Why are nuclear power plants built in a cone shape typically?

What you are showing in the question picture is the cooling tower of a Boiling Water Reactor which has a hyperboloid shape. This geometric figure is widely used in construction and has several advantages. read more

Cofrentes Nuclear Power Plant. In all nuclear power plants, the fission energy released in the reactor is used to produce steam which at the same time is used to drive steam turbines which are coupled to an electric generator. read more

The pressure vessel which houses the nuclear reactor itself is not quite bell shape, and the containment building itself isn't always bell shape either. Here is an image from that article. You can browse through Wikipedia for actual pictures of examples. I typically refer to these containment buildings being"can" shape or"sphere" shape. read more