Catching evildoers armed with nuclear devices is tougher than it sounds. True, unexploded nukes are radioactive, but not face-meltingly so. That makes 100 percent detection impossible. So instead of trying to find a perfect protection scheme (domed cities, anyone?), clever policymakers and technologists are developing a kind of Goldilocks strategy: Neither too much nor too little detection. These admittedly imperfect plans are designed to convince terrorists that even attempting a nuclear attack is futile. Put the ideas into action, the thinking goes, then deploy a smart PR strategy to let the would-be bombers know we’re onto ‘em —security wonks call it “strategic communication.” —Others call it propaganda.