Fukushima: wastewater from nuclear disaster to be discharged into the Pacific Ocean - is it dangerous?

Scientists and nuclear experts say extra radiation from the Fukushima disaster's contaminated wastewater discharged into the ocean "won’t make the tiniest jot of difference" - but the move remains internationally controversial.

Japan could start releasing treated and diluted radioactive wastewater from the Fukushima Daiichi nuclear plant disaster into the Pacific Ocean as early as Thursday (24 August), after Prime Minister Fumio Kishida gave the final go-ahead at a Cabinet meeting on Tuesday.

It has now been more than 12 years since the 9.1 magnitude Tōhoku earthquake - the fourth-strongest ever recorded - hit Japan on 11 March, 2011, causing mass fatalities and huge infrastructure damage.

It triggered deadly tsunamis, which had waves of up to 14 metres high. These seriously damaged generators at the Fukushima Daiichi Nuclear Power Plant - in the Fukushima Prefecture - causing meltdowns and a hydrogen explosion.

When the reactors melted, water - much of it used for cooling the plant - amounting to 1.34 million tonnes was contaminated, and is still stored on-site. Japan's government says it needs to be diluted and discharged so it can finish decommissioning the site, while reducing the risk of its hundreds of tanks leaking into the ground.

Many nuclear experts say they are not concerned, but environmental activists across Japan, South Korea and China have slammed the controversial move - some even protesting in the streets.

Here's everything you need to know:

Why is the move controversial in Japan, and other East Asian countries?

In the days leading up to the wastewater discharge beginning, there have been widespread protests in nearby South Korea.

Opposition leader Lee Jae-myung has called Japan's plan to discharge the water an act of "terror", Reuters reports, while surveys show the public's primary concerns are seafood and ocean contamination.

PA reports the release of the treated wastewater has also faced strong domestic opposition from Japanese fishing organisations - which worry about further damage to the reputation of their seafood amid their ongoing struggle to recover from the nuclear disaster.

China has also expressed concern for the move, making it a political and diplomatic issue. Hong Kong has said it will suspend exports from Fukushima and nine other prefectures if Japan goes ahead with the plan, while China has stepped up radiation testing on Japanese fisheries products - delaying customs clearance.

International environmental organisations like Greenpeace have also weighed in. In a statement, it condemned the decision, which "violates the human rights of communities in Japan and the Pacific region, and is non-compliant with international maritime law".

Greenpeace Japan project manager Hisayo Takada said they were "deeply disappointed and outraged" by the Japanese Government’s announcement. "Despite concerns raised by fishermen, citizens, Fukushima residents, and the international community, especially in the Pacific region and neighbouring countries, this decision has been made."

He also criticised the Japanese government from not learning from the the disaster. "Instead of acknowledging the flaws in the current decommissioning plan, the ongoing nuclear crisis, and the massive amount of public funds required, the Japanese government intends to restart more nuclear reactors despite evidence of major earthquakes and safety risks."

What do scientists think?

Professor Tom Scott, academic lead for the University of Bristol's Sellafield Centre of Expertise for Uranium and Reactive Metals, said from his viewpoint, discharging the water into the sea is the correct decision.

While other alternatives were examined, such as continued storage, he said "ultimately this is a situation where the actual quantity of tritium being discharged per litre of water is incredibly low, and hence the risk posed to the environment and people is incredibly low".

"When released into the Pacific, the tritium is further diluted into a vast body of water and would quickly get to a radioactivity level which is not discernibly different from normal sea water," Professor Scott added. "It poses very little risk and the risk itself decreases with time due to the relatively short radioactive half life... meaning that the amount of tritium - and hence the risk - continually reduces."

Sea water already contains small amounts of tritium, he said. "Tritium is produced naturally in small quantities in the upper atmosphere, and gets into the oceans through rainfall," although some of the atmospheric tritium was also left behind by from nuclear weapons testing.

Over in Australia, Curtin University physics professor Nigel Marks said the radioactivity in the Fukushima water was almost entirely tritium, a type of hydrogen.

"For scale, the Pacific Ocean contains 8,400 grams of pure tritium, while Japan will release 0.06 grams of tritium every year. The minuscule amount of extra radiation won’t make the tiniest jot of difference," he said. "A lifetime's worth of seafood caught a few kilometres from the ocean outlet has the tritium radiation equivalent of one bite of a banana."

Associate Professor Tony Hooker - director of the Centre for Radiation Research, Education and Innovation at the University of Adelaide - said this practice has been carried out for a long time.

“I would like to reiterate that the release of tritium from nuclear facilities into waterways has and is undertaken world-wide with no evidence of environmental or human health implications," he said. "I welcome the news about the impending release and support Japan’s decision to commence disposal."

The government had a robust radiation management plan in place, he said, that had been approved by the radiation protection agencies across the world.

“I think with the likely comprehensive independent monitoring of the environment to occur around the release site of Fukushima, that this will hopefully alleviate some of the fear that has been generated around this issue.”

The government and plant-owner Tepco say the water will be treated and then diluted with seawater to levels even safer than current international standards.

The plan is to release 7,800 tonnes of treated water in the 17 days, 31,200 tonnes by March 2024. This would empty just 10 of the thousand tanks at the site.

Seawater and marine life will be regularly tested throughout this period for any potential impact, and the results will be disclosed on government and Tepco websites.