Japan began the release of wastewater from the Fukushima nuclear plant on Thursday, facility operator TEPCO said.

The process of pumping treated water into the Pacific Ocean through a special kilometer-long tunnel was started despite opposition from fishermen, environmentalists and China.

An earthquake and tsunami caused core meltdowns at the plant in 2011. Since then, the shut-down reactors have had to be cooled with water that was then stored in tanks. 

However, according to TEPCO, capacity is running out. 

What else do we know?

The site has been collecting some 100,000 liters (26,500 gallons) of water every day. Around 1.34 million metric tons are now being stored there.

The water is contaminated not only from cooling the damaged reactors, but also with the seepage of groundwater and rain.

Japan has said that it will discharge at most 500,000 liters per day, with the release of the water planned to take some 30 years to complete.  

Thursday's discharge, which authorities say is on a small scale, is scheduled to be followed by three more between now and March 31.

The International Atomic Energy Agency (IAEA) has said the release of the water is safe.

The Japanese government and TEPCO say the release is necessary to make room for the plant's decommissioning and to prevent accidental leaks.

Japan has said almost all radioactive elements from the water have been filtered out before its release.

The only exception is tritium, which is difficult to filter. But many nuclear experts say tritium poses little risk to human health, as it does not accumulate in the body.

"Nuclear power plants worldwide have routinely discharged water containing tritium for over 60 years without harm to people or the environment, most at higher levels than the 22 TBq per year planned for Fukushima," Tony Irwin, an honorary associate professor at the Australian National University, said
in a note cited by Reuters news agency.

Chinese criticism

China, which has staunchly opposed the release of the wastewater from the start, has slammed Japan for beginning with the discharge.

"The ocean is the common property of all humanity, and forcibly starting the discharge of Fukushima's nuclear wastewater into the ocean is an extremely selfish and irresponsible act that ignores international public interests," the Foreign Ministry said in a statement. 

It said Tokyo had failed to prove that its wastewater purification was reliable in the long term and to give sufficient evidence to support its contention that the discharge was harmless.

Beijing has banned food imports from 10 Japanese prefectures, with Hong Kong following suit.

South Korea said in a statement released Tuesday that it did not necessarily approve the wastewater release plan but that the scientific and technical basis for it appeared to be in order.

tj/rc (AFP, Reuters, AP)