The agency released a statement Tuesday.
" 'This Confirmatory Action Letter formalizes commitments that Southern California Edison has made to ensure that the cause of the tube wear in both steam generators is understood and appropriately addressed in order to ensure safe operation,' NRC Region IV Administrator Elmo E. Collins said. 'Until we are satisfied that has been done, the plant will not be permitted to restart.' "
The order was given in the wake of ongoing safety concerns in two of the nuclear plant's reactors. Units 2 and 3 have been shut down for two months, the longest-ever at the station.
"SCE has identified two causes of the unusual wear: tubes are vibrating and rubbing against adjacent tubes and against support structures inside the steam generators. They are still working to determine why this is occurring," the statement said.
Problems at the San Onofre Nuclear Generating Station were already signaling possible power shortages this summer.
State regulators say the indefinite shutdown of two nuclear reactors at the plant could lead to shortages with the onset of an extended heat wave, unless steps are taken to fill the gap.
Investigators at the plant are trying to determine why hundreds of alloy tubes in massive steam generators are wearing down at an alarming rate. The reactors are operated by Southern California Edison, and 19 percent of power used by SCE customers comes from nuclear generation.
The plant is owned by Edison, San Diego Gas & Electric and the city of Riverside. Southern California Edison serves nearly 14 million residents with electricity in Central and Southern California.