The US has helped five nations completely clear out their stocks of highly enriched uranium since President Barack Obama outlined his plans for securing all weapons-usable materials worldwide, officials say, citing it as progress in the administration\'s efforts to prevent nuclear weapons from getting in terrorists\' hands. Anne Harrington, the National Nuclear Security Administration\'s nonproliferation chief, said that since Obama\'s April 2009 speech in Prague announcing his plans, the US has helped remove enough material from about a dozen countries to make almost 30 warheads. She added that several global leaders are expected to use a nuclear security summit in Seoul, South Korea, which starts Sunday, to announce similar advances. Arms control experts say the most difficult part of building an atomic bomb is acquiring the weapons-grade uranium or plutonium needed for the explosive core of the weapon. Locking up or eliminating these materials is crucial to preventing nuclear-armed terror. Harrington said that\'s the administration\'s top national security concern: “Issue No.1 ... above anything else, keeping this material out of the hands of terrorists,” she said. Over the past three years, officials say, the US has helped Romania, Libya, Turkey, Chile and Serbia completely clear out their stockpiles of weapons-usable uranium. They join 13 other nations that did so previously Brazil, Bulgaria, Colombia, Denmark, Greece, Latvia, the Philippines, Portugal, Slovenia, South Korea, Spain, Sweden and Thailand. For the most part, this has meant shutting down civilian research reactors fueled by weapons-grade uranium, or converting those reactors to use low-enriched uranium.