cs-137 levels to decrease at a fast rate, one order of magnitude faster than the radioactive decay rate. The present study highlights the importance of the effects of vertical and/or horizontal migrations of Cs-137 within the terrestrial area (including decontamination activities), direct deposition of Cs-137 into surface water, behavior of Cs-137 in built-up areas, and Cs-137 behavior in stagnant water bodies (e.g., ponds and lakes). The present study has demonstrated the utility analyzing large amounts of publicly available data for evaluating the environmental behavior of Cs-137.

Many attempts to evaluate exposure doses to the public have been made since the Fukushima accident. Doses to the public from external exposures have been found to be more significant than those from internal exposures. Initiating restrictions on consumption of contaminated foods and drinks fairly promptly after the accident is thought to have helped. Fukushima Prefecture estimated the maximum external effective dose for the first four months after the accident at 25 mSv. Additional external effective doses of Fukushima residents in recent years have all been evaluated at within several mSv per year. Residents are not expected to receive large additional external doses after returning to the evacuated areas after the evacuation orders are lifted. NIRS has estimated the maximum thyroid dose due to inhalation of I-131 in the early phase of the accident at around 30 mSv, which corresponds to an effective dose of 1.2 mSv. Committed effective doses due to internal exposure from radiocesium intake have been estimated to be notably smaller than 1 mSv as a whole. Estimated exposure doses during the early stages of the accident have certain inevitable uncertainties for both internal and external exposures, and further investigation to reduce these uncertainties is urgent.

Many past genetic and ecological studies on wild organisms around Chernobyl have demonstrated significant genetic, physiological, developmental, fitness and population effects resulting from exposure to radioactive contaminants. Similarly, several studies on biological impacts of radiation have also been conducted since the Fukushima accident, including on genetic damage, abnormality rates and population abundance. In Fukushima Prefecture, increased morphological abnormalities have been observed in some wild organisms (e.g., butterflies, gall-forming aphids, firs and red pines). Radionuclide-contaminated soil or marine water has also had physiological impacts on mortality in butterflies and goshawks as well as on blood components in marine birds and Japanese monkeys. Moreover, population censuses of birds, butterflies and cicadas have indicated that the abundance of these species declined significantly with increased levels of radiation. Overall, these impacts on wild organisms from radiation were detected within two to three years after the accident. Apart from the direct impact of radiation, biota monitoring inside and outside the evacuation zone in Fukushima Prefecture has shown that populations of some wild animals and insects have also been affected by decontamination activities and/or the large-scale evacuation. Hereafter, it will be increasingly significant to continue monitoring biota in land and coastal areas of Fukushima Prefecture, and more detailed studies of the impact on wild organisms from radiation are also needed.

Major challenges to implementing full-scale environmental decontamination have included the absence of real-world examples and lack of experience in planning and implementing decontamination technology appropriate to the physical and social boundary conditions in Japan. The Japan Atomic Energy Agency (JAEA) was thus charged with conducting a range of “Decontamination Pilot Projects (DPPs)” to examine the applicability of decontamination technologies, with a special focus on reducing dose rates and thus allowing evacuees to return to re-establish their normal lifestyles as quickly as possible, whilst simultaneously maintaining worker safety. The DPPs provide a good basis for developing recommendations on how to assure decontamination efficiency and worker safety whilst additionally controlling costs and subsequent waste management and environmental impacts. This report revises the report of the DPPs (Nakayama et al., 2014), adding commentary on decontamination technology.

After the accident at the Fukushima Daiichi Nuclear Power Plant, the environment of northeastern Japan was contaminated by fallout containing radioactive cesium isotopes (r-Cs) from the plant, leading to contamination of municipal solid waste (MSW) as well. The contaminated MSW has been handled by various thermal treatment facilities. In order to understand r-Cs behavior during different thermal treatments of MSW, we reviewed earlier research reports on r-Cs concentration in solid residual materials discharged from the treatment plants, the distribution ratio of r-Cs between residue discharged from the bottom of furnaces (bottom ash, incombustibles, or slag) or dust in flue gas (fly ash), and r-Cs leaching rates from these residues. Furthermore, we compared the results with those of our recent investigation into incineration of waste from decontamination activities. In this paper, we present the results and discuss the effect of different types of waste on r-Cs behavior and leaching characteristics, using a thermodynamic equilibrium calculation approach.

Wastes contaminated with radiocesium released from the accident of the Fukushima Daiichi Nuclear Power Station are classified as low-concentration wastes with 8,000 Bq/kg or lower, and high-concentration wastes exceeding 8,000 Bq/kg. The former is called “specified municipal solid waste” or “specified industrial waste,” and the latter, “designated waste.” This paper introduces off-site management, such as landfilling; characteristics of these wastes; and the legal framework for their management. In their management, the key characteristics of these wastes are high leachability of radiocesium from incineration fly ash from specified municipal solid wastes, and low leachability from bottom ash and sewage sludge ash. The radiocesium leachability of cement-solidified designated waste decreases with increasing unconfined compression strength. The significant engineering aspects of landfilling are as follows: soil can inhibit migration of radiocesium in landfill sites, capping over a layer of the radioactively contaminated waste is an effective method of containing radiocesium in landfill sites, and no barrier layer should be installed beneath the waste layer to control migration of radiocesium. Installation of such a barrier would result in production of highly concentrated leachate.

Crowdsourced data and professional scientists’ data were compared for agreement regarding air dose rate levels and trends in air dose rate reduction to assess the value of public initiatives in radiation data collection during nuclear crisis response. This study used seven KURAMA datasets from seven survey periods to represent expert group data. To represent non-expert group data, we used seven datasets from SAFECAST’s database the collection period of which was comparable to the KURAMA survey periods. A simple linear regression model was separately applied to a pair of combination datasets from different sources and also to a pair of first survey period datasets and subsequent datasets from the same source. The R-squared of the models showed the non-scientist group data correlating well with the corresponding expert data. The slopes of all the regression models, however, indicated that the air dose rate values measured by non-expert group were about 40 to 70 percent lower than those of the expert group. The air dose rate reduction trend from the crowd data showed a similar decreasing pattern compared to that of the expert group, although the discrepancy in the magnitude of dose reduction between them was as high as 14 percent. The discrepancy in air dose rate values suggest a careful interpretation of radiation information generated solely from crowdsourced data. Nevertheless, given the strong linear relationship of crowd data with scientist groups’ data, the superior number of data points during a crisis, and the flexibility and agility of self-organization, we argue that the public could be a great partner to scientist groups in radiation data collection.