Nuclear weapons have fallen out of the sky before. No, they won't explode -- it isn't easy to make one go boom.

But I would not be surprised if Green political parties argue that risking a few km^2 contamination is worse than a certain dinosaur killer... :-)

http://en.wikipedia.org/wiki/1966_Palomares_B-52_crash

"The non-nuclear explosives in two of the weapons detonated upon impact with the ground, resulting in the contamination of a 2-square-kilometer (490-acre) (0.78 square mile) area by plutonium. [... another which fell] into the Mediterranean Sea, was recovered intact"