The operator of the Fukushima Daiichi nuclear plant has begun its second round of the release of treated and diluted water from the crippled plant into the sea.

The nuclear plant in northeastern Japan suffered a triple meltdown in the 2011 earthquake and tsunami.

Water used to cool molten nuclear fuel mixes with rain and groundwater, and is stored in some 1,000 tanks within the plant compound.

The accumulated water is treated to remove most radioactive substances, but still contains tritium.

On Tuesday, Tokyo Electric began preparing for the second release by adding about 1,200 tons of seawater to one ton of treated water. The mixture is stored in a pit called a discharge vertical shaft to check the degree of dilution.

The tritium level was found to be 63 to 87 becquerels per liter, well below Japan's environmental release standard of 60,000 becquerels and the utility's own limit of 1,500 becquerels.

The second release started at 10:18 a.m. on Thursday.

Tokyo Electric Power Company started the first round on August 24, after diluting the treated water to reduce tritium to about one-seventh of the guidance level set by the World Health Organization for drinking water quality.

The release was completed on September 11 as planned.

The maximum level of tritium in seawater samples taken within three kilometers of the plant was 10 becquerels per liter, far below the 700 becquerels that would prompt a halt to the release.

This time around, Tokyo Electric plans to discharge about 7,800 tons of treated water from 10 tanks over a period of 17 days. The amount is the same as for the first round.

The treated water at the Fukushima plant amounted to over 1.3387 million tons as of September 28, or about 98 percent of the tanks' capacity.

About 31,200 tons from 40 tanks are planned to be released by the end of March.