About 100 tons of water containing high levels of radioactive material leaked at the damaged Japanese nuclear power plant in Fukushima, the operator of the plant said on Thursday.

Tokyo Electric Power Co (TEPCO) said it had detected the leak from a storage tank at the Fukushima Dai-ichi Nuclear Power Station on Wednesday night.

The leaked water contained 230 million becquerels per litre of beta-ray emitting substances, consisting mainly of strontium 90, broadcaster NHK reported, citing Tokyo Electric officials.

The level is about 7.6 million times the government’s limit for water allowed to be released into the ocean, NHK said.

The cause of the leak is still under investigation, said Tokyo Electric spokesman Masaaki Fukai.

But he said it was likely to have been caused by a faulty valve in the pipes that transfers water from a decontamination facility to storage tanks.

Two other valves leading to the troubled storage tank were also left open, which seems to have resulted in the unexpected flow of water into the tank and caused an overflow, Mr. Masaaki said.

Tokyo Electric said the latest leak had stopped earlier on Thursday, six hours after the trouble was first detected.

The operator did not believe the contaminated water had reached the Pacific Ocean because “the tank is located several hundred metres away from the sea and there are no spillways near the tank,” Mr. Masaaki said.

Tokyo Electric is injecting water into the three reactors to keep them cool, and has been battling leaks from storage containers where the resulting radioactive water is being stored.

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