By The Associated Press

Published: Wednesday, February 13, 2013, 10:06 p.m.
Updated: Wednesday, February 20, 2013

TOKYO — North Korea's underground nuclear test shows it is making big strides toward becoming a true nuclear power. But the test may also reveal key clues the secretive nation might have hoped to hide about how close, or how far away, it is from fielding a nuclear weapon capable of striking the United States or its allies.

Hoping to capitalize on a rare opportunity to gauge North Korea's nuclear capabilities, intelligence and military officials around the region are scrambling to glean data to answer three big questions: how powerful was the device Pyongyang tested, what sort of device was it, and what progress does the test indicate the nation has made.

North Korea hailed Tuesday's test as a “perfect” success, saying it used a device that was stronger and more advanced than those in its past two attempts. Add that to its successful rocket launch in December and the threat of a North Korea ready to strike at the United States, which it sees as its arch-enemy, would appear to be more real than ever.

But just how close is it?

The main thing intelligence officials want to figure out is what kind of device was used. Was it a plutonium bomb, like the ones it tested in 2006 and 2009, or one that used highly enriched uranium?

James Acton, an analyst with the Carnegie Endowment for International Peace, said North Korea's plutonium stockpile is small and it would be difficult and expensive for the North to produce more. But a test using highly enriched uranium, which is cheaper and easier to produce, would raise the threat that North Korea can expand its nuclear arsenal quickly.

“A highly enriched uranium test would be a significant development,” he said. “Unfortunately, we don't yet have any evidence as to the device's design yield or whether it was made from plutonium or highly enriched uranium.”

Finding that out is a race against time.

Joseph De Trani, former head of the National Counterproliferation Center, predicted U.S. intelligence would determine the size and composition of the nuclear device in one to three days based partly on radioactive elements released into the environment.

“Highly enriched uranium is something that degrades quickly, so you would have to collect within a 24-hour period,” especially because the traces from an underground explosion will be minimal, he said.

Neighboring Japan may provide some of those answers.

Its fighter jets were dispatched immediately after the test to collect atmospheric samples. Japan has also established land-based monitoring posts, including one on its northwest coast, to collect similar data.

But experts caution such monitoring doesn't always work because test sites can be sealed to prevent tell-tale leaks. They also note that North Korea has proven it has the ability to mask its tests quite well. No radioactivity was detected after North Korea's test in 2009.

The first indication of the latest test was seismic activity at the test site, which U.S. officials estimated at roughly magnitude 5.1. That would be equivalent to a medium-sized earthquake. North Korea's two previous tests registered at magnitude 4.3 and 4.7.

Working off that data, South Korean officials estimate the yield of the device — a measure of how strong its explosion is in comparison to TNT — to be between 6 and 7 kilotons. The United States has estimated it at “several kilotons.” Either way, it would be North Korea's biggest yield yet but far less than that of the weapon dropped on Hiroshima in 1945, which was about 20 kilotons.

“Because the depth of the test is not known and the geology of the test site is uncertain, translating the seismic magnitude into yield is difficult,” said Acton. “My own back-of-the-envelope calculation suggests a yield of between 4 and 15 kilotons.”

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