Shock waves emanating from North Korea on Monday probably came from the explosion of a nuclear device, but one that did not achieve its full potential because of a failure or a design flaw, U.S. intelligence officials and weapons experts said.
Analysts believe the explosion produced about 3% of the power of the bombs dropped on Hiroshima and Nagasaki more than 60 years ago, raising questions about whether a bigger device malfunctioned, or the regime in Pyongyang was testing only a component of a larger weapon.
The combination of the low strength of the explosion and the failure in July of a missile test over the Sea of Japan is likely to reinforce intelligence assessments that North Korea remains years away from developing a nuclear warhead that could be fitted to a missile and delivered any significant distance, let alone to the shores of the United States.
Still, officials said Monday’s test demonstrated that North Korea had crossed a dangerous technological threshold, and they added that the blast served as an experiment from which Pyongyang could garner valuable information it could use to improve its nuclear designs.
“Often you learn as much from a failure as you do a success,” said Anthony Cordesman, a former Defense Department official and a military expert at the Center for Strategic and International Studies.
“If you have a test, even if it is only partly successful, you’ve proven not only you can do it but that you’re on the way to developing more sophisticated systems.”
Cordesman and others said it could be days before U.S. intelligence agencies and independent scientists are able to reach firm conclusions as to the nature of the test.
Experts will analyze the seismic signature of the explosion, and attempt to collect atmospheric samples by “sniffer” spy planes.
Estimates of the force of the explosion were based on comparisons of seismic readings taken from monitoring stations in nations surrounding North Korea with seismic measures of past nuclear explosions.
The characteristics of the “seismic pulse” have usually enabled officials to rule out earthquakes or natural phenomena. But officials said the relatively small size of Monday’s blast might make it difficult to use seismic data to determine whether the explosion was caused by a nuclear device or conventional explosives. Analysts haven’t ruled out that the seismic shock could have been caused by conventional explosives, but a consensus seemed to be forming Monday that a nuclear device had been detonated.
American and foreign intelligence agencies are expected to focus on detecting and measuring telltale radioactive particles in atmospheric samples collected by planes and unmanned aerial vehicles routinely patrolling the coastline of the Korean peninsula.
The Vienna-based Comprehensive Nuclear Test Ban Treaty Organization, which monitors the globe for possible nuclear activity, said it probably would take about 72 hours to gather all the information necessary to judge whether the seismic event in North Korea was a nuclear explosion.
North Korea claimed that it had completely contained the radioactive fallout from the explosion. But experts were skeptical, saying it would require a level of technical sophistication North Korea was unlikely to possess to prevent the release of any contamination into the atmosphere.
“Sooner or later something escapes,” said Daniela Rozgonova, a spokeswoman for the test ban group. “Then we will know for sure.”
The test was underground and probably carried out in a horizontal tunnel under a mountain, said David Albright, a former weapons inspector who now directs the Institute for Science and International Security. Unlike vertical tests, which leave a massive crater, he said, horizontal tests leave little external trace.
Experts said that using conventional explosives to create a blast of the magnitude recorded Monday -- equivalent to about half a kiloton of TNT -- would be difficult to accomplish.
“A kiloton would be 2 million pounds of TNT,” said Arthur Lerner-Lam, associate director for seismology and geology at Lamont-Doherty Earth Observatory at Columbia University. “It takes a lot of technology to get all of that to go off at once.”
As a result, officials and experts focused on other, more plausible, explanations for the small yield.
Officials said North Korea could have detonated a small device in order to conserve fissile material.
American intelligence agencies believe North Korea probably has enough plutonium for about eight nuclear weapons -- other estimates suggest it could build between four and 13 weapons -- and does not possess any enriched uranium.
Pyongyang also may have been testing a component of a larger system, but experts said such a step would be highly unusual because of its limited experimental value.
As a result, a consensus seemed to be emerging among analysts and experts that North Korea conducted a test of a full-scale device, but one that failed to compress the plutonium fuel at rates capable of producing maximum yield.
“The lower yield could be because of bad luck or bad design,” said Michael Levi, a nuclear weapons expert at the Council on Foreign Relations.
“The fizzle of a 15- to 20-kiloton weapon would be about a kiloton or two,” added Owen Cote, associate director of the security studies program at the Massachusetts Institute of Technology.
American intelligence agencies have struggled to collect meaningful data on North Korea’s nuclear program and have scant information on Pyongyang’s ability to make a warhead small enough to mount on a medium- or long-range missile.
Pakistan’s nuclear bomb, built on a Chinese design, can be put atop a ballistic missile, but it is not clear whether these design details were passed on to the North Koreans as part of the proliferation network operated by Pakistani scientist Abdul Qadeer Khan.
Monday’s test may not provide any answers because the weight of a nuclear weapon -- as opposed to its yield -- would not necessarily be detectible from an underground test.
“You could test a device as big as a house,” said Gary Samore, who handled nuclear proliferation issues on the Clinton administration’s National Security Council. “There is no way to verify that from the seismic activity we’ll get.”
Miller reported from Washington and Kaplan from Los Angeles. Times staff writers Peter Spiegel in Washington and Alissa J. Rubin in Paris contributed to this report.