Three-dimensional Numerical Modeling of Critical Pathways for Radionuclide East of Fukushima Entering China Seas

On August 24, 2023, the Japanese government officially discharged nuclear wastewater into the ocean, causing widespread international concern. Radionuclides in nuclear wastewater pose a serious threat to the marine environment and to the national health of neighboring countries. This paper aims to accurately characterize the diffusion and transport processes of radionuclides in the ocean, which is the key to monitoring and prevention of radionuclides effects. Based on regional ocean modeling system (ROMS), this paper set up a high-resolution regional ocean model of the North Pacific Ocean. The distribution of radionuclides discharged into the ocean from Fukushima over a period of five years was simulated by using passive tracer experiments. The relative concentration of passive tracers are capable of representing the radionuclides distributions and pathways. The results show that the radionuclides mainly move eastward with the Kuroshio extensions after being discharged into the sea, but some radionuclides still arrive at the Chinese offshore in a southwesterly direction due to southwesterly return currents, mesoscale eddy activities and mode water subduction processes. Among the material exchange channels between China Seas and open ocean, radionuclides mainly enter the East China Sea through east of Taiwan channel, and penetrate into the South China Sea through the Luzon Strait, while the other channels are all export radionuclides from China Seas to the open ocean. Shelf circulations play a vital role in the exchange of materials between sea areas, which transport radionuclides into the Yellow Sea and the Bohai Sea particularly by Taiwan warm current and Yellow Sea warm current. Besides, seasonal variations of ocean circulations can significantly affect the transport of radionuclides. The ocean currents on the shelf can even show reverse directions between summer and winter. A new assessment of the impact time of radionuclides at different concentration thresholds in the four Chinese Seas and key channels was finally given. The East China Sea is the first area affected by radionuclides, and the Yellow Sea and the South China Sea are affected by radionuclides at similar times, while the Bohai Sea was the latest with lowest concentration of radionuclides. Radionuclide impact times are more homogeneous vertically in the shallow channels, while the impact times of deep-water channels are different in vertical due to the complex vertical structure. The model results systematically provide the three-dimensional transport pathways and dynamical mechanisms for radionuclides entering the China Seas from the North Pacific Ocean, and confirm that the high concentration radionuclides can not reach the China Seas during the five years of the simulation.