SEOUL — It turns out that South Korea and Japan now have yet one more insoluble, never-ending issue to upset all attempts at patching up their centuries of hostility — and, strangely, it’s in the water.

No sooner had the Japanese begun what promises to be a decades-long process of draining the slightly radioactive water from the Fukushima nuclear plant than protesters — screaming slogans — were attempting to get into the Japanese embassy here.

Sixteen of them were quickly arrested, but the demonstration was just the most visible sign of the response to assurances by both Japanese and Free Korean leaders that the water’s fine, probably less radioactive than all the other water in the sea, and no danger to fish or humans.

Scientific tests, the testimony of experts, and the results of any number of studies aren’t going to stop the critics. They range from North Korea to opposition politicians to fishermen. They won’t be stopped from pillorying the government of President Yoon for endorsing Japan’s decision to begin draining the storage tanks. 

The tanks are brimming with 1.3 million tons of contaminated sea water from the disaster that killed nearly 20,000 persons caught in the tsunami that inundated the plant and surrounding region in March 2011. 

The worst fallout from Mr. Yoon’s decision to go along with Japanese pleas for him to endorse the draining is that it could undermine goodwill displayed in their trilateral summit at Camp David. If Mr. Yoon, President Biden, and Prime Minister Kishida mentioned behind closed doors the need to drain the nuclear plant, they definitely skipped over it in all the statements from the confab.

Plus, too, the problem for Messrs. Yoon and Kishida is that the fallout from the drainage is unlikely to blow away any time soon. It will take at least 30 years to empty the storage tanks, and their foes are not going to let up after a flurry of objections.

The chairman of South Korea’s opposition Democratic Party, Lee Jae-myung, signaled the enduring nature of the protest when he accused Japan of making “the vicious decision to discharge contaminated water into the ocean, which belongs to all humans without scientific proof, understanding from neighboring countries or agreement by the Japanese people.”

That statement contradicts any number of tests showing only minimal risks, but politics trumps science. In the 2022 presidential election, the leftist Mr. Lee lost to the conservative Mr. Yoon by only an eyelash, and he plans to try again in 2027 against another conservative. Mr. Yoon cannot seek a second term under Korea’s constitution.

As might be expected, the most vituperative criticism has come from North Korea, which has denounced “the evil, anti-humanitarian and belligerent action.” The Communist Chinese and Russians joined the chorus, China claiming “Japan has yet to provide sufficient scientific and convincing explanations” and banning imports of seafood from Japan.

Lost in the criticism is that the International Atomic Energy Agency has said the danger from the water, after it’s been processed, will be negligible.

The agency’s director-general, Rafael Mariano Grossi, said his experts were “on the ground to serve as the eyes of the International community and ensure that the discharge is being carried out as planned consistent with IAEA safety standards.”

Also, the agency said its “on-site analysis confirmed that the tritium concentration in the diluted water” was “far below the operational limit of 1,500 becquerels per liter.” The science magazine Nature, though, does not give the released water quite such a clean bill of health.

“Radiation in the water will be diluted to almost-background levels,” Nature’s report says, “but some researchers are not sure this will be sufficient to mitigate the risks.”

The power-station operator, Tokyo Electric Power Company, according to Nature, used an advanced liquid-processing system to treat the water in a five-stage process. “But that process does not remove carbon-14 and tritium,” Nature says, “so the treated water needs to be diluted further to less than one part per 100 parts of seawater.”

Nature quotes an environmental scientist at Britain’s University of Portsmouth, James Smith, as preferring to say the risk is “close to zero,” hesitating to say the risk is zero. “The risk of another earthquake or a typhoon causing a leak of a tank is higher,” he tells Nature. “They’re running out of space.”

None of which is all that reassuring to fishermen and seafood eaters. The release of water from the Fukushima plant “deepened concerns among South Koreans over the safety of seafood,” South Korea’s Yonhap News said.

“Restaurant owners and fishermen,” Yonhap News reports, “are already feeling the pinch of the controversial discharge.” Seoul has “flatly dismissed chances” of lifting a ban on Japanese seafood imports, Yonhap said, “while intensifying monitoring and testing to dispel public jitters.”