North Korea appears to have revamped a nuclear reactor used to produce fuel for nuclear weapons, according to a UN atomic watchdog based in Vienna, Austria. The International Atomic Energy Agency (IAEA) said in its annual report (pdf) that there has been “indications” of operation of a 5-megawatt nuclear reactor at the Yongbyon Experimental Nuclear Power Plant, “including the discharge of cooling water,” since early July 2021. It deems the observation “deeply troubling.” The Yongbyon Experimental Nuclear Power Plant, which North Korea calls “the heart” of its nuclear program and research, is suspected to produce materials that could potentially be used in the country’s nuclear weapons development. The IAEA report, published quietly on Friday, noted that there were “no indications of reactor operation from early December 2018 to the beginning of July 2021″ at the power plant in Yongbyon, north of Pyongyang. The report noted that the steam plant that serves Yongbyon’s radiochemical laboratory …