Why are nuclear reactors shape

[Aero51]

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?

 

[jim hardy]

You already know the answer -

Parabolic towers use natural draft , so driving head is minimal.
Think of it as a converging-diverging nozzle designed to provide most flow for the low differential pressure available.

The shape also lends itself to economical concrete construction .


http://www1.eere.energy.gov/femp/pdfs/waterfs_coolingtowers.pdf
http://che.sharif.edu/~heatlab/Lab/Benefit%20Book%20&%20Journal/Benefit%20book/Cooling%20Tower%20Thermal%20Design%20Manual.pdf


But I sense a misconception here:
the cooling towers you see do not house the reactor,
they cool the water that is used to carry heat away from the plant,
most of which comes from condensing the large amount of steam that exits the turbine.