TOKYO, Aug. 24 (Jiji Press) -- About 170,000 tons of treated water has been released from Tokyo Electric Power Company Holdings Inc.'s meltdown-hit Fukushima No. 1 nuclear plant in northeastern Japan, TEPCO said Monday, three years after the releases began.

According to TEPCO, some 39 trillion becquerels of radioactive tritium contained in the treated water have been discharged into the ocean during the period from the plant devastated by the March 2011 powerful earthquake and tsunami.

As of this month, about 1.23 million tons of water are stored at the plant, including treated water kept in on-site tanks, down by over 7 pct from the level before the discharges began.

In the 2011 accident, scorching hot nuclear fuel that could not be cooled had melted down inside the No. 1, No. 2 and No. 3 reactors of the plant, with some dripping to the bottom of containment vessels.

Water is still being injected into the reactors.

Tainted water, or water injected into the reactors that has come into contact with the melted nuclear fuel, accumulates in the basement of the reactor buildings. The amount of such water also increases due to rainwater and groundwater flowing into the buildings.

For this reason, the amount of tainted water stored at the plant does not drop by as much as the amount of water discharged from the facility into the ocean.

The concentration of radioactive materials in the tainted water is significantly reduced by an advanced liquid processing system, or ALPS. As tritium cannot be removed from the water easily, the treated water has been diluted by mixing seawater before being released into the ocean.

Tritium, a radioactive substance that emits weak beta rays, has a half-life of about 12 years and is found in nature.

TEPCO has set its annual tritium release limit at 22 trillion becquerels. The company said that tritium levels in nearby waters are regularly measured and remain below the operational threshold.