TOKYO -- The operator of Japan's crippled nuclear plant said Saturday that it was moving tons of highly radioactive water from a temporary storage tank to another after detecting signs of leakage, in a blow to the plant's struggles with tight storage space.

Tokyo Electric power Co. said about 120 tons of the water are believed to have breached the tank's inner linings, some of it possibly leaking into the soil. TEPCO is moving the water to a nearby tank at the Fukushima Dai-chi plant - a process that could take several days.

TEPCO detected the leak earlier in the week, when radiation levels spiked in water samples collected in between the inner linings of the tank. Radiation levels in water samples taken outside the tank also have increased, an indication of the water leak, TEPCO spokesman Masayuki Ono said.

Contaminated water at the plant, which went into multiple meltdowns after the March 2011 earthquake and tsunami that devastated northeastern Japan, has escaped into the sea several times during the crisis. Experts suspect there has been a continuous leak into the ocean through an underground water system, citing high levels of contamination among fish caught in waters just off the plant.

The leak is not only an immediate environmental concern, but threatens TEPCO's tight water management situation, Ono said.

The tank contains 13,000 tons of water, which is part of the water that was used to cool melted fuel at the plant's reactors damaged in the twin disasters. So much water has been used that TEPCO is struggling to find storage space.

"The impact (from the leak) is not small, as the space is already tight," Ono said. "We need to revise our water management plans."

More than 270,000 tons of highly radioactive water is already stored in hundreds of gigantic tanks and another underground tank. They are visible even at the plant's entrance and built around the compound, taking up more than 80 percent of its storage capacity.

TEPCO expects the amount to double over three years and plans to build hundreds of more tanks by mid-2015 to meet the demand.

Because of that, TEPCO is anxious to launch a new water treatment system that can purify the contaminated water. The machine, called ALPS, recently started a final test run after six months of delays due to safety requirements by government regulators.

The delay caused TEPCO to use some of seven underground tanks, originally meant for ALPS-treated water, to accommodate the contaminated water backlog as a stopgap measure.

TEPCO officials have indicated they hope to release the water into the ocean, but Ono said the company has no immediate plans to do so without public acceptance.

The plant is being decommissioned but continues to experience glitches. A fuel storage pool temporarily lost its cooling system Friday, less than a month after the plant suffered a more extensive outage.

The underground tank, several times the size of an Olympic-size swimming pool and similar to an industrial waste dump, is dug directly into the ground and protected by two layers of polyethylene linings inside the outermost clay-based lining, with a felt padding in between each layer.

The meltdowns have caused the plant to release radiation into the surroundings and displaced about 160,000 people from around the plant. They do not know when or if they will be able to return home.

• Read more articles by MARI YAMAGUCHI

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