Here is what we know about the discharge, how the water has been treated and concerns about the safety of the exercise:

- Why the operation? -

Around 100,000 litres (26,500 gallons) of water -- contaminated by cooling the plant's wrecked reactors plus groundwater and rain seeping in -- is collected at the site in northeastern Japan every day.

Some 1.34 million tonnes -- equivalent to almost 540 Olympic pools -- are now stored in around a thousand steel containers at the seaside site, and now there is no more space, authorities say.

Japan decided in 2021, after years of discussion, that it would release at most around 500,000 litres per day into the sea via a pipe one kilometre (0.6 miles) long.

The discharge will take around 30 years to complete. The release on Thursday was the first of four scheduled between now and the end of March.

- What has been done to the water? -

Plant operator TEPCO says that a special filtering system called ALPS has removed all radioactive elements -- including caesium and strontium -- except tritium.

TEPCO has said the water is diluted to reduce radioactivity levels to 1,500 becquerels per litre (Bq/L), far below the national safety standard of 60,000 Bq/L.

- Is that safe? -

Tony Hooker, a nuclear expert from the University of Adelaide, said that the level of tritium is well below the World Health Organization drinking water limit of 10,000 Bq/L.

"Tritium is regularly released from nuclear power facilities into waterways worldwide," Hooker told AFP.

"For decades, (there have been) no evidential detrimental environmental or health effects," he said.

UN atomic watchdog the International Atomic Energy Agency (IAEA) has said the release meets international standards and "will not cause any harm to the environment".

- Does everyone agree? -

The overriding consensus among international experts appears to be that the operation is safe.

But Greenpeace says that the filtering technology does not work and that the IAEA "completely ignored the highly radioactive fuel debris that melted down and continues every day to contaminate ground water".

China has accused Japan of treating the Pacific like a "sewer". It said Thursday that the release was "extremely selfish and irresponsible".

Along with Russia, China had proposed vaporising the water and releasing the steam into the atmosphere instead.

Beijing in July banned food imports from 10 Japanese prefectures and imposed stringent radiation tests on food from the rest of the country. This week, Hong Kong and Macau followed suit.

While Seoul has not expressed objections, many South Koreans are alarmed and have been staging demonstrations -- and even panic-buying sea salt.

The release has also hit opposition in Japan itself, in particular from a fishing industry that fears its exports could plummet as consumers and governments shun Japanese seafood.

- What has Japan done to soothe concerns? -

The government has spent months trying to win over sceptics at home and abroad, with everything from study tours of Fukushima to live video of fish living in the wastewater.

Tokyo has also sought to counter disinformation being peddled online about the release, such as misrepresented photos of deformed fish and claims -- denied by Japan -- that it bribed the IAEA.

- What else needs to be done? -

The far more dangerous task is the removal of radioactive debris and highly dangerous nuclear fuel from the three reactors that went into meltdown in 2011.

TEPCO plans to use robots to remove the fuel but there are fears that radiation levels are so high that they could even disable the machines.

The whole gargantuan process is expected to take 30 to 40 years and cost around eight trillion yen ($55 billion).

burs-stu/qan