Here we go again...nukes in the tunnel, into the US. Complete ignorance of the reality of weapons handling, sizes, complexity, etc.
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.