Tokyo Electric Power Co. announced Saturday that about 120 tonnes of water contaminated with radioactive substances leaked from an underground storage facility at its Fukushima No. 1 nuclear power plant.
TEPCO announced the leak late Friday, but said measures to address the problem had not been taken for two days because the cause had not been identified. It assumed the water was still leaking.
The power company estimated that the leaked water contains a total of 710 billion becquerels of radioactive substances. The leak is the largest ever at the plant.
Since Saturday morning, about 13,000 tons of polluted water in the questionable storage facility was being transferred into a neighboring underground storage unit.
The storage facility, which is 60 meters long, 53 meters wide and six meters deep, is pool-like in structure, with a three-layer waterproof sheet and a concrete cover.
Water leaked from nuclear reactors is run through filters and other devices to remove radioactive elements, then stored in facilities for low-level contaminated water.
TEPCO started using the storage facility Feb. 1. As of Friday, 13,000 tons of polluted water was stored there, close to the 14,000-ton limit.
Water samples taken by TEPCO from soil around the facility on Wednesday showed 35 becquerels per cubic centimeter of radioactive substances, indicating an abnormal situation.
However, TEPCO officials did not announce the finding immediately, as no other unusual changes in water quality data, such as chloride concentration, were seen.
On Friday, two days after the problem was noticed, water with 6,000 becquerels per cubic centimeter of radioactive substances sat between the first and second layers of the waterproof sheet, alerting TEPCO officials that a leak had occurred.
As the sheet's layers were joined when the facility was constructed, TEPCO assumed that the sheet may have been damaged, or that a mistake had been made during construction.
An average of about 400 tons a day of groundwater seeped into buildings housing nuclear reactors and turbines, increasing the quantity of polluted water.
The latest incident will reduce the plant's storage capacity for polluted water from about 53,000 tons to 40,000 tons, making it necessary for TEPCO to review measures for handling polluted water, including increasing the number of storage tanks.
TEPCO said it expected the transfer of water would take at least five days to complete.
"As the height of the water storage facility is relatively low, we think it's unlikely that the polluted water mixed into underground water and reached the sea 800 meters away," said Masayuki Ono, acting chief of TEPCO's nuclear facilities department, at a press conference Saturday.