|Fig. 1: Lifecycle emissions of various energy sources|
As global climate change has become a reality that needs to be dealt with by policymakers and energy producers, nuclear energy has emerged as a herald of seemingly low emissions that promises to meet tomorrow's energy demands. Although the actual usage of nuclear power plants emits relatively low carbon emissions, the lifecycle emissions of this energy source needs to be analyzed in order to properly understand the overall impact that nuclear power actually has. Nuclear power plants require a large amount of construction as well as maintenance and using the methodology of life cycle assessment in order to better understand the total environmental footprint will help to uncover the best possible energy solution for the near future.
In order to understand the impact of nuclear power, the methodology of life cycle assessment will be used. Life Cycle Assessment (LCA) is defined as "the compilation and evaluation of the inputs, outputs and the potential environmental impacts of a product system throughout its life cycle."  LCA effectively allows for the establishment of the environmental profile of a given system. A system can be defined as a product and/or the process used to make that product.
The various product stages that a typical LCA spans include, but are not limited to: raw material acquisition, material processing, manufacture and assembly, use and service, retirement and recovery and disposal. Each of these stages can include material and energy inputs as well as waste (liquid, solid, gaseous) outputs. 
In order to better understand the current lifecycle scenario of both the nuclear industry and of other competitive energy sources, two specific studies will be detailed here. First, Sovacool conducted a survey of over 103 different lifecycle studies of the nuclear industry. Specifically, various phases were looked at for the nuclear fuel cycle. This includes uranium mining, milling, conversion, enrichment, fuel fabrication, reactor (construction, operation and decommissioning), fuel processing, fuel conditioning, interim storage to permanent geologic storage. 
What is revealing about Sovacool's study is that the very specific stages being included in the life cycle analysis boundary were explicitly considered. For example, certain studies only looked at the reactor construction, operation and decommissioning while others only considered the uranium fuel processing. Differences in boundary can result in widely different lifecycle values. Due to this, refinements were made to ensure that only studies that were methodologically clear and statistically viable were included in the survey. Roughly 81% of the surveys studied possessed methodological shortcomings that excluded them from the study. The 19% of remaining studies that were recent and methodologically clear varied widely. As stated before, different studies used different elements of the lifecycle inside the analysis boundary, while others assumed different or developing technologies. Another primary difference was in the assumption behind specifically what type of electricity source was being used to power the nuclear power plant, specifically whether that source is renewable or fossil fuel. Due to this, Sovacool found widely varying lifecycle emissions values in the range of 1.4 g CO2/kWh to 288 g CO2/kWh with a mean value of 66 g CO2/kWh. 
Weisser also conducted a similar survey of lifecycle values for nuclear power. Eight different studies were analyzed and then statistically combined to produce a lifecycle emissions value. For nuclear, this same number varied from 5 g CO2/kWh to 25 g CO2/kWh with a mean of roughly 12 g CO2/kWh. 
Since both Sovacool and Weisser conducted surveys of a variety of different LCAs in order to come to their respective conclusions, these two values could be averaged in order to better understand the overall picture of emissions with regard to other energy technologies. Fig. 1 shows a comparison of various renewable and fossil fuel sources with regard to emissions.
From Fig. 1 it is very clear that nuclear power emits significantly less than comparable fossil fuel sources, but still falls short of meeting the majority of renewable energy sources that are seen as viable alternatives. It is also worth discussing how the numbers in Fig. 1 were derived. Values from both Weisser and Sovacool's studies were averaged. When they were averaged, assumptions were made that may have resulted in a more aggregated number. For example, Sovacool provided 6 different values for biomass depending on the specific turbine/engine used and whether or not coal was used. These 6 values were averaged to come to one number for Biomass, which was then averaged with the single value from Weisser. It is also worth pointing out that certain technologies were analyzed in one study and not in the other. Sovacool analyzed biogas, hydroelectric and solar thermal, where Weisser did not. Weisser analyzed Carbon Capture and storage, lignite, and energy storage where Sovacool did not.
However, most importantly there were wide variations in the mean value of nuclear emissions between Sovacool and Weisser. These variations largely arose from the differences previously mentioned, namely boundary, technology, electricity mix, and geography. However, given that the numbers are so widely different, Sovacool proposed adopting LCA protocols that are in line with ISO 14040-4 that are specifically customized towards the nuclear industry.
Nonetheless, nuclear power has a real and tangible emissions footprint. Although very little or no emissions are released during the actual usage of the nuclear power plant, the upstream and downstream material impacts and energy usage to get the plant running carry a true environmental burden that must be kept in mind when assessing the energy technology landscape for the optimization of the electricity system to reduce carbon output.
© Subhan Ali. 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.
 "Environmental Management - Life Cycle Assessment - Principles and Framework," International Organisation for Standardization, ISO 14040:2006a, 30 Jun 06.
 S. M. Ali, Life Cycle Assessment, Physics 240, Stanford University, Fall 2010.
 B. K. Sovacool, "Valuing the Greenhouse Gas Emissions From Nuclear Power: A Critical survey," Energy Policy 36, 2940 (2008).
 D. Weisser, "A Guide to Life-Cycle Greenhouse Gas (GHG) Emissions From Electric Supply Technologies," Energy 32, 1543 (2007).