North Korea has vowed to restart the country's only nuclear reactor, which it shuttered in 2007 as part of an international deal. The plant can produce enough plutonium for about one small nuclear weapon per year.
The announcement would certainly seem like bad news, a sign that North Korea wants to expand its already-dangerous nuclear weapons program, another provocation after weeks of threats. And it is all those things. But there's potentially a major silver lining to this development: it suggests that North Korea has probably not learned how to develop nuclear weapons from uranium, as had been feared.
Obviously, there is no such thing as a "good" kind of North Korean nuclear warhead. But a plutonium bomb is not as bad as a uranium bomb. We already knew they had plutonium bombs, maybe the equivalent of six to eight warheads. Although the country has at least tried to develop a bomb out of highly enriched uranium, as it revealed in 2010, it has not had any proven successes. The fact that it's reopening the plutonium plant, even though plutonium is a less attractive bomb material, suggests that it's recent suggestions that it had developed a uranium bomb may have been mere bluffs.
When North Korea conducted its third-ever nuclear weapons test in February, it dropped some strong hints that it had used highly enriched uranium. State media, for example, declared the military had "diversified" the nuclear program. But Pyongyang worked hard to keep the world from finding out for sure whether the bomb had been uranium, and it has so far succeeded.
A uranium bomb would have been a significant upgrade for North Korea, and a scary development for the rest of the world, for four reasons:
1. North Korea would have two ways to build a bomb, which means a potentially larger arsenal.
2. The country has a natural supply of uranium and can enrich it to bomb-making levels in secret; plutonium is limited and is much tougher to hide. So weaponized uranium would be tougher to keep track of and easier to make in larger quantities.
3. Iran uses uranium in its nuclear program, so North Korea could share research and lessons from the nuclear test with Tehran.
4. Uranium is easier to ship abroad, meaning North Korea could more easily sell it to, for example, Iran.
It's still possible that North Korea did succeed in developing a uranium weapon and that its February test was a uranium warhead. We don't know for sure. But the big effort to rebuild and relaunch its plutonium plant (it destroyed the reactor in 2010 as part of the peace deal), despite the diplomatic costs this will certainly incur Pyongyang in any future international talks, suggests that the country really, really wants more plutonium.
Enriching uranium would be a much better way for Kim Jong Un to fill his nuclear appetite than re-opening the plutonium plant. If he wants plutonium this badly, it's a reasonable bet that he doesn't have the ability to enrich uranium. Or not yet, anyway.
Update: Jeffrey Lewis, a nuclear proliferation expert who specializes in East Asia, says he thinks that it's too strong to infer from this development that North Korea has "probably" not learned to develop highly enriched uranium. Paul Carroll of the Ploughshares Fund suggests that the diplomatic costs of restarting the plant are precisely the point: "It shows that DPRK is playing every belligerent card they can short of direct attacks," he says.