There is no perfect energy source. Every energy source has its own advantages and disadvantages. According to the Nuclear Energy Institute, the power produced by the world's nuclear plants would normally produce 2 billion metric tons of CO2 per year if they depended on fossil fuels. In fact, a properly functioning nuclear power plant actually releases less radioactivity into the atmosphere than a coal-fired power plant. And then there are the negatives.
Historically, mining and purifying uranium hasn't been a very clean process. Even transporting nuclear fuel to and from plants poses a contamination risk. And once the fuel is spent, you can't just throw it in the city dump. It's still radioactive and potentially deadly.