Honestly, there is no real reason for S. Korea to be fearful of NK's nuclear arms. Jung didn't develop these weapons to gain leverage over South Korea. He used these weapons to obtain a bargaining tool with the U.S. America's "commie-itis" is what Jung is appealing towards. Any communist nation that is just as powerful as America is a threat, and, while the policy of "not bargaining with terrorists" is [more or less] in affect, America has no other choice.
Simply put: "You don't demolish a country your trying to re-unite with. It just doesn't work."