The problem is massive: the Fukushima Daiichi plant, where several reactors melted down after the 2011 tsunami overwhelmed cooling systems, generates 100,000 liters of contaminated water per day.

The mix of groundwater and rainwater that seeps into the area is used for cooling, then treated by plant operator TEPCO and stored on site. But after filling 1.33 million cubic meters, the plant has nearly run out of space.

TEPCO and Japan's government want to release the treated liquid, which has been diluted with seawater, via a pipe extending a kilometer from the coast where the plant sits.

They say filtration removes most radionuclides, elements that emit radiation, and renders the water no different from what is regularly released by nuclear plants elsewhere, a view endorsed by experts and the UN nuclear watchdog.

The release is a "robust plan," according to Tony Hooker, an associate professor at the University of Adelaide's Centre for Radiation Research, Education and Innovation.

"No environmental or human health impacts are likely to be observed," he said, though he noted there is growing debate about the global practice of ocean dumping.

Claims about risks "are not founded in scientific evidence," added Jim Smith, professor of environmental science at the University of Portsmouth.

"Those making claims... should consider the negative impact of their, I think scientifically baseless, claims on the communities in Japan who have been affected by the Fukushima accident," he added.

'Radiation risks'

Still, Japan's government and TEPCO have faced persistent concern and criticism, with some pointing to missteps around the initial accident as grounds for distrust.

Anti-nuclear group Greenpeace has been among the loudest voices condemning the plan, accusing the government of having "discounted the radiation risks."

And neighbors from China to some Pacific nations have expressed varying degrees of concern.

Fishing communities that spent years battling stigma and even bans on their catches now fear that rebuilt customer confidence will be destroyed.

It is no surprise then that dozens of government officials have been drafted in to work on swaying domestic and international opinion.

Regional and international delegations and media have been led on plant tours, sometimes stopping at tanks where fish swim in treated and diluted wastewater, an experiment that was streamed on YouTube.

There have also been technical briefings for neighbors like South Korea, though government officials said multiple invitations for such talks with China had gone unanswered.

Perhaps the most important plank of Japan's campaign is a review by the International Atomic Energy Agency, which has endorsed the release plan and will monitor its implementation.

"This is the organization which can provide reassurance to not only the Japanese people but also to the international community... they provide the gold standard," said Shinichi Sato, director of the international nuclear cooperation division at Japan's foreign ministry.

'Negligible impact'

The IAEA, whose chief is visiting Tokyo and the Fukushima plant this week, said on Tuesday the plan meets international standards and will have "a negligible radiological impact on people and the environment."

Still, it is likely to remain controversial, with Pacific Islands Forum Secretary General Henry Puna last month urging "more time and an abundance of caution."

Seoul has tempered earlier opposition and diplomatic ties between the neighbors warm, but that hasn't stopped panic buying of salt by some convinced the release will contaminate local seawater.

It is a painful moment for Fukushima's fishing communities, whose livelihoods have been battered.

A spokesman for the Fukushima Prefectural Federation of Fisheries Co-operative Associations said TEPCO's measures inspired little confidence about radiation risks.

But "the reputational damage is the bigger concern," he told AFP, declining to be named.

"There are still concerns raised both at home and abroad... and we want the government to do more."

The release is expected to unfold over several decades, and Japan's government says its campaign is far from over.

"We understand that this requires a lot of explanation, and we are ready to do that," said Sato.

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