US and allied spy agencies have found no traces of telltale nuclear-related particles from North Korea's February 12 nuclear bomb test, leaving unresolved basic questions about the device's design, according to officials in the United States, Europe and South Korea.
This lack of scientific evidence suggests that key questions may remain unanswered about the type of fissile material used in the test, which was detected by seismic sensors. It also leaves unaddressed questions about how far the North has advanced in its bomb design.
After the test, the U.S. Air Force Technical Applications Center in Florida dispatched WC-135 "sniffer" airplanes to look for traces of gas residue that could offer clues to the device's design, but those efforts apparently turned up empty, the officials said.
An Air Force spokesperson confirmed that the planes were dispatched but said no results from the missions could be released. A U.S. intelligence official said analysis from the tests "was continuing."
Based on seismic evidence, both officials and private experts say there is little doubt that the North Korean device was several times more powerful than those tested in 2006 and 2009.
While estimates of the explosive power of the latest test vary widely, most officials and experts estimate it was at least five kilotons, which is smaller than the power of the atomic bomb the United States dropped on Hiroshima in World War Two.
In a statement about the test issued through its official news agency, North Korea declared that it had used "a miniaturized and lighter nuclear device with greater explosive force than previously (and which) did not pose any negative impact on the surrounding ecological environment."
One critical question is what kind of fissile material North Korea used in the latest test.
In the two earlier tests, North Korea is believed to have used plutonium as the fissile core of its test devices.
Following international diplomatic pressure, North Korea in 2007 abandoned plutonium production. But it later acknowledged that it had built facilities to produce highly enriched uranium, another fissile material that can be used in bombs.
While plutonium is a by-product of nuclear reactors, experts say it can be difficult to