By combining long monitoring data (> halftime of Cesium 137 after the Chernobyl Accident in 1986, 15 years after the Fukushima Accident in 2011, and other events), we can improve our knowledgebase on the environmental behavior of radioactive materials and its environmental/biological impact. This should lead to improved monitoring systems in the future including emergency response systems, acute sampling/measurement methodology, and remediation schemes for any future nuclear accidents. Furthermore, as part of the decommissioning of the Fukushima Daiichi Nuclear Power Station, the discharge of ALPS-treated water is being carried out, which has attracted international attention. The discharge rate is published in real time and monitoring is being conducted, providing a valuable opportunity for analyzing the behavior of radionuclides in the ocean. In addition, past nuclear contamination events and other data sets also welcome.
The following specific topics have traditionally been discussed:
(a) Atmospheric Science (emissions, transport, deposition, pollution);
(b) Hydrology (transport in surface and ground water system, soil-water interactions);
(c) Oceanology (transport, bio-system interaction);
(d) Soil System (transport, chemical interaction, transfer to organic system);
(e) Forestry;
(f) Natural Hazards (warning systems, health risk assessments, geophysical variability);
(g) Measurement Techniques (instrumentation, multipoint data measurements);
(h) Ecosystems (migration/decay of radionuclides).
Orals: Tue, 5 May, 14:00–15:45 | Room -2.92
Overview of the observation and simulation studies regarding radiocesium resuspension from contaminated land surfaces in Fukushima is presented based on our previous papers, Kajino et al., ACP (2016), Kajino et al., ACP (2022), Watanabe et al., ACP (2022). The long-term atmospheric behaviors of radiocesium have been understood based on the long-term measurements of concentration and deposition of radiocesium in Fukushima city (Watanabe et al., 2022) and numerical simulations considering radiocesium resuspension from soil and vegetation (Kajino et al., 2022). However, there is still one unresolved issue remains: exceptionally high monthly cumulative deposition amounts in January in Fukushima city even though the monthly atmospheric concentrations are not very large. We therefore hypothesized that the giant aerosol resuspension due to snow removal work or passing vehicles that carried radiocesium deposited in the vicinity of the observation site into the deposition sampler, but not into the air sampler, since the gravitational velocity of such giant aerosols is too high to collect by the air sampler. This additional source is referred to as secondary resuspension. The numerical assessment and field observations of the secondary resuspension will also be presented at the conference.
How to cite: Kajino, M.: Resuspension of radiocesium from contaminated land surfaces in Fukushima: source contributions from soil, vegetation, and other sources, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8529, https://doi.org/10.5194/egusphere-egu26-8529, 2026.
Wildfires in radioactively contaminated regions, such as the Chernobyl Exclusion Zone, pose a growing environmental threat by resuspending long-lived radionuclides into the atmosphere. However, accurately quantifying the redistribution of these radionuclides remains challenging. Existing top-down inversion studies often oversimplify source terms by assuming fixed particle sizes and release altitudes, which hinders the precise evaluation of transport mechanisms and deposition footprints.
To address this gap, this study proposes a novel multi-component source term inversion framework to simultaneously reconstruct the time-varying release profiles of 137Cs across multiple particle sizes (0.4, 8, and 16 μm) and seven vertical layers (0-3000 m). We improved the Projected Alternating Minimization with L1-norm and Total variation regularization (PAMILT) algorithm by incorporating a TV-regularized initialization and a Bayesian optimization scheme for hyperparameter tuning to ensure robust convergence. These retrieved source terms were then coupled with the WRF-Chem model using size-resolved microphysics to conduct a high-resolution simulation of the April 2020 Chernobyl wildfires.
Validation results demonstrated exceptional agreement between the simulated and observed concentrations, achieving a Pearson correlation coefficient of 0.996 and reducing maximum relative biases from over 106 to generally below 102. The inversion estimates a total 137Cs release of approximately 836 GBq. This release was dominated by fine particles (0.4 μm, ~54%) and low-altitude injections, with 58.1% occurring below 1 km. Crucially, our WRF-CHEM simulations reveal a decoupling between emission abundance and deposition impact. Although fine particles dominate the source term, coarse particles (16 μm) control the near-field deposition flux due to rapid gravitational settling. These coarse particles exhibit a "transport plateau" beyond roughly 800 km, whereas fine particles show a linear growth in transport distance constrained only by meteorological dispersion. Furthermore, we identified distinct deposition signatures. Dry deposition manifests as a continuous spatial accumulation or "creeping" effect. In contrast, wet deposition drives "step-wise" long-range transport, triggering sudden and pulse-like removal events far from the source.
These findings provide critical insights into the complex mechanics of radionuclide redistribution and offer a refined methodology for assessing the environmental impact of future wildfire events in contaminated zones.
How to cite: Xu, Y. and Fang, S.: Unraveling size-resolved 137Cs resuspension and deposition from the 2020 Chernobyl Wildfires via multi-component inversion and WRF-Chem simulation, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2436, https://doi.org/10.5194/egusphere-egu26-2436, 2026.
Nuclear accidents contaminate large terrestrial areas with long-lived radionuclides, and river systems play a key role in their redistribution. The concentration of dissolved radiocaesium (¹³⁷Cs) in river water is influenced by catchment-scale physical and geochemical characteristics. After the Chernobyl accident, environmental radionuclide concentrations generally declined over time; however, systematic inter-river comparisons remain limited, and the key factors controlling long-term differences in dissolved ¹³⁷Cs concentrations are still poorly understood.
In this study, we investigated the environmental behavior of dissolved ¹³⁷Cs in river systems affected by the Fukushima Daiichi Nuclear Power Plant accident and compared it with long-term observations from major European rivers impacted by the Chernobyl accident. In Fukushima, river water samples were seasonally collected between 2021 and 2024 from headwater catchments in the Yamakiya and Kuchibuto River basins. Samples were filtered through 0.22 µm membranes, and dissolved ¹³⁷Cs was measured using high-purity germanium detectors. Major ions (K⁺, NH₄⁺), stable ¹³³Cs, and dissolved organic carbon (DOC) were also analyzed. Univariate and multivariate regression analyses were applied to identify dominant release mechanisms. Catchment land cover, topographic gradients, and precipitation were analyzed using GIS, and groundwater residence times were estimated. These results were compared with long-term monitoring data and additional field measurements from nine European river catchments in Ukraine, Finland, Austria, and Italy, incorporating climatic, vegetation, and anthropogenic factors into an international comparison framework.
In Fukushima headwater catchments, dissolved ¹³⁷Cs concentrations increased from summer to autumn, coinciding with rising temperatures, enhanced organic matter decomposition, and increased K⁺ availability. Multiple regression analysis identified ¹³³Cs and K⁺ as significant explanatory variables, indicating that ion exchange plays a key role in ¹³⁷Cs mobilization. In contrast, DOC showed only a weak relationship with ¹³⁷Cs in Fukushima rivers. Comparative analysis of dissolved ¹³⁷Cs trends since 1986 revealed that European rivers have maintained higher concentrations over longer periods. Correlation analysis demonstrated that DOC and ¹³³Cs were significant scaling factors controlling dissolved ¹³⁷Cs concentrations across European river systems, whereas K⁺ and NH₄⁺ contributed little to concentration variability.
These results indicate that differences in the long-term behavior of dissolved ¹³⁷Cs between Fukushima and European rivers are associated with contrasting DOC- and ¹³³Cs-related controls at the catchment scale. This study suggests that accounting for regional variability in biogeochemical controls should be useful for long-term river environment and also can inform environmental modeling of radionuclide transport under nuclear emergency conditions, contributing to improved preparedness and long-term risk assessment.
How to cite: Onda, Y., Igarashi, Y., Smith, J., Sakaguchi, A., Fan, S., and Takahashi, J.: Why Are Dissolved ¹³⁷Cs Concentrations Lower in Fukushima Rivers? A Comparative Study with European Catchments, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18568, https://doi.org/10.5194/egusphere-egu26-18568, 2026.
Following the Fukushima Daiichi Nuclear Power Plant accident, radioactive substances such as radiocaesium (137Cs) were widely dispersed and contaminated soils, raising concerns about their transfer from soil to crops. 137Cs transfer is primarily regulated by exchangeable potassium (KEx), a chemically analogous element, but its effectiveness varies across environmental conditions such as soil type and land-use. Recent studies suggest that soil exchangeable 137Cs (137CsEx) dynamics and its solid–liquid partitioning play key roles in predicting 137Cs transfer irrespective of regional differences. In contrast, current direct methods for measuring 137Cs are costly and time-consuming, making them unsuitable for rapid risk assessment. As an alternative approach for risk management, mid-infrared spectroscopy (MIRS) may provide a rapid and cost-effective means of estimating soil properties. Recently, models for predicting soil KEx concentrations from spectral data have been reported. However, their applicability to 137Cs transfer remains unclear. In this study, we aimed to construct prediction models for the ratio of soil 137CsEx to soil total 137Cs (137CsTotal) using MIRS spectra and to evaluate the variability of model performance among soil or land-use categories.
1249 soil samples collected in Fukushima Prefecture, Japan, from 2015 to 2020, were analyzed for soil properties, including soil total C, 137CsEx, and 137CsTotal, through MAFF and NARO in Japan. Each soil sample was analysed after drying at 37°C for at least 12 hours and being sieved to less than 0.2 mm before measurement. Mid-infrared spectra for these samples were obtained at the FAO/IAEA Soil and Water Management and Crop Nutrition Laboratory over the wavenumber range of 650–4000 cm–1 using four replicate measurements per sample. Using noise-removed spectral data, partial least squares regression models were developed to predict the ratio of soil 137CsEx to 137CsTotal. In addition, prediction models were constructed for different soil types (andosol, brown forest soil, lowland soil, and peat soil) and land-use categories (upland fields and paddy fields), and their differences in model performance were evaluated.
Prediction models were constructed and achieved moderate predictive performance (R² around 0.6). In contrast, by stratifying prediction models by soil type, prediction accuracy improved for all soil types except for peat soil relative to the non-stratified model. In particular, andosol showed the highest prediction accuracy. Comparison of variable importance in projection (VIP) scores among these models showed that the contributions of specific wavenumber ranges to model performance differed among soil types. In andosols, VIP scores were higher in wavenumber ranges associated with carbohydrates, quartz, and clay minerals compared with the model constructed using all data. These results suggest that soil type specific mineralogical composition and carbon content may play roles in improving prediction performance. Furthermore, predictions stratified by land-use showed higher accuracy in upland fields than in paddy fields. Differences of VIP scores between them were also observed in wavenumber ranges associated with carbohydrates and clay minerals. These results suggest that environmental conditions, such as soil redox status, may influence prediction accuracy through their effects on soil minerals and carbon.
How to cite: Murashima, K., Iwai, J., Dercon, G., Vezzone, M., Vlasimsky, M., Albinet, F., Maruyama, H., and Shinano, T.: Performance Variability of Mid-Infrared Spectroscopy–Based Predictions of Soil Radiocaesium Dynamics across Diverse Soil and Land Use Conditions, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17315, https://doi.org/10.5194/egusphere-egu26-17315, 2026.
Since the commencement of the ALPS-treated water discharge from Fukushima Daiichi on 23 August 2023, an operational forecasting system developed in Taiwan has been established to provide daily seven-day predictions of tritium dispersion in the North Pacific. The system integrates the real-time CWA-OCM with particle tracking and grid-based concentration diffusion modules, driven by hourly discharge data reported by TEPCO. The computational domain covers the Kuroshio–Kuroshio Extension and adjacent marginal seas, with refined resolution near the outlet to capture dispersion within approximately 3 km. Validation against TEPCO tritium monitoring data at 12 sites across three representative batches (1, 2 and 12) demonstrated that the system successfully reproduced both the spatial distribution and temporal evolution of tritium concentrations, with modeled maxima typically within the observed range of 10–20 Bq/L. However, the model slightly underestimated the peak values, and simulated concentrations decreased more rapidly than observed during the five-day post-discharge period. This discrepancy is likely attributed to the absence of the jet effect in the current model. Therefore, we will continue to refine the model and integrate these improvements into our operational forecasting system.
How to cite: Zeng, H.-T., Teng, J.-H., and Chiang, Y.: Validation of the Refined Daily Forecasting System for ALPS Treated Water Dispersion Against Observation Data near the Fukushima Outlet, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4581, https://doi.org/10.5194/egusphere-egu26-4581, 2026.
The Fukushima Daiichi Nuclear Power Plant (FDNPP) disaster, triggered by the tsunami after the massive earthquake on March 11, 2011, which led to the accumulation of vast quantities of contaminated water used for emergency cooling. Although containment measures were implemented to prevent leakage, the on-site storage facilities approached full capacity. Consequently, Japan announced plans to disposal the Advanced Liquid Processing System (ALPS) treated wastewater, which removes most radionuclides except tritium. TEPCO officially began discharging the treated water on August 24, 2023, diluted with seawater, into the Pacific Coast via a submerged outfall located 1 km offshore at a depth of 13 meters. This decision raised significant concerns among the publics of neighboring countries regarding marine safety. In response, Taiwan established a specialized task force to monitor and to predict the consequences by developing an operational forecast model system to monitor the discharge and provide daily predictions of radioactive dispersion in the Western North Pacific.
The system integrates a three-dimensional hydrodynamic model (CWA-OCM-FH), an extension of the existing operational model CWA-OCM, with a transport model driven by the simulated currents. In order to capture the influence of the Kuroshio Current and the Extension on the transport of tritiated water, the model domain was expanded to 180°E. An unstructured mesh is employed to resolve complex topographic features. The grid resolution varies from approximately 1 km in the coastal zone to less than 20 meters near the discharge outfall, ensuring a representation of spatiotemporal variations in the near-field flow.
To ensure the reliability of the flow fields driving the dispersion, the hydrodynamic model underwent rigorous validation using tide gauge data and ADCP observations. Harmonic analysis on both the observed and simulated data for data for calibration and verification.
Driven by the verified flow fields, a 3D Lagrangian particle tracking model simulates the dispersion pathways of the tritiated water. These computed trajectories provide the essential spatial distribution data required for calculating subsequent concentration. Simulation results indicate that while the primary transport direction follows the Kuroshio Extension and North Pacific Current eastward, mesoscale eddies induce significant cross-stream transport. Therefore, the contaminated particles could potentially influencing waters near Taiwan.
The model has been verified with observations utilize quantitative metrics such as the Pearson correlation coefficient (R value), coefficient of determination (R²), and Root Mean Square Error (RMSE) over a period exceeding one year. Validation using data from tide gauge stations, ARGO drifter profiles, AVISO satellite altimetry geostrophic currents, and GHRSST sea surface temperature satellite data will be presented and discussed in the paper.
How to cite: Wang, C.-H., Cheng, H.-Y., Yu, J. C. S., Zeng, H.-T., and Teng, J.-H.: An Operational Modeling System Forecasting the Disposal of Fukushima Tritiated Water and Transport: A Lagrangian Particle Tracking System Driven by High-Resolution Hydrodynamics., EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14564, https://doi.org/10.5194/egusphere-egu26-14564, 2026.
The Fukushima Dai-ichi Nuclear Power Station (F1NPS) accident released radionuclide was believed to circulate in the North Pacific Ocean and was suggested to arrive at East China Sea ECS (Aoyama et al., 2022). Temporal fraction of 137Cs from F1NPS was estimated to be 0.5 mBq l-1 at ECS in 2023 (Inomata et al., 2023). Appeared 137Cs radioactivity 1.4 mBq l-1 off Okinawa seawater in 2025 seems to suggest still having contribution of 0.6 mBq l-1 as F1NPS originated fraction even after 14 years of the accident, compared to assumed global fallout originated level 0.8 mBql-1 in ECS at 2025 (ENVRDB, 2025). This level was suggested to be caused by recirculation of 137Cs within the north western Pacific waters by Subtropical Mode Water (STMW) and Central Mode Water (CMW)(Kumamoto et al., 2025). Similarly, F1NPS-137Cs may be brought by North Equatorial Current (NEC) within 10-18 years (Chen et al., 2023). However, in contrast, there is other possibility as depuration delay of global fallout-137Cs in surface water by depression of vertical mixing to deeper layer due to high surface water temperature after 2010 as observed global warming. Since F1NPS-orginated 134Cs originated F1NPS almost decayed and being difficult to detect, it is still unknown the precise contribution rate of F1NPS-137Cs compared to global fallout 137Cs fractions. Possible method to derive F1NPS fraction may be using fish muscle which has approximately 50-100 times greater radioactivity in equivalent sample size. Successful detection of F1NPS-originated radio-caesium will is expected not to understand up-to date ocean circulation environment, but also to find the usefulness of bioindicator as oceanic tracer.
How to cite: Tateda, Y., Takata, H., Inomata, Y., Hamajima, Y., and Bezhenar, R.: Possibility of radionuclide originated from the Fukushima accident as oceanic tracer by fish muscle as bio-indicator, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15352, https://doi.org/10.5194/egusphere-egu26-15352, 2026.
The size distribution (SZ) of radioactive aerosols emitted after nuclear accident at nuclear power plants plays a crucial role in assessment of the subsequent atmospheric transport and deposition. However, in reality this distribution in the source is usually unknown. The SZ of particles in the plume also changes with travel time of the plume, because the coarser particles fall out more rapidly than the finer particles. Hence when the measurements of SZ are undertaken at certain distances from the source the SZ could be already altered by plume travel time while it is SZ in the source which is required by atmospheric transport models (ATMs) for simulation of radionuclides atmospheric dispersion and deposition. Also, SZ measurements are usually not available in real time during the accident. More readily available measurements are airborne concentrations. Hence when concentration measurements are available, the SZ parameters of ATMs could be fitted to achieve better agreement between model and measurements.
In this work, the inverse problem is stated to identify the optimal set of size distribution parameters of the Fukushima source term – activity-averaged mean aerodynamic diameter (d) and geometric standard deviation (σ) which best fit results of FLEXPART ATM to both, local and global measurements datasets. The problem is formulated as multi-objective optimization in which two objective functions. The first objective function J1 corresponds to model deviations from measurements in the territory of Japan, while the second objective function J2 corresponds to model deviations from the global observations of CTBTO measurement stations. The combined cost function J=J1J2 , characterizing model deviation against measurements in both datasets was also considered. In this way, the estimate of the unknown SZ parameters, which fits both local and global concentration observations is to be found. The method of finding Pareto solution of such multi-objective optimization problem was developed and preliminary results of comparisons of the estimated SZ parameters with SZ measurements, performed following Fukushima accident were obtained.
The solution of the stated problem leads to reasonable results. The simulations with small values of 1≤σ≤2 led to excellent agreement of estimated mean aerodynamic diameter d of emitted particles between 2 and 3 μm with available measurements of SZ. At the same time if large values of σ were allowed the resulting estimated mean aerodynamic diameter could significantly deviate from the observed values. The use of the small values of mean aerodynamic diameter (d <1μm) in turn did not allow for the minimization of the combined cost function J.
How to cite: Jung, K. T., Kim, J.-H., and Kovalets, I.: Inverse estimation of size-distribution parameters of emitted aerosols following the Fukushima accident using FLEXPART simulations and measurements, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15588, https://doi.org/10.5194/egusphere-egu26-15588, 2026.
The radionuclides 129I and 137Cs released during the 2011 Fukushima nuclear accident led to contamination of forested environments. The concentrations of these nuclides in precipitation, as well as their subsequent environmental behavior, are critical for assessing internal radiation exposure. In this study, deposition records of atmospheric 129I and 137Cs following the accident were reconstructed using a borehole drilled between 2012 and 2014 at Koriyama, located approximately 60 km from the accident site. After subtraction of contributions from global fallout and nuclear reprocessing facilities, the inventories of 129I and 137Cs in forest soil at Koriyama, integrated to a depth of 50 cm, were estimated to be 4.80 × 105 and 81.7 mBq m−2, respectively. The 129I/137Cs radioactivity ratio derived from Fukushima-derived deposition in litter and soil (0–50-cm depth) was 1.71 × 10−7, which is consistent with the ratio observed in atmospheric aerosols at the time of the accident. The 129I/137Cs radioactivity ratio in the litter layer was marginally lower than that in the underlying topsoil. This difference is attributed to the higher solubility and mobility of 129I relative to 137Cs in litter, resulting in preferential washout from the surface layer. It is therefore inferred that a fraction of 129I originally retained in the litter layer has migrated from the forest surface toward riverine systems.
How to cite: Ohta, T., Mahara, Y., Matsuzaki, H., Hayami, H., and Tsumune, D.: Deposition of 129I in the forests of Koriyama and 129I/137Cs ratio originated from the Fukushima Daichi Nuclear Power Plant, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16674, https://doi.org/10.5194/egusphere-egu26-16674, 2026.
Chornobyl remains the world’s longest-running field-scale experiment of how societies and ecosystems respond to persistent, spatially heterogeneous contamination. Yet sustainability-relevant synthesis across environmental compartments—soils, forests, surface waters, groundwater, and the evolving exposure landscape—remains fragmented, often separated into radiological, ecological, or regulatory discussions. Here we integrate four decades of observations in and around the Chornobyl Exclusion Zone to evaluate what has changed, what has not, and what this implies for sustainable land and resource use under long-lived hazards. We assess four compartment-linked insights: (i) soils as the primary long-term reservoir of fallout: inventories of 137Cs and 90Sr have declined but remain highly heterogeneous, while vertical redistribution and particle-associated processes increasingly govern mobility and bioavailability; (ii) forests as both sink and pathway: radionuclides are continuously recycled through litter and biomass, and contrasting within-tree distributions of 137Cs versus 90Sr impose distinct constraints on wood utilization and circular-economy strategies; (iii) aquatic systems as delayed but persistent exporters: multi-decadal river records exhibit long tails and sensitivity to disturbances (e.g., floods, fires), while groundwater pathways—especially for 90Sr—represent enduring, often weakly observed legacy with clear management relevance; and (iv) exposure landscapes that evolve nonlinearly: spatiotemporal changes in dose fields complicate re-zoning decisions that depend on both scientific evidence and societal acceptance. We synthesize these findings into a sustainability framework that links environmental dynamics to governance choices, including conditional resource use, monitoring prioritization, and intergenerational risk trade-offs. These lessons generalize to other nuclear accidents and to broader classes of persistent contaminants where returning to baseline is unrealistic and sustainability must be designed under enduring constraints.
How to cite: Igarashi, Y., Yoschenko, V., Onda, Y., Protsak, V., Laptev, G., Holiaka, D., Samoilov, D., Kirieiev, S., Konoplev, A., and Smith, J.: Four decades after Chornobyl: long-lived radionuclide legacy and sustainable land and resource use , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8971, https://doi.org/10.5194/egusphere-egu26-8971, 2026.
The society acquired vast amounts of data from past major nuclear accidents, then learned the causes of those accidents, the methods to mitigate their adverse effects, and accident-prevention measures. However, it is challenging to store and organize highly technical knowledge related to nuclear accidents and share it in ways that meet our purposes. That is one reason we still do not have a centralized public database of nuclear incidents, despite efforts by international organizations such as the IAEA and academic institutions. Internet searches and AI queries return answers based on publicly available data sources without curation, thereby posing a risk of biased knowledge representation.
We aim to develop a prototype nuclear accident knowledge base using an ontology-based approach to establish the structured management system of nuclear accident-related knowledge. The top-level classes of the Basic Formal Ontology (BFO) are reviewed and utilized to design the base ontology hierarchy of the entities involved in nuclear accidents. The past ontology work in the nuclear and non-nuclear industries is reviewed, and some of their proposed classes and relationships were imported into the nuclear accident knowledge base structure. The classes, entities, and relations among those entities, and data properties relevant to the knowledge base are defined and are entered in protégé ontology editing software, whose ontology design can be shared digitally with interested parties.
During the development of the ontology structure, five knowledge-ambiguity factors were identified as potential focal points for developing the nuclear accident knowledge base. The ambiguity factors include: 1) terminology definition, 2) location definition, 3) temporal change in knowledge needs, 4) contamination definition, and 5) accident cause definition. When sharing nuclear accident knowledge, these factors must be considered to minimize confusion during the user’s knowledge-finding endeavour. By dissecting those ambiguity factors and providing a logical structure for nuclear accident-related data, this prototype knowledge base will assist in developing a public centralized nuclear accident knowledge base that can serve as a trustworthy data depository for preventing future accidents as well as enabling prompt recovery from the adverse effects of those accidents.
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Booshehri, M., Emele, L., Flügel, S., Förster, H., Frey, J., Frey, U., 2021. Introducing the open energy ontology: Enhancing data interpretation and interfacing in energy systems analysis. Energy and AI 2021; 5: 100074. https://doi.org/10.1016/j.egyai.2021.100074
Rashdan, A., Browning, J., Ritter, C., 2019. Data Integration Aggregated Model and Ontology for Nuclear Deployment (DIAMOND): Preliminary Model and Ontology. Idaho National Laboratory. https://doi.org/10.2172/2439922
Sorokine, A., Schlicher, B.G., Ward, R.C., Wright, M.C., Kruse, K.L., Bhaduri, B., Slepoy, A., 2015. An interactive ontology-driven information system for simulating background radiation and generating scenarios for testing special nuclear materials detection algorithms. Engineering Applications of Artificial Intelligence 43. 157-165. https://doi.org/10.1016/j.engappai.2015.04.010
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How to cite: Yasumiishi, M. and Bittner, T.: Developing Ontology-Based Nuclear Accident Knowledge Base, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-22532, https://doi.org/10.5194/egusphere-egu26-22532, 2026.
Since the accident at the Fukushima Daiichi Nuclear Power Station in March 2011, numerous studies have examined the behavior of radioactive cesium (¹³⁷Cs) in the ocean. Recent studies suggest that large amounts of particulate ¹³⁷Cs deposited on land are transported to coastal waters via rivers, becoming a major source of coastal ¹³⁷Cs input. Although numerical simulations and conceptual studies indicate that particulate ¹³⁷Cs entering coastal waters can be transported offshore through sedimentation, resuspension, and lateral transport, long-term, high-frequency observational studies remain limited. In this study, we evaluated the transport of particulate ¹³⁷Cs in coastal area off Fukushima using one year of continuous measurement data from multiple moored systems.
Moored systems were deployed at three sites near the mouth of the Ukedo River, where current velocity, current direction, and turbidity were continuously measured from February 2017 to February 2018. These data were combined with regularly collected measurements of suspended solid concentrations (mg/L) and particulate ¹³⁷Cs concentrations (Bq/L) to estimate hourly lateral fluxes of particulate ¹³⁷Cs (Bq/h). The study area is influenced by ¹³⁷Cs inputs transported via the Ukedo River, and the relationships between particulate ¹³⁷Cs fluxes and seasonal variability, meteorological conditions (waves, precipitation, and wind), and river discharge were analyzed. Furthermore, by focusing on differences in fluxes among the observation sites, we examined the factors controlling riverine input and transport variability from the coastal area toward offshore.
This study uses long-term monitoring data off Fukushima to improve understanding of particulate ¹³⁷Cs transport processes in coastal waters and to provide observational constraints for future numerical modeling studies.
How to cite: Satoh, S., Yoshimura, K., Misonou, T., and Tsumune, D.: Transport of Particulate ¹³⁷Cs in the Coastal Area off Fukushima Based on Long-Term Continuous Measurement, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3623, https://doi.org/10.5194/egusphere-egu26-3623, 2026.
The Fukushima Daiichi Nuclear Power Plant (FDNPP) accident contaminated large areas of the Pacific Ocean with different radionuclides. However, not all areas are studied equally. For example, the Sea of Okhotsk is one of the least studied regions, with almost no measurement data available. In the current study, we applied the Lagrangian particle tracking model Parcels V3.0 to simulate the trajectories of virtual particles containing radionuclides in the Pacific Ocean. Here, the output from the KIOST-MOM circulation model is used. It includes monthly mean climatic data for 3D currents (U, V, W components of water velocity) and vertical diffusivity coefficients. Coefficients for horizontal diffusion are calculated using the Smagorinsky formula.
Virtual particles were emitted at the location of the FDNPP during 31 days (26 Mar to 25 Apr 2011), when 96.6% of the total amount of radionuclides was released directly to the ocean. Each particle initially contained a certain activity of radionuclides (137Cs, 134Cs, 90Sr, 3H, 129I) proportionally to the estimated total release of each radionuclide. The activity of each radionuclide inside the particle decreased according to radioactive decay with the corresponding half-life. The atmospheric deposition of radionuclides on the sea surface was not considered here.
Model results were validated on the 134Cs concentrations in the Northeastern Pacific in areas with measurement data after 2012, when the impact of atmospheric deposition decreased. For the Sea of Okhotsk, the concentrations of 5 radionuclides were calculated and analyzed. For particles that reached the Sea of Okhotsk, we calculated statistical characteristics based on Lagrangian trajectories: visitation frequency, mean age, and representative trajectory, which demonstrated the pathways of water masses transporting radioactivity from FDNPP to the Sea of Okhotsk.
How to cite: Bezhenar, R., Tateda, Y., Inomata, Y., Kim, K. O., and Kim, H.: Lagrangian trajectories of Fukushima Daiichi NPP originated water, transported by large-scale circulation in the North Pacific Ocean, and reached the Sea of Okhotsk, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9903, https://doi.org/10.5194/egusphere-egu26-9903, 2026.
The Luzon Strait serves as a critical conduit between the Western North Pacific and the South China Sea (SCS), through which water-mass exchange plays a key role in regulating regional heat budgets and primary productivity. While surface exchange processes have been known well, subsurface intrusion dynamics—particularly those associated with Subtropical Mode Water (STMW)—remain poorly understood. In this study, we investigate the pathways and transport timescales of STMW intrusion through the Luzon Strait by employing radiocesium 137Cs released during the 2011 Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident as a transient tracer. A three-dimensional Regional Ocean Modeling System (ROMS) was used to simulate the long-term dispersion of 137Cs from the North Pacific into the SCS. The results show that the 137Cs within the STMW layer reached the Luzon Strait approximately seven years after the accident, notably earlier than surface circulation. The net flux of 137Cs into the SCS exhibits seasonal variability, with enhanced inflow during winter, primarily driven by horizontal advection and variations in Kuroshio intrusion behavior. A comparison of different intrusion modes indicates that the leaking path yields a substantially larger net inflow of radiocesium into the SCS than either the looping or leaping paths. Given that the SCS serves as a gateway to downstream marginal seas—including the East China Sea, Yellow Sea, and Japan/East Sea—these findings provide important insights into basin-scale transport processes of Pacific-derived tracers and their potential ecological implications.
How to cite: Lee, S.-T., Cho, Y.-K., Kim, K. O., and Seo, S.: The Role of Subtropical Mode Water in the Subsurface Transport of Fukushima-derived 137Cs into the South China Sea, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15000, https://doi.org/10.5194/egusphere-egu26-15000, 2026.
Fifteen years after the Fukushima Daiichi Nuclear Power Plant (F1NPP) accident, 137Cs and 3H activities in coastal waters near the plant remain elevated compared to surrounding regions, indicating persistent radioactive inputs. While concentrations within the port are highest, recent estimates suggest that the leakage rate outside the port exceeds that inside, implying the presence of an additional or previously unrecognized source outside the F1NPP site. However, the mechanisms governing these releases remain unclear.
The 3H/137Cs activity ratio is a useful tracer for identifying contamination sources, as it remains relatively stable in seawater over short timescales. Since approximately 2016, 137Csconcentrations near the FDNPP have shown little decline, while spatial contrasts in the 3H/137Cs ratio have become more pronounced. Although both radionuclides’ concentrations peak within the port, the ratio is consistently lower there and higher offshore. This suggests the potential existence of sources outside the harbor governing the distribution pattern of the radionuclides.
To investigate these patterns, we applied a color-classified 3H/137Cs ratio analysis and conducted release-rate estimations for the port and adjacent coastal waters. In addition, we collected independent samples of seawater, river water, groundwater, and spring water near the F1NPP. The 3H/137Cs ratios of river water, groundwater, and spring water were used in an end-member mixing analysis to evaluate potential terrestrial and subsurface contributions. Preliminary results indicate that the end members for groundwater and spring water (excluding river water) show trends similar to the 3H/137Csratio in seawater, potentially explaining the observed increase in the ratio offshore.
This integrated analysis improves constraints on radionuclide sources and transport pathways in the F1NPP coastal environment and contributes to a better understanding of long-term radioactive contamination dynamics.
How to cite: Godse, N. S., Tsumune, D., Kato, H., Iino, H., Onda, Y., and Otosaka, S.: Radionuclide Dynamics in the Coastal Ocean off the Fukushima Daiichi Nuclear Power Plant Using Radioactivity Ratios., EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15360, https://doi.org/10.5194/egusphere-egu26-15360, 2026.
Large amounts of radioactive cesium (134Cs and 137Cs) were released as a result of the Fukushima Daiichi Nuclear Power Plant (F1NPP) accident. Even 15 years after the accident, 137Cs concentrations in the marine environment have not returned to pre-accident levels, indicating that leakage from areas outside of the F1NPP site may still be ongoing.[1] Although 137Cs concentrations in sandy beach groundwater outside the F1NPP site have been reported to be higher than those in seawater, suggesting groundwater as a major leakage pathway, no observational data are available from areas in close proximity to the plant.[2] Based on these considerations, this study aims to estimate discharge flux of 137Cs originating from groundwater in the coastal waters surrounding the F1NPP.
Groundwater-derived 137Cs discharge flux (Bq day-1) was estimated by dividing the inventory (Bq) by the residence time of groundwater (day). Residence times following groundwater discharge to the coastal ocean were estimated using changes in the short-lived radium isotope activity ratios (223Ra/224Ra) between groundwater and seawater. Radium isotopes were selected as groundwater tracers for three reasons: (i) they were scarcely released from the F1NPP, such that the influence of the accident on Ra isotopes can be considered negligible[3]; (ii) radium isotopes (223Ra, 224Ra, 226Ra, and 228Ra) exhibit pronounced concentration differences between groundwater and seawater; and (iii) the wide range of half-lives and multiple isotopes enables their application to the estimation of water residence times as well as to the quantification of nutrient fluxes transported via submarine groundwater discharge.[4] In addition, the spatial area representative of 137Cs leakage for inventory estimation was defined based on the variability of Ra isotopes and 3H. The mean 137Cs concentration within the target domain was determined using seawater sampling data of 137Cs concentrations conducted by Tokyo Electric Power Company Holdings, Inc. (TEPCO HD). The 137Cs inventory (Bq) was then calculated by multiplying the mean 137Cs concentration (Bq m⁻³) by the volume (m³) of the target domain.
The calculated discharge flux is from 2.1×109 to 8.6×109 (Bq day⁻¹). These values are comparable to the flux required to sustain coastal ¹³⁷Cs concentrations (2.0 × 10⁹ Bq day⁻¹)[1], indicating that submarine groundwater discharge may explain why 137Cs concentrations in the vicinity of the FDNPP have not returned to pre-accident levels.
[1]Tsumune et al., J Environ Radioact , 2024
[2]Sanial et al., Proc Natl Acad Sci, 2017
[3]Buesselar et al., Ann Rev Mar Sci , 2017
[4]Garcia-Orellana et al., Earth-Science Review, 2021
How to cite: Iino, H., Tsumune, D., Kato, H., Godse, N., Onda, Y., and Otosaka, S.: Quantifying groundwater-derived 137Cs fluxes to surrounding coastal waters using radium isotope, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15358, https://doi.org/10.5194/egusphere-egu26-15358, 2026.
The discharge of ALPS-treated water from the Fukushima Daiichi Nuclear Power Plant (FDNPP) in August 2023 renewed concerns regarding radionuclide dispersion in the North Pacific, particularly in the waters surrounding Taiwan. This event highlighted the need to assess not only releases from Fukushima but also the cumulative influence of multiple nuclear power plants operating within the region. To investigate potential dispersion patterns under simultaneous multi-source discharges, this study employed a particle tracking model coupled with the Semi-implicit Cross-scale Hydroscience Integrated System Model (SCHISM), together with a two-dimensional Gaussian diffusion model, to simulate tritium dispersion in surface seawater from six facilities located in the western North Pacific region during 2023–2024: FDNPP, Wolsong, Qinshan, Fuqing, Daya Bay, and the Maanshan Nuclear Power Plant (NPP3 in Taiwan). Also, the modeled tritium concentrations in the Pacific area were compared with background seawater levels reported in the IAEA Marine Radioactivity Information System (MARIS) database. This comparison provided a baseline consistency check to examine whether the simulated tritium distributions were influenced by large-scale ocean circulation and cumulative multi-source discharges.
To further evaluate potential local impacts around Taiwan, seven representative monitoring sites were selected to capture spatial variability across different coastal sectors and offshore regions, including Kinmen, Matsu, the Tamsui River Estuary, Cijin, the Zhuoshui River Estuary, Guishan Island, and FRI-ST-15 (a Fisheries Research Institute monitoring station). These sites were used to examine seasonal concentration responses associated with eastern, western, northern, and southern waters, as well as offshore island environments. The results indicate that tritium released from multiple sources was transported northward by the Kuroshio Current, reaching southern Japan and extending eastward to approximately 180°E. In the northwestern waters of Taiwan, including Kinmen and Matsu, contributions from Fuqing and Qinshan were dominant. At Kinmen, Fuqing’s contribution reached maximum values immediately after discharge and remained significant into early spring, whereas the contribution from Qinshan was comparatively smaller. At Matsu, Qinshan’s contribution increased approximately one month after discharge, decreased by late winter, and reached a secondary maximum in the subsequent winter, while Fuqing’s contribution increased during late winter and maintained a moderate influence thereafter.
Finally, some sensitivity analyses assuming a 50-fold increase in discharge concentrations were conducted to assess potential variability and relative influence among sources. The results indicated negligible influence from Wolsong and FDNPP, whereas discharges from Qinshan, Fuqing, Daya Bay, and NPP3 produced more pronounced, seasonally modulated signals that diminished with increasing distance from Taiwan.
How to cite: Chiang, Y. and Huang, P.-C.: Modeling the Regional Dispersion of Continuous Multi-Source Tritiated Water Discharges in Surface Waters Around Taiwan, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3203, https://doi.org/10.5194/egusphere-egu26-3203, 2026.
The discharge of ALPS treated water from the Fukushima Daiichi Nuclear Power Station into the North Pacific Ocean has necessitated a detailed assessment of long-term dispersion pathways. The transport of this ALPS treated water is primarily governed by the Kuroshio Extension (KE) system. However, the upstream Kuroshio has been experiencing a persistent “Kuroshio Large Meander (KLM)” event since August 2017. Since the variability of the KE is dynamically linked to the path of the Kuroshio south of Japan, understanding how the KLM modulates the downstream flow field is critical for evaluating environmental impacts.
In this study, we investigate the influence of the KLM on the dispersion of tritium-treated water by Lagrangian particle tracking model with a continuous release scheme. The model was forced by ocean current data from the Hybrid Coordinate Ocean Model (HYCOM) to capture the spatiotemporal variability of the Kuroshio Current. We specifically examined the differences in transport patterns during the KLM period (2017–2022) versus non-meander period (2011–2016).
Preliminary results indicate that the presence of the upstream Large Meander induces specific downstream responses in the Kuroshio Extension that distinctively deviate from the reference non-meander period. We focus on how the KLM modulates the stability and position of the KE jet, thereby altering the initial advection pathways of the ALPS treated water. The analysis aims to clarify whether these KLM-induced changes in the KE system act to retard zonal transport or enhance regional retention, creating significant discrepancies in tracer arrival times and concentration fields between the two periods.
This study quantifies these deviations and discusses the implications of the “Kuroshio-Kuroshio Extension coupling” mechanism in determining the dispersion patterns and concentration distributions of passive tracers. The findings highlight the necessity of incorporating low-frequency climate variability into environmental risk assessments for oceanic discharges.
How to cite: Chu, Y.-J., Zeng, H.-T., Teng, J.-H., and Wang, C.-H.: Assessment of the Kuroshio Large Meander’s Impact on the Dispersion Pathways of Fukushima Tritium-treated Water, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5031, https://doi.org/10.5194/egusphere-egu26-5031, 2026.
Validating the reproducibility of ocean dispersion models used in prior environmental impact assessments of ALPS-treated water release is essential for evaluating their applicability. In this study, we conducted reproduction simulations using actual release records together with observed meteorological and oceanographic conditions, and quantitatively compared the results with seawater monitoring data.
Model results were compared with tritium monitoring data collected by TEPCO, the Ministry of the Environment, the Nuclear Regulation Authority, and Fukushima Prefecture. The release was assumed to instantaneously disperse within a model grid (147 m × 186 m), with release scenarios prescribed for both the surface layer and the near-bottom layer at a depth of 10 m. As the actual discharged water is expected to rise upward from the seabed, results from the surface-release simulation are mainly discussed. Geometric means were used for model–observation comparisons to reduce the influence of outliers. Since background tritium concentration is not explicitly represented in the model, a constant background of 100 Bq m⁻³ was added to the modeled concentrations to ensure consistency with observations.
For the entire one-year period, the correlation coefficient between annual geometric means of modeled and observed concentrations was R = 0.30, indicating moderate reproducibility of temporal variability. In contrast, the mean log(Model/Obs) was −0.035, corresponding to a Model/Obs ratio of 0.92, demonstrating very good agreement in annual mean concentration levels. When the comparison was restricted to release periods, the correlation improved (R = 0.64), while the mean Model/Obs ratio increased to 1.37, suggesting a tendency toward overestimation associated with uncertainties in local release representation and model resolution near the outlet.
These results indicate that, although the model has limitations in reproducing short-term concentration variability, it reliably reproduces annual mean tritium concentrations that are critical for radiological dose assessment. The present validation demonstrates that the ocean dispersion model used in the prior environmental impact assessment has sufficient reliability for evaluating the dispersion behavior of ALPS-treated water, while highlighting the need for further improvements in the treatment of background concentrations and near-field processes.
How to cite: Tsumune, D., Misumi, K., and Tsubono, T.: Reproducibility of ocean dispersion simulations for ALPS-treated water release off Fukushima: comparison with one-year monitoring data, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10166, https://doi.org/10.5194/egusphere-egu26-10166, 2026.
