On 11 March 2011, radioactive water from the Fukushima Daiichi Nuclear Power Plant in Japan began being released into the Pacific Ocean. This happened after a nuclear disaster caused by the Tōhoku earthquake and tsunami. Three of the plant’s reactors experienced meltdowns, leaving behind melted fuel debris. Water was added to stop the meltdowns from getting worse. When cooling water, groundwater, and rain mixed with the melted fuel debris, they became contaminated with radioactive materials, such as iodine-131, caesium-134, caesium-137, and strontium-90.
More than 500,000 tonnes of untreated wastewater (including 10,000 tonnes released to create space for storage) entered the ocean shortly after the accident. Also, leaks into groundwater continued for years before the plant operator admitted the problem in 2013. The radioactivity from these sources was higher than allowed by law.
Since then, contaminated water has been stored in tanks and treated using the Advanced Liquid Processing System (ALPS) to remove most radioactive materials. However, tritium, a radioactive substance with a half-life of 12.32 years, remains in the water because it is chemically bonded to it. In 2021, the Japanese government approved the release of ALPS-treated water containing tritium. To reduce its radioactivity, the treated water will be mixed with seawater before being released.
A report by the International Atomic Energy Agency (IAEA) states that releasing diluted ALPS-treated water into the ocean follows international safety standards. The report also notes that the decision to release the water is a choice made by the Japanese government and does not support or recommend the action.
On 24 August 2023, the power plant began releasing treated wastewater into the Pacific Ocean. At that time, the storage tanks held over a million tonnes of wastewater. Because new wastewater continues to form and treated water must be released slowly, the entire process may take more than 30 years. Some countries and international groups have expressed concerns about this decision.
As of the fourth round of wastewater release in March 2024, no higher-than-normal levels of tritium have been found in nearby waters.
Initial atmospheric release
Radioactive materials were spread into the air right after the disaster and make up most of the materials that entered the environment. According to a 2020 report by UNSCEAR, 80% of the radioactive materials first released into the atmosphere eventually settled over rivers and the Pacific Ocean. The report states that the total amounts of iodine-131 and caesium-137 released into the air were generally between about 100 to 500 PBq and 6 to 20 PBq, respectively. These amounts represent about 2% to 8% of the total amount of iodine-131 and about 1% to 3% of the total amount of caesium-137 in the three operating units (Units 1–3).
The radioactive materials that settled in rivers came from earlier releases into the air. The same 2020 UNSCEAR report notes that about 5 to 10 TBq of caesium-137 are released each year into rivers that flow from areas affected by the disaster.
Discharge to ocean, untreated water (2011)
On April 5, 2011, the company that operated the nuclear plant, Tokyo Electric Power Company (TEPCO), released 11,500 tons of water that had not been treated to remove radioactive materials into the Pacific Ocean. This action was taken to create space for storing water that had even higher levels of radioactivity. The water that was released had lower radioactivity than the stored water, but it still contained 100 times more radioactive material than was legally allowed. TEPCO estimated that 520,000 tons of untreated radioactive water had already entered the ocean before the company installed barriers to stop further leaks.
A report from the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) in 2020 found that during the first three months after the incident, between 10 and 20 petabecquerels (PBq) of iodine-131 and between 3 and 6 PBq of caesium-137 were released directly into the environment. Approximately 82 percent of this radioactive material had flowed into the ocean by April 8, 2011.
Discharge to soil and groundwater by leakage
Scientists thought radioactive elements might still be leaking into the ocean. High levels of caesium-134 were found in local fish, even though this isotope has a shorter half-life. At the same time, radiation levels in nearby seawater did not decrease as expected. After many denials, the nuclear plant operator, Tokyo Electric Power Company (TEPCO), admitted on 22 July 2013 that groundwater leaks had been happening. Some groundwater samples contained 310 Bq/L of caesium-134 and 650 Bq/L of caesium-137, which is higher than the World Health Organization’s maximum guideline of 10 Bq/L for drinking water.
Later, it was found that some leaks came from storage tanks for wastewater. Since then, TEPCO has had a history of providing incorrect information and has lost public trust. For example, in 2014, TEPCO claimed its measuring method was flawed and changed the recorded level of strontium in a groundwater well from 900,000 Bq/L to 5,000,000 Bq/L in July 2013. This level was 160,000 times higher than the standard for discharge.
Soil can absorb some radioactive elements like caesium, but others like strontium and tritium move through the ground more easily. At one time, nearly 400 tonnes of radioactive water were being created each day (150,000 tonnes per year). TEPCO has since tried to stop or redirect groundwater from flowing into the damaged reactor areas and prevent contaminated water from reaching the ocean.
A 2020 report by the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) stated that about 60 terabecquerels (TBq) of caesium-137 were released directly into groundwater draining from the site up to October 2015, when steps were taken to reduce these releases. After that, about 0.5 TBq of caesium-137 was released each year.
In February 2024, a leak at the power plant was found by a contractor and later fixed by TEPCO. The company estimated that 5.5 tonnes of water, which may have contained 22 billion becquerels of radioactive materials like caesium and strontium, escaped from an air vent. The water pooled outside and seeped into the surrounding soil but did not leave the plant area. TEPCO said this happened because 10 out of 16 valves were left open when they should have been closed for maintenance.
Discharge to ocean, treated water
To stop the reactor meltdowns from getting worse, a continuous supply of new water is needed to cool the melted fuel debris. In 2013, 400 metric tonnes of water became radioactively contaminated each day. This water is pumped out and combined into the reactor-cooling loop, which includes strontium–cesium removal (KURION, SURRY) and reverse osmosis desalination processes.
In October 2012, TEPCO introduced the "Advanced Liquid Processing System" (ALPS), designed to remove radionuclides other than tritium (which is chemically bonded to water) and carbon-14. ALPS works by first pre-processing water through iron coprecipitation (removes alpha-emitting nuclides and organics) and carbonate coprecipitation (removes alkali earth metals, including strontium). The water is then passed through 16 absorbent columns to remove nuclides.
Wastewater is pumped to ALPS along with concentrated saltwater from desalination. Since some tritium remains, even treated water would need dilution to meet drinkable standards. Although carbon-14 is not removed, its content in pre-treated water is low enough to meet drinkable standards without dilution.
Japan's Nuclear Regulation Authority (NRA) approved ALPS in March 2013. ALPS is to be operated in three independent units, each able to purify 250 tons of water per day. Unit "A" started operation in April. In June, unit A was found leaking water and shut down. In July, the cause was identified as chloride and hypochlorite corrosion of water tanks; TEPCO added a rubber layer to the tanks. By August, all systems were shut down for repairs. One unit was expected to restart by September, with full recovery planned by the end of 2013.
By September 2018, TEPCO reported that 20% of the water had been treated to the required level. By 2020, the daily buildup of contaminated water was reduced to 170 metric tonnes due to groundwater isolation installations. TEPCO reported that 72% of the water in its tanks, some from early ALPS trials, needed repurification. By 2021, the portion of ready-to-discharge water reached 34%, and by 2023, it was 35%.
Some scientists expressed concerns about potential bioaccumulation of ruthenium, cobalt, strontium, and plutonium, which sometimes slip through the ALPS process and were present in 71% of the tanks.
Since the 2011 Fukushima Daiichi nuclear disaster, the plant has accumulated 1.25 million tonnes of wastewater, stored in 1,061 tanks on the site as of March 2021. Experts projected the site would run out of space for water tanks by 2022. Some suggested the government could have allocated more land for tanks, as the surrounding area was already deemed unsuitable for human use. However, the government did not act. Mainichi Shimbun criticized the government for showing "no sincerity" in "unilaterally pushing through" the plan to address storage space shortages.
On 13 April 2021, Japan's Cabinet approved TEPCO's plan to release stored water into the Pacific Ocean over 30 years. The Cabinet stated the water would be treated and diluted to meet drinkable standards. The idea of releasing water was proposed by Japanese experts as early as 2016. A mascot named "Tritium-kun" was created to improve public opinion but was later scrapped due to backlash.
In April 2023, Japan's NRA announced a Comprehensive Radiation Monitoring Plan to monitor radionuclide concentrations in food, soil, water, and air across Japan. The NRA also set up a system to monitor ALPS-processed water to verify TEPCO's readings.
An IAEA task force visited Japan in 2021 and released its first report in February 2022. Among other findings, TEPCO demonstrated to the IAEA that its pump setup thoroughly mixes water in tanks.
In May 2023, 3 IAEA laboratories and 4 national laboratories participated in an interlaboratory comparison to verify TEPCO's testing of ALPS-treated water. Of the 30 radionuclides TEPCO regularly tests for, 12 were found above detection limits. 52 out of 53 results agreed with the combined result; the only discrepancy was in I-129, where Korea's Institute of Nuclear Safety reported a value too low compared to the weighted average. TEPCO's methodology was found to be suitable for its purpose: although less sensitive for actinides than some labs, detection limits were far from regulatory limits, and alpha-emission screening tests appeared accurate. TEPCO's testing method for Am-141 may need further review. Japan's NRA tested the same sample and found no disagreements.
The tritium not filtered out has a radioactivity of 148,900 Bq/L, compared to 620,000 Bq/L before treatment. TEPCO plans to dilute it to 1,500 Bq/L or less before release.
On 22 August 2023, Japan began releasing treated wastewater into the Pacific Ocean, despite opposition from neighbors. Japan claims the water is safe after ALPS treatment, which removes nearly all radiation, with tritium as the main exception. Japan committed to diluting tritium to meet IAEA standards. The IAEA stated the plan meets safety standards, but critics argue more studies are needed. On 24 August, Japan started the discharge, sparking protests and prompting China to expand its ban on Japanese aquatic imports. Over 1 million tonnes of treated wastewater will be released over 30 years.
On 25 August 2023, TEPCO reported that tritium levels in seawater
Environmental effects
A large amount of caesium entered the ocean after the initial release of radioactive materials. By 2013, the levels of caesium-137 in the waters near Fukushima had returned to levels similar to those before the accident. However, the levels of caesium-137 in coastal sediments (the material at the bottom of the ocean) decreased more slowly than in the water. By 2020, the amount of caesium-137 stored in sediments was likely greater than the amount in the water. These sediments may continue to release caesium-137 into the seawater over a long time. According to Buesseler, the release of strontium-90 could be more concerning because, unlike some other radioactive materials, it can enter a person’s bones.
Data on marine food shows that its radioactive levels are decreasing and getting closer to original levels. In 2011, 41% of fish samples caught near Fukushima had caesium-137 levels above the legal limit (100 becquerels per kilogram). By 2015, this had dropped to 0.05%. In 2021, the United States Food and Drug Administration stated, "FDA has no evidence that radionuclides from the Fukushima incident are present in the U.S. food supply at unsafe levels." However, simply sharing scientific information has not helped restore public trust in eating fish from the Fukushima region.
The most common radioactive substance in the wastewater is tritium. A total of 780 terabecquerels (TBq) of tritium will be released into the ocean at a rate of 22 TBq per year. Tritium is regularly released into the ocean by operating nuclear power plants, sometimes in much larger amounts. For example, the La Hague nuclear processing site in France released 11,400 TBq of tritium in 2018. Additionally, about 60,000 TBq of tritium is naturally produced in the atmosphere each year by cosmic rays.
Other radioactive materials in the wastewater, such as caesium-137, are not usually released by nuclear power plants. However, the levels in the treated water are very small compared to allowed limits.
Michiaki Kai, a Japanese nuclear expert, said, "Scientists agree that the health impact is very small, but the risk cannot be completely ruled out, which causes controversy." David Bailey, a physicist who studies radioactivity, stated that with tritium at low concentrations, "there is no problem for marine life, unless fish populations drop significantly."
Ferenc Dalnoki-Veress, a scientist at the Middlebury Institute of International Studies, said that considering living organisms makes the situation more complicated. Robert Richmond, a biologist from the University of Hawaiʻi, told the BBC that incomplete studies on radiation and ecosystems raise concerns that Japan may not be able to detect changes in the environment or "get the genie back in the bottle." Dalnoki-Veress, Richmond, and three other experts advising the Pacific Islands Forum wrote that dilution may not account for how tritium can build up in living things or how it enters the environment through organic pathways.
According to a 2023 survey by the Yomiuri Shimbun, 57% of respondents supported the government’s plan to release treated water into the ocean, more than the 32% who opposed it.