Why do nuclear power plants have such wide chimneys?

If you are talking about this than This is not a chimney. Large power plants produce an enormous amount of heat but use up only about 50% of it. They must release the remaining 50% heat to avoid melting, so they blow it into the environment. If you release that moist air at a low level. read more

Cooling towers vary in size from small roof-top units to very large hyperboloid structures that can be up to 200 metres (660 ft) tall and 100 metres (330 ft) in diameter, or rectangular structures that can be over 40 metres (130 ft) tall and 80 metres (260 ft) long. The hyperboloid cooling towers are often associated with nuclear power plants. read more