* Manufacturer's report showed safety modifications were rejected -lawmakers
* Report says rejection contributed to reactors' shutdown -lawmakers
* Mitsubishi Heavy says design decisions followed accepted industry practice
LOS ANGELES, Feb 6 (Reuters) - The owner of a shuttered Southern California nuclear power plant was aware of problems with its new steam generators before they were installed, two lawmakers said on Wednesday, citing a report by the maker of the equipment.
In a letter to U.S. Nuclear Regulatory Commission Chairman Allison Macfarlane, Senator Barbara Boxer and Representative Edward Markey said a 2012 document by Mitsubishi Heavy Industries Ltd, maker of the San Onofre power plant's steam generators, found that some safety modifications were rejected by both Mitsubishi Heavy Industries and plant owner Southern California Edison due to "unacceptable consequences."
The potential changes may have raised the possibility that the plant would need to seek a license amendment because of changes to the steam generators' design, the lawmakers said the report showed.
"SCE is fully cooperating with the NRC review process and is complying with all requests for information and documents related to the company's San Onofre Nuclear Generating Station," Southern California Edison spokeswoman Jennifer Manfre said in an emailed statement. "SCE leadership takes very seriously all allegations raised by the letter."
In the letter, the lawmakers said the report also showed that Edison's and Mitsubishi's decision to reject those modifications contributed to problems with the faulty steam generators and the shutdown of two nuclear reactors at the San Onofre nuclear power plant.
"This newly obtained information concerns us greatly," Boxer and Markey wrote in the letter.
The report is confidential and Reuters was not able to review it.
"All design decisions for 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," Mitsubishi Heavy Industries said in a statement issued to Reuters.
Both reactors at the 2,150-megawatt San Onofre plant have been shut down for more than a year due to tube wear problems in the units' new steam generators. Mitsubishi Heavy built the generators and installed them in 2010 and 2011.
The Japanese company said it has been cooperating with the utility and the NRC in investigations since the shutdown of the steam generators.
The NRC is reviewing Southern California Edison's plan to restart one of those units. Southern California Edison is a unit of Edison International.
"We have received the letter from Senator Boxer and Congressman Markey and will respond in the normal course of business," NRC spokesman Victor Dricks said in an email. "As an independent safety agency, we will review all available information in making a judgment as to whether the plant would meet our safety standards if restart were permitted."
San Onofre sits on the California coast halfway between Los Angeles and San Diego. It is the largest power plant in Southern California.