The Japanese utility giant Tepco is considering a plan to dump roughly 1 million cubic meters of treated radioactive water -- enough to fill 400 Olympic-size swimming pools -- from the wrecked Fukushima Dai-Ichi nuclear power plant into the Pacific Ocean, part of its nearly $200 billion effort to clean up the worst atomic accident since Chernobyl. Storage tanks at the site are forecast to be full by mid-2022, and space for building more is scarce. Scary as it sounds, discharges are common practice in the industry and would likely meet global guidelines. That hasn’t assuaged angry locals or neighboring South Korea.

1. Where does the water come from?

A 2011 earthquake, the strongest ever recorded in Japan, and ensuing tsunami caused structural damage to Fukushima’s reactor buildings, about 220 kilometers (135 miles) north of Tokyo. While Tepco cycles in water to keep fuel and debris cool, about 100 cubic meters of groundwater flows in daily and becomes contaminated. The tainted water is pumped out and run through something called the Advanced Liquid Processing System, or ALPS, then stored in one of roughly 1,000 tanks at the site. The processing removes most of the radioactive elements except for tritium. Before being released, the so-called tritiated water would be reprocessed to ensure all of it meets safety standards, according to the Ministry of Economy, Trade and Industry.

2. What is tritium?

A form of hydrogen that has two extra neutrons, making it weakly radioactive. It is naturally produced in the upper atmosphere and also is a common byproduct of nuclear power generation. It has various applications including in making nuclear weapons, in medicine as a biological tracer, and in producing such glow-in-the-dark items as exit signs and watch dials.

3. Is it dangerous?

It can be carcinogenic at high levels. While tritium’s beta particles (those emitted during radioactive decay) are too low-energy to penetrate the skin, they can build up in the body if inhaled or consumed (usually via tainted water). Yet according to the Canadian Nuclear Safety Commission, a human would need to ingest billions of units of becquerels (a measure for radioactivity) before seeing any health effects. The Tepco tank with the highest concentration has 2.5 million becquerels per liter, according to data from March 31. For comparison, a banana has 15 becquerels and 1 kilogram (2.2 pounds) of uranium has 25 million.

4. How is it handled?

Most nuclear power plants discharge small amounts of tritium and other radioactive material into rivers and oceans, according to David Hess, a policy analyst at the World Nuclear Association, an industry group. In the U.S., such “authorized releases” of so-called tritiated water are done “routinely and safely” and are fully disclosed, according to the U.S. Nuclear Regulatory Commission. The International Commission on Radiological Protection’s recommendations, which form the basis for rules globally, limit liquid radioactive waste so that public radiation doses annually are less than 1 millisievert (a unit for measuring radiation exposure, abbreviated as mSv). For comparison, the World Nuclear Association says background radiation in the natural environment typically exposes people to an average 2.4 mSv a year, while a CT scan of the pelvis results in an effective dose of 10 mSv.

5. Why not build more tanks?

Tepco, or Tokyo Electric Power Company Holdings Inc., is essentially out of room on the facility grounds. It has already felled 500 square meters (5,400 square feet) of trees next to a bird sanctuary to make room for about 1,000 tanks.

6. Who’s against a release? For it?

Fishing groups in Fukushima prefecture are strongly opposed, fearing it could further taint the reputation of their catch and affect their livelihoods. (More than 20 countries still have import restrictions imposed after the disaster on some Japanese food products.) South Korean officials also have expressed concern about the possible release, though ocean currents are unlikely to bring any contaminated water near its shores. Tepco Chairman Takashi Kawamura and former Nuclear Regulation Authority Chairman Shinichi Tanaka have both voiced support for releasing the water into the ocean. Prime Minister Shinzo Abe’s government has yet to announce its position.

7. How will the decision be made?

A draft report in December by a panel under the Ministry of Economy, Trade and Industry proposes releasing the water into the sea, decreasing the volume through evaporation, or a combination of the two. (Earlier ideas included injecting the water into the ground or mixing it with concrete and burying it.) After the panel submits its final report, likely in early 2020, Abe’s governments will choose an option to pursue, which Tepco will then implement. The final plan also has to be cleared by the nation’s nuclear watchdog. The International Atomic Energy Agency said in a September report that a decision should be made urgently and engage all stakeholders.

8. How’s the cleanup going otherwise?

The March 11, 2011, quake off Japan’s northeast coast and ensuing tsunami caused about 16,000 confirmed deaths and extensive damage, including the meltdowns at Fukushima. Eight years on, there’s been steady progress in the cleanup at the plant, which Tepco estimates will take 30 to 40 years more. In early 2019 the utility sent a robot to touch melted fuel at the bottom of one of the reactors for the first time -- a necessary step toward developing a device to remove and dispose of it. An underground ice wall and drainage system was installed to reduce the amount of groundwater flowing into the wrecked reactors by more than half. The life of cleanup workers has improved as well. A thin surgical-style mask is all that’s needed to walk around most of the grounds, as opposed to a full body suit with a hard plastic mask covering the entire face. Radiation levels on the grounds have dropped, allowing for more work around the plant. A convenience store opened in 2016 on the grounds, while a new cafeteria offers hot meals.

--With assistance from Jason Gale, Isabel Reynolds and Aya Takada.

To contact the reporters on this story: Aaron Clark in Tokyo at;Stephen Stapczynski in Singapore at

To contact the editor responsible for this story: Ramsey Al-Rikabi at

©2019 Bloomberg L.P.