The U.S., South Korea and Japan are sniffing the air in a coordinated intelligence effort to determine what type of nuclear device was detonated by North Korea’s secretive regime.
The country’s two previous underground nuclear tests, in 2006 and 2009, drew on its limited reserves of plutonium. Any evidence that North Korea used highly enriched uranium this time would signal that it has developed a second source of fissile material, expanding its potential warhead capabilities and raising the risk that the cash-strapped nation may sell such uranium to would-be nuclear weapons states such as Iran.
Air-sampling equipment, some in aircraft and some at ground-based facilities, is checking for residual radioactive emissions from the underground test that would confirm it was a nuclear explosion and provide markers for the type of fissile material used. A similar inquiry succeeded in 2006, while one in 2009 failed because that blast didn’t vent radioactive material.
“One important question is whether the nuclear test used only plutonium or involved highly enriched uranium, either alone or in combination with plutonium,” said David Albright and Andrea Stricker of the Institute for Science and International Security in comments on the website of the Washington-based research group that tracks nuclear issues.
North Korea tested “a smaller and light” nuclear device Feb. 12, the official Korean Central News Agency said, two months after test-firing a long-range rocket. The device had a yield of 6 to 7 kilotons, South Korea’s Defense Ministry estimated. That was bigger than the previous two North Korean detonations, though less than half the explosive power of the uranium-fueled bomb that the U.S. dropped on Hiroshima in 1945.
“There is no question that North Korea constitutes a threat to the United States, to regional stability, and to global security,” outgoing U.S. Defense Secretary Leon Panetta said yesterday at a news conference in Washington. “A combination of a recent missile test combined with what apparently was this nuclear test we believe represents a real threat to the United States of America.”
Much of the policy debate since the blast has focused on indications that North Korea was testing a miniaturized nuclear device as the regime seeks to develop a warhead that could be placed on a long-range missile. The U.S. intelligence community assesses that North Korea remains some years from achieving the capability to threaten the continental U.S., though it may pose a nuclear threat to its regional neighbors sooner.
South Korea doubts that Kim Jong Un’s regime has perfected the miniaturization technology, Defense Ministry spokesman Kim Min Seok told reporters yesterday in Seoul.
“It took Pakistan about seven nuclear tests to weaponize, so relatively speaking, the North doesn’t seem to have conducted enough tests,” Kim said.
North Korea said in its statement that it now has achieved a “diversified” nuclear capability -- a possible signal that highly enriched uranium was used.
That would be a significant development, according to James Acton, a nuclear policy specialist at the Carnegie Endowment for International Peace, a Washington-based policy group.
“North Korea’s plutonium stockpile is small and has been depleted somewhat by earlier tests,” Acton said in a statement on the group’s website. “Given the dilapidated state of its plutonium-production infrastructure, it would be both difficult and expensive to produce more. If, however, North Korea has mastered uranium enrichment, it could expand its arsenal relatively cheaply and quickly.”
North Korea’s plutonium supply is limited because it shut down and disabled the sole source, a reactor complex at Yongbyong, under a short-lived 2007 denuclearization accord with the U.S., South Korea, Japan, China and Russia.
North Korea was estimated last year to have 30 to 50 kilograms of separated plutonium produced before the shutdown, enough for at least a half-dozen nuclear weapons, according to a U.S. Congressional Research Service report.
“While North Korea’s weapons program has been plutonium- based from the start, in the past decade, intelligence emerged pointing to a second route to a bomb using highly enriched uranium,” according to the report last year.
After repeatedly denying such an effort, North Korea in November 2010 showed visiting American specialists early construction of a 100-megawatt light-water reactor and a newly built gas centrifuge uranium-enrichment plant, both at its Yongbyong site.
In late 2011, North Korea said the facility was producing uranium enriched to as much as 3.5 percent, suitable for use in a power reactor but short of the 90 percent enrichment for a nuclear device. Iran has natural uranium reserves that can supply an enrichment program.
“The North’s disclosure supports the United States’ longstanding assessment that North Korea has pursued a uranium- enrichment capability,” U.S. Director of National Intelligence James Clapper told Congress a year ago.
The production of highly enriched uranium is much easier to hide than is the case for plutonium, according to Victor Cha and Ellen Kim, writing on the website of the Center for Strategic and International Studies, a Washington policy group. “It can be made in from centrifuges operating in buildings the size of a warehouse unlike the big and easily identifiable footprint of a plutonium nuclear plant facility.”
The post-explosion detection effort involves the U.S. and its allies, as well as global monitoring stations established under the Comprehensive Nuclear Test-Ban Treaty, which prohibits testing in an effort to slow the spread of atomic weapons.
“If radioactivity is detected, it should be possible to distinguish between a uranium and a plutonium device,” Acton said. “It may also be possible to assess what other materials were present in the device and hence make educated guesses about its design.”
Secretary of State John Kerry said yesterday that North Korea’s nuclear test poses a threat to the U.S. and to global peace.
“The international community needs to come together for a swift, clear response,” Kerry said in Washington at a press conference with Jordanian Foreign Minister Nasser Judeh. Kerry said the response to North Korea would have to be swift and clear because it also would send a message to Iran.
“This is about proliferation, and it’s also about Iran because they’re linked,” Kerry said.
The U.S. has said that Iran is using its uranium-enrichment technology as part of an effort to develop nuclear weapons capability, and the U.S. and Israel have looked for evidence that Iran is cooperating with North Korea in weapons development.
Iran denied such cooperation in December, following a report by Japan’s Kyodo news agency saying Iran had stationed defense staff in North Korea for joint work on missile and nuclear development.
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