Explainer - Fukushima: Why is Japan releasing water and is it safe?

The water was distilled after being contaminated from contact with fuel rods at the reactor, destroyed in a 2011 earthquake and tsunami.

Tanks on the site hold about 1.3 million tonnes of the water - enough to fill 500 Olympic-sized swimming pools.

The utility responsible for the plant, Tokyo Electric Power (Tepco), has been filtering the contaminated water to remove isotopes, leaving only tritium, a radioactive isotope of hydrogen that is hard to separate. Tepco will dilute the water until tritium levels fall below regulatory limits before pumping it into the sea from the site on the coast north of Tokyo.

Water containing tritium is routinely released from nuclear plants around the world, and regulatory authorities support dealing with the Fukushima water in this way.

Tritium is considered to be relatively harmless because its radiation is not energetic enough to penetrate human skin. When ingested at levels above those in the released water it can raise cancer risks, a Scientific American article said in 2014.

The water disposal will take decades to complete alongside the planned decommissioning of the plant.

Japan and scientific organisations say the water is safe, but environmental activists argue that all possible impacts have not been studied. Japan says it needs to start releasing the water as storage tanks are full.

The International Atomic Energy Agency (IAEA), the U.N. nuclear watchdog, gave the plan a green light in July, saying it met international standards and the impact on people and the environment would be "negligible".

Greenpeace said on Tuesday that the radiological risks have not been fully assessed, and the biological impacts of tritium, carbon-14, strontium-90 and iodine-129 - to be released with the water - "have been ignored".

The filtering process will remove strontium-90 and iodine-129, and the concentration of carbon-14 in the contaminated water is far lower than its regulatory standard for discharge, according to Tepco and the government.

Japan said tritium levels in the water will be below those considered safe for drinking under World Health Organization standards.

The government said in a document it would take "appropriate measures, including immediate suspension of the discharge" if unusually high concentrations of radioactive materials were detected.

The South Korean government has concluded from its own study that the water release meets international standards and said it respected the IAEA assessment.

Tepco has been engaging with fishing communities and other concerned groups and is promoting agriculture, fishery and forest products to reduce any reputational harm to produce from the area.

Fishing unions in Fukushima have urged the government for years not to release the water, arguing it would undo work to restore the damaged reputation of their fisheries.

Masanobu Sakamoto, the head of the National Federation of Fisheries Cooperative Associations, said on Monday the group understood the release could be scientifically safe but still feared reputational damage.

Neighbouring countries have also expressed concern. China has been the most vocal, calling Japan's plan irresponsible, unpopular and unilateral. China is the biggest importer of Japanese seafood.

Shortly after the 2011 tsunami and earthquake damaged the Fukushima plant, China banned imports of food and agricultural products from five Japanese prefectures. China later widened its ban to 10 of Japan's 47 prefectures.

The latest import restrictions were imposed in July after the IAEA approved Japan's plans to discharge the treated water.

On March 11, 2011, a magnitude 9.0 quake hit off the coast of northeast Japan, triggering a tsunami that devastated towns and villages and sparked the world's worst nuclear accident since Chernobyl.

The tsunami swamped backup power and cooling systems at the Fukushima plant, eventually causing meltdowns at three of six reactors. More than 160,000 people were eventually evacuated from the area.