( CNN ) -- A small amount of radioactive gas escaped from a steam generator at Southern California 's San Onofre nuclear power plant during a water leak , but there was no threat to public health , federal regulators said Wednesday . Operators shut down the plant 's No . 3 reactor on Tuesday after the water leak was discovered , Nuclear Regulatory Commission spokesman Victor Dricks said . The gas was vented from the steam generator into an auxiliary building , where it triggered a radiation monitor , he said . The amount of gas that leaked was not immediately known , but it was a small amount that wo n't endanger the public or plant workers , Dricks said . He said the water leak was about six-hundredths of a gallon per minute , far below the levels required to declare an `` unusual event '' -- the lowest of four NRC alert levels . NRC inspectors will conduct a follow-up review of the incident , Dricks said . The plant 's owner , Southern California Edison , reported the problem Tuesday , but had no new comment on Wednesday . The water leak occurred in the thousands of tubes that carry heated water from the reactor core through the steam generator , a 65-foot-tall , 640-ton piece of equipment that boils water used to drive the unit 's turbines . Though leaking tubes periodically occur in older units , Dricks said , Southern California Edison replaced the steam generators at San Onofre between 2009 and 2011 . CNN 's Karan Olson and Matt Smith contributed to this report .
A small amount of radioactive gas escaped from a steam generator , the NRC says . Comment: Operators shut down the No . 3 reactor at California 's San Onofre plant as a result .