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The operators of the San Onofre nuclear plant and the manufacturer of its flawed steam generators knew of serious design problems before the generators were installed, according to a 2012 report described Wednesday by two members of Congress.

The report also appears to show that the plant operator, Southern California Edison, and the manufacturer, Mitsubishi Heavy Industries in Japan, chose to avoid making some "safety modifications" that could have triggered additional safety review by the Nuclear Regulatory Commission.

The generators were built and installed as part of a $671 million operation without the higher-level review, known as a license amendment. But the design flaws have kept both reactors offline for more than a year. The utility is seeking permission to start one reactor at lower power.

The 2012 report from Mitsubishi is described in a letter sent to the NRC on Wednesday by Sen. Barbara Boxer and Rep. Ed Markey, D-Mass.

Boxer is chairwoman of the Senate Committee on Environment and Public Works, and Markey is a ranking member of the House Natural Resources Committee.

The letter quotes parts of the report, but the entire report has not been released.

"This newly obtained information concerns us greatly, and we urge the NRC to immediately conduct a thorough investigation into whether (Southern California Edison) and (Mitsubishi Heavy Industries) did in fact fail to make needed safety amendments to avoid the license amendment process," the letter states. "All people in our nation, including the 8.7 million people who live within 50 miles of the San Onofre plant, must have confidence in the NRC's commitment to put safety before any other concern."

In a statement Wednesday, Edison officials said they were aware of the letter, were cooperating with the NRC review process, and were complying with requests for documents and other information about the San Onofre plant.

"SCE is strongly committed to the transparent review of its operations at San Onofre and the safety of the public and its employees," the statement said.

Mitsubishi said in a statement that all design decisions on the steam generators "were made in accordance with well-established and accepted industry standards and practices, along with our own and third-party operating data and experience."

An NRC spokesman said the agency knew about the 2012 report before receiving the letter from Boxer and Markey, but chose not to release it to the public "because it contains proprietary information."

The release of the letter prompted demands from environmental activists and others for release of the entire 2012 report.

"It appears to indicate Edison was aware that the significant design changes they were contemplating making could lead to the very kinds of problems that manifested themselves," said Damon Moglen, energy and climate director for Friends of the Earth, a group that has long contended that design changes in the generators should have triggered a higher level of review known as a license amendment process.

Revelation of the report comes at a sensitive time as the NRC considers whether to allow a lower-power restart for one of the San Onofre reactors.

Both of the plant's reactors have been shut down for more than a year because of problems with their massive steam generators, two for each reactor.

The Unit 2 reactor was shut down for routine maintenance early in January 2012. But Unit 3 was shut down on Jan. 31 of that year after sensors at the plant picked up a small release of radioactive gas.

Later inspections revealed unexpected wear among the thousands of tubes inside each of the generators. The tubes carry water heated by the reactors. The tube wear was traced to design flaws in the steam generators, installed between 2009 and early 2011.

Edison has proposed restarting the Unit 2 reactor, where the tube wear was less extensive, at 70 percent power. Operating at lower power should eliminate the vibrations believed to have caused the unexpected wear, the company said.

The NRC has said it could decide whether to allow the lower-power restart as soon as late April. The agency will hold a public meeting on steam generator issues today in Rockville, Md. A public meeting on San Onofre also will be held by the NRC from 6 to 9 p.m. Tuesday at Capo Beach Church, 25975 Domingo Ave., Capistrano Beach.

According to the Boxer and Markey letter, the 2012 report says the two companies accepted some changes in the steam generators, but other safety modifications were rejected because of their "unacceptable consequences."

"Among the difficulties associated with the potential changes was the possibility that making them could impede the ability to justify the RSG (replacement steam generator) design" without triggering a license amendment, the letter quotes the report as saying.