North Korea's recent threat to conduct an underground nuclear weapons test, its third, is provocative enough on its own. The North Korean nuclear weapons program is illegal, dangerous and destabilizing, has been widely condemned by the rest of the world and is even causing some tension (alas, probably relatively minor and temporary tension) in Pyongyang's all-important relationship with China.

Some analysts fear, though, that an upcoming test could feature a uranium-fueled weapon, rendering it potentially even more provocative. North Korea has in the past used plutonium. Why would the switch to uranium matter? Here are four reasons.

1) Uranium enrichment is easier to hide. "It doesn't need a reactor like plutonium, and can be carried out using centrifuge cascades in relatively small buildings that give off no heat and are hard to detect," Mark Fitzpatrick, who as director of the International Institute for Strategic Studies' non-proliferation program often focuses on North Korea, told an Australian news outlet. The country revealed it had a uranium enrichment facility in 2010 and is suspected of having more. Nuclear analyst Siegfried Hecker wrote in Foreign Policy recently that, based on his observations during a 2010 trip to the North, he has concluded that "Pyongyang must have a covert centrifuge facility" and probably possesses enough highly enriched uranium for a weapons test.

2) Weapons-grade uranium is easier to ship abroad. "Highly enriched uranium is the preferred currency of rogue states or terrorist groups," Paul Carroll, who works for the Ploughshares Fund, told the same Australian outlet. "It's the easiest fissile material to make a crude bomb out of and the technical know-how and machinery for enriching uranium is more readily transferred and sold." A North Korean nuclear weapon is bad enough for Northeast Asia, but proliferation is a potentially global problem.

3) Iran might be able to build a bomb without a nuclear test. North Korea could share its experience from the uranium-bomb test with Iran, as it did with missile technology, according to Hecker. This means that if Iran decides it wants to build a uranium-fueled weapon, it might not necessarily need to conduct its own weapons test to do so. That puts Iran potentially one step closer to "break-out" capability and means the world would have one less signal that Tehran had decided to go ahead.

4) North Korea would have two different ways to build a bomb. The first way, using plutonium, is limited by the country's stockpile, which Hecker estimates is only enough for "four to eight primitive devices." Plutonium is hard to make in secret because it requires a big plant. Highly enriched uranium, on the other hand, can be produced in greater secrecy and greater quantities, particularly given North Korea's access to uranium deposits. It would be that much easier for Pyongyang to squirrel away more nuclear weapons if it had two ways to make them.