Rainfall from recent typhoons caused partial melting of the “ice wall” at the Fukushima No. 1 nuclear plant, allowing highly radioactive water to leak from around the damaged reactor buildings, the plant’s operator said Sept. 1.

Tokyo Electric Power Co. said melting occurred at two sections of the ice wall, which is designed to divert groundwater away from the reactor buildings.

TEPCO officials believe that during the latest typhoon, contaminated water from around the reactor buildings flowed through openings of the ice wall created by the deluge and reached downstream toward the sea.

The groundwater level near a seaside impermeable wall temporarily rose to 28 centimeters below the ground surface when Typhoon No. 10 passed the area on Aug. 30.

Before the typhoon hit, the water level was 35 cm below the surface.

Around 5.5 cm of rainfall a day fell in the area when the typhoon hit.

The groundwater level, however, actually rose by 7 cm, although 740 tons of groundwater was pumped out of the section.

“If there had been an additional 15 cm of rain, (the contaminated water) could have poured out over the ground surface” and spilled into the sea, a TEPCO official said Sept. 1.

The Meteorological Agency’s initial forecast said Typhoon No. 10 would bring a maximum 20 cm of rain a day at some locations in the Tohoku region.

The 34.5-billion-yen ($335 million) frozen wall was completed in spring to prevent groundwater from entering the reactor buildings and mixing with highly radioactive water.

TEPCO admitted the underground wall of frozen dirt is not working.

The company said the temperatures at the two sections of the frozen wall have climbed above zero since Typhoon No. 7 approached Fukushima Prefecture on Aug. 17.

The company believes that the partial melting was caused by the influx of water brought by the typhoons and heavy rain in between.

TEPCO plans to freeze the wall again by pouring chemicals into pipes that extend underground.