Updated

A transformer exploded at the Indian Point nuclear power plant Sunday night, leading to an automatic emergency shutdown of one of its reactors, authorities said. No one was injured and no radioactive materials leaked.

The explosion happened after 6:30 p.m. and triggered an alert at one of the plant's two main electrical transformers, said officials with Entergy Corp., the plant's owner. An alert is the second-lowest of four classification levels for emergency events.

The alert ended at around 10:20 p.m., but workers would continue to monitor the transformer, Entergy spokesman Jim Steets said.

The affected reactor, Indian Point 2, would remain offline until investigators determined the cause of the explosion, Steets said. The reactor began operating in 1973 and generates about 1,000 megawatts of electricity.

The plant's other main reactor, Indian Point 3, was operating normally, he said. Indian Point 1 was shut down in 1974 because the emergency core cooling system failed to meet regulatory requirements.

It was the second shutdown within the hour at an Entergy-owned plant.

The Vermont Yankee nuclear plant in Vernon, Vt., shut down at about 7 p.m. after workers detected radioactive water seeping from a leaky pipe in the complex.

The cause of that leak wasn't immediately known. The Nuclear Regulatory Commission said the public wasn't in danger.

NRC spokesman Neil Sheehan called the two shutdowns "complete coincidence."

He said the risk of a radiological emergency at Indian Point were mitigated by the transformer being located outdoors on the power production side of the plant, not the nuclear side.

The plant's fire brigade responded but did not report any flames, Sheehan said. Foam was sprayed as a precaution to prevent flare-ups.

The Indian Point plant is located in Buchanan, about 25 miles north of New York City.

In 2007, a transformer failure and fire led to the unplanned shutdown of Indian Point 3.