Are nuclear "bunker busters" really a good idea?

This simulation by the Union of Concerned Scientists details the potential problems of adding a nuclear element (depleted uranium) to conventional bunker busters. One key point is that if the penetration depth of the bunker is at all uncertain, the repercussions could be unconscionable--this simulation claims that up to 3 million people could die from the explosion and radiation effects if a nuclear bunker buster were dropped on the Isfahan nuclear facility. Aside from the potential fallout, cost-benefit analysis of these weapons reveals that they are developed at great expense even though they may not even be effective. Shouldn't the government be more certain to develop a weapon that clearly works rather one that only might work? Be sure to turn up the sound to hear the commentary.