Contaminated water overflowed from a storage tank at the Fukushima station as heavy rains compounded Tokyo Electric Power Co. (9501)’s difficulties managing irradiated water at the wrecked atomic plant.

The estimated 430-liter leak occurred as crews transferred rainwater that had collected at the plant into a storage tank, Masayuki Ono, an official at the utility’s plant siting department, said today at a press conference in Tokyo.

The tank was already partially filled with water that had been used to cool nuclear fuel in the Fukushima Dai-Ichi station’s reactors, Mayumi Yoshida, a spokeswoman for the company known as Tepco, said by phone.

Beta radiation levels of 200,000 becquerels per liter were found near the leak that was confirmed at 9:55 p.m. yesterday, Tepco said in a statement early this morning. Beta radiation includes strontium-90, which safety rules require to be kept under 30 becquerels at atomic plants.

Heavy seasonal rainfall is adding to Tepco’s troubles in its effort to manage contaminated water filling tanks at the plant at a rate of 400 metric tons a day, as groundwater seeping into basements mixes with cooling water that has been in contact with highly radioactive melted reactor cores.

Long-Term Issues

“There are long-term issues that we need to solve and respond to firmly,” Ono said. “We are very sorry that something like this happened.”

The Japan Meteorological Agency’s Fukushima monitoring station recorded 174.5 milliliters (6.9 inches) of precipitation last month and 2.5 millimeters so far this month.

The rainwater being transferred when the accident occurred had collected between a contaminated water storage tank and a cement barrier around the tank built to contain possible leaks, Tepco said in a statement.

The water may have overflowed because the 450-ton-capacity tank had been built on a slope and was at a slight tilt, Ono said. Tepco said in the statement that it can’t rule out the possibility that some of the water flowed to the sea.

Fukushima Governor Yuhei Sato sent monitoring crews to the coast today to measure radiation levels at the outlet of a drainage ditch that may have carried contaminated water to the ocean, Shigeru Ito, a prefectural official in charge of monitoring radiation, said by phone. The results are expected Oct. 7, Ito said.

‘Under Control’

While the accident shows Tepco’s measures to control leaks have been insufficient, the utility seems able to manage the situation, Chief Cabinet Secretary Yoshihide Suga said today at a press conference.

“Overall, we believe things are under control,” he said, adding the Japanese government would continue to work with Tepco to prevent further leaks.

The government last month announced plans to spend 47 billion yen ($481 million) to stop leaks of radioactive water, saying it would be involved more closely in the site’s cleanup. Tepco reported a leak of about 300 tons of water from a storage tank on Aug. 20.

The Fukushima site has hundreds of thousands of tons of water stored in more than 1,000 tanks, with additional water remaining untreated in reactor basements and service tunnels.