President Barack Obama has hailed the success of the Washington summit on nuclear security, saying it's made the world a safer place. He said the possibility of terrorists getting hold of nuclear material to make a crude weapon remains the greatest threat to world security. Our North America editor … 奥巴马总统为华盛顿核安全峰会取得成功叫好,并认为本次峰会为世界营造一个更加安全的环境。他表示,恐怖组织可能获得核材料来制作粗制武器,这是世界安全的最大威胁。下面是我台驻北美记者马克·马代尔发回的详细报道。 奥巴马称本次峰会在面对全球安全方面取得了前所未有的进展。与会的47个国家纷纷表示将在四年内确保易流…