After the Nuclear Summit, What Happens Now?

Nobody knows how likely it is that some terrorist group could set off a nuclear weapon.  But the consequences are so catastrophic that even a small chance justifies urgent action to reduce the risk. That was the idea behind President Obama's nuclear summit. Ukraine, Mexico, Chile, Kazakhstan, Vietnam and Canada have agreed to dispose of highly enriched uranium that might be made into bombs. But those and other commitments at the summit were voluntary, and "locking down" nuclear materials will be easier said than done. Even supporters call the 47-nation meeting a "first step." Opponents say it did nothing to put the brakes on Iran, which will hold its own conference this coming weekend. Others ask if nuclear terrorism is exaggerated in the first place.