The operator of the Fukushima Daiichi nuclear power plant is set to begin its second round of the release of treated and diluted water from the crippled plant into the ocean on Thursday.

Accumulated water is treated to remove most radioactive substances, but still contains tritium. The water is stored in some 1,000 tanks within the plant compound.

Tokyo Electric Power Company started its first round of discharge on August 24 after diluting the 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.

Seawater samples taken within 3 kilometers of the plant showed that the concentration of tritium was below the minimum detectable level of 10 becquerels per liter, and well below 700 becquerels per liter, the level set for the utility to suspend the release.

Ahead of the release, TEPCO on Tuesday checked the concentration of tritium in one ton of treated water mixed with 1,200 tons of seawater stored in a pit called a discharge vertical shaft.

Officials found that the tritium levels were between 63 and 87 becquerels per liter, far below Japan's environmental release standards of 60,000 becquerels per liter and TEPCO's own limit of 1,500 becquerels.

The second release will begin as early as Thursday morning. Some 7,800 tons of treated water stored in 10 tanks will be released simultaneously over a period of 17 days.

TEPCO officials maintain no safety issues have emerged and say the utility is resolved to remain engaged in the undertaking with utmost caution.