On August 24, 2023, the Japanese government started discharging the Fukushima Nuclear Contaminated Water (FNCW) into the North Pacific. This process is bound to pose radiation risks for the marine ecological environment. In this study, we analyzed the concentrations of major artificial radionuclides in the FNCW and estimated their inventories. Based on the data provided by the Tokyo Electric Power Company, we found that the concentrations of ³H in FNCW tanks as of March 2023 ranged from 1.9×10⁵ to 25.0×10⁵ Bq/L, significantly exceeding the maximum release concentration for ³H (6×10⁴ Bq/L) allowed by Japanese law. In addition, the concentrations of ⁹⁰Sr and ¹²⁹I in some FNCW tanks were higher than the corresponding maximum release concentrations (30 Bq/L for ⁹⁰Sr and 9 Bq/L for ¹²⁹I) allowed by Japanese law. The inventories of ³H and ¹²⁹I in the FNCW before the discharge were estimated to be 0.9 and 6.2×10⁹ Bq, respectively, i.e., comparable to the leakage amounts of ³H (0.1~1.0 PBq) and ¹²⁹I (6.9×10⁹ Bq) to the ocean during the nuclear accident stage. We further discuss the migration and behavior of typical Fukushima radionuclides (e.g., ³H, ¹⁴C, ⁶⁰Co, ⁹⁰Sr, ¹²⁹I, ^{134, 137}Cs, and ^{239, 240}Pu) in marine environments from three aspects: \mathrm{①} transport of Fukushima radionuclides by ocean currents in the Pacific; \mathrm{②} sediment adsorption to radionuclides; and \mathrm{③} marine biota uptake of radionuclides. This study is expected to provide scientific foundations and insights for radiation monitoring and risk assessment, which may be required for an appropriate response to the discharge of the FNCW.