Simulation of the impact assessment of the discharge of ALPS treated water into the ocean

As part of the decommissioning work at the Fukushima Daiichi Nuclear Power Plant, the discharge of ALPS-treated water began in August 2023. ALPS-treated water contains not only H-3, but also other nuclides, albeit at extremely low concentrations. ALPS-treated water is treated so that the total of the concentration ratios of the 30 target nuclides is less than 1, and then diluted so that the H-3 concentration is less than 1500 Bq/L before being discharged. Prior to discharge, a radiation impact assessment was carried out by TEPCO. This assessment used a marine dispersion model that had been verified using observation results for Cs-137 that had leaked out as a result of the Fukushima Daiichi Nuclear Power Plant accident.

The ocean dispersion model was a 1 km x 1 km ROMS model with a variable mesh that was refined to 200 m x 200 m in the vicinity. It was driven by meteorological reanalysis data from the Japan Meteorological Agency, and data assimilation was performed using JCOPE2M ocean reanalysis data to reproduce the Kuroshio and mesoscale eddies.

In this assessment, a H-3 concentration of 0.1 Bq/L was used as the background concentration from atmospheric nuclear testing. However, a concentration of 1 Bq/L of H-3 was observed even before the release, and this is thought to be due to the influence of the supply from the power plant site or surrounding areas, or from rivers. After taking into account the influence of these background concentrations, the results of the model simulations were verified against the results of the monitoring of H-3 concentrations. Because the rate of release of the ALPS processed water was small, the range of influence of the monitoring of H-3 concentrations was limited, but the verification results were consistent. It is necessary to determine the average distribution by continuing to monitor in the future.