The shutdown occurred at 5:34 p.m. Wednesday when an automatic safety system tripped in the reactor's secondary, or non-nuclear, section, said company spokeswoman Alyson Fuqua. Nuclear plants are designed to turn themselves off if monitoring equipment detects anything out of the ordinary.
Engineers are trying to determine the cause of the shutdown and no decision has been made about when to restart the unit, she said, adding that Unit 2 has experienced no problems and continues to operate at 100 percent capacity.
The idled unit is in a safe, stable condition, she said.
The shutdown of Unit 1, she added, was not expected, but it also not an emergency. The U.S. Nuclear Regulatory Commission has been notified of the incident, as a precaution.
Vogtle has two existing units and is also in line to be the first site in the U.S. where new commercial power reactors will be built in almost 30 years. Site work for units 3 and 4 has been under way since last year, and the company is awaiting final permits from the NRC before moving ahead with construction.