The plant was partially shut down in Jan 2012 following the discovery.
On Wednesday, Sen. Barbara Boxer asked federal regulators to investigate new information she uncovered in a confidential report written by Mitsubishi. The Democratic senator said the report suggests Edison was aware of design problems with its steam generators before the equipment was installed. Boxer accused Edison of rejecting safety modifications.
The utility has since released a statement denying Boxer's claims, stating "it is simply not accurate." Edison also said the equipment carried a 20-year warranty against defects.
"SCE would never, and did not, install steam generators that it believed would not perform safely," the company said. Edison "sought to purchase replacement steam generators that would meet or improve upon the safety standards and performance of the original steam generators."
The NRC said it received the letter and "will review all available information in making a judgment as to whether the plant would meet our safety standards if restart were permitted."
The Associated Press contributed to this report.