If it’s just for steam why make it different?
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They are shaped differently because they are being used for different things. A smokestack is used just to exhaust gasses up high enough to be away from people on the ground. The nuclear plant doesn’t have smokestacks though, they have *cooling towers*. The aim there is to draw in air at the bottom and make it carry heat out the top.
Hot water is sprayed into the tower and allowed to fall through the cool air, becoming more cool water when it lands at the bottom. Only a small amount escapes as the steamy mist out the top. This is the whole aim of the cooling towers, to make cool water which can then be used to cool the water that actually flows into the reactor.
Are you sure you’re talking about smoke stacks in both cases? Usually the very high and slim towers are smoke stacks, to reach high enough winds to spread the pollutants thin enough and not just around the plant.
The fat parabolic towers are cooling towers and are also present around refineries.
Nuclear power plants do not have smoke stacks. That is something you find on coal, oil and gas power plants. What you are likely confusing for smoke stacks is cooling towers. These are a feature of some nuclear, coal, oil and gas power plants. Others use a nearby body of water for cooling or even remote heating of houses as a form of cooling. The cooling towers use the evaporation of water to cool steam so it condenses back into liquid. This requires a lot of surface area. So a cooling tower is wide to provide lots of surface area for the evaporation. However a smoke stack tends to be narrow and much taller.
The tall structures you see that have a tighter waist are cooling towers. Even normal power power plants have them (like coal fired). These are not smoke stacks to release exhaust products.
Nuclear plants don’t have smoke stacks.
A smoke stack is a tall chimney that carries harmful or annoying gasses or smoke high into the air so that by the time it falls to the ground it’s spread out enough that people won’t sue for making them sick or hurting their property.
The big fat stubby towers at nuclear plants are called “cooling towers”, and they allow hot water to cool. Sometimes hot moist air from the tower can rise above the tower and form clouds.
Many modern cooling towers are wide at the bottom, narrower in the middle, and wide at the top. This called a Hyperboloid.
This has two advantages. It is easier to construct and takes less material. This shape also accelerates the normal rising airflow.
Plants use cooling towers to recirculate most of the water they use. A plant might use up to 100,000 cubic meters an hour. That could kill millions of fish per year who get sucked into the intake filters. The warm exhaust water can alter the lake/rivers ecosystem.
The wide hourglass-shaped towers you’re thinking of are not smokestacks, and are not unique to nuclear power plants. They are cooling towers. Every power plant needs to dump heat into the environment. Cooling towers work by spraying mist over pipes carrying hot water; the mist cools the pipes and evaporates into water vapor that rises up the tower. The hourglass shape of the tower increases the speed of the rising air, making cooling more efficient.
There are other ways to cool a power plant, though: one of the most common is to run river or seawater over the hot pipes instead. Both nuclear and fossil fuel power plants can use this method, but it only works if you’ve got a good supply of water.
Here are some nuclear power plants that don’t have cooling towers, because they use river or ocean water to cool off:
[https://www.epa.gov/npdes-permits/pilgrim-nuclear-power-station](https://www.epa.gov/npdes-permits/pilgrim-nuclear-power-station)
[https://en.wikipedia.org/wiki/Diablo_Canyon_Power_Plant](https://en.wikipedia.org/wiki/Diablo_Canyon_Power_Plant)
[https://en.wikipedia.org/wiki/Indian_Point_Energy_Center](https://en.wikipedia.org/wiki/Indian_Point_Energy_Center)
Here’s a coal power plant that does have cooling towers, because the local community complained that the hot water it was dumping into the harbor was killing fish. So they built huge cooling towers at enormous expense, and then coal power became unprofitable so they demolished the whole thing including the brand-new towers.
[https://www.powermag.com/new-englands-largest-coal-and-oil-power-plant-to-close/](https://www.powermag.com/new-englands-largest-coal-and-oil-power-plant-to-close/)
They’re cooling towers like others have mentioned but in addition to that, not all nuclear plants have cooling towers. Some just rotate water from e.g. a lake or a sea.
For example, the Olkiluoto nuclear power plant: [https://en.wikipedia.org/wiki/Olkiluoto_Nuclear_Power_Plant#/media/File:Olkiluoto_Nuclear_Power_Plant_2015-07-21_001_(cropped).jpg](https://en.wikipedia.org/wiki/Olkiluoto_Nuclear_Power_Plant#/media/File:Olkiluoto_Nuclear_Power_Plant_2015-07-21_001_(cropped).jpg)
The high towers there aren’t for smoke but for exhausting the ordinary air from inside the power plant. In case of a radioactive leak, any contaminated air is pushed high enough that it’s spread to the atmosphere instead of contaminating the site. Under normal operating conditions they don’t really offer any particular benefit though.
The “smoke stacks” at nuclear plants are for “gaseous effluent discharge”. You can see these very clearly in this photo: https://imgur.com/a/JAstQXj
During normal operation of a nuclear plant, the entire building is kept under slight vacuum. This ensures that air only leaks in – and the air coming in from outside follows a specific path from clean areas, into low radiological risk areas, then medium risk, then high risk, then finally into giant fans which go into special filters – then finally, the air after being filtered goes out of the “smoke stack”.
This ensures that if there is a leak of radioactive material in the reactor vault, the only place that leak can go is into the filters, and in the unlikely event that anything that gets through the filters gets released high up above the plant, so it doesn’t contaminate the rest of the plant. Because the air flow is carefully designed to go from low risk to high risk, a reactor vault leak won’t contaminate the offices or generator room.
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