The US, 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 did not vent radioactive material.
North Korea tested “a smaller and light” nuclear device on Tuesday, the official Korean Central News Agency said, two months after the test-firing of a long-range rocket.
The device had a yield of between 6 and 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 US dropped on Hiroshima in 1945.
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 US intelligence community assesses that North Korea remains some years from achieving the capability to threaten the continental US, 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 on Tuesday 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 Min-seok 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.
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 US, South Korea, Japan, China and Russia.
North Korea was estimated last year to have between 30kg and 50kg of separated plutonium produced before the shutdown, enough for at least six nuclear weapons, according to a US 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,” the report stated last year.
After repeatedly denying such an effort, North Korea in November 2010 showed visiting US 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 required for a nuclear device.