Okay, let's distinguish between the low tech Dirty Bomb (which is really nothing more than radiological waste product wrapped around conventional high explosive) and a real nuclear device. Setting off a small nuke within a few thousand yards of the Superbowl wipes out the game and mostly everybody within a five square mile area, depending on the mass of the explosion. More KT (Kilotons) means more destruction. More threat of the truly deadly gamma radiation. More effect of the EMP on area electronic systems.
The biggest threat for the so called "dirty bomb" is the blast itself for those right on top of it. The rest depends on wind drift and the DECON plans for first responders. The big consequence is the propaganda value and that would affect a perceived impact on the economy.
I'm more worried about the State of the Union Address.
Thanks for the clarification. I did a little research into the kind of destruction a dirty bomb could produce, and I found quite a range. I guess, like you said, it depends on many factors including wind, etc... But I agree with you that even a conventional bomb would be a phsycological big strike. I also wonder how the 65,000 fans would react? Many with cell phones (I guess) would get instant info from relatives, etc.. perhaps wrong or distorted info. I could see a panic and stampede. That would not be good.