Tokyo(CNN) Japan will start releasing more than 1 million metric tons of treated radioactive water from its destroyed Fukushima nuclear plant into the ocean in two years, the government said Tuesday -- a plan that faces opposition at home and has raised "grave concern" in neighboring countries.
The decision to release the wastewater comes more than a decade after the nuclear disaster at the Fukushima Daiichi plant in 2011, having been repeatedly delayed due to safety concerns and strong opposition from local fishermen still reeling from the fallout of the crisis.
Work to release the water into the Pacific Ocean will begin in about two years, and the whole process is expected to take decades, according to the Japanese government.
Prime Minister Yoshihide Suga said dealing with the treated water is "an unavoidable issue" in order to decommission the nuclear plant.
"We have decided that guaranteeing safety far above the accepted standard, and ensuring the entire government's best efforts to prevent reputational damage, means releasing it to the ocean is a realistic option," he said.
In 2011, a powerful earthquake and tsunami cut off power supply and cooling systems for the Fukushima plant. To prevent its three damaged reactor cores from melting, cooling water was pumped in continuously, and was thus contaminated by uranium fuel rods. The water then leaked into damaged basements and tunnels, and mixed with groundwater.
The plant's operator, Tokyo Electric Power Co., has built massive tanks on the site to store the accumulating water after treatment. The tanks have a storage capacity of 1.37 million metric tons, and are expected to be filled up late next year, according to the company.
Amid safety concerns, the Japanese government has stressed that "water stored in tanks will not be discharged as it is." Instead, it will have been treated through a system that removes most of the radioactive material except for tritium, an isotope of hydrogen not harmful to humans in small amounts.
"Before the discharge, the water in tanks will also be sufficiently diluted so that the concentration of tritium will be much lower than Japan's national regulatory standards, which is compliant with international standards," the Prime Minister's office said in a statement.
Tritium only "emits weak radiation" and its impact on health is "very low," the statement said, adding that operators of nuclear reactors around the world routinely discharge tritium into the sea and air.
Japan's Ministry of Economy, Trade and Industry said on its website that the previously contaminated water will only be released once any chemicals are diluted to levels much lower than national and international standards.
It added that the diluting and discharging process will be monitored by third parties, including the International Atomic Energy Agency (IAEA).
IAEA Director General Rafael Mariano Grossi told CNN there is "no harm" in releasing treated water into the sea.
"It is not like you are going to see the sea glowing in purple or green, and all fish will be dead, and the Pacific Ocean will be killed. Of course not," Grossi said. "This has been done ... in the North Atlantic and the Mediterranean, in many parts of the world, and there is no adverse environmental impact whatsoever.
"There wouldn't be any authorization or any endorsement, if I can put it like this, from the IAEA to an operation that is causing harm or that is not environmentally neutral."
Japan's safety assurances have failed to assuage the fears of its neighbors South Korea and China.
On Tuesday, South Korea voiced "grave concerns" at Japan's water release plan, saying the decision could "directly or indirectly affect the safety of the Korean people and the surrounding environment in the future."
"The government has been emphasizing that the decision needs to be made through transparent disclosure of information and consultations with neighboring countries. If the Japanese government decides to discharge contaminated water from the Fukushima nuclear power plant without sufficient consultation, it is difficult for us to accept this," South Korean Foreign Affairs Ministry spokesman Choi Young-sam said at a news briefing.
In response, he said the South Korean government plans to double down on radioactivity monitoring and strengthen cooperation with the IAEA and international community.
China has also expressed "grave concern," calling on Japan to handle the wastewater release "in a responsible manner."
In a statement Tuesday, the Chinese Foreign Ministry said Japan had not exhausted safe disposal methods and did not conduct full consultation with neighboring countries and the international community.
"Japan has ... unilaterally decided to discharge nuclear wastewater from the Fukushima nuclear power plant accident into the sea, which is extremely irresponsible and will seriously damage international public health and safety and the vital interests of people in neighboring countries," the statement said, calling for Japan to "re-examine the issue."
The United States, meanwhile, showed support for its ally's decision.
"In this unique and challenging situation, Japan has weighed the options and effects, has been transparent about its decision, and appears to have adopted an approach in accordance with globally accepted nuclear safety standards," the US State Department said in a statement.
"We look forward to the (Japanese government's) continued coordination and communication as it monitors the effectiveness of this approach."