Twelve years after a nuclear catastrophe triggered by a massive earthquake and tsunami, workers at the Fukushima Daiichi plant in northeast Japan are preparing to release treated wastewater into the sea.

Operator TEPCO says the water has been filtered to remove most radioactive elements, and calls the release both safe and necessary, but there has been domestic and international opposition.

- Why does the water need to be released? -

The site produces 100,000 litres  of contaminated water daily. It is a combination of groundwater, rainwater that seeps into the area, and water used for cooling.

The water is filtered to remove most radionuclides, and more than 1.32 million tons of treated water was being stored at the site as of February.

That accounts for 96 percent of storage capacity, so TEPCO is keen to start releasing the water soon.

Under a plan approved by the central government, the process is expected to begin this spring or summer.

- Is it safe? -

TEPCO says several filtering systems, including in its ALPS facility, remove most of the 62 radioactive elements in the water, including caesium and strontium, but tritium remains.

Experts say tritium is only harmful to humans in large doses, and TEPCO plans to dilute the water to reduce radioactivity levels to 1,500 becquerels per litre, far below the national safety standard of 60,000 becquerels per liter.

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

Neighboring countries, including China and South Korea, along with activist groups such as Greenpeace and some local residents are strongly opposed to the release.

Local fishermen fear the release would once again make consumers wary of buying their catch.

"We have suffered reputational damage since the disaster, and we will go through that all over again, starting from zero," fisherman Masahiro Ishibashi, 43, told AFP.