As North Korea threatens to conduct a “high-level” nuclear test, speculation has been raised over its method, type, intensity and venue, as well as how far its military nuclear technology has come.

Seoul officials and experts say Pyongyang may conduct an underground test using highly enriched uranium. The first two tests in 2006 and 2009 used plutonium-based fissile material.

They largely concur that the explosive power of a future test will be greater than the past ones given that the communist state seeks to show off its “nuclear deterrence” capability to the international audience and bolster its negotiating power.

“As the North’s previous tests did not have a great political impact with the U.S. downplaying it, Pyongyang could conduct a much stronger test,” An Jin-soo, senior adviser at the state-funded Korea Institute of Nuclear-nonproliferation and Control, told The Korea Herald.

“Pyongyang could conduct multiple tests all at once as India and Pakistan did. The international criticism it would face would be the same anyway, whether it conducts a single test or multiple ones all together.”

An added that the North did not carry out multiple nuclear detonations simultaneously in the past as its stockpile of weapons-grade plutonium was limited.

Since the U.N. Security Council adopted a new resolution condemning the North’s December rocket launch last week, Pyongyang has hinted at conducting another provocative test through a series of official statements.

Seoul believes the North has already completed preparations for an underground test at the Punggye-ri test site in the country’s northeast, where it carried out the two atomic tests.

“With the current level of preparedness for another test, we judge the North can carry out the detonation experiment at any time. Therefore, we can detect signs of the test or cannot (as there may not be a significant change in activities at the site when the test occurs),” Defense Ministry spokesperson Kim Min-seok told reporters on Tuesday.

Stressing the importance of precluding the test, experts said another nuclear test would focus on enhancing the detonation power and technology to miniaturize and lighten warheads.

Some nuclear experts said that in a future test, the North could use a device with the explosive power of up to 15 kilotons, close to that of the atomic bomb the U.S. dropped in Hiroshima in 1945. A kiloton is equivalent to 1,000 tons of TNT.

In the North’s first nuclear test in 2006, its explosive power was about 1 kiloton. Due to its weak explosion, experts evaluated the test as a failure. But the second one recorded the explosion of between 2 kiloton and 6 kiloton, which was regarded as a “half success.”

As to the type of fissile material, experts forecast Pyongyang may use highly enriched uranium rather than plutonium, given that its operation of 5-megawatt reactors ― needed to yield plutonium-based fissile material ― has been halted.

“As Pyongyang is a state whose behavior is hard to predict, we can’t draw an easy conclusion,” said An of the KINC. “But considering the North can hardly produce additional plutonium and could have secret facilities for uranium enrichment, it is likely that the next test will use HEU.”

As the multilateral aid-for-denuclearization talks progressed in 2007 and 2008, Pyongyang disabled part of its facilities that yield plutonium. After the six-party talks were suspended in the late 2008, it is thought to have begun restoring the facilities.

As for the North’s stockpiles of weapons-grade plutonium, it is believed to have accumulated some 40 kg of plutonium after it reprocessed spent fuel rods at least three times in 2003, 2005 and 2009, according to Seoul officials. To produce one nuclear bomb, around 6 kg of plutonium is required.

Regarding the North’s uranium enrichment program, Pyongyang has claimed to have some 2,000 operational centrifuges capable of producing some 40 kg of HEU each year. To produce one HEU bomb, more than 15 kg of HEU is required.

To produce warheads mountable on long-range missiles, their miniaturization remains a critical task for the North.

Since 1980, the North is presumed to have conducted more than 100 experimental high-explosive detonations as well as the two nuclear tests, all of which have helped it gradually enhance its miniaturization technology.

To mount a nuclear warhead on its SCUD-B missile, the North should reduce its weight to 1,000 kg and its diameter to 90 centimeters, Seoul officials said. Seoul believes the North’s nuclear warhead still weighs some 2-3 tons.

As for foreign examples, the U.S. has developed a nuclear warhead weighing around 110 kg whose explosive power is around 150 kilotons, while Russia has a nuclear warhead weighing 255 kg whose power amounts to 200 kilotons, according to reports. China’s warhead weighs 600 kg and boasts an explosive power of 200-500 kiloton.

U.S. warheads are mountable on cruise missiles while the other countries’ warheads are mounted on submarine-launched ballistic missiles.

To enhance its delivery capability, the North has steadfastly developed long-range missiles under the name of space technology development. The successful launch in December of its rocket indicated that the North’s missile is capable of traveling some 10,000 km to strike the U.S. mainland.

After the North has conducted a nuclear test, South Korea and the U.S. authorities will be able to gain information on the explosive power of its nuclear detonation by analyzing seismic waves, sound waves and radioactive gases such as xenon and krypton, experts said.

But they said that confirming the level of miniaturization technology will be difficult unless one looks into the scale of the actual weapons tested.

By Song Sang-ho (sshluck@heraldcorp.com)