TOKYO (AP) -- Workers at a crippled Japanese nuclear plant are investigating why highly radioactive water leaked from a new storage tank, amid concerns that the problem could further hamper cleanup efforts.
The Fukushima Dai-ichi plant suffered multiple meltdowns after a massive earthquake and tsunami in March 2011 knocked out power. It is currently using a fragile makeshift cooling system that creates large amounts of highly radioactive water.
Plant operator Tokyo Electric Power Co. said Thursday that the leak stopped after some of the water in the faulty tank was moved to two adjacent containers, but that its cause is still unknown.
The leak occurred in one of nearly 40 steel tanks TEPCO hastily assembled last month to hold radioactive water from several underground storage pools that were defective and also leaked. They are a portion of about 300 similar tanks of different sizes spread across the plant to contain the growing amounts of contaminated water.
A worker on Wednesday spotted water dripping from a seam on the 500-ton tank. The total leakage was about 1 liter (a quarter gallon), according to TEPCO spokesman Masayuki Ono.
Three other tanks of the same design had similar leaks last year.
TEPCO says the tanks are not intended for long-term use. Ono said additional tanks are being built with welded seams that are more watertight.
TEPCO has been hit by a series of problems in recent weeks, including a rat-induced blackout, adding to concerns about its ability to safely conclude the decades-long process of decommissioning the reactors.
The massive amount of radioactive water is among the most pressing issues affecting the plant's cleanup process.
The Nuclear Regulation Authority, Japan's nuclear watchdog, urged TEPCO on Wednesday to do more to halt leaks and to try to stop underground water runoff by 2020.
"The handling of the contaminated water is an extremely pressing issue. But the ongoing measures are still inadequate and uncertain," authority Chairman Shuichi Tanaka said. "We must make sure that the contaminated water storage plans will not fall apart."
A team of experts dispatched by the International Atomic Energy Agency earlier urged TEPCO to step up efforts to solve the contaminated water problem, including measures to detect early signs of leakage.
Runoff from the three reactors that melted in the aftermath of the March 2011 disasters and a steady flow of groundwater into the basements of the damaged buildings produce about 400 tons of contaminated water daily. TEPCO says about 300,000 tons of contaminated water has been stored in about 1,000 tanks at the plant, and that the amount will double within a few years.
TEPCO plans to build additional tanks to increase the water storage capacity to as much as 800,000 tons by 2016.
It has developed a new water processing system that can remove all but one radioactive material, tritium, from the contaminated water. Officials say the machine will reduce the risk of radiation exposure for workers and reduce the environmental impact.
The government recently set up a special panel to deal with the contaminated water problem. It instructed TEPCO last week to minimize or prevent the groundwater flow by building a wall of solidified soil around the reactor buildings. TEPCO also plans to reduce the amount of underground water runoff into the reactor buildings by pumping some out from a nearby hillside before it runs into the buildings and becomes contaminated there.