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 Washington and others are bracing for the possibility that if punished for a successful long-range rocket launch on Dec. 12 that the U.N. considers a cover for a banned ballistic missile test, North Korea's next step might be its third nuclear test.
Rocket and nuclear tests unnerve Washington and its allies because each new success puts North Korean scientists another step closer to perfecting a nuclear warhead small enough to put on a missile that could hit the mainland United States.
Another nuclear test, which North Korea's Foreign Ministry hinted at on the day of the rocket launch, would fit a pattern. Pyongyang conducted its first and second atomic explosions, in 2006 and 2009, weeks after receiving U.N. Security Council condemnation and sanctions for similar long-range rocket launches.
North Korea is thought to have enough plutonium for a handful of crude atomic bombs, and unveiled a uranium enrichment facility in 2010, but it must continue to conduct tests to master the miniaturization technology crucial for a true nuclear weapons program.
"With an additional nuclear test, North Korea could advance their ability to eventually deploy a nuclear weapon on a long-range missile," said Daryl Kimball, executive director of the nongovernment Arms Control Association.
Analysts caution that only so much can be determined from satellite imagery, and it's very difficult to fully discern North Korea's plans. This is especially true for nuclear test preparations, which are often done deep within a mountain. North Korea, for instance, took many by surprise when it launched its rocket this month only several days after announcing technical problems.
Although there's no sign of an imminent nuclear test, U.S. and South Korean officials worry that Pyongyang could conduct one at any time.
Analysis of GeoEye and Digital Globe satellite photos from Dec. 13 and earlier, provided to The Associated Press by 38 North, the website for the U.S.-Korea Institute at Johns Hopkins School of Advanced International Studies, said scientists are "determined to maintain a state of readiness" at the Punggye-ri Nuclear Test Facility after repairing flood damage.
The nuclear speculation comes as South Korea's conservative president-elect, Park Geun-hye, prepares to take office in February, and as young North Korean leader Kim Jong Un marks his one-year anniversary as supreme commander.
Kim has consolidated power since taking over after his father, Kim Jong Il, died Dec. 17, 2011, and the rocket launch is seen as a major internal political and popular boost for the 20-something leader.
Some analysts, however, question whether Kim will risk international, and especially Chinese, wrath and sure sanctions by quickly conducting a nuclear test.
The election of Park in South Korea and Barack Obama's re-election to a second term as U.S. president could "prompt North Korea to try more diplomacy than military options," said Chang Yong-seok, an analyst at the Institute for Peace Affairs, a private think tank in Seoul. "I think we'll see North Korea more focused on economic revival than on nuclear testing next year."
The 38 North analysis said the North "may be able to trigger a detonation in as little as two weeks, once a political decision is made to move forward." But the report by Jack Liu, Nick Hansen and Jeffrey Lewis also said it was unclear whether water seepage from a tunnel entrance at the site was under control. Water could hurt a nuclear device and the sensors needed to monitor a test.
The analysis also identified what it called a previously unidentified structure that could be meant to protect sensitive equipment from bad weather.
"We don't have a crystal ball that will tell us when the North will conduct its third nuclear test," said Joel Wit, a former U.S. State Department official and now editor of 38 North. "But events over the next few months, such as the U.N. reaction to Pyongyang's missile test and the North's unfolding policy toward the new South Korean government, may at least provide us with some clues."
Another unknown is how China, the North's only major ally, would respond to calls for tighter sanctions. Washington views more pressure from Beijing as pivotal if diplomatic pressure is going to force change in Pyongyang.
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 Even if Beijing signs on to U.N. punishment if the North conducts a test, there may be less hurt for Pyongyang than Washington wants.
The impact of tougher sanctions would be "a drop in the bucket compared with the tidal wave of China-North Korean trade" that has risen sharply since 2008, even as inter-Korean trade has remained flat, said John Park, a Korea expert at the Massachusetts Institute of Technology.
Trade figures show North Korea's deepening dependence on China. Pyongyang's trade with Beijing surged more than 60 percent last year, reaching $5.63 billion, according to South Korea's Statistics Korea. China accounted for 70 percent of North Korea's annual trade in 2011, up from 57 percent in 2010.
North Korea's 2006 nuclear test had an estimated explosive yield of 1 kiloton. The Los Alamos National Laboratory estimated in 2011 that the North's test on May 25, 2009, which followed U.N. condemnation of an April long-range rocket launch, had a minimum yield of 5.7 kilotons. The atomic bomb that hit Nagasaki at the end of World War II was about 21 kilotons.
Both North Korean tests used plutonium for fissile material. Without at least one more successful plutonium test, it's unlikely that Pyongyang could have confidence in a miniaturized plutonium design, according to an August paper by Frank Pabian of Los Alamos and Siegfried Hecker of Stanford University.
North Korea's small plutonium stockpile is sufficient for four to eight bombs, they wrote, but it may be willing to sacrifice some if it can augment information from the previous tests. Pabian and Hecker predicted that Pyongyang may simultaneously test both plutonium and highly enriched uranium devices.
A uranium test would worry the international community even more, as it would confirm that North Korea, which would need months to restart its shuttered plutonium reactor, has an alternative source of fissile material based on uranium enrichment. North Korea unveiled a previously secret uranium enrichment plant in November 2010.
"Whether and when North Korea conducts another nuclear test will depend on how high a political cost Pyongyang is willing to bear," Pabian and Hecker wrote.
Another test would also undermine Pyongyang's assertion that its long-range rocket launches are for a peaceful space program and not what outsiders see as the development of ballistic missiles that could eventually deliver nuclear weapons.
On the same day as this month's rocket launch, an unidentified North Korean Foreign Ministry spokesman told state media that a hostile U.S. response to a failed launch in April of this year had forced Pyongyang "to re-examine the nuclear issue as a whole."
The statement was a clear threat to detonate a nuclear device ahead of any U.N. Security Council action, said Baek Seung-joo, an analyst at the state-run Korea Institute for Defense Analyses in Seoul.
[Associated
Press; By FOSTER KLUG and MATTHEW PENNINGTON]
Pennington reported from Washington. Associated Press writers Hyung-jin Kim and Sam Kim contributed from Seoul.
Copyright 2012 The Associated
Press. All rights reserved. This material may not be published,
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