Tokyo Electric Power Co. said Wednesday night that it found chinks in a storage tank from which 300 tons of highly radioactive water escaped with barely a trace last month.

It is highly possible the small openings are what caused the massive leak, but more testing of the flange-type storage tank is required to confirm that, said Akira Ono, chief of the Fukushima No. 1 power plant, during a news conference at the plant streamed live over the Internet.

The water is thought to have entered the ground or made its way into the sea.

Tepco made the discovery Wednesday by conducting a vacuum test on the bottom of the tank after coating the bolts inside with foam. When the vacuuming caused some of the foam inside to disappear, it revealed chinks around two bolts.

Ono said it is not yet known how the chinks formed because the two bolts are usually not loosened.

“The fact that the bubbles were vacuumed means there are spaces, and it is highly likely that they caused the leakage. But we will eventually need to disassemble the tank and examine it to be thorough,” said Ono.

The tank in question is one of the plant’s 300 or so flange-type tanks, which consists of steel plates bolted together and sealed with waterproof packing at the seams. They are not as sturdy or watertight as welded tanks.

Last month, Tepco announced that 300 tons of tainted water had vanished from the tank in about a month without anyone noticing, an incident the Nuclear Regulatory Authority classified as level 3 on the International Nuclear and Radiological Event Scale, which tops out at 7.

Bumbling Tepco had been unable to locate any leaks in the tank until Wednesday.

There are about 300 flange-type tanks storing radioactive water that was used to cool the reactors’ melted fuel. Although the water is processed to remove most of the cesium, it still contains other hazardous radioactive materials, including strontium, which can cause bone cancer.

The utility is planning to transfer the tainted water in the flange-type tanks to the welded ones.

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