One thing that has been bothering me since I started using nuclear power is the lack of something to vent from the reactor, as in an excuse to build smoke-stacks. That's why I purpose that nuclear reactors produce smoke when they turn uranium ingots into power. This smoke wouldn't just be cheesy little particles coming from the reactor though, but would instead be a "block" of smoke that generates directly above the reactor.
Smoke would behave like a liquid (specifically water) but would instead flow up eventually disappearing into the sky. Also, whenever the smoke gathers in one particular block to the point of producing a source block (think water behavior) it would make a solid smoke block instead that would allow flowing smoke to build upon until it fills the room. And if that wasn't enough, if you let smoke fill up your reactor room (to the point of occupying 3 or more blocks adjacent or diagonal to the reactor) or if you let it touch any adjacent water blocks (as in you cover the top of the reactor with water and don't let the smoke vent) it would more than double the chance of a reactor meltdown. It would also be un-breathable.
Now this idea is built on one big assumption though: that water can be recoded (with only a tolerable headache) to flow up instead of down. If this is not the case than the whole idea becomes unrealistic(coding wise) and can be disregarded.