VIENNA: If North Korea forges ahead with its announced plan to test a nuclear device, it will likely be conducted underground to avoid revealing secrets about its program and spreading radiation, an analyst said.
An above-ground, atmospheric test can yield the trademark, and terrifying, mushroom cloud and large amounts of radioactive fallout, while an underground test shakes the earth but is usually designed to avoid fallout.
John Pike, from the Washington-based think tank Global Security, said North Korea was not expected to carry out an atmospheric test, although he admitted this would have "a theatrical impact."
"The current thinking is that they have a horizontal tunnel in the side of a hill," at a site called Punggye-yok in the northeast of the country, he said.
Pike said the idea of an underground nuclear test is to do it "deep enough underground so that it does not vent," that is send radioactive debris to the surface.
This would be for two reasons. The first is to avoid radiation contaminating the atmosphere.
The second, Pike went on, "is that if you have a lot of venting, you give technical intelligence to enemies as to how it works. You want to have many hundreds of feet of rock above you and you might have to tunnel in over 1,000 feet (over 300 metres) to get that rock above you," he said.
Then there would a "room at the far end of the tunnel" where the nuclear device would be placed, with diagnostic sensors to measure neutrons and x-rays positioned at an appropriate distance.
"Then you put cable running out of the room back out the tunnel to the outside and once you have that set up, you fill the tunnel and set the thing off," Pike said.
South Korean lawmaker Chung Hyung-Keun, who sits on the Seoul parliament's intelligence committee, has said a deep shaft and a nearby horizontal tunnel have been constructed at Mount Mantap, near Punggye-yok.
North Korea, which last year declared itself a nuclear power, announced on Tuesday that it planned to carry out a test. It did not specify a date.
The UN Security Council, which has been meeting amid international concern, will most likely adopt Friday a non-binding statement urging North Korea to halt its plans, but without explicitly mentioning a threat of sanctions, its president said on Thursday.
Underground nuclear testing made up the majority of nuclear tests by the United States and the Soviet Union during the Cold War.
Other forms of nuclear testing, such as atmospheric and underwater tests, were banned by an international treaty in 1963.
The Federation of American Scientists said on its website that a diagnostic canister to measure the data from a test could be two metres (eight feet) in diameter and 30 metres (120 feet) long, containing all the instruments needed to receive the burst of information at the time of the explosion.
"The diagnostic canister might contain lead and other materials as shielding for the detectors," the federation said.
It said that "when an underground nuclear device is detonated, the energy release almost instantaneously produces extremely high temperatures and pressure that vaporizes the nuclear device and the surrounding rock."
At first, within a fraction of a second after detonation, "a generally spherical cavity is formed at the emplacement position," the federation said.
"After a period of minutes to hours, as the gases in the cavity cool, the pressure subsides and the weight of the overburden causes the cavity roof to collapse, producing a vertical, rubble-filled column known as a rubble chimney" which actually is a huge crater, according to the federation.
Once the crater cools, "a post-shot hole is usually drilled into the point of the explosion in order to retrieve samples of the debris. These samples are highly radioactive but provide important information on the test." —AFP
