Fukushima disaster: What happened at the nuclear plant?

On Thursday, Japan will release treated radioactive wastewater from its Fukushima nuclear power plant - the site of a meltdown disaster 12 years ago.

The United Nations atomic energy regulator says the discharge of filtered water into the Pacific Ocean is safe and will have "negligible" impact on people and the environment.

But the move has drawn fierce criticism from citizens of countries in the region. It is the latest development in the continuing saga of one of the world's worst-ever nuclear disasters.

It began on 11 March 2011, when the most powerful earthquake ever recorded in Japan struck off the country's eastern coast.

The 9.0-magnitude quake was so forceful it shifted the Earth off its axis. It triggered a tsunami which swept over Japan's main island of Honshu, killing more than 18,000 people and wiping entire towns off the map.

At the Fukushima nuclear power plant, the gigantic wave surged over coastal defences and flooded the reactors, sparking a major disaster. Authorities set up an exclusion zone which grew larger and larger as radiation leaked from the plant, forcing more than 150,000 people to evacuate from the area.

More than a decade later, that zone remains in place and many residents have not returned. Authorities believe it will take up to 40 years to finish the work of decontamination, which has already cost Japan trillions of yen.

The Fukushima Disaster is classified as a level seven event by the International Atomic Energy Agency, the highest such event and only the second event to meet this classification after the 1986 Chernobyl disaster.

The Fukushima Daiichi Nuclear Power Plant is in the town of Okuma, in Fukushima Prefecture. It sits on the country's east coast, about 220km (137 miles) north-east of the capital Tokyo.

On 11 March 2011 at 14:46 local time (05:46 GMT) the earthquake - known as the Great East Japan Earthquake, or the 2011 Tohoku earthquake - struck east of the city of Sendai, 97km north of the plant.

Residents had just 10 minutes warning before the tsunami hit the coast.

Overall almost half-a-million people were forced to leave their homes as a result of the earthquake, tsunami and nuclear accident.

Systems at the nuclear plant detected the earthquake and automatically shut down the reactors. Emergency diesel generators turned on to keep coolant pumping around the cores, which remain incredibly hot even after a shutdown.

But soon after a wave over 14 metres (46ft) high hit Fukushima. The water overwhelmed the defensive sea wall, flooding the plant and knocking out the emergency generators.

Workers rushed to restore power, but in the days that followed the nuclear fuel in three of the reactors overheated and partly melted the cores - something known as a nuclear meltdown.

The plant also suffered a number of chemical explosions which badly damaged the buildings. Radioactive material began leaking into the atmosphere and the Pacific Ocean, prompting evacuations and an ever-widening exclusion zone.

At least 16 workers were injured in the explosions, while dozens more were exposed to radiation as they worked to cool the reactors and stabilise the plant. Three people were reportedly taken to hospital after high-level exposure.

A number of people were later confirmed to have died in the evacuation, including dozens of hospital patients who had to be moved due to fears of radiation.

The long-term effects of radiation exposure are a matter of debate. The World Health Organization (WHO) released a report in 2013 that said the disaster will not cause any observable increase in cancer rates in the region. Scientists both inside and outside Japan believe that aside from the region immediately around the plant, the risks of exposure remain relatively low.

On 9 March 2021, ahead of the 10-year anniversary, a UN report said there had been "no adverse health effects" documented among Fukushima residents directly related to the radiation from the disaster. Any future radiation-related health effects were "unlikely to be discernible", it said.

But many believe the dangers are far greater, and residents remain wary. Though officials have lifted restrictions in many areas most people have not returned to their homes.

In 2018, the Japanese government announced that one worker had died after exposure to radiation and agreed that his family should be compensated.

Critics blamed a lack of preparedness for the event, as well as a muddled response from both the plant operator Tokyo Electric Power (Tepco) and the government.

An independent investigation by Japan's parliament concluded that Fukushima was "a profoundly man-made disaster", blaming the energy company for failing to meet safety requirements or to plan for such an event. However, in 2019 a Japanese court cleared three former Tepco executives of negligence in what was the only criminal case to come out of the disaster.

In 2012, Japan's then prime minister Yoshihiko Noda said the state shared the blame for the disaster. A court ruled in 2017 that the government bore partial responsibility and should pay compensation to evacuees.

These days, several towns in north-eastern Japan remain off limits. Authorities are working to clean up the area so residents can return.

Some have decided never to return because they fear radiation, have built new lives elsewhere or don't want to go back to where the disaster hit.

Major challenges remain. Tens of thousands of workers will be needed over the next 30 to 40 years to safely remove nuclear waste, fuel rods and more than one million tons of radioactive water still being kept at the site.

The water used to cool the highly radioactive reactors was mixed with groundwater and rain and stored in tanks at the plant but Japan says it's run out of space.

As such the wastewater is being released into the Pacific Ocean - heavily filtered to reduce radioactivity.

Operators argue the huge ocean will dilute the water and that it would pose a low risk to human and animal health. However, there remain critics and sceptics of the plan.