An Operational Modeling System Forecasting the Disposal of Fukushima Tritiated Water and Transport: A Lagrangian Particle Tracking System Driven by High-Resolution Hydrodynamics.

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.