Consequences of marine ecological environment and our preparedness for Fukushima radioactive wastewaterdischarge into the ocean

Ten years after the Fukushima nuclear accident (FNA), Japan announced the planned discharge of over one million tons of Fukushima radioactive wastewater (FRW) into the Pacific Ocean in two years. This decision regarding FRW disposal has aroused worldwide concerns and public fears, which may be exacerbated by reputational damage and the lack of a clear public understanding of the possible adverse impacts of the FRW. As one of the countries surrounding the Pacific Ocean, China is a stakeholder in this decision regarding the FRW disposal. In this study, we compared the FRW source and its associated radionuclide components with the liquid effluent from routine operation of the nuclear power plant and its associated radionuclides. The activity concentrations of 13 radionuclides in the pre- and post-treated FRW by the Advanced Liquid Processing Systems (ALPS) were quantitatively compared with limits for radionuclide concentrations required by Japan law, guidance levels for radionuclides in drinking water provided by the World Health Organization (WHO), and baseline concentrations of radionuclides in surface seawater from the Pacific Ocean before the FNA. Sediment-seawater distribution coefficients and bioconcentration factors are also shown to provide insights into the mobility and biological availability of radionuclides derived from the FRW in the marine environment. Although 62 radionuclides can be recovered from the FRW by ALPS according to a report from the Tokyo Electric Power Company (TEPCO), a large amount of ³H remains in the ALPS-treated FRW. The total amount of ³H in the ALPS-treated FRW was approximately 8.6×10¹⁴ Bq (by October 31, 2019), with an average concentration of 7.3×10⁵ Bq/L, higher than the concentration limit (6×10⁴ Bq/L) required by Japan law and guidance level (10⁴ Bq/L) provided by the WHO. The amount of ³H in the ALPS-treated FRW (8.6×10¹⁴ Bq by October 31, 2019) is continually increasing, and is already higher than the amount of ³H (3×10¹⁴‒7×10¹⁴ Bq) released into the Pacific Ocean immediately after the FNA. Additionally, other radionuclides (e.g., ¹⁴C, ⁹⁰Sr, ¹²⁹I, etc.) in the ALPS-treated FRW with high bioconcentration factors and high activity-dose conversion factors relative to ³H should also be carefully monitored and evaluated. The measured results provided by the TEPCO indicated that ~70% of the current ALPS-treated FRW should be repurified to reduce concentrations of other radionuclides to meet Japan’s legal requirements. Despite several unresolved factors (e.g., the FRW source terms, discharging plan, hydrodynamic and biogeochemical processes, etc.) simultaneously influencing the fate of FRW in the marine environment, we qualitatively described the hydrodynamically driven passive transport pathway and the biologically driven active transport pathway of the FRW. The transport of the FRW should be comprehensively investigated from the perspective of physical-biogeochemical processes at multiple scales (e.g., large-scale wind-driven circulation, mesoscale eddies, small-scale turbulence) and three-dimensional (e.g., vertical and horizontal vectors) oceans. Key gateways and transport pathways relevant to the FRW entry into the China seas have been suggested to include the Luzon Strait, the outer continental shelf and cross-shelf penetrating fronts in the East China Sea, the Yellow Sea Warm Current, and the Korean Coastal Current. Under specific conditions, the neglected biologically driven active transport pathway may significantly accelerate transport speed for the FRW-derived radionuclides via migratory animals (e.g., Pacific bluefin tuna) and may impose relatively high radiological risk to humans via seafood consumption. Finally, the consequences of marine ecological environment and our preparedness for the FRW release were discussed from the perspectives of total radioactivity, radionuclide components in the FRW, transport pathways of the FRW, and enhancing capacity to meet radiological risk assessment needs. Nuclear power plants located near the coastal seas are gradually developing in China and play a significant role in China’s national strategy of “Carbon Neutrality”. Technical systems of measurement, tracer, and assessment of radionuclides in the marine environments should be given more attention and be continually enhanced in line with the development of nuclear power plants. Several directions including extremely low minimum detection activity for the analytical methods of radionuclides, buoy-based online and real-time measurement technology for marine radioactivity, construction and validation of numerical models for marine radioactivity, key marine biogeochemical processes for radionuclides, radiation dose-effect for marine biotas, radiological assessment models, and remediation technology for radionuclides in marine environments are emphasized as key technologies in nuclear emergency preparedness to protect marine environment security.