In tests taken by several Japanese government agencies and Tepco, the water released starting last week contained scant amounts of tritium, far below the standard set by the World Health Organization. There is more tritium in water being discharged by nuclear power plants in China and in South Korea, where protesters have also condemned the Japanese release.

With a monitoring network that includes the International Atomic Energy Agency and experts from numerous countries, “the international pressure is really high on the government in Japan,” said Kai Vetter, a professor of nuclear engineering at the University of California, Berkeley, who has studied the environmental and social impacts of the Fukushima disaster.

Hirokazu Matsuno, the chief cabinet secretary to Japan’s prime minister, Fumio Kishida, said on Monday that Japan had “made counterarguments many times against information, including contents which are not factual, that have been released from China.”

Part of the challenge for Japan, where the foreign ministry is using the hashtag #LetTheScienceTalk on X, the social media platform formerly known as Twitter, is that the science is difficult for average citizens to comprehend and that people often react emotionally to such events.

“It’s understandable that people worry and are fearful of something they don’t know well,” said Ittaka Kishida, a professor at Aoyama Gakuin University in Tokyo who studies the sociology and history of nuclear physics. “They just have to trust what experts explain, even though they haven’t seen it or can’t confirm it with their own eyes.”