By Kim Eun-jung
SEOUL, Feb. 4 (Yonhap) -- North Korea is believed to have detonated a nuclear device in 2009 inside a mountain tunnel elaborately designed with several traps to prevent radioactivity from escaping, Seoul's defense ministry said Monday, releasing video footage from its state-run television.
The latest revelation comes as South Korean and American officials have been closely analyzing satellite imagery on the Punggye-ri site in the North's northeastern tip, where three tunnels are dug into the 2,200-meter-tall Mt. Mantap. The area was used as the site for previous tests.
The four-part documentary film series aired on the North's Korean Central Television in October 2010 shows a horizontal tunnel with three "traps" between nine "doors," which are designed to block radioactivity and debris from escaping from the western tunnel. It was used for a second test in May 2009.
"The intertwined or spiral design seems to be aimed at preventing a nuclear blast and debris from going outside the tunnel," a defense ministry official said, asking for anonymity. "It may also have absorbed shock to prevent (the tunnel) from collapsing."
It was not immediately known what kind of materials were used to build the tunnel, presumed to be 2-3 meters wide and less than one kilometer long.
The first door, which is thicker than the others, is believed to be composed of high-intensity steel, and materials may be stuffed between the section to withstand shock from the nuclear explosion, the official said.
"(The North) may have filled up the area between the doors with earth or rocks to absorb shock and prevent leakage of radioactivity," the official said.
The first test is considered to have yielded an explosion of about 1 kiloton, and the second has generated a yield of 2-6 kilotons, according to nuclear experts.
While other countries have typically used vertical underground tunnels, North Korea has made a sophisticated horizontal-tunnel design, taking advantage of the mountainous terrain. It is expected to use a similarly designed tunnel for the upcoming test, military officials said.
"As Mt. Mantap is composed of granite, rocks can be melted down due to high heat after a nuclear test," another ministry official said, on the condition of anonymity. "The horizontal tunnel seems to be built sturdily to prevent (a meltdown)."
When the North conducted its first ever nuclear test in October 2006, radioactive materials such as xenon flew southward in the wake of the explosion inside the straight-line tunnel on the east side.
The picture's release came as earthquake monitoring stations and military officials have been on high alert to detect seismic tremors and radioactivity in the air that would point to imminent signs of a third nuclear test. It is the first time that the interior design of the tunnel was disclosed to the media.
According a government source in Seoul, senior North Korean officials have recently visited the western tunnel at the nuclear site, which was recently covered by a camouflage net to evade satellite monitoring, further fueling speculation about an imminent test.
"After vehicles believed to be containing senior officials visited the western tunnel, a mine car was seen moving in and out of the tunnel," the source said asking anonymity, as he was not authorized to speak to the media.
If a nuclear explosion takes place, earthquake monitoring stations will be the first to catch seismic tremors and sound waves within 15 minutes as it creates different patterns from those of natural earthquakes.
To determine the kind of nuclear devices, scientists will have to collect air samples to examine radioactivity, which could take days and is only possible if southward wind delivers enough materials.
While North Korea exploded plutonium bombs in 2006 and 2009, it is not yet known whether Pyongyang will test a uranium-based bomb this time or explode a mix of uranium and plutonium devices simultaneously in separate tunnels.
If progress has been made after the isolated state revealed its uranium enrichment centrifuge plant to an American nuclear expert in November 2010, Seoul officials believe the North will be capable of testing a bomb out of uranium, which could open a second route to conduct an atomic test and make weapons.
After Pyongyang successfully launched a long-range missile in December, the main question now is whether the North has accumulated technology to develop a nuclear warhead small enough to be mounted on a delivery device that can fly as far as the United States.
Examining the debris from the rocket booster, South Korean experts concluded the impoverished nation under heavy United Nations sanction independently built most of its key parts, and the rocket could fly as far as 10,000 kilometers, enough to western U.S.
If Kim Jong-un defies international calls to drop the test plan, Seoul and Washington will prepare a strengthened nuclear deterrence scenario, including the early deployment of an advanced missile interception system and a pre-emptive strike against the nuclear-armed North, in the worst case, according to senior officials.
Consultations have been underway after defense chiefs of the two sides in October of last year agreed to draft out a tailored nuclear deterrence scenario by the end of this year, but the recent nuclear tension shed fresh light on the growing threat from the defiant state under its young leader.
"As North Korea's nuclear threat will become more clear if it conducts a third test, South Korea and the U.S. will consider a tougher (nuclear strategy)," said a senior ministry official, who has been working with American counterparts on the scenario.
The military measure is expected to add pressure on the reclusive state in addition to U.N. sanctions that were already strengthened after its long-range rocket launch. China, its main ally, also endorsed the tighter sanctions and indicated it would cut aid to Pyongyang in the case of further provocations.