April 7, 2012 -- Updated 0036 GMT (0836 HKT) At the time, federal regulators said there was no threat to public health, though they could not identify how much gas leaked or exactly why it had happened. The water leak occurred in thousands of tubes that carry heated water from the reactor core through the plant's steam generators. Leaks occur periodically in older units, but plant owner Southern California Edison replaced the four steam generators at San Onofre in 2010 and 2011 as part of a $680 million project. They are in units 2 and 3 of the nuclear facility; unit 1 went out of service in 1992. Each of the 65-foot-tall, 640-ton generators -- built by Japan's Mitsubishi Heavy Industries -- are packed with thousands of narrow tubes that carry hot, pressurized water from the reactors. The heat produces steam in a separate loop that drives the plant's turbines and generators. "Tubes are vibrating and rubbing against adjacent tubes and against support structures inside the steam generators," the agency noted. Eight of the more than 9,700 tubes in one of the unit 3 generators failed a pressure test, while six tubes in unit 2's reactor needed to be plugged, the NRC has found. Another 186 tubes in unit 2, which was shut down for refueling at the time of the leak, were plugged "as a precautionary measure." In addition to driving the turbines to create electricity, the steam generators are "one of the barriers between the radioactive material in the reactor core and ultimately the external environment," Jaczko noted. When operational, the facility -- which is owned by that utility, San Diego Gas and Electric, and the city of Riverside -- supplies power for 1.4 million households at any given time.