North Korea on Tuesday carried out an underground test of what it claimed to be a high-level, miniaturized nuclear device.

After the test, the North’s official media said the explosion showed off the “superior capability of its diversified nuclear deterrence,” suggesting that it may have tested a uranium-based atomic device unlike its two previous tests in 2006 and 2009.

Officials and experts have suspected that the North has developed a highly enriched uranium bomb technology which is simpler in structure and easier to hide and reduce in size than a plutonium-based device.

To determine whether Pyongyang used highly enriched uranium or plutonium, experts need to analyze radioactive gas, which takes two to four days.

Uranium-235, the core ingredient for the uranium bomb, is acquired from uranium found in nature. As the concentration of 235U is only 0.7 percent natural uranium, 1,000 tons of uranium must be excavated to produce 1 ton of nuclear fissile grade uranium. 



Uranium is refined into yellow lumps, which is called yellow cake. Yellow cake is then combined with fluoride and processed in a centrifuge, which separates 235U ― the nuclear fissile material ― from the non-nuclear fissile materials. This process is called enrichment. Through the enrichment process, 235U concentration is raised.

Low-enriched uranium has 3-4 percent 235U concentration and is used by nuclear power plants. Highly enriched uranium has over 93 percent concentration of U235 and is used to construct nuclear weapons.

It requires 15 to 25 kilograms of HEU to build one nuclear weapon. Little Boy, which was dropped over Hiroshima in 1945, contained 25 kg of highly enriched uranium. Pyongyang has claimed to have some 2,000 operational centrifuges capable of producing approximately 40 kg of HEU each year.

The communist country’s two previous tests in 2006 and 2009 used plutonium for fissile material.

Plutonium, meanwhile, is not found in nature. Plutonium-239 is obtained from spent fuel rods of a nuclear power plant. This process is conducted in a nuclear reprocessing facility, which North Korea possesses in its northern area of Yongbyon. North Korea 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.

A plutonium bomb requires less fissile material than a uranium version. But analysts point out that HEU is favored, as uranium does not require a reactor as plutonium does. Uranium enrichment can be conducted in centrifuge cascades in relatively small structures, and does not give off telltale signs of the tests.

In addition, HEU is said to be the easiest material to make a bomb with, and technical expertise and equipment for HEU are more easily accessible on the international black market. Many observers believe North Korea has been enriching weapons-grade uranium at its numerous secret facilities.

North Korea is believed to possess a 5-ton nuclear weapon and is developing a Daepodong-2 inter-continental ballistic missile, which can carry a 1-ton nuclear weapon. By December 2012, North Korea had already successfully launched a rocket. Many are wary that the communist state will eventually create a nuclear warhead that can be fitted onto an intercontinental ballistic missile, which could strike the continental United States.

However, experts forecast that North Korea still faces numerous technical hurdles to developing genuine ICBM capacity.

Yields from the communist country’s first and second tests were estimated to be 1 kiloton and 2-6 kilotons, respectively. A kiloton has the exploding force of 1,000 tons of TNT. North Korea has also been working to enrich uranium, which was revealed to the world in 2010 when Pyongyang invited the famous U.S. nuclear physicist Siegfried S. Hecker to tour its uranium enrichment facility and centrifuges in Yongbyon.

Despite earlier concerns that North Korea may be on the verge of testing a boosted fission weapon, preliminary assessments suggest that North Korea failed to produce such force. The latest test had a force of 6-7 kilotons, which falls well short of the 10 kilotons expected from a nuclear weapon test. The nuclear bombs dropped on Nagasaki and Hiroshima had 18 kilotons of force.

Boosting is a technique for increasing the efficiency of a small, lightweight fission bomb by introducing a modest amount of deuterium-tritium mixture inside the fission core. Though the boosted fission weapon in question is not at the level of a hydrogen bomb, it is much more powerful than uranium or plutonium ammunition.

North Korea began its nuclear program directly after the Korean War in 1953. Pyongyang built its first nuclear research institute in 1955.

The U.S. first detected Pyongyang’s nuclear development in 1989 through satellite pictures. North Korea has consistently used nuclear development as a political strategy to maintain its communist regime.

By Lee Sang-ju
(sjlee370@heraldcorp.com)