Unlike chemical energy, which derives from electron transactions between chemicals, nuclear energy is a result of processes that occur deep inside atoms. In a nuclear reactor the cores (nuclei) of uranium atoms are broken apart in a process called fission, which releases about a million times more energy than a comparable weight of chemical fuel. This enormous energy is used to produce steam, which in turn powers huge electricity-generating turbines.
Nuclear power plants do not burn fuels; therefore, they release no airborne pollutants. What is mistaken as billowing white clouds of "smoke" emanating from stacks at nuclear plants is actually steam carrying heat released in the reactors' cores.
If something goes seriously wrong with the power generation process, perhaps resulting from a design or equipment failure, or human error, consequences can be catastrophic. In extreme circumstances high levels of dangerous radioactive pollutants can be released into the air, into surrounding bodies of water, and onto the ground where they can find their way into biological media and ground water.
Nuclear energy has an exemplary safety record. Considered in context, the potential dangers are probably best viewed as a good trade-off for its advantages.