Comparative impact of nuclear and renewables on CO2 emissions

October 9, 2020

Countries which are heavily invested in nuclear energy remain higher CO2 emitters, on average, than countries which have invested at the same level in renewable energy. This is the main finding of a study recently published in the journal Nature Energy. The results also tend to confirm the hypothesis that it is difficult to commit both to nuclear and renewables due to a systemic incompatibility between these two approaches.

The study, led by Benjamin K. Sovacool of the University of Sussex, England, is not based on simply on an analysis of various national case studies, but on a database encompassing 123 countries over 25 years (30 favouring nuclear and 117 renewables, with some overlap between the two). The information spans two periods, 1990 to 2004 and 2000 to 2014, which makes it easier to better track the impact of the emergence of renewables in the 21st century. The data was used to verify the existence of correlations, which often turned out to be very strong (a p value of less than 0.05, or even 0.001). The results of these regressions are presented in the form of rather dry tables.

The work aimed to assess three hypotheses. The first is that the greater a country’s nuclear power generation capabilities, the lower its greenhouse gas emissions are. The second is that the greater a country’s renewable energy generation capabilities, the lower its emissions are. The third is that nuclear and renewables coexist uneasily within a national energy system and that the dominance of either delays or prevents the adoption of the other.

Nuclear climate mitigation hypothesis

The proposition that nuclear energy can help reduce emissions appears self-evident, since electricity from nuclear sources has a low carbon footprint than electricity from coal. The International Energy Agency, for instance, is proposing a massive increase in production up to 1,200 GW of installed capacity in 2050, which would represent an expenditure of USD 4000 billion dollars, higher than all other proposed investments.

The study does not find any overall demonstrable correlation between high nuclear adoption and low emissions. This is true for the two periods under study. A closer look, however, reveals a correlation between  nuclear and reduced emissions in countries with high GDP but, conversely, higher emissions in countries with lower GDP. Gains and losses therefore cancel each other out in a generally inconclusive balance sheet.

Although the study does not dwell on this subject, these results are less surprising than they might appear. Several heavily nuclear countries are also rely greatly on coal for their electricity needs – think of Russia, China, the United States, India, and even Germany before 2010. The example of France, where nuclear energy has almost completely superseded fossil fuel power generation, remains atypical.

Renewable climate mitigation hypothesis

This hypothesis postulates that extensive adoption of renewable energy will yield reduced emissions. Several forms of renewable energy suited to local conditions are possible. The study shows that this hypothesis is true and that CO2 emissions decrease as a result of investment in renewable energy, in either of the time periods considered and regardless of a given country’s GDP.

The crowding-out effect hypothesis

This hypothesis suggests that countries invested in nuclear power are less invested in renewables and vice versa. The researchers did not invent this hypothesis, which is often mentioned in the scientific literature. In short, the two options would tend to be mutually exclusive for various reasons. Nuclear countries, for example, tend to have highly centralized power transmission networks, optimized for large-scale power generation. These networks are not easily adapted to renewable electricity production, which tends to be decentralized and widely distributed across a given territory. Also, regulations and financial structures favoring large projects developed over long periods may not be suitable for smaller scale renewable projects that are quickly implemented.

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The study partially confirms this hypothesis of the crowding-out effect. There is a negative correlation between the scale of nuclear power generation and that of renewables in all the countries studied, regardless of GDP. However, it is statistically significant only in countries strongly committed to renewables. In other words, countries more focused on renewable energy have a strong tendency not to be nuclear, while countries focusing on nuclear energy have a slightly less exclusive approach. The researchers believe this could be due to a partial overlap, with some countries belonging to both categories.

Explanatory factors

What explains these rather unfavorable results for nuclear power? Data collected by the researchers shows that, on average, the delivery time is 90 months for nuclear projects, compared to 40 months for solar and wind projects. Only hydropower has longer lead times. Nuclear and hydropower projects are more prone to delays and cost overruns than smaller-scale renewable projects, which yield low carbon energy more quickly.

Renewables are also associated with a positive learning curve whereby each completed project decreases the costs and increases the performance of subsequent projects. In comparison, nuclear power exhibits a negative learning curve. The study specifically cites the case of France, where each new generation of reactors has involved increased costs or lower performance. The tightening of safety measures after each major accident (Three Mile Island, Chernobyl, Fukushima) has greatly contributed to these increased costs in every country.

The researchers regret the lack of granularity of the available data, which has forced them to lump all nuclear technologies and all renewable technologies into two poorly differentiated categories. They wish they could compare various nuclear reactor technologies or be able to distinguish between solar and wind power, but the data provided by the various international agencies is not detailed enough to carry out this kind of survey. The researchers would also like to have more recent data, that would allow them to push the analysis beyond 2014.

The study concludes that renewables have a demonstrable record of reducing greenhouse gas emissions. Nuclear power has a more mixed record, due to the different nature of the energy systems in which it operates. Finally, the results tend to confirm the hypothesis of mutual exclusion already widely noted in the scientific literature. According to the researchers, countries that think they can obtain emission reductions by investing in nuclear energy may actually be forgoing even greater reductions that could be achieved by renewables.


Benjamin K. Sovacool et al. Differences in carbon emissions reduction between countries pursuing renewable electricity versus nuclear power, in Nature Energy, October 5, 2020 % 3D


Teaser photo credit:

Philippe Gauthier

Philippe Gauthier is a a science journalist, translator and activist in Québec’s degrowth movement. His article in this issue is based on a talk delivered at the Great Transition conference in Montréal on May 20, 2018. He maintains a blog (in French) on energy and the environment:

Tags: clean energy transition, nuclear energy, Renewable Energy