I guess it depends where the nuke gets hit. If it hit any of the detonation devices around charges chances are the fission won't happen (as it needs very precis

I guess it depends where the nuke gets hit. If it hit any of the detonation devices around charges chances are the fission won't happen (as it needs very precise condition or that it will be severely reduced in force). If it hits propulsion of the missile, then falls on the ground and the detonation circuit goes "right, that's good enough, let's explode"...