Tokyo Electric Power Company has begun the release of treated and diluted water from the crippled Fukushima Daiichi nuclear plant into the ocean. It is a milestone in a decades-long effort to decommission the plant.

Prime Minister Kishida Fumio says the International Atomic Energy Agency analyzed the water to confirm the concentration of tritium was well within safety standards ahead of the release.

Kishida said: "We expect the marine discharge of the treated water will be conducted in an even more transparent manner. The Japanese government will do its utmost to ensure this."

Fukushima Daiichi suffered a triple meltdown in the 2011 earthquake and tsunami.

Since then, water used to cool molten fuel at the plant is mixing with rain and groundwater, seeping into the damaged reactor buildings.

The volume has been accumulating. TEPCO says because of the roughly 1,000 tanks of water, there is not enough room to store fuel debris to be extracted from the reactor containment vessels, as well as radioactive waste.

The water is being treated to remove most radioactive substances, but still contains tritium. Before the release, the operator is diluting the treated water to reduce tritium levels to about one-seventh of the World Health Organization's guidelines for drinking water.

Tritium is measured in units called "becquerel.'' Before releasing the water, TEPCO confirmed the diluted water contained between 43 to 63 becquerels per liter. That is far below Japan's environmental release standards of 60,000 becquerels per liter.

The water will then be moved through a tunnel under the seabed and discharged one kilometer off the coast. The first round of the process will take around 17 days, and involve the release of about 7,800 tons of treated water.

People in the local fishing and tourism industries are concerned about the impact it could have on the region. The government has promised to work hard to prevent reputational damage.