SEOUL, South Korea (AP) — North Korea unveiled for the first time a nuclear-powered submarine under construction, a weapons system that can pose a major security threat to South Korea and the U.S. The state media agency on Saturday released photos showing what it called “a nuclear-powered strategic guided missile submarine,” as it reported leader Kim Jong Un’s visits to major shipyards where warships are built. The Korean Central News Agency, or KCNA, didn't provide details on the submarine,