around 7,000 tons of t.n.t. but that's not large by atomic bomb standards. seth doane begins our coverage. >> reporter: "a perfect success" was how north korea's state media put it... defending its nuclear arsenal as a powerful deterrent against the u.s., which it called "the chieftain of aggression." the underground test registered as a 5.1 magnitude quake. it was close to the site of three previous nuclear tests. the news sparked cheers in north korea, and rattled nerves in south korea, which questioned whether the explosion was big enough to be a hydrogen bomb. north korea is known for its saber-rattling, but possessing a hydrogen bomb would be a major and surprising step forward. when we were in north korea last october, it appeared relations