“Even though we have not broken ground on a new nuclear plant in nearly 30 years, nuclear energy remains our largest source of fuel that produces no carbon emissions." ... “We’ll have to build a new generation of safe, clean nuclear power plants in America,” ... “To meet our growing energy needs and prevent the worst consequences of climate change, we’ll need to increase our supply of nuclear power. It’s that simple.”
~President Barack Obama
Nuclear power is sometimes described as being free of greenhouse gas emissions, and that’s true of the nuclear fission reactions themselves. But here is a list of all the stages of the nuclear power cycle at which greenhouse gases are emitted: uranium mining, uranium milling, conversion of uranium ore to uranium hexafluoride, uranium enrichment, fuel fabrication, reactor construction, reactor decommissioning, fuel reprocessing, nuclear waste disposal, mine site rehabilitation, and transport throughout all stages.
During these stages, greenhouse gases are emitted directly (for instance, by trucks) but also indirectly (such as through the use of materials such as steel and cement, which are manufactured using emissions-intensive processes).
Quantifying all these emissions is a complicated prospect, but we can attempt to do it using a method called “life-cycle assessment”. The result of one such estimate (with which I agree) is quoted in the Intergovernmental Panel on Climate Change’s Special Report on Renewable Energy Sources and Climate Change Mitigation
This shows that despite the long list of stages at which greenhouse gases are emitted, and based on what researchers have been able to take into account so far, the overall life-cycle emissions for nuclear power are likely to be lower than for fossil fuels.
My review of various estimates suggests that the greenhouse emissions from nuclear power vary from 10 to 130 grams of CO2 per kilowatt hour of power, with an average of 65 g per kWh – or roughly the same as wind power. For comparison, coal power has emissions of about 900 g per kWh, and gas-fired power about 450 g per kWh. About 15-25% of nuclear’s greenhouse emissions come from building, maintaining and decommissioning the nuclear power plant.
Another analysis, by energy consultants Jan Willem Storm van Leeuwen and Philip Smith, forecast that the emissions from nuclear power could ultimately rival those from natural gas. But this high estimate is a clear outlier among the results shown in the IPCC assessment, and this was largely because it assumed that uranium mines will need to be fully restored to “greenfield sites” – something that most analysts have not considered necessary.
But why is the range of accepted estimates so broad? To answer that, the author goes on to discuss multiple factors that can influence the calculation of lifecycle greenhouse gas emissions to include: supply chain processes, the initial grade (purity) of the ore; the uranium enrichment method; the age and efficiency of the reactor; emissions from cleaning up the uranium ore mine, and the assumptions used in the calculation method.