N Korean secrecy on N-test fuels speculation about bomb design

North Korea took unusual steps to conceal details about its latest nuclear weapon test, fuelling suspicions that it has achieved a bomb design that uses highly enriched uranium as the core, according to a media report.
Analyses of the February 12 nuclear tests confirmed that the effects of the blast were remarkably well contained, with few radioactive traces escaping into the atmosphere where they could be detected, The Washington Post reported citing unnamed US officials and weapons experts who have studied the data.
North Korean had claimed it successfully conducted an underground nuclear weapons test, its third in seven years.

US intelligence agencies had positioned special aircraft in the region in hopes of picking up radioactive isotopes from the blast to determine the material used in the device.
However, they 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 Post said.
"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," the Post said.

"There's very little information, which suggests that the North Koreans are doing a good job of containing it," one of the officials was quoted as saying by the Post.

The absence of physical data could suggest a deliberate attempt by North 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 US analysts briefed on assessments of the tests.
An 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."
A device based on highly enriched uranium also would deepen concerns about cooperation between Pyongyang and Iran, the paper said, adding that American officials have so far no direct evidence of nuclear cooperation between them.

North Korea has long possessed plutonium, but its enrichment of uranium is a more recent development. On the other hand, Iran has been concentrating on uranium enrichment, which it says is for civilian purposes, the paper said.
In its first two nuclear tests, North Korea was thought to have used plutonium extracted from a stockpile of fissile material that it developed in the late 1990s, the report said.
The bomb that the US dropped on Hiroshima in 1945 used highly enriched uranium as its core, and the one dropped three days later on Nagasaki was a plutonium device.
North Korea recently warned that US bases in Hawaii and Guam would be targeted in what could turn into "an all-out war, a nuclear war". The communist regime has declared a "state of war" with South Korea and threated to.