The South Korean military and intelligence agencies are concentrating their efforts on finding out what kind of nuclear test North Korea is preparing to conduct. They believe that will tell them how close the North is to developing a nuclear warhead small enough to fit on a missile.
The question is whether the North will use highly enriched uranium or plutonium. "Now the hope is that an underground blast will answer several mysteries. Can the North Koreans produce a bomb out of uranium?" the New York Times wrote on Thursday.
The North's National Defense Commission seemed to hint at a uranium bomb on Jan. 24 when it threatened a "high-level" nuclear test.
The North extracted about 40 kg of plutonium, enough to make six to seven nuclear bombs, at a reprocessing plant in Yongbyon, North Pyongan Province, but it used up a considerable portion for its first and second nuclear tests.
But the country has the world's largest uranium ore deposits, and there is a chance that it has produced enough highly enriched uranium for four to six nuclear bombs at an enrichment facility over the last two years.
Once the North goes ahead with a fresh nuclear test, South Korean and U.S. intelligence agencies can analyze radioactive gases such as xenon and krypton to discover how the bomb was made.
South Korean security authorities regard the yield or explosive power as the most important criterion for judging the success or failure of the test. The first nuclear test was considered a failure because its yield was just 1 kt of TNT or less. The second test with a yield of 2 to 6 kt was regarded as half-successful.
If the North's nuclear weapons development proceeded without a hitch over the past four years, the upcoming nuclear test's yield could be close to 15 or 20 kt, intelligence authorities speculate.
The yields of the atomic bombs that fell on Hiroshima and Nagasaki in 1945 were 15 kt and 22 kt.
A security official said, "It's relatively easy to measure the yield by analyzing seismic waves and airborne sound waves that occur when a nuclear test is conducted."
What is militarily and strategically most significant is whether the North has succeeded in miniaturizing the bomb. It succeeded in test-launching a rocket with a range of 10,000 km on Dec. 12 last year, which takes the same technology as an intercontinental ballistic missile. If it succeeds in miniaturizing the payload, it would have overcome most of the technical barriers to producing nuclear ICBMs.
Pyongyang would then likely push the U.S. to recognize it as a nuclear state threatening the U.S. mainland, which could also increase its leverage with China and Japan. South Korea and the U.S. feel this would be a security disaster for the Korean Peninsula.
If it wants to mount nuclear warheads on Scud or Rodong missiles, the North needs to reduce their size to less than 88 cm and their weight to less than 1 ton.
Seoul and Washington have so far officially said that the North has not yet reached that level and its nuclear bombs still weigh 2 to 3 tons.
Verifying whether the North has succeeded in miniaturizing nuclear warheads will be difficult without a first-hand look at the weapon that the North puts into the underground nuclear test tunnel, experts say.