SEOUL – At a busy intersection in Seoul, South Korea, this summer, a banner from the main opposition Democratic Party barked “No!” to Japan’s plan to dump tritiated water from its destroyed Fukushima nuclear power plant into the Pacific.

​Across the street, a placard from the governing People Power Party said the real threat was the opposition spreading conspiracy theories that would scare people away from seafood: “The Democratic Party is killing the livelihoods of our fishermen!”

Japan’s decision to release more than 1.3 million tons of ​treated water at Fukushima Daiichi, the power plant that was destroyed by an earthquake and tsunami in 2011, has raised alarms across the​ Pacific.

But in South Korea, it has triggered a particularly raucous political debate, with the government of President Yoon Suk-yeol and its enemies slugging it out through banners, YouTube videos, news conferences and protests.

​What sets South Korea apart from other critics in the region is that its government has endorsed Japan’s discharge plan despite widespread public misgiving, only asking Japan to provide transparency to ensure the water is discharged properly.

The authorities are running online advertisements and holding daily news briefings to dispel what they call fear-mongering by the opposition and to convince people that the water will do no harm.

But the continued uproar in South Korea over the ​discharge has threatened to complicate the progress the United States, Japan and South Korea have made in recent months to build a stronger trilateral partnership. Prime Minister Fumio Kishida visited the Fukushima site on Sunday.

Government critics accuse Mr Yoon of agreeing to the Fukushima water release plan for the sake of improving relations with Japan, South Korea’s historical enemy, and at the behest of the US, a strong ally of both nations.

Mr Yoon’s recent attempts to mend ties with Japan by burying long-time historical feuds have pleased Washington, which has pushed to align Seoul and Tokyo more closely together in a broader effort to counter China, North Korea and Russia.

“We need to improve ties with Japan, but also important is to protect our people’s health,” National Assembly majority whip Park Kwang-on, a member of the Democratic Party, said in an interview. “I cannot help suspecting that President Yoon made a compromise on this to improve relations with Tokyo.”

A majority of South Koreans were sceptical when Mr Yoon’s government said it was time to improve ties with Japan, according to recent surveys. When his government said not to worry about the Fukushima plan, they baulked at Japan’s ability to successfully filter the contaminated water and be transparent about its safety.

Japan has 1,000 large tanks to hold water that has been used to cool the destroyed reactor cores at the Fukushima plant. As tank capacity runs out, Japan wants to gradually release the water into the ocean over the next 30 years, after filtering and diluting it to meet Tokyo’s regulatory standards.

When the plan was first announced in 2021, the US Food and Drug Administration said it saw “no impact to human and animal health” if the treated wastewater was discharged as proposed. Independent experts appointed by the United Nations’ Human Rights Council, however, warned of “considerable risks” to millions of lives and livelihoods in the Pacific region.

In July, the International Atomic Energy Agency, the United Nations’ nuclear watchdog, endorsed Japan’s plan, calling the water’s radiological impact “negligible”. Weeks later, environmental regulators in Massachusetts denied a similar request to release treated wastewater from a shuttered nuclear power plant into Cape Cod Bay.

Like Japan, other nations filter cooling water from their nuclear power plants and release the treated water into the ocean. But critics say the water from Fukushima has been contaminated with more hazardous radioactive materials than what is typical.

“Scientifically speaking, the issue at stake is simple: whether enough radioactive materials would reach our country to affect us,” Dr Chung Bum-jin, president-elect of the Korean Nuclear Society, said in an interview. “But when politics gets into the mix, the question gets complicated, with more than one answer.”

“What matters is whether Japan releases its water according to international standards. All else is demagoguery,” Dr Chung added. “We can’t really meddle as long as Japan releases its water below regulatory limits.”

Marine discharge is the “surest” way that the water can be disposed of safely, said Dr Jeong Yong-hoon, a professor of nuclear engineering at the Korea Advanced Institute of Science and Technology. Other disposal options only make its eventual route to the sea – and the process of assessing the environmental impact – more complicated, he said.

To add assurances, South Korea vowed to ramp up efforts to monitor seawater and fisheries for any rise in radioactive substances after the water is released. It also said that its ban on seafood from around Fukushima, first imposed following the 2011 disaster, will remain until people feel confident that the water is safe.

Some governing party lawmakers went as far as to drink water from fish tanks in a local fish market to prove their point.

“What Japan is trying to do is unprecedented: It’s no ordinary cooling water from a normal nuclear power plant that it wants to dump into the sea; it’s laced with all kinds of hazardous radionuclides from the meltdown reactor cores,” said Dr Seo Kyun-ryul, a professor emeritus of nuclear engineering at Seoul National University.

Japan has dismissed other long-term disposal options, such as keeping the water on land by adding more tanks, digging an artificial lake or mixing it into mortar, angering critics in South Korea, China and Pacific island countries.

“Japan made the cheapest choice – simply dumping it into the ocean,” said Mr Park, the lawmaker in Seoul. “It may gain economic benefits from that, but it loses the trust of people in neighbouring countries.” NYTIMES