|Fig. 1: Sequoyah Nuclear Power Plant near Chattanooga, TN (Source: Wikimedia Commons)|
The US and world energy consumption is constantly increasing.  As our global power budget increases and as we seek to curb our emission of greenhouse gases (GHGs) it is important that we accurately evaluate the carbon footprint of various energy sources as GHGs may contribute to an unstable rise in global temperatures. 
Controversy often accompanies nuclear power over issues of safety during operation as well as waste disposal, but another area of contention in nuclear energy is how much GHG nuclear energy emits. A simple nuclear reaction only emits heat and waste as byproducts and doesn't emit any greenhouse gases. However, nuclear power is not completely green because the process by which nuclear fuel is obtained and refined emits GHGs. This is process of mining and refining nuclear fuel is called the "nuclear fuel cycle" or "nuclear life cycle." The two elements used in nuclear energy are Uranium and Plutonium. Extracting these elements from the earth requires large amounts of energy that often burn fossil fuels. Furthermore, refining them requires large amounts of energy that emit GHG. Since the nuclear fuel cycle is such a long process it is difficult to obtain an accurate accounting carbon emissions and these numbers may be skewed by political involvement. In order to quantify the carbon emissions of nuclear power a standardized methodology should be developed.
Nuclear power plants (as seen in Fig. 1) are the facilities where electrical power is generated using nuclear energy. A nuclear reaction takes place releasing thermal energy which is used to heat water inside a pressurized water reactor (PWR). This water is radioactive and is heated to about 570°F but does not boil since it is under pressure. The hot water flows through a heat exchanger which causes water in a secondary system to heat and boil. This steam is fed through a turbine which turns an electric generator producing AC electricity.  Nothing in this process emits greenhouse gases. Electricity is produced and the only byproducts are steam, heat, and nuclear fuel waste.
|Fig. 2: Mary Kathleen Uranium Mine in Queensland, Australia. (Source: Wikimedia Commons)|
Uranium that is used in nuclear reactors must be extracted from mines such as the Mary Kathleen Uranium Mine (shown in Fig. 2). In order to obtain this uranium heavy machinery mines and digs into the earth and much of this machinery burns fossil fuels.
A nuclear reaction is fairly simple process but the process to obtain the fuel and have facilities available for a nuclear reaction consists of many steps. A comprehensive list of the steps of the nuclear fuel cycle consists of pre-reaction, post-reaction, and construction and decomissioning of nuclear facilities. 
The enumerated steps of the nuclear fuel cycle are mining, milling, conversion, enrichment, production of fuel elements, transport, storage and disposal of the fuel.  The power plant facility also is responsible for GHGs during construction, maintenance, and decommissioning. 
Throughout the lifespan of a nuclear reactor it will consume a certain amount of fuel as well as generate a certain amount of power. Based on the amount of nuclear fuel consumed and how much GHG was emitted during its construction (normalized for its lifespan) we can determine how much GHG the reactor is accountable for. We simply divide the amount of GHG by the amount of electricity generated to get an average amount of CO2 per kilowatt hour.
Since the nuclear fuel cycle consists of many steps it is difficult to obtain an accurate accounting of GHG emissions. Sovacool et. al. in 2008 performed a survey of 103 different articles that quantified the GHG emission of nuclear power. They found that these studies ranged from 1.4 g CO2/kWh to 288 g CO2/kWh. This spans more than two orders of magnitude and is not very accurate. Sovacol et. al. claimed that 81% of the studies were outdated and had flaws in methodology or methodologies that were inaccessible. The remaining 19% of the studies that were up-to-date varied greatly in range of comprehensiveness. 
The life cycle reports for these reactors need to be more transparent, open, and accountable. There is no industry standard for reporting the GHG emissions of a nuclear reactor.  Thus, it is easy for a nuclear power company to devise its own methodology for CO2 emissions that reports they are emitting few GHGs when in reality they aren't accounting for all the CO2 that is emitted.
With the nuclear fuel cycle being so long and nuclear energy being used extensively in the United States it is essential that we establish a methodology that is appropriate to accurately evaluate GHG emissions and standardize the industry and isolate reported numbers from political manipulation.
As energy demand increases continually and as our concern for the environment also is taken into consideration we need to be able to accurate evaluate different energy sources. Some sources such as wind energy are economically expensive but also very green. Oil and coal are very economically friendly but they emits large volumes of GHG as well as other pollutants. As we weigh these economic and environmental costs it is important that we have a clear understanding of the impact of each energy source. And in order to make a specific energy source more efficient we need a comprehensive breakdown of each expense. Nuclear energy seems to strike a good balance.
There is still a need to standardize the way we calculate GHG emissions from nuclear power.
© Dustin Gerrard. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 "BP Statistical Review of World Energy," British Petroleum, June 2014.
 P. M. Cox et al., "Acceleration of Global Warming Due to Carbon-Cycle Feedbacks in a Coupled Climate Model," Nature 408, 184 (2000).
 R. Meiswinkel, J. Meyer, and J. Schnell, Design and Construction of Nuclear Power Plants (Ernst and Son, 2013).
 M. Lenzen, "Life Cycle Energy and Greenhouse Gas Emissions of Nuclear Energy: A Review," Energ. Convers. Manage. 49, 2178 (2008).
 J. Beerten, et al., "Greenhouse Gas Emissions in the Nuclear Life Cycle: A Balanced Appraisal," Energy Policy 37, 5056 (2009).
 B. K. Sovacool, "Valuing the Greenhouse Gas Emissions From Nuclear Power: A Critical Survey," Energy Policy 36, 2950 (2008).