Federal regulators are pointing to design flaws over defective materials or improper installation as the primary cause of premature tube wear that caused San Onofre Nuclear Generating Station to enter emergency shutdown over safety concerns.

“It looks primarily [like] we are pointed toward the design,” Nuclear Regulatory Commission Regional Administrator Elmo Collins told the Associated Press on Sunday.

Local activists have long pointed to design changes as a potential cause for the failure of the generators, accusing the plant’s engineers of deliberately avoiding scrutiny from regulators when implementing major design changes such as the addition of 400 tubes per generator and the installation of newly designed braces that had been intended to reduce vibration.

Collins did not rule out the possibility that at least one of the generators might need to be replaced, after eight more tubes failed in a pressure test at the Unit 3 reactor. “We have not seen that in the industry before,” Collins noted. The new generators were installed in 2009 and 2010 at a cost to ratepayers upwards of $670 million.

Plant operator Southern California Edison says its own engineers have determined that vibration in the system is responsible for premature tube damage, though they have not identified a cause or fix for the vibration. A previous, short-lived plan to continue operation under reduced load never got off the ground.

“That doesn't make sense: If your car overheats on the freeway, you don't drive a little slower and hope the problem goes away. You stop the car, figure out what's wrong and don't drive until it's fixed. If the problem persists, you get rid of the car,” said former Sacramento Municipal Utility District general manager S. David Freeman of the proposed fix.