Nuclear power plants primarily use uranium as a fuel, and several steps are needed to make natural uranium into a suitable fuel. Natural uranium contains 99.3% U-238, and only 0.7% of the isotope that is relevant for nuclear energy: U-235. Since reactors require 4-5% U-235 content, natural uranium must be enriched, requiring energy and thus leading to CO2 emissions. However, due to the high energy density of uranium, total CO2 emissions are still low compared to fossil fuels.
First uranium must be mined, either mechanically, or by leaching it from the ground with chemical solutions. After mining, the natural uranium (U3O8) is converted to a gaseous form (UF6), which is the input for enrichment plants. In Western Europe, uranium is enriched by highly efficient centrifuges, which force the heavier U-238 away from the U-235. Subsequently, the uranium is converted to solid form (UO2) and fuel rods for the power plant are produced. After use, spent fuel rods are reprocessed, separating the uranium and plutonium from the other materials (which are considered as waste, and need to be stored).
Energy used during construction and decommissioning of the power plant, and construction of the waste facilities is not considered here. Cooling energy during temporary waste storage is included, as is transportation.
Nuclear power plants generate high-level radioactive waste, which remains dangerous for thousands of years. The waste is now temporarily stored near nuclear facilities, but no permanent storage facility has yet been completed anywhere.
In addition, nuclear energy is associated with risk of nuclear incidents. Severe incidents do not occur often, but may have serious impacts on environment and human health, as the disasters in Chernobyl and Fukushima have illustrated. The plutonium generated during plant operation poses a proliferation risk, as it can be used in nuclear weapons.
Like all mining operations, uranium mining is associated with negative environmental impacts. Mechanic mining methods (underground and open-pit mining) create accident risks and radiation exposure for miners. In addition, large quantities of waste rock (tailings) are produced, which emit low levels of radiation and contain toxic substances. In-situ leaching poses fewer risks for miners, but may cause groundwater pollution with acids found in the leaching fluids.
Finally, large quantities of depleted uranium are generated during the enrichment process. These low-radioactive materials are now temporarily stored (e.g. near enrichment facilities), but it is unclear where to leave them in the long term.
The numbers and references used to quantify the fuel chain emissions for uranium can be found on our 'Reference Manager'. The Fuel Chain Emissions documentation can be downloaded from here (will open in a new tab).