I am an aeronautical engineer and I am curious why do NE's design the reactor smokestacks to be shaped like elongated rocket nozzles? In general, the nozzle shape offers less drag and a cleaner exit of the flow upstream and downstream from the nozzle exit. In supersonic flow, the nozzle shape is used to accelerate the exiting velocity faster than Mach 1. The real question is here: I do not see why such a shape would be necessary on a nuclear power plant. My guess would be it has something to do with maintaining a specific flow attribute near the cooling rods. Even then, you would want the water near the cooling rods to be a fast as possible to convect the heat away most quickly. An alternative guess would be the fact that the evaporating water should at first have a very high initial velocity (near the interface), but decrease as it cools climbing the smoke stack. Perhaps the shape then is to maintain a constant vapor rising speed?