system
Given the following post, respond as if you're commenting on Reddit:
human
Why is a cooling tower in a Nuclear power plant, the shape that it is?
gpt
The towers you are talking about aren’t exclusive to nuclear power plants. In the US, more nuclear units use once through cooling than cooling towers. You also see these with fossil plants like the Michigan city coal plant in Indiana. 

The large cooling towers are called natural draft cooling towers. The shape creates a chimney effect, where the rising steam and moisture creates negative pressure as it passes through the contours of the tower. This sucks cold air into the bottom which supplies additional cooling. 

If you didn’t have the large hyperboloid shape, you would need to use fans to create that air draft to create the same cooling effect. Look up Columbia Generating Station which uses 6 forced draft cooling towers that use forced air fans for contrast. 

The purpose of the cooling towers is to cool the water which circulated through the condenser. Recall that the condenser takes the waste steam and makes it a liquid again for re-use in the plant. If you don’t use cooling towers you need to push 500,000 gallons of lake water per minute through the condenser and back to the lake. If you use cooling towers you just need to draw to make up for evaporation (10,000 gallons per minute). 

Hope this helps.