Operators have pumped water to cool the nuclear reactors at the Fukushima Daiichi Nuclear Power Plant (FDNPP) since the accident in 2011 and treated this cooling water with the Advanced Liquid Processing System (ALPS), which is a state-of-the-art purification system that removes radioactive materials, except tritium. As part of the water molecule, tritium radionuclide, with a half-life of 12.32 years, is very costly and difficult to remove. The ALPS-treated water was accumulating and stored at the FDNPP site and there is limited space to store this water. Therefore, in 2021, the Government of Japan announced a policy that included discharging the ALPS-treated water via approximately one kilometre long tunnel into the ocean. Planned releases of the ALPS-treated water diluted with ocean water began in August 2023 and will be completed by 2050.

In a new numerical modeling study, researchers from the Institute of Industrial Science, The University of Tokyo, in collaboration with the Fukushima University have revealed that the simulated increase in tritium concentration in the Pacific Ocean due to the tritium originating from the ALPS-treated water is about 0.1 % or less of than the tritium background concentration of 0.03-0.2 Bq/L in the vicinity of the discharge site (within 25 km) and beyond, which is below detection limits (i.e., so small that the difference due to the presence or absence of ALPS-treated water added to the original seawater cannot be measured). This is far below the WHO international safety standard of 10,000 Bq/L and consistent with results of tritium concentration monitoring in seawater conducted in conjunction with the discharge of ALPS-treated water.

"Since the government's announcement in 2021 to discharge the ALPS-treated water, several studies have investigated the radiological impact of ALPS-treated water discharge on tritium concentration in seawater and marine biota, but there were no global ocean simulations with anthropogenic tritium concentration using a realistic discharge scenario and for a period long enough to consider long-term impacts such as global warming" explains lead author of the study Alexandre Cauquoin. "In our global ocean simulations, we could investigate how ocean circulation changes due to the global warming and representation of fine-scale ocean eddies influence the temporal and spatial distribution of tritium originating from these treated-water releases."

Climate change and eddies in the water currents speed up the tritium movement through the ocean. However, the researchers found that the concentrations of tritium from ALPS-treated water discharge remain similar and very low. "Our simulations show that the anthropogenic tritium from the discharge of ALPS-treated water would have negligible impact on the tritium concentration in the ocean, both in the short and long term" says Maksym Gusyev from the Institute of Environmental Radioactivity, Fukushima University.

This study may help in building models to understand how tritium, as tritiated water molecule, moves through water vapor and ocean water. Tritium is useful to trace the dynamics of the water cycle, so climate models able to simulate tritiated water can help studies of precipitation patterns, atmospheric and oceanic circulation, moisture sources, river catchments, and groundwater flow in the future.

The article, "Ocean general circulation model simulations of anthropogenic tritium releases from the Fukushima Daiichi nuclear power plant site," was published in Marine Pollution Bulletin at DOI: 10.1016/j.marpolbul.2025.118294.

Research Contact

Cauquoin Alexandre, Project Research Associate
Institute of Industrial Science, the University of Tokyo
Tel：+81-4-7136-6965　
E-mail：cauquoin (Please add "@iis.u-tokyo.ac.jp" to the end)

Kei Yoshimura, Professor
Institute of Industrial Science, the University of Tokyo
Tel：+81-4-7136-6965
E-mail：kei (Please add "@iis.u-tokyo.ac.jp" to the end)