The idea of using nukes would not be to blow it to bits. That would result in a huge mass of smaller chunks moving along the same orbit ready to lay the smack onto a wide area of Earth, with the same fireballs and explosions and dead bodies and all that sort of funky stuff.
The way to use these nukes would be to detonate them nearby. The resulting shockwave would nudge the asteroid into a slightly different orbit - even if it's altered by a few degrees, given the distances involved altering the course a few years from impact would result in this big rock missing us completely. Or they'll put into action plans involving mining and inserting big flare-like things, or using neutron bombs or gynormous lasers to vapourise the surface rock to create a big blast that moves the asteroid etc.
Again I wonder just how long people will pay serious attention to this. Especially with the scientists now saying there's a "slim" chance, as if to not frighten the public into running amok. All that will result in is governments saying they don't have to do anything for the next few decades because it's only a "slim" chance.
