TOKYO -- Since April, North Korea has shown signs of preparing for its fourth nuclear test. It is not easy to pick up slight indications of nuclear explosions conducted deep underground, or to press for facts from the regime. Nevertheless, scientists are moving ahead with analysis and are concluding that "there is a possibility they have succeeded in developing a small nuclear weapon."
It is thought that if North Korea were to test such a weapon, it would trigger seismic readings equivalent to a magnitude-5 earthquake. The Comprehensive Nuclear-Test-Ban Treaty Organization has been building a worldwide observation network for detecting vibrations from underground nuclear tests and as part of that has 10 monitoring facilities in Japan. These sites pick up seismic signatures indicative of underground nuclear tests.
By analyzing the waveform of such vibrations, it is possible to estimate the location and scale of an explosion. However, it is impossible with this alone to distinguish between nuclear explosions and explosions made from conventional weapons.
The key is xenon. Radioactive substances produced by nuclear explosions leak from test sites. If the specific wave signatures are detected in conjunction with radioactive xenon in the atmosphere, the conclusion must be that a nuclear test has been conducted.
Nuclear weapons, in contrast to conventional weapons, are like precision instruments with complex structures. These bombs require advanced technology to develop, build and maintain. To be practical, these weapons need to be tested to confirm whether they work and if they yield as much power as hypothesized.
The U.S. and the former Soviet Union conducted their early nuclear tests in deserts and at sea. But since the 1960s, such tests have been regulated by treaties. Testing moved underground. If sufficient test data can be accumulated, it is possible to substitute these tests with simulated tests on computers and subcritical nuclear tests that stop prior to a nuclear detonation. Industrialized nations these days rarely conduct even underground nuclear tests.
North Korea launched nuclear development in earnest in the 1990s. It has so far conducted three tests at an underground facility at Punggye-ri in the northeastern part of the country. Although the size of the explosions have been confirmed from seismic readings, the exact type of nuclear weapons being developed can only be guessed at from North Korea's statements and apparent level of technology.
An analysis has been made of these tests by Akihiro Kuroki, managing director at the Institute of Energy Economics, Japan who has served as head of the nuclear safety and quality assurance department at the Korean Peninsula Energy Development Organization (KEDO).
"There is a high likelihood that they have succeeded in developing an implosion-type weapon," Kuroki said.
Implosion type bombs are one of the earliest nuclear weapon designs. It is the same mechanism used in the bomb dropped on Nagasaki. The design places a mass of plutonium in its core. Conventional explosive charges around this plutonium pit are set off at the same time, creating an implosion that envelops the plutonium and compresses it until a fission chain reaction is triggered, causing a nuclear explosion.
These conventional charges must go off at the exact same time. If one is off by even a fraction of a second, the compression on the plutonium core will be incomplete and a full chain reaction will not be triggered.
Based on analysis of seismic waves, North Korea's first nuclear test, conducted in October 2006, was smaller in size than what the country reported to China. Kuroki says it is believed that the bomb failed to complete its chain reaction.
North Korea is rumored to be rushing to develop a boosted fission weapon. This type of nuclear bomb is smaller and lighter than an implosion type for the yield it can produce. It would be compact enough to fit on a Rodong missile, which can reach virtually anywhere in Japan.
Boosted fission bombs utilize a fusion reaction caused by tritium and deuterium in the device's core. This reaction emits high-energy neutrons into a surrounding shell of fission material, which boosts the material's explosive yield. The key to developing this type of weapon is mastering the nuclear fusion technology.
In May 2010, North Korea announced that it had achieved small-scale nuclear fusion. According to Kuroki, who has visited North Korea numerous times, where he met with nuclear engineers, the country has abundant knowledge, advanced engineering and also precision machine tools.
"They have adequate capability to create a boosted type weapon," he said.
The threat posed by boosted type weapons does not stop with the fact that they can be carried great distances by missiles.
"With these, a nuclear bomb is possible even without high-grade plutonium or highly enriched uranium," said Tetsuo Sawada, an assistant professor at the Tokyo Institute of Technology.
The older type of nuclear weapons, such as the implosion type, require time to create weapons-grade enriched nuclear materials, such as plutonium. Boosted types, however, can use even lowly enriched uranium. Moreover, enrichment is possible in facilities small enough to fit on a truck, making easier to evade observation.