Chimneys?
You mean these things?
Those aren't chimneys. They're called cooling towers. Large power plants produce enormous amounts of heat, but only use about half of it. They have to release the remaining heat to prevent melting, so they vent it into the environment.
How do they release this heat?
If there's a sufficiently large river nearby, they use that water. If not, they build these large, chimney-like towers to release the heat through EVAPORATING WATER. Yes, these chimneys release steam and vapors, not smoke or radiation. And why these large towers? The hot water raining down the bottom of the tower causes the air inside to heat up and rise, much like a hot air balloon. This "natural draft" draws in fresh air through the zigzag openings at the base, which cools the falling water through evaporation, so the remaining puddle at the bottom stays nice and cool and returns to the power plant. The tower has to be this large to accommodate the enormous volume of air needed to cool the vast amount of water heated by the power plant.
Water, in the quantities consumed by power plants, is not cheap, partly because it is highly purified. In most cases, it is cheaper to recycle it than to obtain more of it.