Technically, he wouldn' be able to. It is impossible to survive a nuclear explosion. Only Cockroaches can. Also, if he didn't die from the immediate explosion, the radiation would mutate his cells and kill him.
His insides are not actually made from steel as this would physically impossible. He would be stuck still - not able to move. The only way he could move would be if it was molten steel, but then he would die anyway..
Dude it's Superman! A guy who is from a differnet planet, draws his powers from the sun, has x-ray and heat ray vision. He can blow ice cold breath, pick up just about anything, Run nearly as fast as light and can fly! So why are you talkin about impossibilities?
In terms of the physical, mechanical shockwave, I believe that Superman would, in fact, survive if he had a nuke up his a**.
My evidence for this is the latest film in which he was shot, at point blank, in the eye, and in slow motion, the bullet hit his eye, squashed and splintered.
But what I'm curious about is exactly what it was about kryptonite that debilitated him so, and whether it would be possible to replicate these effects with any other material.
You know, Superman started out way back when as just a superhero who could run really fast and jump really high and was really strong. You know the whole "Faster than a speeding bullet, able to leap tall buildings in a single bound..." Then they just kept giving him more and more powers like some kind of 8 year old playing with his friends who just keeps making up more superpowers. If a villain ever did shove a nuke up Superman's rectum, they writers would probably decide that he had some sort of rectal bunker with a lead-based anti radioactive colon!
Superman has the strength in his hand to turn a lump of coal into a diamond which means he's tougher than a diamond. A diamond is the densest thing known to man and that wouldn't be destroyed by a nuke.
=> Superman, even with a nuke up his ass will always survive.
