VIENNA (Reuters) – A reactor at North Korea’s Yongbyon nuclear complex appears for the first time to be using atomic fuel, the U.N. nuclear watchdog said late on Thursday, which would mean the country has a new potential source of plutonium for its nuclear weapons. North Korea has for years used spent fuel from a 5 megawatt nuclear reactor at Yongbyon to produce plutonium for its nuclear arsenal but a telltale discharge of warm water from a larger light-water reactor suggests that is now coming online, too, the International Atomic Energy Agency said. “The discharge of warm water is indicative…