Seven-year temporal variation of Caesium-137 discharge inventory from the port of Fukushima Daiichi Nuclear Power Plant: continuous monthly estimation of Caesium-137 discharge in the period from April 2011 to June 2018

Improving the estimation of direct release rates and transport processes from the Fukushima Daiichi Nuclear Power Plant accident using higher-resolution oceanic dispersion model

A series of accidents at the Fukushima Daiichi Nuclear Power Station (F1NPS), following the 11 March 2011 earthquake and tsunami, resulted in the release of radioactive substances into the ocean. In particular, the release of large amounts of radioactive caesium has damaged the fishing industry, leading to voluntary restrictions on fishing and shipping. Oceanic dispersion simulations based on estimates of the pathways and fluxes of radioactive materials provide useful information for assessing the environmental impacts and formulating measures to mitigate the effects of the accident. For the direct release rate from the F1NPS site, an estimation method was developed using the results from nearby monitoring, and the seawater exchange rate was estimated in target volume using a numerical simulation. However, the influence of volume on the seawater exchange rate was not considered. Appropriate volumes must be considered when estimating the effects of future accidents. In addition, the directional coastal transport was underestimated in the simulations of the F1NPS accident because of the low resolution. To estimate the pathways and fluxes of radioactive material to the ocean and understand the distribution of the concentration of radioactive material based on ocean dispersion simulations, a study was conducted using a higher-resolution model. The horizontal resolution of the conventional ocean dispersion model Regional Ocean Modelling System (ROMS), was increased from 1 km to 200 m. The optimal settings of the seawater exchange rate were investigated, and the radioactive caesium transport process in the coastal direction was more accurately reflected. We found that the conventional volume for determining the seawater exchange rate, including the locations of release sources and observation points, is optimal. The validity of this finding was confirmed using experimental equations from previous oceanic tracer release experiments. To estimate future release rates, it an appropriate volume must be defined, for example, depending on the distance between the locations of the release sources and the observation point. In addition, improvements in the accuracy of the simulation of the coastal transport process were observed owing to the higher resolution, which increased reproducibility. However, with a horizontal resolution of 200 m, problems with repeatability near the harbours arose. A higher resolution, achieved using nesting or other methods, would be desirable to deal with releases smaller than those in the F1NPS accident.

Environmental recovery from 137Cs contamination in Japanese coastal waters shown by comparison of temporal distributions with European seas

We compared the spatiotemporal distributions of ¹³⁷Cs in the European Baltic Sea (semi-enclosed) and the North and Norwegian Seas (open to the ocean) after the Chernobyl Nuclear Power Plant (CHNPP) accident, with those in Japanese coastal regions, including the waters off Miyagi, Fukushima, and Ibaraki prefectures, open to the western North Pacific, after the Fukushima Nuclear Power Plant accident. The effective half-lives 1-9 years after each accident were shortest (1.6-4.7 y) in Japanese coastal waters, 4.9 y in the North Sea, and 14.4 y in the Baltic Sea, suggesting that decreases in ¹³⁷Cs concentrations are largely dependent on the local geography, and that the dilution-diffusion effect of seawater was greater in the Japanese coastal waters. The effective half-lives of ¹³⁷Cs in the surface waters of European seas, based on 30 years of data after the CNPP accident, became longer, ranging from 8.4 to 11.9 y. This may be due to the influence of rivers, and a delay in the decrease in ¹³⁷Cs levels caused by the small difference in radioactivity concentrations between the seas and diluting waters. These results could contribute to the prediction of contamination levels in Japanese coastal waters in the future.

Temporal variability of 137Cs concentrations in coastal sediments off Fukushima

Large amounts of radiocesium were released into marine environments following the Fukushima Daiichi Nuclear Power Plant accident in March 2011. Released radiocesium influenced not only marine environment but also marine biota in Fukushima. Since marine biota as fisheries products is important for Japanese market, it is important to assess the distribution of radiocesium in coastal environment off Fukushima for safety concerns of radioactive contamination. Radiocesium concentrations in sediments are important for understanding fishing ground conditions and for proving the safety of fisheries products in Fukushima. In this study, monthly monitoring data collected from May 2011 to March 2020 were analyzed to describe the temporal variability of ¹³⁷Cs concentrations in coastal sediments off Fukushima (total of 3647 samples from eight lines at depths of 7-125 m off Fukushima, and three sites in Matsukawa-ura Lagoon). The ¹³⁷Cs concentration in sediment showed a decreasing trend, but our nonlinear model fitting suggested that this rate of decrease had slowed down. Additionally, ¹³⁷Cs concentrations were up to 4.08 times greater in shallow sampling sites (7, 10, 20 m depth) following heavy rainfall events (before five months vs. after five months), such as typhoons. These observations were consistent with increasing input from particulate ¹³⁷Cs fluxes from rivers and increasing dissolved ¹³⁷Cs concentrations in seawater. Finally, our numerical modeling suggested that riverine ¹³⁷Cs input could maintain ¹³⁷Cs concentrations in coastal sediment. These results indicate that riverine ¹³⁷Cs input following heavy rainfall events is the main factor for maintaining ¹³⁷Cs concentrations in coastal sediments near the Fukushima Daiichi Nuclear Power Plant.

Current status of the environmental monitoring database on the accident at Fukushima Daiichi Nuclear Power Plant

An enormous amount of environmental monitoring data has been acquired by various organisations for the evaluation and implementation of countermeasures to mitigate the effects of the accident at the Fukushima Daiichi Nuclear Power Plant. However, it is difficult to collate, compare, and analyse this data because it was published in different formats at different sites according to the respective objectives of the publishing organisations. Moreover, these organisations have been accumulating data in large volumes for over nine years after the accident. We established procedures to collect this data, convert them into a unified format, classify them according to categories, and make the data accessible on a web-based database system. The database contains environmental monitoring data on air dose rates, ground deposition densities, and concentrations in various environmental samples such as soil, water, and food. This data is being provided not only in numerical format for quantitative analysis but also as distribution maps and time-series graphs for visual understanding. The database system enabled us to spatially and temporally compare large volumes of monitoring data. By using the database functions, characteristics of some representative data in the database was clarified.

Behavior of Radiocesium in Sediments in Fukushima Coastal Waters: Verification of Desorption Potential through Pore Water

Concentrations of ¹³⁷Cs in seawater, seabed sediment, and pore water collected from the area around Fukushima were investigated from 2015 to 2018, and the potential of coastal sediments to supply radiocesium to the bottom environment was evaluated. The ¹³⁷Cs concentration in the pore water ranged from 33 to 1934 mBq L^-1 and was 10-40 times higher than that in the overlying water (seawater overlying within 30 cm on the seabed). At most stations, the ¹³⁷Cs concentrations in the overlying water and the pore water were approximately proportional to those in the sediment. The conditional partition coefficient between pore water and sediment was [0.9-14] × 10² L kg^-1, independent of the year of sampling. These results indicated that an equilibrium of ¹³⁷Cs between pore water and sediment has been established in a relatively short period, and ¹³⁷Cs in the pore water is gradually exported to seawater near the seabed. A simple box model estimation based on these results showed that ¹³⁷Cs in the sediment decreased by about 6% per year by desorption/diffusion of ¹³⁷Cs from the seabed.