Japan will soon begin releasing treated radioactive water into the ocean following approval from the United Nations' nuclear watchdog for a controversial plan that comes 12 years after the Fukushima nuclear meltdown.

The plan to release wastewater has been in the works for years, with the environment minister declaring in 2019 there were "no other options" as space runs out to contain the contaminated material.

Rafael Grossi, chief of the International Atomic Energy Agency (IAEA), arrived in Japan on Tuesday to visit Fukushima and present the UN body's safety review to Prime Minister Fumio Kishida.

But the UN's approval has done little to reassure rattled residents in neighboring countries, and local fishermen who still feel the impact of the 2011 disaster.

Some have cast doubt on the IAEA's findings, with China recently arguing that the group's assessment "is not proof of the legality and legitimacy" of Fukushima's wastewater release.

Here's what you need to know.

Why are they doing this?

The devastating 2011 earthquake and tsunami damaged the Fukushima nuclear plant's power supply and cooling systems -- causing the reactor cores to overheat and contaminate water within the plant with highly radioactive material.

Since then, new water has been pumped in to cool fuel debris in the reactors. At the same time, ground and rainwater have leaked in, creating more radioactive wastewater that now needs to be stored and treated.

The state-owned electricity firm Tokyo Electric Power Company (TEPCO) has built over 1,000 massive tanks to contain what is now 1.32 million metric tons of wastewater -- enough to fill more than 500 Olympic pools.

But space is quickly dwindling. The company says building more tanks isn't an option, and it needs to free up space in order to safely decommission the plant -- a process that involves decontaminating facilities, dismantling structures and fully shutting things down.

What are the risks?

Radioactive wastewater contains some dangerous elements, but the majority of these can be removed from the water, said TEPCO.

The real issue is a hydrogen isotope called radioactive tritium, which cannot be taken away. There is currently no technology available to do so.

But Japan's government and the IAEA say the contaminated water will be highly diluted and released slowly over decades.

That means the concentration of tritium being released would be on par or lower than the amount other countries allow, and meet international safety and environmental regulations, they say.

TEPCO, Japan's government, and the IAEA also argue that tritium occurs naturally in the environment, from rain to sea water to tap water, and even in the human body -- so releasing small amounts into the sea should be safe.

In the IAEA report, Grossi said discharging treated water into the sea would have a "negligible radiological impact on people and the environment."

But experts are divided on the risk this poses.

The Canadian Nuclear Safety Commission says tritium itself is too weak to penetrate the skin -- but can increase the risk of cancer if consumed in "extremely large quantities." Meanwhile, the US Nuclear Regulatory Commission acknowledged that "any exposure to radiation could pose some health risk" -- but added that "everyone is exposed to small amounts of tritium every day."

Robert H. Richmond, director of the Kewalo Marine Laboratory at the University of Hawaii at Manoa, is among a group of international scientists working with the Pacific Island Forum to assess the wastewater release plan -- including visits to the Fukushima site, and meetings with TEPCO, Japanese authorities and the IAEA. After reviewing the details of the plan, Richmond called it "ill-advised" and premature.

One concern is that diluting the wastewater might not be enough to reduce its impact on marine life. Pollutants like tritium can pass through various levels of the food chain -- including plants, animals, and bacteria -- and be "bioaccumulated," meaning they will build up in the marine ecosystem, he said.

He added that the world's oceans are already under stress from climate change, ocean acidification, overfishing and pollution. The last thing it needs is to be treated like a "dumping ground," he said.

And the potential risks won't just affect the Asia-Pacific region. One 2012 study found evidence that bluefin tuna had transported radionuclides -- radioactive isotopes like the ones in nuclear wastewater -- from Fukushima across the Pacific to California.

How will the water be released?

First, the wastewater will be treated to filter out all the removable harmful elements. The water is then stored in tanks and analyzed to measure how radioactive it still is; much of it will be treated a second time, according to TEPCO.

The wastewater will then be diluted to 1,500 becquerels of tritium -- a unit of radioactivity -- per liter of clean water.

For comparison, Japan's regulatory limit allows a maximum of 60,000 becquerels per liter. The World Health Organization allows 10,000, while the US has a more conservative limit of 740 becquerel per liter.

The diluted water will then be released through an undersea tunnel off the coast, into the Pacific Ocean. Third parties including the IAEA will monitor the discharge during and after its release.

"This will ensure the relevant international safety standards continue to be applied throughout the decades-long process laid out by the government of Japan and TEPCO," Grossi said in the report.

What have other countries said?

The plan has met a mixed reaction, with support from some corners and skepticism from others.

The US has backed Japan, with the State Department saying in a 2021 statement that Japan had been "transparent about its decision" and seems to be following "globally accepted nuclear safety standards."

Taiwan's Atomic Energy Council said the amount of tritium being released is estimated to be "below the detection limit, and the impact on Taiwan will be minimal." The island is located southwest of Japan.

But there is more resistance from Japan's closer neighbors.

In March, a prominent Chinese official warned the wastewater could cause "unpredictable harm to the marine environment and human health," adding: "The Pacific Ocean is not Japan's sewer for discharging its nuclear contaminated water."

The Secretary General of the Pacific Islands Forum, an inter-governmental group of Pacific islands including Australia and New Zealand, also published an op-ed in January voicing "grave concerns."

"More data is needed before any ocean release should be permitted," he wrote. "We owe it to our children and grandchildren to work toward ensuring that their futures are secured and safe."

South Korean Prime Minister Han Duck-soo showed support for the plan in June, saying he could drink the wastewater after it had been treated to meet international standards, according to Yonhap -- a statement ridiculed by the country's opposition leader.

Don't other countries release wastewater too?

Many bodies, including the IAEA, point out that nuclear plants around the world routinely and safely release treated wastewater containing low levels of tritium.

A spokesperson from the US' Nuclear Regulatory Commission, a government body, confirmed to CNN that "virtually all nuclear plants in the US discharge water containing low levels of radioactivity to the waterway on which they are located."

"Tritium cannot be filtered out, but a member of the public would have to ingest a significant amount of it for there to be even the possibility of a health concern and radioactive water released is greatly diluted by the flows in the waterway," the spokesperson added.

Many scientists aren't reassured. Tim Mousseau, a biological sciences professor at the University of South Carolina, pointed out that even if this is common practice among nuclear plants, there just isn't enough research into the impact of tritium on the environment and on our food items.

Richmond, from the University of Hawaii, added that "other people's bad behavior" was not an excuse to continue releasing wastewater into the ocean. "This is an ultimate opportunity for (Japan and the IAEA) to change the way in which business is being done for the better," he said.

How does the public feel?

There has been much more skepticism from residents in the region -- prompting some shoppers to stock up on seafood and sea salt, for fear these products may be impacted by the wastewater release.

In South Korea, sea salt prices have jumped, with store owners saying their sales had doubled recently, Reuters reported. It cited a viral tweet in Korean that claimed to have bought three years' worth of seaweed, anchovies and salt.

The Korean fisheries authority also said it would ramp up efforts to monitor salt farms for radioactivity, and maintain a ban on seafood from waters near Fukushima, Reuters reported.

Members of the Korean public have also staged protests against the plan, with some donning gas masks outside the Japanese embassy in Seoul.

Opinion is mixed among the Japanese public, too. A survey by Asahi Shimbun in March found that 51% of 1,304 respondents supported the wastewater release, while 41% opposed it. Earlier this year, residents in the capital Tokyo took to the streets to protest the plan.

In Fukushima, the prefecture where the disaster occurred, local fishermen have been vocal against the plan from day one. For many years after the meltdown, authorities suspended their fishing operations and other countries introduced import restrictions.

Even after the surrounding water and fish returned to safe levels, consumer confidence was never fully restored, and Fukushima's fishing industry is now worth just a fraction of what it once was.

The release of wastewater could further damage Fukushima's global and regional reputation -- once again hurting fishermen's livelihoods, many argue. Earlier this year, one told CNN: "It really feels like they made this decision without our full consent."