Then enlighten us. Bear in mind that the US did create artillery fired tactical nukes, and the largest land artillery for the US is maybe 10" in diameter.
However, even the rocket based atomic warheads aren't very big. This page shows three warheads on a minuteman missle, but without a size reference. ttp://www.gwu.edu/~nsarchiv/nsa/NC/mirv/mirv.html This page shows men installing a warhead on an minuteman missile: http://www.geocities.com/minuteman_missile/photo_gallery1.htm The warhead appears to be about 4 feet across, which makes the mirv smaller warheads at most two foot across.
It'd be easy to get one of the mirv'd warheads through that tunnel.
As for complexity, if they stole or bought a loose Russian nuke, they need to know as much about the complexity of the bomb as a car thief needs to know about the complexity of a Maserati: only enough to set it off.
Tell me what kind of nuke you think they have and I'll explain it to you. I don't think it's likely they have a US field artillery tactical weapon or warhead from a MIRV. I guess you're left with the mythical loose Russian nuke.
This nuke has now been purchased, stored, maintained, driven through Mexico, hoisted down a tunnel, hoisted out of the tunnel, and is now on the way to the Super Bowl in a panel truck. Why use a tunnel? Why not a boat or a small plane? Why wait until now? Was it stolen recently? Purchased from whom? Note that noone bothered to check the tunnel for residual radiation, since the idea of dragging it through of a tunnel is ludicrous.