The operators of the Salem I nuclear power plant are investigating what caused a reactor at the southern New Jersey facility to unexpectedly shut down, triggering fire sirens.

The plant declared an "unusual event," the lowest level of emergency, on Monday morning because it was unable to investigate all the alarms within 15 minutes as required by safety regulations.

Joe Delmar, a spokesman for the reactor's operator, PSEG Nuclear, tells the Today's Sunbeam newspaper that the cause of the shutdown remains under investigation. But he adds the plant was testing the reactor's emergency cooling system at the time.

Delmar says no radiation was released, adding it is unclear when the reactor might return to service. It is one of three reactors at the site in Lower Alloways Creek Township.

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