Thermoelectric power use emits CO2, a principal greenhouse gas that, along with others, produces acid rain. All of these send thousands of tons of ash, residues from coal and heavy metals, and even concentrates amounts of radioactive material into the atmosphere.
For their part, modern nuclear reactors emit almost no contaminants into the air, although they periodically emit small quantities of radioactive gases. Their residues are smaller in volume (to the order of a million times) and are better controlled than those of thermoelectric power.