Associated Press: The nation’s largest nuclear power plant has made "substantial improvements" since a number of safety issues led regulators to downgrade its safety rating three years ago, according to a letter released Tuesday by the U.S. Nuclear Regulatory Commission. The Palo Verde Nuclear Generating Station, 50 miles west of Phoenix, has had the lowest safety rating of any licensed U.S. reactor since 2006, when inspectors found that an emergency backup generator had been inoperative for 18 days …