Flounder at Fukushima Shows No Harm From Tritium in Water

Japan Forward

10-06-2025

A trial at the Fukushima Daiichi Nuclear Power Plant testing the biological effects of tritium, a naturally occuring radioisotope of hydrogen, ended in late March with promising results. It involved raising flounder and abalone in water containing the substance over a three-year period.
The project aimed to demonstrate the safety of releasing ALPS treated water containing tritium, a radioactive form of hydrogen, into the ocean.
Images of healthy flounder and abalone raised in tritium-containing water were widely shared in Japan and abroad, helping to ease public concerns and reduce reputational damage.
At first glance, the experiment may seem minor. But no other study in the world has examined the biological effects of tritium with such precision, over such a long period, and using so many specimens.
Some of the flounder from the trial will later be used in research in a field closely linked to nuclear fusion energy.
At Fukushima Daiichi, groundwater continues to seep into the damaged reactor buildings, becoming contaminated after contact with fuel debris.
Most radioactive substances are removed by the Advanced Liquid Processing System (ALPS), but tritium, a hydrogen isotope, cannot be filtered out. However, tritium emits only very weak radiation and does not build up in the body, even if ingested.
Releasing water containing tritium into the ocean is standard practice at nuclear power plants around the world. Nevertheless, at Fukushima Daiichi, the same process, even after filtration and dilution, still sparks public concern and fears of reputational harm.
Explaining safety with data alone means little if the public remains unconvinced.
To address this, TEPCO launched its own marine life rearing trial. It began in September 2022, nearly a year before the first release of treated water in August 2023.
The company set up two test tanks, each holding about 400 juvenile flounder. One tank contained seawater mixed with treated water, while the other held only regular seawater. Flounder were raised in tanks like this one, filled with seawater mixed with treated water. (Courtesy of TEPCO)
The study found no difference in survival or growth between the two groups.
Additionally, tritium levels in the bodily fluids of flounders raised in the treated water never exceeded those in the water itself. These levels dropped to undetectable within 24 hours after the fish were returned to normal seawater.
Tritium was also detected in their muscle tissue, but those levels rose slowly over two years and stabilized at just one-fourth to one-sixth of the treated water concentration.
TEPCO is also conducting related studies on abalone and hondawara seaweed to monitor how tritium is absorbed and released.
The flounder trial began in March 2022, including the preparation phase. None of the TEPCO staff had experience raising flounder. This made the task especially difficult in land-based, closed-system tanks. For the first six months, they used only regular seawater to practice and refine their techniques.
Two months in, the flounder began dying suddenly, and the cause was unclear. With help from outside experts, the team eventually discovered blood-sucking parasites attached to the fish's gills. Once the fish were treated, the deaths stopped.
Kazuo Yamanaka, who led the project, and Etsushi Kashiwagi, who oversaw risk communication about the ALPS treated water, now share a knowing glance when they recall that period.
The team documented their daily challenges and progress through an online rearing diary and livestreams, often adding a touch of humor. These updates attracted a wide audience. Seeing healthy, growing flounder and abalone provided a kind of reassurance that numbers alone could not. An Ezo abalone extends its tentacles. This post drew 46,000 views, which was 30 times more than usual. (February 9, 2023, TEPCO Marine Life Rearing Log)
The trial provided more than just data on safety. It also advanced scientific understanding of tritium's effects on marine life.
Few studies have used tritium directly, especially before TEPCO's rearing experiment. Even at low levels, its radioactivity limits handling to controlled environments such as nuclear facilities.
As a result, most past research relied on deuterium, the non-radioactive counterpart of tritium. Although deuterium is safe to use in standard laboratories, its high cost has made long-term studies difficult to carry out.
In Canada, where nuclear power plants discharge relatively high amounts of tritium, some studies have examined its effects on nearby lakes. However, since these studies were conducted in uncontrolled natural environments, the results often varied too much to draw firm conclusions.
Over the three-year rearing trial, the TEPCO team also developed and patented a system to predict outbreaks of blood-sucking parasites. This technology could benefit land-based marine aquaculture, a field that has been growing in recent years.
Another key outcome of the trial is a group of about 120 flounder containing organically bound tritium, molecules stored in their tissues.
Once dried, these fish can serve as standard reference samples to help improve methods for measuring organically bound tritium. Some of the samples will also be shared with the International Atomic Energy Agency (IAEA).
Tritium, along with deuterium, is a key fuel for nuclear fusion. As Japan ramps up its efforts to develop fusion energy, understanding tritium's biological effects is becoming increasingly important.
The flounder raised in treated water will support this broader scientific effort. Findings from the trial will be presented at the International Conference on Tritium Science and Technology in Canada in September.
( Read the article in Japanese . )
Author: Shohei Nagatsuji, The Sankei Shimbun