Governor Mitt Romney's description, caught on video, of what he considered the real nuclear threat from Iran has further undermined his national security credentials, showing a fundamental misunderstanding of nuclear threats. Iran's nuclear program has nothing to do with dirty bombs. Terrorists would not use uranium -- from Iran or anywhere else -- in a dirty bomb. It is unclear if Gov. Romney was just riffing, or if his advisors had fed him this line of attack. But it is dead wrong.