Japanese nuclear regulators raised the severity of the latest leak of radiation-contaminated water at a damaged plant to the level of “serious incident” on Wednesday.

Plant operator Tokyo Electric Power Co said last week about 300 tons of highly contaminated water had leaked from a storage tank at the Fukushima Daiichi plant.

The Nuclear Regulation Authority raised the severity of the leak on the International Nuclear Event Scale from level 1 — an “anomaly” — to level 3.

The leak was the worst since the start of the nuclear disaster in 2011, the operator said.

Level 7 — “major accident” — was applied when three of the reactors suffered meltdowns.

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