International Atomic Energy Agency chief Rafael Grossi called it "a very special night," as he handed Japanese Prime Minister Fumio Kishida the final report after a two-year review.

It said Japan's plans were consistent with global safety standards and that they'd have a, quote, "negligible radiological impact to people and the environment."

Grossi later told reporters at the Japan National Press Club, where he was met by a small group of protesters, that he'd seek to allay lingering concerns.

"The IAEA will have a continued presence at the site, we will continue the review if the plan moves forward because as you know this is a decision by the government."

Japan's government maintains the process is safe, insisting it will act with transparency, and saying it has treated the water used to cool the fuel rods of the Fukushima plant after it was damaged by the 2011 earthquake and resulting tsunami.

Japan hasn't specified a date to start the release of the water - enough to fill 500 Olympic-sized swimming pools.

Japanese fishing unions have long opposed the plan.

They say it would undo work to repair reputations after several countries banned some Japanese food products after the disaster.

Some neighboring countries have also complained over the years about the threat to the marine environment and public health, with Beijing emerging as the biggest critic, calling for the suspension of the plan.