Highly radioactive water overflowed barriers into Japan's Fukushima Daiichi nuclear power plant, its operating utility said on Monday, after it underestimated how much rain would fall at the plant and failed to pump it out quickly enough.
The utility, Tokyo Electric Power Co, also known as Tepco, has been battling to contain radioactive water at the nuclear complex, which suffered meltdowns and hydrogen explosions following a devastating earthquake and tsunami in March 2011.
Dealing with hundreds of tonnes of groundwater flowing through the wrecked nuclear plant daily is a constant headache for the utility and for the government, casting doubt on Prime Minister Shinzo Abe's promises that the Fukushima water "situation is under control."
After heavy rain on Sunday, water with high levels of radioactive strontium overflowed containment areas built around some 1,000 tanks storing tonnes of radioactive water at the plant, Tepco said. The radioactive water is a by-product of an improvised cooling system designed to keep the wrecked reactors under control in case of further disaster.
Tepco said it had planned to pump out the accumulating rainwater into empty tanks, check it for radioactivity, and if it was uncontaminated, release into the sea. But the company was overwhelmed by the amount of rainwater.
"Our pumps could not keep up with the rainwater. As a result, it flowed over some containment areas," said Tepco spokesman Yoshikazu Nagai. The company had planned for 30 to 40 millimeters of rainfall on Sunday, but by late afternoon the rainfall already stood at around 100 millimeters, he said.
The ongoing crisis at the Fukushima Daiichi plant, 220 km (130 miles) north of Tokyo, highlight the immensity of the task of containing and controlling radioactive water and eventually decommissioning the plant, processes expected to take decades.
Earlier this year, Tepco lost power to cool spent uranium fuel rods at the plant after a rat shorted wiring at the plant.
In the latest incident, containment areas surrounding 12 of 23 groups of tanks overflowed, with one of them containing Strontium-90 as highly concentrated as 710 Becquerels per liter - 71 times higher than the level set by the company as safe for release.
Strontium-90 is a by-product of the fission of uranium and plutonium in nuclear reactors as well as nuclear weapons, the U.S. Environmental Protection Agency says on its website.
Tepco said it will prepare some 30 extra pumps and lay additional 10 kilometers of pipes to prevent overflowing from happening again.
The utility has come under increased scrutiny after it found in August that 300 tonnes of highly radioactive water had leaked from one of the hastily built storage tanks at the Fukushima site. Japan stepped up support for the embattled utility in September, pledging half a billion dollars to help contain contaminated water at Fukushima.
Tepco is seeking permission to restart its only remaining viable plant - Kashiwazaki Kariwa, the world's largest nuclear power station, to cut high fuel costs and restore its finances.