When set off, a ring of explosives around the inner core creates a shock wave that compresses the soccer ball-size core of plutonium into a lump the size of a baseball, inducing fission.
(Related photos: living with the bomb.)
North Korea is believed to have been accumulating plutonium since the mid-1980s, according to the Institute for Science and International Security in Washington, D.C.
North Korean government officials stopped stockpiling the radioactive element in 1994 as part of an agreement with the United States.
But production ramped up again when that agreement broke down in late 2002. The country's current stockpile is estimated to be between 44 and 116 pounds (20 and 53 kilograms) of refined plutonium.
Now the international community is struggling to verify North Korea's claims of a successful nuclear test.
The bomb was most likely tested underground in a mountainside tunnel filled with concrete and sand, says Philip Coyle, a senior advisor for the Center for Defense Information.
Intelligence officials have been going over seismic signals, isotope air samples collected by aircraft, aerial photographs, and water samples taken since the announcement.
Results, however, are not instantaneous, and it could take several weeks for officials to come to a conclusion, CSIS nuclear expert Wolfsthal says.
"There are some tests for which we never get a definitive answer," he said.
For example, North Korean military officials claimed earlier this week that the bomb they tested yielded the energy equivalent of four kilotons of TNT.
But U.S. officials estimate that the bomb, if it existed, was much smaller—around 0.5 kiloton, Wolfsthal says.
Still, a small nuclear device dropped into a crowded urban environment—such as downtown New York City or Seoul, South Korea—could kill hundreds of thousands of people through the destruction of buildings, he says.
"There's a fireball, a blast effect, then a radiation impact," he said.
By comparison, the nuclear implosion bomb dropped by the U.S. on Hiroshima, Japan, in 1945 was estimated to yield 12 kilotons.
That blast killed about a hundred thousand people instantly.
Korea's "Nuclear" Winter
No matter the bomb's actual potency, North Korea has succeeded in grabbing the world's attention, policy experts say.
But the sanctions being proposed in response to the alleged test might devastate the already poor and isolated nation, says Hyeong Jung Park, a North Korea expert with the Brookings Institution in Washington, D.C.
Trade sanctions would force many impoverished citizens to face starvation, he says.
"This winter would be the most tragic and horrible winter for the ordinary and powerless people in North Korea," Park said.
Just how the situation reached this boiling point is a matter of textbook psychology, he says.
The country's physical proximity to superpowers China, Russia, and the U.S. has kept its communist leaders fearful of external influences, such as political and economic reform.
"On the other hand, the U.S. tried to isolate North Korea from any Western countries" after the Korean War started in 1950, Park said. And today North and South Korea "are still technically at war," he added.
(Related news: "U.S. Veterans Day Marked by Release of Vets' Stories" [November 10, 2004].)
"Because North Korean leadership was so afraid of outside influences, North Korea and its people have lost the opportunity to adapt itself to the changing world," Park said.
"The leadership in a economically ruined country surrounded by the biggest countries could not accommodate the outside world with normal relations," he said.
Paranoia about national security set in, and "they need psychological relief: a nuclear weapon."
"By doing this, they have gained for themselves greater leverage if they return to the negotiating table," said Michael Swaine, an Asian policy expert at the Carnegie Endowment for International Peace in Washington, D.C.
And "one downside consideration about the possibility that what they've just exploded is not a fully successful nuclear test is that they'll test again," Swaine said.
"They will want to test until they get it right."
Free Email News Updates
Best Online Newsletter, 2006 Codie Awards
Sign up for our Inside National Geographic newsletter. Every two weeks we'll send you our top stories and pictures (see sample).
SOURCES AND RELATED WEB SITES