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Washington - U.S. officials and independent experts say North Korea appears to have taken unusual steps to conceal details about the nuclear weapon it tested last month, fueling suspicions that its scientists shifted to a bomb design that uses highly enriched uranium as the core.
At least two separate analyses of the Feb. 12 detonation confirmed that the effects of the blast were remarkably well contained, with few radioactive traces escaping into the atmosphere where they could be detected, according to U.S. officials and weapons experts who have studied the data.
U.S. officials anticipated the test and monitored it closely for clues about the composition of the bomb, which was the third detonated by North Korea since 2006. The first two devices were thought to have used plutonium extracted from a dwindling stockpile of the fissile material that North Korea developed in the late 1990s.
A successful test of a uranium-based bomb would confirm that Pyongyang has achieved a second pathway to nuclear weapons, using its plentiful supply of natural uranium and new enrichment technology. A device based on highly enriched uranium, HEU, also would deepen concerns about cooperation between the hermetic regime and Iran.
North Korea's belligerent threats in recent weeks have increased concerns among American and South Korean officials and ratcheted up worries about the level of progress on long-range missiles and nuclear weapons by Pyongyang.
There are two paths to a nuclear weapon. The bomb that the United States dropped on Hiroshima in 1945 used HEU as its core, and the one dropped three days later on Nagasaki was a plutonium device. North Korea has long possessed plutonium, but its enrichment of uranium is a more recent development. Iran has been concentrating on uranium enrichment, which it says is for civilian purposes.
Although North Korea and Iran have cooperated on missile technology, U.S. officials said there is no direct evidence of nuclear cooperation.
"We're worried about it, but we haven't seen it," said a former senior Obama administration official, who, like others interviewed, spoke on the condition of anonymity to discuss intelligence assessments. "They cooperate in many areas, especially missiles. Why it hasn't yet extended to the nuclear program is frankly a mystery."
The prospect of a third nuclear test prompted heightened scrutiny of the Korean Peninsula by intelligence agencies from the United States and other countries. Despite the intense focus, U.S. analysts acknowledged that they did not pick up enough physical evidence to draw firm conclusions about the fissile material used in the device.
In fact, in the days following the detonation, U.S. and South Korean sensors failed to detect even a trace of the usual radioactive gases in any of the 120 monitoring stations along the border and downwind from the test site, the officials said. A Japanese aircraft recorded a brief spike of one radioactive isotope, xenon-133, but it was seen as inconclusive, the analysts said. Xenon-133 is released during nuclear weapons tests but also given off by nuclear power plants.
The absence of physical data could suggest a deliberate attempt by North Korea to prevent the release of telltale gases, presumably by burying the test chamber deep underground and taking additional steps to prevent any radioactive leakage, according to two U.S. analysts briefed on assessments of the tests.
"There's very little information, which suggests that the North Koreans are doing a good job of containing it," one of the officials said.
A second analyst familiar with the data said it appeared that North Korea "went to some length to try to contain releases. One possible reason to try to contain releases is secrecy, so we don't know very much about their nuclear testing."
The second analyst added that North Korea also appears to be worried about the reaction from China, its most important ally, in the event radioactivity drifts across the border and causes panic among residents.
Officials and analysts said North Korea's second nuclear test, which occurred in 2009, also left no detectable traces. Some experts pointed out that finding evidence of a nuclear blast is often a matter of luck because of the dependence on air currents and geological features at the test site. Still, it would not be surprising for North Korea to take extra steps to prevent outsiders from gaining insights into its nuclear capability, said a third U.S. official with access to classified data on the tests.
"Any country conducting a nuclear test works hard to contain it," the official said.
U.S. intelligence agencies had positioned special aircraft in the region in hopes of picking up two or more types of radioactive isotopes from the blast. Comparing ratios of isotopes can help determine the material used in the device.
Seismology readings confirmed that the explosion occurred under a mountain near North Korea's border with China. The readings indicated it was roughly as powerful as the bomb dropped on Hiroshima. Statements released by U.S. intelligence agencies have described the Feb. 12 event as a "probable" nuclear test.
North Korea's state-run news agencies said the country had "diversified" its nuclear stockpile with the new test. The declaration underscored concerns that the North had mastered a design that uses the country's ample supply of uranium. North Korea's plutonium stockpile consists of only a few dozen pounds of the gray metal, enough to build a handful of bombs. But recent visits to North Korea by U.S. nuclear experts confirmed that Pyongyang operates at least one uranium-enrichment factory, described by visitors as large, sophisticated and fully operational.
The United States was already concerned about an agreement between North Korea and Iran pledging technical and scientific cooperation. The pact was signed in Tehran in September at a ceremony attended by Iranian President Mahmoud Ahmadinejad and Fereydoun Abbasi, the head of Iran's nuclear energy program.
Representing the North Koreans at the signing was Kim Yong Nam, the country's second-highest-ranking official. A decade earlier, Kim had signed a similar pact with the government of Syria, an agreement that U.S. officials think led to the construction of a secret plutonium-production reactor near the Syrian city of Deir al-Zour. The nearly finished reactor was destroyed by Israeli warplanes in 2007.
Although Iran and North Korea have signed technical pacts before, the September accord was seen as particularly worrisome because it appeared to imply nuclear cooperation.
In the past, North Korea and Iran assisted each other in missile development, sharing parts and data and perhaps even conducting surrogate tests for one another at times when either nation was under international pressure, said Leonard Spector, a former Energy Department official who has studied technical ties between the two countries.
Further, both countries bought black-market enrichment technology from A.Q. Khan, the rogue Pakistani scientist accused of selling nuclear secrets to foreign governments. The two countries would almost certainly benefit from exchanging data on nuclear subjects ranging from centrifuge design to uranium metallurgy, said Spector, deputy director of the Center for Nonproliferation Studies at the Monterey Institute of International Studies.
Unlike sharing missile technology, which would require the movement of actual hardware, Olli Heinonen, a former senior U.N. nuclear official who inspected the programs of both countries, said the sharing of enrichment know-how would be harder to spot.
"It would be meetings between individuals, with very little hard evidence," said Heinonen, now a senior fellow at Harvard's Belfer Center.