Global ocean model affirms Fukushima wastewater release is safe

Most monitoring of the Fukushima Daiichi Nuclear Power Plant’s release of tritiated wastewater has thus far focused on local coastal waters. No global-scale study has tracked how tritium from the planned 30-year discharge might move through a warming Pacific Ocean.

To fill this gap, researchers from University of Tokyo and Fukushima University recently combined accurate estimates of the amount and timing of tritium release with an ocean circulation model called COCO 4.9. The model solved the basic physics of ocean flow on a grid with 62 vertical layers and two alternative horizontal meshes, about 1º for low-resolution simulations runs and 0.25º for experiments that need to resolve patterns up to the level of eddies.

The researchers ‘injected’ tritium into the model in two phases. First, they added 179-181 TBq for the 2011-2019 period into the model’s surface layer to mimic the Fukushima disaster’s effects. Then they added 480 TBq over 2023-2051 some 11 m below the surface. This dose is the highest possible the Tokyo Electric Power Company is expected to release during the discharge.

Then they explored three scenarios: present-day conditions, a strong warming pathway (SSP5-8.5), and mesoscale eddies. Across each simulation, the team found that tritium levels in the open Pacific remained below current detection limits, except immediately next to the plant during the 2011 accident spike. The peaks from the routine discharge never exceed 0.002 Bq/L, which is 25x lower than natural background radiation levels.

The team also reported that warmer oceans might shift the important Kuroshio Current a little north and strengthen eastward flow, cutting the time the first trace of tritium takes to travel to the mid-Pacific and Asian subtropical coasts by roughly three years. Even then, the modelled concentration of tritium stayed three orders of magnitude under the detection threshold.

The simulation that resolved the eddies showed that small quantities of tritium might reach western North America or the South China Sea sooner than expected.

Because the facility is releasing tritiated water so slowly, the radiation it’s responsible for is lower than that due to natural and historical sources. And because tritium has a half life of around 12 years, the modelled levels of the element remained undetectable across the wider Pacific Ocean through 2099, even under extreme warming or a worst-case eddy transport scenario.

The team’s findings are to be published in the November 2025 edition of Marine Pollution Bulletin.