South Korea confirmed increased radiation levels following the North's 2006 nuclear test but didn't find anything in 2009.
And if North Korea decides to conduct a so-called subcritical test, there would be no release of radioactivity at all.
A sub-critical test only works on the properties of plutonium but stop short of creating a critical mass, the point at which a self-sustaining nuclear reaction occurs. Such an experiment requires a "very difficult technology" that only a few countries like the U.S., Russia and England have acquired, said nuclear expert Whang Joo-ho of Kyung Hee University.
"I believe North Korea's technology has not reached that level," Whang said.
North Korea said its upcoming atomic explosion will be a "high-level" test and many analysts said that refers to a device made from highly enriched uranium, which gives the country a second source for manufacturing bombs in addition to plutonium.
Whether North Korea detonates a uranium- or plutonium-based device, there won't be much difference in how easily scientists can detect the tests. The only difference is that they produce different radioactive gases, Whang said.
He also said a uranium-based test explosion would mean that North Korea's nuclear stockpile can continue to be enlarged at a time when there is no evidence of continued production of plutonium at its main Yongbyon nuclear complex.
North Korea watchers in South Korea are speculating various dates for a possible nuclear test, with some predicting it could happen as early as this week and others choosing days just before the Feb. 16 birthday of late North Korean leader Kim Jong Il.
There is no way to determine when North Korea will conduct a nuclear test, said analyst Shim BeomChul at the state-run Korea Institute for Defense Analyses in Seoul. U.S. spy satellites "can detect objects 15 centimeters (5.9 inches) in size on the ground but they cannot detect what's happening underground," he said.