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Journal Articles

Evaluation of the impact of the $$^{137}$$Cs supply from rivers to coastal waters off Fukushima on the $$^{137}$$Cs behavior in seabed sediment

Ikenoue, Tsubasa; Nakanishi, Takahiro; Shimadera, Hikari*; Kawamura, Hideyuki; Kondo, Akira*

E3S Web of Conferences (Internet), 530, p.02005_1 - 02005_10, 2024/05

https://doi.org/10.1051/e3sconf/202453002005

The accident at the Fukushima Daiichi Nuclear Power Plant (FDNPP) caused a radioactive contamination in seabed sediment. The $$^{137}$$Cs supply from rivers could be an important process for the long-term behavior of $$^{137}$$Cs in seabed sediment. In this study, a ten-year simulation of the $$^{137}$$Cs behavior in seabed sediment was conducted using an oceanic dispersion model combined with a prediction model of $$^{137}$$Cs behavior in land and river. In the waters north of FDNPP, the simulation results suggested that the $$^{137}$$Cs supply from rivers had a great impact on the concentrations in coastal sediment due to the initial low concentrations in seabed sediment and the large supply of $$^{137}$$Cs from rivers. In the waters near FDNPP and south of FDNPP, the simulation results suggested that the impact of the $$^{137}$$Cs supply on the temporal variation of $$^{137}$$Cs concentration in coastal sediment was relatively small due to the large initial adsorption from seawater. Overall, these results indicated that $$^{137}$$Cs supply from rivers had an impact on the spatiotemporal distribution of $$^{137}$$Cs concentrations in seabed sediment on a decadal time scale and the impact was especially great in the waters north of FDNPP.

Journal Articles

Thirty-year prediction of $$^{137}$$Cs supply from rivers to coastal waters off Fukushima considering human activities

Ikenoue, Tsubasa; Shimadera, Hikari*; Nakanishi, Takahiro; Kondo, Akira*

Water (Internet), 15(15), p.2734_1 - 2734_18, 2023/08

https://doi.org/10.3390/w15152734
 Times Cited Count：0 Percentile：0.00(Environmental Sciences)

The Fukushima Daiichi Nuclear Power Plant accident caused an accumulation of $$^{137}$$Cs in coastal sediment. The $$^{137}$$Cs supply from rivers to the ocean can affect the long-term fate of $$^{137}$$Cs in coastal sediment. Since the Fukushima coastal river basins include large decontaminated and evacuation order areas, considering the decontamination work and resumption of agriculture is important for predicting the $$^{137}$$Cs supply. We conducted a 30-year prediction of the $$^{137}$$Cs supply from the Fukushima coastal rivers to the ocean using a distributed radiocesium prediction model, considering the effects of human activities. In river basins with decontaminated and evacuation order areas, human activities reduced the total $$^{137}$$Cs outflow from agricultural lands, urban lands, and forest areas to the rivers and the $$^{137}$$Cs supply to the ocean by 5.0% and 6.0%, respectively. These results indicated that human activities slightly impacted the $$^{137}$$Cs outflow and supply. The $$^{137}$$Cs supply from rivers impacted by the accident to the coastal sediment was estimated to correspond to 11-36% of the total $$^{137}$$Cs in the coastal sediment in the early phase of the accident. Therefore, the $$^{137}$$Cs supply from rivers to the ocean is important for the long-term behavior of $$^{137}$$Cs in coastal sediment.

Journal Articles

Thirty-year simulation of environmental fate of $$^{137}$$Cs in the Abukuma River basin considering the characteristics of $$^{137}$$Cs behavior in land uses

Ikenoue, Tsubasa; Shimadera, Hikari*; Nakanishi, Takahiro; Kondo, Akira*

Science of the Total Environment, 876, p.162846_1 - 162846_12, 2023/06

https://doi.org/10.1016/j.scitotenv.2023.162846
 Times Cited Count：4 Percentile：46.73(Environmental Sciences)

We conducted 30 years simulation of environmental fate of $$^{137}$$Cs in the Abukuma River basin considering the characteristics of the $$^{137}$$Cs behavior in land uses. Overall, in the Abukuma River basin, the $$^{137}$$Cs transported into the ocean for 30 years was estimated to correspond to 4.6% of the initial deposition in the basin, and the effective half-life of $$^{137}$$Cs deposited in the basin was estimated to be 3.7 years shorter (by 11.6%) than its physical half-life. These results suggested that $$^{137}$$Cs deposited from the accident could still remain for decades. Based on the analysis of the $$^{137}$$Cs behavior in land use, in 2011, the contribution of $$^{137}$$Cs export to the ocean from urban lands was estimated to correspond to 70% of the total $$^{137}$$Cs export. Meanwhile, from 2012 to 2040, the contribution of $$^{137}$$Cs export from agricultural lands was estimated to correspond to 75% of the total $$^{137}$$Cs export. The reduction ratios excluding radioactive decay of $$^{137}$$Cs remained in areas with and without human activities for 30 years after the accident, defined as the ratios of the total outflow to the initial deposition, were estimated to be 11.5%-17.7% and 0.4%-1.4%, respectively. These results suggested that human activities enhance the reduction of $$^{137}$$Cs remaining in land in the past and future.

Journal Articles

Statistical analysis of simulated oceanic dispersion of dissolved radionuclide hypothetically released from the Fukushima Dai-ichi Nuclear Power Plant using long-term oceanographic reanalysis data

Ikenoue, Tsubasa; Kawamura, Hideyuki; Kamidaira, Yuki

Journal of Nuclear Science and Technology, 60(1), p.61 - 71, 2023/01

https://doi.org/10.1080/00223131.2022.2079569
 Times Cited Count：0 Percentile：0.00(Nuclear Science & Technology)

We conducted numerical simulations on the oceanic dispersion of dissolved radionuclide hypothetically released from the Fukushima Dai-ichi Nuclear Power Plant using long-term oceanographic reanalysis data. We evaluated the characteristics and trends of dissolved radionuclide behavior in the ocean using statistical analysis based on the simulation results. The surface meridional current at the release point in the Fukushima coastal ocean and the Kuroshio Extension significantly affected the north-south transport of the surface radionuclide in the Fukushima coastal ocean and the eastward transport of the offshore surface radionuclide, respectively. Because the surface kinetic energy in the coastal to the offshore area was larger, the range of the dispersed surface radionuclide tended to be larger. In summer (July-September), the increased frequency radionuclide entrainment by the Kuroshio Extension because of the surface southward radionuclide transport in the Fukushima coastal ocean and the large surface kinetic energy caused a large dispersed surface radionuclide. In winter (January-March), the decreased frequency radionuclide entrainment by the Kuroshio Extension because of the surface northward radionuclide transport in the Fukushima coastal ocean and the small surface kinetic energy caused a small dispersed surface radionuclide.

Journal Articles

Distribution, dynamics, and fate of radiocesium derived from FDNPP accident in the ocean

Otosaka, Shigeyoshi*; Kamidaira, Yuki; Ikenoue, Tsubasa; Kawamura, Hideyuki

Journal of Nuclear Science and Technology, 59(4), p.409 - 423, 2022/04

https://doi.org/10.1080/00223131.2021.1994480
 Times Cited Count：5 Percentile：50.16(Nuclear Science & Technology)

After the accident at Fukushima Daiichi Nuclear Power Plant (FDNPP), many oceanographic observations were carried out from various perspectives. Numerical simulations were also effectively applied to understand the distribution of radionunclides. By integrating these results, the dynamics of the accident-derived radionuclides in the ocean were highlighted. The transport processes of the accident-derived radionuclides to the ocean include (1) direct discharge to the ocean, (2) deposition in the ocean via the atmosphere, and (3) inflow from rivers after deposition to the land surface. In the case of $$^{137}$$Cs, one of major accident-derived radionuclides, most of the supply to the ocean immediately after the accident (8-21 PBq) was estimated to be due to processes (1) and (2). The amount of $$^{137}$$Cs accumulated on the seafloor is only about 1% (0.2PBq) of the amount carried to the ocean, but it remains in the sediments in the coastal area for a long period of time and gradually migrates to the seawater and ecosystems near the seafloor.

Journal Articles

Impact of soil erosion potential uncertainties on numerical simulations of the environmental fate of radiocesium in the Abukuma River basin

Ikenoue, Tsubasa; Shimadera, Hikari*; Kondo, Akira*

Journal of Environmental Radioactivity, 225, p.106452_1 - 106452_12, 2020/12

https://doi.org/10.1016/j.jenvrad.2020.106452
 Times Cited Count：5 Percentile：18.67(Environmental Sciences)

This study focused on the uncertainty of the factors of the Universal Soil Loss Equation (USLE) and evaluated its impacts on the environmental fate of $$^{137}$$Cs simulated by a radiocesium transport model in the Abukuma River basin. The USLE has five physically meaningful factors: the rainfall and runoff factor (R), soil erodibility factor (K), topographic factor (LS), cover and management factor (C), and support practice factor (P). The simulation results showed total suspended sediment and $$^{137}$$Cs outflows were the most sensitive to C and P among the all factors. Therefore, land cover and soil erosion prevention act have the great impact on outflow of suspended sediment and $$^{137}$$Cs. Focusing on land use, the outflow rates of $$^{137}$$Cs from the forest areas, croplands, and undisturbed paddy fields were large. This study indicates that land use, especially forest areas, croplands, and undisturbed paddy fields, has a significant impact on the environmental fate of $$^{137}$$Cs.

Oral presentation

Analysis of suspended solids discharge in the Kako River basin during flood events by using water quality model incorporated with soil erosion process

Koike, Taichi*; Ikenoue, Tsubasa; Koga, Yutaro*; Matsuo, Tomohito*; Shimadera, Hikari*

no journal, , 

Accurate estimation of nutrient load from land is necessary for nutrient management in the enclosed coastal sea of Harimanada with oligotrophication problem. We aim to accurately estimate the nutrient load including the rapid increase during flood events in the Kako River basin, which has the largest basin area among the rivers flowing into the Harimanada. In this study, we estimated suspended solid (SS) load during flood events using a water quality model incorporated with the soil erosion process and evaluated its accuracy. As a result, although there is room for improvement in the height of the peak and the timing, the increase in SS load during Flood Events was generally reproduced.

Oral presentation

Uncertainty assessment of environmental fate of radioactive cesium in the river basins of Fukushima Prefecture through long-term simulation

Nittono, Akira*; Ikenoue, Tsubasa; Shimadera, Hikari*; Matsuo, Tomohito*; Kondo, Akira*

no journal, , 

The Fukushima Daiichi Nuclear Power Plant accident caused extensive $$^{137}$$Cs contamination. Long-term simulations have been conducted to predict the amount of $$^{137}$$Cs outflow from river basins to the ocean. However, there are uncertainties about the influences of rainfall, tree types and dams to the $$^{137}$$Cs outflow. This study focused on how much these three factors influence the $$^{137}$$Cs outflow in eastern Fukushima river basins from 2011 to 2040. The $$^{137}$$Cs outflow varied by -10 to +17% depending on precipitation, and by -16 to +19% depending on tree species, decreased by 21% by dams.

Oral presentation

Prediction of radiocesium behavior near the seafloor in coastal areas considering the impacts of rivers

Ikenoue, Tsubasa

no journal, , 

We report the results of a recent simulation study by JAEA on the behavior of radiocesium ($$^{137}$$Cs) released by the Fukushima Daiichi Nuclear Power Plant accident (1F accident) near the coastal seafloor. The $$^{137}$$Cs supply from rivers could be an important process for the long-term behavior of $$^{137}$$Cs in seabed sediment. In this study, we conducted a simulation to predict the behavior of $$^{137}$$Cs in seabed sediment by combining an ocean dispersion model with a river model. Based on the simulation results, we explain the impact of $$^{137}$$Cs supply from rivers on the long-term behavior of $$^{137}$$Cs in seabed sediment.

Oral presentation

Analysis of oceanic dispersal of $$^{137}$$Cs derived from Fukushima Dai-ichi Nuclear Power Plant using the short-term emergency assessment system of marine environmental radioactivity

Ikenoue, Tsubasa; Kawamura, Hideyuki; Kamidaira, Yuki

no journal, , 

In this study, we analyzed the characteristics and trends of oceanic dispersal of $$^{137}$$Cs by performing the oceanic dispersion simulations using past oceanographic data received on the Short-Term Emergency Assessment system of Marine Environmental Radioactivity (STEAMER) developed at JAEA. Oceanic dispersion simulations in 1461 cases were carried out in the northwestern Pacific Ocean for 60 days from every day between 2015 and 2018. It was assumed that $$^{137}$$Cs was released into the ocean from the Fukushima Dai-ichi Nuclear Power Plant. In all calculation cases, the maximum concentration in 30 days at each computational grid (maximum concentration distribution) was calculated. $$^{137}$$Cs in the ocean surface in winter tended to be not dispersed so much. In the ocean surface, the seasonal variation was greater than the annual variation though the maximum concentration distributions have little annual variation. The average of the maximum concentration distribution in all calculation cases was inclined to be high from the coast of Fukushima to the offshore (140$$^{circ}$$E-145$$^{circ}$$E) and near the Kuroshio Extension.

Oral presentation

Analysis of environmental fate of radiocesium in the Abukuma River basin for 30 years after the Fukushima Dai-ichi Nuclear Power Plant accident

Ikenoue, Tsubasa; Shimadera, Hikari*; Kondo, Akira*

no journal, , 

We evaluated the environmental fate of $$^{137}$$Cs in the Abukuma River Basin from 2011 to 2040 by using a distributed radiocesium prediction model. The model well reproduced interannual variations in the concentration of $$^{137}$$Cs in the suspended sediment and the total outflow of $$^{137}$$Cs at Iwanuma near the estuary. Thirty years after the Fukushima Daiichi Nuclear Power Plant accident, it was estimated that the concentration of $$^{137}$$Cs in the suspended sediment decreased to 1.9% of the concentration in the earliest phase after the accident and the total outflow of $$^{137}$$Cs was equivalent to 4.6% of the initial deposition in the Abukuma River basin.

Oral presentation

Uncertainty analysis on soil erosion parameters for radiocesium transport in the Abukuma River basin

Ikenoue, Tsubasa; Shimadera, Hikari*; Kondo, Akira*

no journal, , 

This study focused on the uncertainty of the factors of the USLE and evaluated its impacts on the environmental fate of $$^{137}$$Cs simulated by a radiocesium transport model in the Abukuma River basin. In the USLE, soil loss is expressed as a product of factors representing rainfall (R factor), geology (K factor), topographic (L $$times$$ S factor) and land cover and soil erosion prevention act (C $$times$$ P factor). The simulation results showed total outflows of suspended sediment and $$^{137}$$Cs were the most sensitive to C $$times$$ P factor among the all factors. Therefore, land cover and soil erosion prevention methods have the great impact on outflow of suspended sediment and $$^{137}$$Cs.

Oral presentation

Long-term prediction of environmental fate of $$^{137}$$Cs in ocean considering migration process of $$^{137}$$Cs in a lower trophic level ecosystem

Ikenoue, Tsubasa; Nakanishi, Takahiro; Kawamura, Hideyuki

no journal, , 

The accident at the Fukushima Daiichi Nuclear Power Plant in March 2011 resulted in the release of radionuclides in the marine environment. Among the radionuclides released, since large quantities of $$^{137}$$Cs were released and $$^{137}$$Cs has a long half-life, there is concern about the impacts of $$^{137}$$Cs on marine environment. Most of $$^{137}$$Cs is dissolved in seawater, but some $$^{137}$$Cs is taken in and adsorbed on biogenic particles (phytoplankton, zooplankton, and detritus). In this study, we aim to predict the long-term fate of $$^{137}$$Cs in ocean considering migration process of $$^{137}$$Cs in a lower trophic level ecosystem. We improved an oceanic dispersion model by coupling with a lower trophic level ecosystem model and conducted the long-term simulation of $$^{137}$$Cs in ocean. This improvement enabled the long-term prediction of $$^{137}$$Cs concentrations in biogenic particles and organic seabed sediment as well as in seawater. The calculation results suggested that the adsorption process from seawater to organic sediment had a much greater impact on $$^{137}$$Cs concentration in organic seabed sediment than the sedimentation process of biogenic particles.

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