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In this Feb. 14, 2008 photo released by U.S. researchers, Siegfried Hecker, a Stanford University physicist, and Joel Wit, a former State Department expert on the North, who visited North Korea, the empty inner structure of a cooling tower at the Nyongbyon Nuclear Center in Nyongbyon, also known as Yongbyon, North Korea, is shown. The North's plutonium reactor began operations in 1986 but was shut down as part of international nuclear disarmament talks in 2007 that have since stalled. North Korea vowed Tuesday, April 2, 2013, to restart a nuclear reactor that can make one bomb's worth of plutonium a year, escalating tensions already raised by near daily warlike threats against the United States and South Korea. (AP Photo/S.S. Hecker)