TOKYO (AP) — Japanese officials plan to start discharging treated radioactive wastewater from the damaged Fukushima Daiichi nuclear power plant into the Pacific Ocean on Thursday, a contentious step more than 12 years after a massive earthquake and tsunami set off a battle against ever-increasing amounts of radioactive water at the plant.

The government and plant operator say the release is an unavoidable part of its decommissioning and will be safely carried out, but the plan faces opposition in and outside Japan. Here is a look at the controversy.

WHY IS THERE SO MUCH WASTEWATER?

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 (TEPCO), 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.

WHY IS TEPCO RELEASING THE WATER NOW?

The government and TEPCO say they need to make room for the plant’s decommissioning and prevent accidental leaks from 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.

WHAT’S IN THE TREATED WATER’?

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 retreated 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.

HOW SAFE IS IT?

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 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.

HOW WILL IT BE RELEASED?

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

The diluted water will enter an undersea tunnel and be released 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.

Final preparation for the release began Tuesday when just 1 ton of water was sent for dilution with 1,200 tons of seawater, and the mixture was to be kept in the primary pool for two days for final sampling to ensure safety, Matsumoto said. A batch of 460 tons will be sent to the mixing pool Thursday for the actual discharge.

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.

WHY ARE PEOPLE WORRIED?

Fukushima’s badly hit fisheries, tourism and economy are still recovering from the 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.

WHAT IF SOMETHING GOES WRONG?

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.