Your plot is too complicated. The foremost motive to steal a nuclear warhead is *not* to obtain a out-of-a-box working bomb - no - what you are after is plutonium metal. Everything else is rubbish. All bombs have very sophisticated dial-a-yield mechanisms and conditional detonation systems which render them useless for terrorists. So - forget the 50-man team making an epic firefight with ex-navy seals etc. If nuclear warheads are actually transported without convoys then you need two, maybe just one man to steal it.
Here is how:
1) A roadside bomb so powerful that it will destroy the tractor and the container. Two other under road bombs blocking all access to the site.
3) Small aircraft stored nearby where you have 500 yards of reasonably solid and even surface, a field will do in most cases. A good and simple plane with autopilot, like Cessna 182, is a good choice.
4) Fire extinguishers, metal cutting tools, hand trolley.
5) A lorry, a van, something parked 100km away. Just make sure it is well outside of 30min scrambling radius of any AF base.
What to do:
- Wait until the nuclear transport arrives.
- Detonate IEDs and eat popcorn while watching how the fireball kills the truck driver and ex navy seal "nuclear guard". Watch how the container breaks open and the secondary explosions finish the show. Other IEDs have blocked all direct road access to the scene giving you the 10 minutes you need.
- Locate warhead from the container and put out fires, if any. Use metal tools, if needed, to extract the warhead.
- Carry the warhead to the aircraft. (warheads are usually less than 200kg)
- Take off, set the autopilot course to the direction where your getaway vehicle is parked
- Parachute both you and the warhead when you reach the vicinity of you getaway car - remember to set the autopilot before you leave.
- Enjoy your freedom and laugh when goverment X is making the greatest firework show out of that autopiloted Cessna.
Now you have most sought out metal of this universe and can do all sorts of business with it.
Cost of all this is less than 30 000USD - then again with that amount of money you can probably bribe some Pakistani dude giving you the keys to that nuclear transporting lorry. Add a zero, or two zeros, to that number and you have an American nuclear bomb. Hell, it is still a bargain if it is intact and costs only $3M. :)