You mean these things?
These are not chimneys. Large power plants produce huge amount of heat, but use up only about half of it. They must release the remaining heat to avoid melting, so they blow it into the environment.
How do they release it?
If there is a big enough river nearby, they use the water. But if not, they build these big chimney-LIKE towers to release heat by EVAPORATING WATER. Yes, these chimneys release steam and vapor, not smoke or radiation. And why such big towers? To allow the rising steam to cool down and be able to recollect and in the next turn use it in the machinery and turbines again.
Water, in the amounts the power plants use it, is not cheap. In most cases, it’s cheaper to reprocess it than to get more of it.