HOUSTON (Reuters) - The U.S. Nuclear Regulatory Commission on Tuesday said it will not allow Southern California Edison's San Onofre nuclear generator in California to restart until the agency is sure that the company has addressed premature degradation of tubes in the plant's steam generators. Both reactors at the 2,150-megawatt plant near San Diego have been shut since January due to the discovery of premature wear on tubes inside giant steam generators made by Mitsubishi Heavy Industries and installed in 2010 and 2011. ...