North Korea’s threats may not be so empty. Earlier this month, it strategically took an American nuclear scientist on a tour of a huge new facility designed to enrich uranium. The plant, built quickly and secretly, could mean that North Korea is planning to build more powerful atomic weapons and stands as a direct challenge to the United States. The American scientist was Siegfried S. Hecker, a Stanford professor who ran the Los Alamos National Laboratory. He said that he was “stunned” by the plant’s sophistication and that the North Koreans told him they already have 2,000 centrifuges up and running. Still, like Iran, North Koreans maintain that the nuclear fuel they are producing is for peaceful purposes and that it will be used to deliver electricity. But American officials are not convinced. They think that North Korea, under its new leader, is trying to flex its muscles by showing its capacity for building weapons.