Explainer: Fukushima's Radioactive Wastewater: What You Need to Know

It's treated and diluted, but still slightly radioactive.

This aerial view shows the tanks containing treated radioactive wastewater at the Fukushima Daiichi nuclear power plant in Fukushima, northern Japan, on Aug. 22, 2023.
This aerial view shows the tanks containing treated radioactive wastewater at the Fukushima Daiichi nuclear power plant in Fukushima, northern Japan, on Aug. 22, 2023.
Kyodo News via AP

OKUMA, Japan (AP) β€” Treated and diluted but still slightly radioactive wastewater from the wrecked Fukushima Daiichi nuclear power plant is being discharged into the Pacific Ocean.

The first batch being released over the next 17 days is a small part of the tons of water that have accumulated at the plant. But the Japanese government and the plant's operator say releasing the water is an unavoidable step in the decommissioning of the plant, which already will take decades. They say the water is treated and diluted to levels that are safer than international standards. But the plan faces opposition in and outside Japan. Here is a look at the controversy.


The March 2011 earthquake and tsunami destroyed the plant's cooling systems, causing three reactors to melt. Highly contaminated cooling water applied to the damaged reactors has leaked continuously to building basements and mixed with groundwater.

The plant operator, Tokyo Electric Power Company Holdings, has taken steps to limit the amount of groundwater and rainwater entering the reactor area, and has reduced the increase in contaminated water to about 100 tons a day, 1/5 of the initial amount. The water is collected and partly recycled as cooling water after treatment, with the rest stored in around 1,000 tanks, which are already filled to 98% of their 1.37 million-ton capacity.


The government and TEPCO say they need to empty the tanks and clear them from the plant complex to make room for the plant's decommissioning and prevent accidental leaks of the tanks.

Japan has obtained support from the International Atomic Energy Agency to improve the transparency and credibility of the release and ensure it meets international safety standards. The government has also stepped up a campaign promoting the plan's safety at home and through diplomatic channels.


The water is being treated by what's called an Advanced Liquid Processing System, which can reduce the amounts of more than 60 selected radionuclides to government-set releasable levels, except for tritium, which officials say is safe for humans if consumed in small amounts.

About 70% of the water held in the tanks still contains cesium, strontium, carbon-14 and other radionuclides exceeding government-set levels. It will be re-treated until the concentrations meet those limits, then diluted by more than 100 times its volume of seawater before it is released. That will bring it way below international safety limits, but its radioactivity won't be zero.


IAEA concluded in a report that the plan, if conducted as designed, will have negligible impact on the environment and human health. IAEA chief Rafael Grossi visited the plant in July and said he was satisfied with preparations.

Japan's government says the release of tritium into the sea is a routine practice by nuclear plants around the world and that the amount will be several times lower than from plants in China and South Korea.

Scientists generally support the IAEA's conclusion, while some call for more attention to dozens of low-dose radionuclides that remain in the water, saying data on their long-term effects on the environment and marine life are insufficient.

Experts say the release of treated water from Fukushima is much less challenging than the daunting task of removing deadly radioactive debris that remains in the damaged reactors.


TEPCO executive Junichi Matsumoto says the release began Thursday with the least radioactive water to ensure safety. After samples are analyzed in final testing, the water is sent through a thin black pipe to a coastal area where it is diluted with hundreds of times its volume in seawater.

The diluted water will enter an undersea tunnel and be discharged a few minutes later from a point 1 kilometer (0.6 mile) off the coast. The release will be gradual and will continue for decades until the decommissioning of the plant is finished, TEPCO officials say. Matsumoto said the slow release will further reduce the environmental impact.

The company plans to release 31,200 tons of treated water by the end of March 2024, which would empty only 10 tanks because of the continued production of wastewater at the plant. The pace will later pick up.


Fukushima's fisheries and tourism industries and its economy are still recovering from the 2011 disaster. Fisheries groups worry about a further damage to the reputation of their seafood. Fukushima's current catch is only about one-fifth its pre-disaster level due to a decline in the fishing population and smaller catch sizes.

The head of the National Federation of Fisheries Cooperatives, Masanobu Sakamoto, said on Monday that "scientific safety and the sense of safety are different."

Groups in South Korea and China have also raised concerns, turning the release into a political and diplomatic issue. China has stepped up radiation testing of fishery and agricultural products from Fukushima and nine other prefectures, halting exports at customs for weeks, Fisheries Agency officials say.


The Japanese government says potential risks from the release of treated water are limited to reputational damage resulting from rumors, rather than scientific study. It has allocated 80 billion yen ($550 million) to support fisheries and seafood processing and combat potential reputation damage. TEPCO has also promised to deal with reputational damage claims.

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