WASHINGTON, March 8 (Kyodo) — Mitsubishi Heavy Industries Ltd. and a local utility firm in California were aware of a design flaw in Mitsubishi-made steam generators for a California nuclear reactor before a radiation leak in January 2012, according to a document recently released by the U.S. nuclear regulator.

A report submitted by Mitsubishi Heavy to the U.S. Nuclear Regulatory Commission on the accident showed that the Japanese manufacturer and Southern California Edison Co., the operator of the nuclear reactor, explored the possibility of changing designs in 2005 to curb the vibration of the fine tubing in the steam generators.

But the two companies decided not to change the design, according to the report. A steam generator is a critical part of a pressurized water reactor.

A minor radiation leak occurred in January 2012 at the Unit 3 reactor of San Onofre nuclear power plant as a result of abnormal wear on the tubes shortly after the recently repaired generators were installed.

In the report, Mitsubishi Heavy says the company believed it was possible to ensure safety even without altering the design, but that unpredictable phenomena occurred.

The Unit 3 reactor was shut down following the radiation leak. Following an investigation of Unit 3 and of the Unit 2 reactor, which was not in operation, about 15,000 cases of unusual wear were found in a total of four steam generators.

The steam generators were replaced between 2009 and 2011 by those with a greater number of tubes. The subsequent NRC probe found that the velocity of the steam passing through the tubes caused vibration, wearing them down and causing cracks.

Following the accident, operation of the nuclear reactors has been suspended for more than a year. Southern California Edison has submitted a plan to the NRC in which the utility firm will repair the Unit 2 steam generators first and resume operations at low power output to avoid vibrating the steam generators' tubes.