( CNN ) -- A large Southern California nuclear plant is out of commission indefinitely , and will remain so until there is an understanding of what caused problems at two of its generators and an effective plan to address the issues , the nation 's top nuclear regulator said Friday . Gregory Jaczko , chairman of the U.S. Nuclear Regulatory Commission , refused to give a timetable as to when the San Onofre nuclear plant could resume operation . He said only that his agency had `` set some firm conditions '' as to when that could happen . `` We wo n't make a decision ( to approve the facility 's restart ) unless we 're satisfied that public health and safety will be protected , '' Jaczko told reporters . `` They have to demonstrate to us that they understand the causes , and ... that they have a plan to address them . '' The power plant has been shut down since this winter , when a small amount of radioactive gas escaped from a steam generator during a water leak . 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 . Located near San Clemente , the San Onofre nuclear plant 's twin reactors are `` Southern California 's largest and most reliable sources of electricity , '' according to Southern California Edison 's website . 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 . Anti-nuclear activists gathered Friday , not far from where Jaczko , Sen. Dianne Feinstein , D-California , and Rep. Darrell Issa , R-California , toured the power plant , to question the need for nuclear energy and raise alarms about a potential environmental catastrophe . Gary Headrick , founder of the group San Clemente Green , said that such public pressure was needed in order to guard against a nuclear crisis along the lines of what happened last year at Japan 's Fukushima Daiichi plant . `` If we were to let things go as they 've gone in the past , it 's very likely that we 'd experience a Fukushima right here in Southern California , '' Headrick said at the rally . `` And that 's why we 're here today . '' CNN 's Greg Botelho contributed to this report .
NEW : Anti-nuclear activists warn of a potential environmental catastrophe . The San Onofre nuclear plant has been shut down since radioactive gas escaped . Score: 6 The head of the NRC says the plant wo n't restart until a cause and plan is put forward .