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People watch a TV news program showing North Korea's announcement, at the Seoul Railway Station in Seoul, South Korea, Wednesday, Jan. 6, 2016. North Korea said Wednesday it had conducted a hydrogen bomb test, a defiant and surprising move that, if confirmed, would put Pyongyang a big step closer toward improving its still-limited nuclear arsenal. The letters read " Will not use nuclear weapon if autonomy secured."  (AP Photo/Ahn Young-joon)
People watch a TV news program showing North Korea's announcement, at the Seoul Railway Station in Seoul, South Korea, Wednesday, Jan. 6, 2016. North Korea said Wednesday it had conducted a hydrogen bomb test, a defiant and surprising move that, if confirmed, would put Pyongyang a big step closer toward improving its still-limited nuclear arsenal. The letters read " Will not use nuclear weapon if autonomy secured."  (AP Photo/Ahn Young-joon)

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N.Korea likely aiming for 'smaller and lighter' warheads

Nuclear scientist Siegfried Hecker tells Christiane Amanpour North Korea's nuclear test was most likely aimed at making nuclear weapons smaller and lighter

N.Korea likely aiming for 'smaller and lighter' warheads

Nuclear scientist Siegfried Hecker tells Christiane Amanpour North Korea's nuclear test was most likely aimed at making nuclear weapons smaller and lighter