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Nuclear energy and renewable power: which is the best climate change mitigation option?

Nuclear Monitor Issue: 
Benjamin Sovacool

This article assesses different lifecycle studies of greenhouse gas equivalent emissions for nuclear and renewable power plants to identify a subset of the most current, original, and transparent studies. It calculates that mean value for greenhouse gas emissions for nuclear energy over the lifetime of a plant are quite high at about 66 carbon dioxide equivalent per kWh (gCO2e/kWh). Offshore wind power has less than one-seventh the carbon equivalent emissions of nuclear plants; large-scale hydropower, onshore wind, and biogas, about one-sixth the emissions; small-scale hydroelectric and solar thermal one-fifth. This makes these renewable energy technologies seven-, six-, and five-times more effective on a per kWh basis at fighting climate change. Policymakers would be wise to embrace these more environmentally friendly technologies if they are serious about producing electricity and mitigating climate change.

Advocates of nuclear power have recently framed it as an important part of any solution aimed at fighting climate change and reducing greenhouse gas emissions. Opponents of nuclear power have responded in kind. Which side is right?

I. Introduction
To find out which side is right, this paper screened 103 lifecycle studies of greenhouse gas equivalent emissions for nuclear power plants to identify a subset of the most current, original, and transparent studies. It begins by briefly detailing the separate components of the nuclear fuel cycle before explaining the methodology of the survey and exploring the variance of lifecycle estimates. It calculates that while the range of emissions for nuclear energy over the lifetime of a plant reported from qualified studies examined is from 1.4 grams of carbon dioxide equivalent per kWh (gCO2e/kWh) to 288 gCO2e/kWh, the mean value is 66 gCO2e/kWh.

The article then explains some of the factors responsible for the disparity in lifecycle estimates, in particular identifying errors in both the lowest estimates (not comprehensive) and the highest estimates (failure to consider co-products). It should be noted that nuclear power is not directly emitting greenhouse gases, but rather that life-cycle emissions account for fossil fuel emissions occurring elsewhere and indirectly attributable to nuclear plant construction, operation, uranium mining and milling, and plant decommissioning.

II. Nuclear Lyfecycle
Engineers generally classify the nuclear fuel cycle into two types: “once-through” and “closed.” Conventional reactors operate on a “once-through” mode that discharges spent fuel directly into disposal. Reactors with reprocessing in a “closed” fuel cycle separate waste products from unused fissionable material so that it can be recycled as fuel. Reactors operating on closed cycles extend fuel supplies and have clear advantages in terms of storage of waste disposal, but have disadvantages in terms of cost, short-term reprocessing issues, proliferation risk, and fuel cycle safety.

Despite these differences, both once-through and closed nuclear fuel cycles involve at least five interconnected stages that constitute a nuclear lifecycle: the “frontend” of the cycle where uranium fuel is mined, milled, converted, enriched, and fabricated; the construction of the plant itself; the operation and maintenance of the facility; the “backend” of the cycle where spent fuel is conditioned, (re)processed, and stored; and a final stage where plants are decommissioned and abandoned mines returned to their original state.

III. Review of lifecycle studies
To assess the total carbon dioxide-equivalent emissions over the course of the nuclear fuel cycle, this study began by reviewing 103 lifecycle studies estimating greenhouse gas emissions for nuclear plants. These 103 studies were narrowed according to a three-phase selection process.

  • First, given that the availability of high quality uranium ore changes with time, and that mining, milling, enrichment, construction, and reactor technologies change over the decades, the study excluded surveys more than ten years old (i.e., published before 1997). Admittedly, excluding studies more than a decade old is no guarantee that the data utilized by newer studies is in fact new. One analysis, for instance, relies on references from the 1980s for the modeling of uranium mining; data from 1983 for modeling uranium tailing ponds; 1996 data for uranium conversion; and 2000 data for uranium enrichment. Still, excluding studies more than ten years old is an attempt to hedge against the use of outdated data, and to ensure that recent changes in technology and policy are included in lifecycle estimates. Still, 40 studies analyzed are excluded by their date.
  • Second, this study excluded analyses that were not in the public domain, cost money to access, or were not published in English. Nine studies excluded for lack of accessibility.
  • Third, 35 studies were excluded based on their methodology. These studies were most frequently discounted because they either relied on “unpublished data” or utilized “secondary sources.” Those relying on “unpublished data” contained proprietary information, referenced data not published along with the study, did not explain their methodology, were not transparent about their data sources, or did not detail greenhouse gas emission estimates for separate parts of the nuclear fuel cycle in gCO2e/kWh. Those utilizing “secondary sources” merely quoted other previously published reports and did not provide any new calculations or synthetic analysis on their own.

Excluding detailed studies that rely on unpublished or non-transparent data does run the risk of including less detailed (and less rigorous) studies relying on published and open data. Simply placing a study in the public domain does not necessarily make it “good.” However, the author believes that this risk is more than offset by the positives benefits of transparency and accountability. Transparency enhances validity and accuracy; public knowledge is less prone to errors, and more subject the process of debate and dialogue that improves the quality of information, tested against other propositions in the marketplace of ideas. Furthermore, transparency is essential to promoting social accountability. Society simply cannot make informed decisions about nuclear power without public information; since the legitimacy of nuclear power is a public issue, the author believes that only results in the public domain should be included.

The survey conducted here found 19 studies that met all criteria: they were published in the past 10 years, accessible to the public, transparent about their methodology, and provided clear estimates of equivalent greenhouse gas emissions according to the separate parts of the nuclear fuel cycle. These studies were “weighed” equally; that is, they were not adjusted in particular for their methodology, time of release within the past ten years, or how rigorously they were peer reviewed or cited in the literature.

A somewhat rudimentary statistical analysis of these 19 studies reveals a range of greenhouse gas emissions over the course of the nuclear fuel cycle at the extremely low end of 1.4 gCO2e/kWh and the extremely high end of 288 gCO2e/kWh. Accounting for the mean values of emissions associated with each part of the nuclear fuel cycle, the mean value reported for the average nuclear power plant is 66 gCO2e/kWh. The frontend component of the nuclear cycle is responsible for 38 percent of equivalent emissions; decommissioning 18 percent; operation 17 percent; backend 15 percent; and construction 12 percent.

IV. Assessing the disparity in estimates
What accounts for such a wide disparity among lifecycle estimates of greenhouse gas emissions associated with the nuclear fuel cycle? Studies primarily differ in terms of their scope; assumptions regarding the quality of uranium ore; assumptions regarding type of mining; assumptions concerning method of enrichment; whether they assessed emissions for a single reactor or for a fleet of reactors; whether they measured historical or marginal/future emissions; assumptions regarding reactor type, site selection, and operational lifetime; and type of lifecycle analysis.

4.1 Scope
Some studies included just one or two parts of the nuclear fuel cycle, whereas others provided explicit details for even subcomponents of the fuel cycle. One study, for example, analyzed just the emissions associated with construction and decommissioning for reactors across the world, where another assessed the carbon equivalent for the construction of the Sizewell B nuclear reactor in the United Kingdom. Their estimates are near the low end of the spectrum, at between 3 and 11.5 gCO2e/kWh. In contrast, another study looked at every single subcomponent of the fuel cycle, and produced estimates near the high end of the spectrum at 112 to 166 gCO2/kWh.

4.2 Quality of Uranium Ore
Studies varied in their assumptions regarding the quality of uranium ore used in the nuclear fuel cycle. Low-grade uranium ores contain less than 0.01% yellowcake, and is at least ten times less concentrated than high-grade ores, meaning it takes ten tons of ore to produce 1 kg of yellowcake. Put another way, if uranium ore grade declines by a factor of ten, then energy inputs to mining and milling must increase by at least a factor of ten . This can greatly skew estimates, as uranium of 10% U3O8 has emissions for mining and milling at just 0.04 gCO2/kWh, whereas uranium at 0.013% grade has associated emissions more than 1,500 times greater at 67 gCO2/kWh. The same trend is true for the emissions associated with uranium mine land reclamation. With uranium of 10 percent grade, emissions for reclamation are just 0.07 gCO2e/kWh, but at 0.013%, they are 122 gCO2/kWh.

4.3 Open Pit or Underground Mining
The type of uranium mining will also reflect different CO2e emissions. Open pit mining often produces more gaseous radon and methane emissions than underground mines, and mining techniques will release varying amounts of CO2 based on the explosives and solvents they use to purify concentrate. They also point out that the carbon content associated with acid leeching used to extract uranium can vary, as well as the emissions associated with the use of lime to neutralize the resulting leached tailings. The emissions associated with uranium mining depend greatly on the local energy source for the mines. In Canada, uranium extracted from mines closer to industrial centers rely on more efficient, centrally generated power. In contrast, remote mines there have relied on less efficient diesel generators that consumed 45,000 tons of fossil fuel per year/mine, releasing up to 138,000 tons of carbon dioxide every year.

4.4 Gaseous Diffusion or Centrifuge Enrichment
Another significant variation concerns the type of uranium enrichment. Gaseous diffusion is much more energy-intense, and therefore has higher associated carbon dioxide emissions. Gaseous diffusion requires 2,400 to 2,600 kWh per seperative work unit (a function measuring the amount of uranium processed proportioned to energy expended for enrichment), compared to just 40 kWh per SWU for centrifuge techniques. The energy requirements for these two processes are so vastly different because gaseous diffusion is a much older technology, necessitating extensive electrical and cooling systems that are not found in centrifuge facilities.

Emissions will further vary on the local power sources at the enrichment facilities. One study calculated 9 gCO2e/kWh for Chinese centrifuge enrichment relaying on a mix of renewable and centralized power sources, but up to 80 gCO2e/kWh if gaseous diffusion is powered completely by fossil fuels.

4.5 Individual or Aggregate Estimates
Some studies look at just specific reactors, while others assess emissions based on industry, national, and global averages. These obviously produce divergent estimates. One study, for instance, looked at just two actual reactors in Switzerland, the Gosgen Pressurized Water Reactor and Liebstadt Boiling Water Reactor and calculate emissions at 5 to 12 gCO2e/kWh, whereas other studies look at global reactor performance and reach estimates more than 10 times greater.

4.6 Historical or Marginal/Future Emissions
Yet another difference concerns whether researchers assessed historic, future, or prototypical emissions. Studies assessing historic emissions looked only at emissions related to real plants operating in the past; studies looking at future average emissions looked at how existing plants would perform in the years to come; studies analyzing prototypical emissions looked at how advanced plants yet to be built would perform in the future. One study, for example, found historical emissions for light water reactors in Japan from 1960 to 2000 to be rather high at between 10 and 200 gCO2e/kWh. Others looked at future emissions for the next 100 years using more advanced Pressurized Water Reactors and Boiling Water Reactors. Still other studies made different assumptions about future reactors, namely fast-breeder reactors using plutonium and thorium, and other Generation IV nuclear technology expected to be much more efficient if they ever reach commercial production.

4.7 Reactor Type
Studies varied extensively in the types of reactors they analyzed. More than 30 commercial reactor designs exist today, and each differs in its fuel cycle, output, and cooling system. The most common are the world’s 263 Pressurized Water Reactors, used in France, Japan, Russia and the U.S., which rely on enriched uranium oxide as a fuel with water as coolant. Boiling Water Reactors are second most common, with 92 in operation throughout the U.S, Japan, and Sweden, which also rely on enriched uranium oxide with water as a coolant. Then come Pressurized Heavy Water Reactors, of which there are 38 in Canada, that use natural uranium oxide with heavy water as a coolant. Next comes 26 gas-cooled reactors, used predominately in the United Kingdom, which rely on natural uranium and carbon dioxide as a coolant. Russia also operates 17 Light Water Graphite Reactors that use enriched uranium oxide with water as a coolant but graphite as a moderator. A handful of experimental reactors, including fast breeder reactors (cooled by liquid sodium) and pebble bed modular reactors (which can operate at fuel load while being refueled), still in the prototype stages, make up the rest of the world total.

To give an idea about how much reactor design can influence lifecycle emissions, CANDU reactors are the most neutron efficient commercial reactors, achieving their efficiency through the use of heavy water for both coolant and moderator, and reliance on low-neutron absorbing materials in the reactor core. CANDU reactors thus have the ability to utilize low-grade nuclear fuels and refuel while still producing power, minimizing equivalent carbon dioxide emissions. This could be why CANDU reactors have relatively low emissions (~15 gCO2e/kWh) compared to the average emissions from qualified studies as described by this work (~66 gCO2e/kWh).

4.8 Site Selection
Estimates vary significantly based on the specific reactor site analyzed. Location influences reactor performance (and consequential carbon equivalent emissions). Some of the ways that location may influence lifetime emissions include differences in:

  • Construction techniques, including available materials, component manufacturing, and skilled labor;
  • Local energy mix at that point of construction;
  • Travel distance for materials and fuel cycle components;
  • Associated carbon footprint with the transmission and distribution (T&D) network needed to connect to the facility;
  • Cooling fuel cycle based on availability of water and local hydrology;
  • Environmental controls based on local permitting and siting requirements.

Each of these can substantially affect the energy intensity and efficiency of the nuclear fuel cycle.

Consider two extremes. In Canada, the greenhouse gas-equivalent emissions associated with the CANDU lifecycle are estimated at about 15 gCO2e/kWh. CANDU reactors tend to be built with skilled labor and advanced construction techniques, and they utilize uranium that is produced domestically and relatively close to reactor sites, enriched with cleaner technologies in a regulatory environment with rigorous environmental controls. By contrast, the greenhouse-gas equivalent emissions associated with the Chinese nuclear lifecycle can be as high as 80 gCO2e/kWh. This could be because Chinese reactors tend to be built using more labor-intensive construction techniques, must import uranium thousands of miles from Australia, and enrich fuel primarily with coal-fired power plants that have comparatively less stringent environmental and air-quality controls.

4.9 Operational Lifetime
How long the plants at those sites are operated and their capacity factor influences the estimates of their carbon-dioxide equivalent intensity. A 30-year operating lifetime of a nuclear plant with a load factor of 82 percent tends to produce 23.2 gCO2/kWh for construction. Switch the load factor to 85 percent and the lifetime to 40 years, and the emissions drop about 25 percent to 16.8 gCO2/kWh. The same is true for decommissioning. A plant operating for 30 years at 82 percent capacity factor produces 34.8 gCO2/kWh for decommissioning, but drop 28 percent to 25.2 gCO2/kWh if the capacity factor improves to 85 percent and the plant is operated for 40 years.

Most of the qualified studies referenced above assume lifetime nuclear capacity factors that do not seem to match actual performance. Almost all of the qualified studies reported capacity factors of 85 to 98 percent, where actual operating performance has been less. While the nuclear industry in the U.S. has boasted recent capacity factors in the 90-percent range, average load factors over the entire life of the plants is very different: 66.3 percent for plants in the UK and 81 percent for the world average.

4.10 Type of Lifecycle Analysis
The type of lifecycle analysis can also skew estimates. Projections can be “top-down,” meaning they start with overall estimates of a pollutant, assign percentages to a certain activity (such as “cement manufacturing” or “coal transportation”), and derive estimates of pollution from particular plants and industries. Or they can be “bottom-up,” meaning that they start with a particular component of the nuclear fuel cycle, calculate emissions for it, and move along the cycle, aggregating them. Similarly, lifecycle studies can be “process-based” or rely on economic “input-output analysis.” “Process-based” studies focus on the amount of pollutant released—in this case, carbon dioxide or its equivalent—per product unit. For example, if the amount of hypothesized carbon dioxide associated with every kWh of electricity generation for a region was 10 grams, and the cement needed for a nuclear reactor took 10 kWh to manufacture, a process analysis would conclude that the cement was responsible for 100 grams of CO2. “Input-output” analysis looks at industry relations within the economy to depict how the output of one industry goes to another, where it serves as an input, and attempts to model carbon dioxide emissions as a matrix of interactions representing economic activity.

V. Conclusion
The first conclusion is that the mean value of emissions over the course of the lifetime of a nuclear reactor (reported from qualified studies) is 66 gCO2e/kWh, due to reliance on existing fossil-fuel infrastructure for plant construction, decommissioning, and fuel processing along with the energy-intensity of uranium mining and enrichment. Thus, nuclear energy is in no way “carbon free” or “emissions free,” even though it is much better (from purely a carbon equivalent emissions standpoint) than coal, oil, and natural gas electricity generators, but worse than renewable and small scale distributed generators (See Table 1).

Table 1: Lifecycle greenhouse gas emission estimates for various electricity generators



Estimate (gCO2e/kWh)


2.5 MW, Offshore



3.1 MW, Reservoir



1.5 MW, Onshore



Anaerobic Digestion



300 kW, Run-of-River


Solar Thermal

80 MW, Parabolic Trough



Forest Wood Co-combustion with hard coal



Forest Wood Steam Turbine



Short Rotation Forestry Co-combustion with hard coal



Forest Wood Reciprocating Engine



Waste Wood Steam Turbine


Solar Photovoltaic

Polycrystalline silicone



Short Rotation Forestry Steam Turbine



80 MW, Hot Dry Rock



Short Rotation Forestry Reciprocating Engine



Various reactor types


Natural Gas

Various combined cycle turbines


Fuel Cell

Hydrogen from gas reforming



Various generator and turbine types


Heavy Oil

Various generator and turbine types



Various generator types with scrubbing



Various generator types without scrubbing



Source: This article is based on B.K. Sovacool, “Valuing the Greenhouse Gas Emissions from Nuclear Power: A Critical Survey,” Benjamin K. Sovacool. Energy Policy 36 (8) (August, 2008), pp. 2940-2953.

Contact: B.K. Sovacool is with the Lee Kuan Yew School of Public Policy at the National University of Singapore, 469C Bukit Timah Rd., Singapore, 259772.
Tel: +65 6516 501;