A given energy source doesn't simply pollute—or not—as it's converted into electricity. There's also mining, plant construction, transportation, and other factors to consider. Looking at the big picture is the point of life-cycle numbers like these, expressed in grams of carbon dioxide equivalent per kilowatt-hour of electricity created.
Lifetime carbon emissions by source (g/kwh)
Natural gas: 466
Solar: 17 to 39
Nuclear: 16 to 55
The U.S. Nuclear Breakdown
Mining and milling: 1.7 (g/kwh)
Includes unearthing and transportation of uranium to refining and enrichment facilities
By far the most carbon-intensive part of the fuel cycle, enrichment is needed to concentrate uranium isotope U-235 from its natural level of 0.7 percent to the 4 percent required for reactor fuel. The United States uses a gaseous-diffusion enrichment method that gobbles 40 times more energy than the gas-centrifuge process used in Europe, but a planned switch to centrifuge enrichment will shrink nuclear's domestic carbon footprint significantly. Once enriched, the uranium is shaped into small pellets and inserted into fuel rods.
Building a nuclear power plant takes time, hundreds of thousands of metric tons of steel and concrete, and billions of dollars. It also requires lots of diesel trucks and industrial machinery, which translates into greenhouse gas emissions. Fortunately, a nuclear plant should last 40 years or more, which helps reduce its overall emissions per kilowatt-hour.
Plant operation: 3.9
Fission doesn't produce CO2, but replacement of spent fuel rods, plant heating, and other routine procedures do put a small load on the environment.
Temporary storage: ~1
Nuclear plants must store their own waste on-site or at an approved temporary site until Yucca Mountain is allowed to receive it. Transporting and storing the waste creates its own minor carbon footprint.
Nuclear total: ~21 g/kwh
If the industry switches entirely to centrifuge enrichment, the total will fall to around 12 g/kwh.