The Japanese utility giant Tepco is planning to dump more than 1 million cubic meters of treated radioactive water -- enough to fill 500 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 (137 miles) north of Tokyo. While Tepco cycles in water to keep fuel and debris cool, about 100 cubic meters (840 barrels) 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.

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 Dec. 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. Japan could move toward more long-term storage on nearby land by investing in petroleum reserve tanks, the biggest of which can hold some 2.4 billion liters (20 million barrels) of liquid. It’s unlikely anyone will want to live in areas around the plant for a long time. But it would also require a political decision.

6. When will it be released, and how?

Not for about another 2 years as Tepco gets ready, according to an April report from the Ministry of Economy, Trade and Industry. The water will be diluted and reprocessed before release into the ocean, it said, and the government will strengthen efforts to monitor radioactivity. Exactly how it will be released has yet to be decided, and must be approved by the nation’s nuclear regulator. Some nuclear safety experts in Vienna, where the IAEA is based, suggest it might be preferable to pump the water at depth in the middle of the ocean rather than along littoral coastlines where marine life breeds. That could be a boon for climate scientists studying ocean circulation, since tritium has been used before as a tracer. Most of our knowledge currently is of surface-level circulation. Less is know about the deeper sea. Some radiochemists say the idea has some merit, but note that International Maritime Organization laws prohibit the intentional release of radioactive material in the open ocean -- rules that were created following Russian low-level dumping in the Sea of Japan during the 1990s.

7. 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 renewed their “grave concern” about the release in April, though ocean currents are unlikely to bring any contaminated water near its shores. China urged Japan to act prudently, while the U.S. said the release was in line with global standards.

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. Since then, there’s been steady progress in the cleanup at the plant, which Tepco estimates will take 30 to 40 years more. In 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.

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