First of all, we use many kinds of energy from fossil fuels to nuclear power. Fossil fuels are limited and ultimately we are bound to deplete this resource. If we conserve the amount of fossil fuels we use, our reserves will last longer. More importantly though, fossil fuels, when burned, emit sulfur compounds and hydrocarbons which cause acid rain, smog, and contribute to global warming. By reducing the amount of fossil fuels we burn, we directly reduce the amount of pollution emitted into our atmosphere.
Nuclear power is used to generate electricity.
However, the radiation associated with nuclear power is very dangerous. It can cause cancer and a host of health problems If we could guarantee that no radiation would leak from these nuclear plants, that would be one thing. However, no one can make this guarantee. A nuclear disaster, such as Chernobyl can have devastating effects which last for hundreds of years, making contaminated regions virtually uninhabitable. Additionally, nuclear reactors almost always generate nuclear waste, which most be disposed of.