Has anyone considered their approach to unexploded nuclear weapons?

I was told sometime ago that because of the extreme environment that warheads are exposed to that some third of them are expected not to explode and probably would remain intact on impact.

At a conference I asked a bomb disposal officer for New York how they would deal with them, his reply, "we have been told to kick them and if they don't go off cart them away"

I suppose if reentry and/or impact does not set off a warhead it may be presumed safe.

Just how robust are the triggers?