Small modular nuclear reactors (SMRs) are advanced reactors designed to produce low-carbon electricity using smaller units that are factory-fabricated. SMRs aim to overcome the safety, cost, and scalability challenges of traditional large-scale nuclear power. They offer benefits such as passive safety systems, lower capital investment, and the potential to be deployed flexibly in remote or underserved regions. However, commercial deployment is limited, the costs remain uncertain, and long-term nuclear waste and proliferation concerns persist. We conclude that deploying SMRs is "Worth Watching” as a promising climate solution still in development that has not yet proven its readiness for large-scale implementation.
Based on our analysis, SMRs are a plausible and potentially impactful climate solution, but they are not yet ready for widespread deployment. The core technology is credible and carries significant potential for reducing GHG emissions. However, readiness, cost certainty, and deployment evidence are still lacking. For now, SMRs are "Worth Watching."
| Plausible | Could it work? | Yes |
|---|---|---|
| Ready | Is it ready? | No |
| Evidence | Are there data to evaluate it? | No |
| Effective | Does it consistently work? | Yes |
| Impact | Is it big enough to matter? | Yes |
| Risk | Is it risky or harmful? | No |
| Cost | Is it cheap? | No |
Small modular nuclear reactors (SMRs) are advanced reactors that produce low-carbon electricity by harnessing the heat from nuclear fission, an established and well-understood physical process. The innovation of SMRs lies primarily in their design. Typically smaller than traditional reactors, with a capacity of less than 300 megawatts (MW), SMRs are factory-built for enhanced quality control. This design allows them to be delivered to installation sites more quickly and potentially at a lower cost compared to conventional reactors, which typically range from about 700 MW to over 1,600 MW. While SMRs are generally considered "utility-scale" in their capacity, their smaller size makes them a viable option for smaller-scale applications, such as large micro-grids. These reactors can be assembled in a modular fashion, allowing incremental capacity additions. Additionally, some SMR designs boast enhanced safety features, including passive cooling systems that can function without external power sources, reducing the risks associated with reactor overheating or meltdowns. Currently, several countries are planning the deployment of SMRs, particularly China and the United States. Given their modular nature, several African countries, such as Ghana, are also looking toward SMRs to address their energy access deficits. Based on current plans, the International Energy Agency expects several countries to have multiple SMRs installed and operational by around 2030.
The physics behind SMRs is sound, and their potential as low-carbon energy sources is also scientifically valid, as they do not emit GHG emissions during operation. Several pilot SMR projects have also been launched. SMRs have yet to move beyond the demonstration phase to widespread commercial adoption. No SMR is currently deployed at the scale necessary to reduce global emissions measurably. Furthermore, independent, peer-reviewed empirical data on long-term operational performance, scalability, and cost remain sparse. While several countries, including the United States, Hungary, China, and Ghana, have announced plans or are discussing deploying SMRs within the next decade, those plans are still in the preparatory stages.
SMRs have several features that make them appealing as a potential climate solution. If scaled appropriately, they could displace fossil-fuel-based power generation and reduce carbon emissions significantly. Projected deployment scenarios by the Nuclear Energy Agency suggest that by 2050, the global SMR market could reach 375 gigawatts of installed capacity, avoiding up to 15 Gt of cumulative CO₂ emissions. Their smaller size and modular nature reduce financial risk, making them potentially more accessible to developing countries or smaller utilities. They are also flexible in siting and can complement variable renewable energy sources like solar and wind by providing reliable baseload or backup power. Additionally, SMRs could help decarbonize hard-to-electrify sectors like process heat in industry or remote energy systems. These attributes have prompted excitement among proponents who see SMRs as a scalable, flexible, and resilient solution for emissions-free power.
Despite their promise, SMRs face several challenges that limit their readiness for large-scale deployment. Safety remains a concern – not necessarily because of design flaws, but because any nuclear reactor carries inherent risks. Waste disposal and the potential for proliferation of nuclear materials remain persistent issues. Regulatory hurdles are also significant, as existing frameworks are often geared toward conventional reactors and may slow the licensing of newer designs. The cost of SMRs is another outstanding question. Recent analyses by Wood Mackenzie suggest that SMRs could cost US$6,000 to US$8,000 per kilowatt of capacity, which is well above the costs of utility-scale solar (US$1,448) or onshore wind (US$2,098). Deployment timelines also pose a challenge. Given the urgency of climate action, technologies that cannot be deployed at scale within the next 10–15 years may offer limited near-term benefits. A recent study by the Institute for Energy Economics and Financial Analysis opines that SMRs are still too costly, too time-consuming to construct, and too risky to significantly impact the transition away from fossil fuels in the next decade. While peer-reviewed academic studies have been conducted, a comprehensive, independent evaluation of large-scale deployment remains absent.
Asuega, A., Limb, B. J., & Quinn, J. C. (2023). Techno-economic analysis of advanced small modular nuclear reactors. Applied Energy, 334, 120669. https://doi.org/10.1016/J.APENERGY.2023.120669
Hussein, E. M. A. (2020). Emerging small modular nuclear power reactors: A critical review. Physics Open, 5, 100038. https://doi.org/10.1016/J.PHYSO.2020.100038
IEA. (2025). The Path to a New Era for Nuclear Energy. https://www.iea.org/reports/the-path-to-a-new-era-for-nuclear-energy
Midgley, E. (2023). Decarbonizing Industries with the Help of Small and Micro Nuclear Reactors | IAEA. https://www.iaea.org/bulletin/decarbonizing-industries-with-the-help-of-small-and-micro-nuclear-reactors
Sam, R., Sainati, T., Hanson, B., & Kay, R. (2023). Licensing small modular reactors: A state-of-the-art review of the challenges and barriers. Progress in Nuclear Energy, 164, 104859. https://doi.org/10.1016/J.PNUCENE.2023.104859
Sovacool, B. K., Andersen, R., Sorensen, S., Sorensen, K., Tienda, V., Vainorius, A., Schirach, O. M., & Bjørn-Thygesen, F. (2016). Balancing safety with sustainability: assessing the risk of accidents for modern low-carbon energy systems. Journal of Cleaner Production, 112, 3952–3965. https://doi.org/10.1016/J.JCLEPRO.2015.07.059
Van Hee, N., Peremans, H., & Nimmegeers, P. (2024). Economic potential and barriers of small modular reactors in Europe. Renewable and Sustainable Energy Reviews, 203. https://doi.org/10.1016/j.rser.2024.114743
Vanatta, M., Patel, D., Allen, T., Cooper, D., & Craig, M. T. (2023). Technoeconomic analysis of small modular reactors decarbonizing industrial process heat. Joule, 7(4), 713–737. https://doi.org/10.1016/J.JOULE.2023.03.009
World Nuclear Association. (2024). Small Nuclear Power Reactors. https://world-nuclear.org/information-library/nuclear-fuel-cycle/nuclear-power-reactors/small-nuclear-power-reactors
Greenhouse gas quantity expressed relative to CO₂ with the same warming impact over 100 years, calculated by multiplying emissions by the 100-yr GWP for the emitted gases.
Greenhouse gas quantity expressed relative to CO₂ with the same warming impact over 20 years, calculated by multiplying emissions by the 20-yr GWP for the emitted gases.
Reducing greenhouse gas concentrations in the atmosphere by preventing or reducing emissions.
The process of increasing the acidity of water or soil due to increased levels of certain air pollutants.
Benefits of climate solutions that extend beyond their ability to reduce emissions or store carbon (e.g., benefits to public health, water quality, biodiversity, advancing human rights).
The extent to which emissions reduction or carbon removal is above and beyond what would have occurred without implementing a particular action or solution.
An upper limit on solution adoption based on physical or technical constraints, not including economic or policy barriers. This level is unlikely to be reached and will not be exceeded.
The quantity and metric to measure implementation for a particular solution that is used as the reference unit for calculations within that solution.
Farming practices that work to create socially and ecologically sustainable food production.
Addition of trees and shrubs to crop or animal farming systems.
Spread out the cost of an asset over its useful lifetime.
A crop that live one year or less from planting to harvest; also called annual.
black carbon
Made from material of biological origin, such as plants, animals, or other organisms.
A renewable energy source generated from organic matter from plants and/or algae.
An energy source composed primarily of methane and CO₂ that is produced by microorganisms when organic matter decomposes in the absence of oxygen.
Carbon stored in biological matter, including soil, plants, fungi, and plant products (e.g., wood, paper, biofuels). This carbon is sequestered from the atmosphere but can be released through decomposition or burning.
Living or dead renewable matter from plants or animals, not including organic material transformed into fossil fuels. Peat, in early decay stages, is partially renewable biomass.
A type of carbon sequestration that captures carbon from CO₂ via photosynthesis and stores it in soils, sediments, and biomass, distinct from sequestration through chemical or industrial pathways.
A climate pollutant, also called soot, produced from incomplete combustion of organic matter, either naturally (wildfires) or from human activities (biomass or fossil fuel burning).
High-latitude (>50°N or >50°S) climate regions characterized by short growing seasons and cold temperatures.
The components of a building that physically separate the indoors from the outdoor environment.
Businesses involved in the sale and/or distribution of solution-related equipment and technology, and businesses that want to support adoption of the solution.
A chemical reaction involving heating a solid to a high temperature: to make cement clinker, limestone is calcined into lime in a process that requires high heat and produces CO₂.
A four-wheeled passenger vehicle.
Technologies that collect CO₂ before it enters the atmosphere, preventing emissions at their source. Collected CO₂ can be used onsite or in new products, or stored long term to prevent release.
A greenhouse gas that is naturally found in the atmosphere. Its atmospheric concentration has been increasing due to human activities, leading to warming and climate impacts.
Total GHG emissions resulting from a particular action, material, technology, or sector.
Amount of GHG emissions released per activity or unit of production.
A marketplace where carbon credits are purchased and sold. One carbon credit represents activities that avoid, reduce, or remove one metric ton of GHG emissions.
A colorless, odorless gas released during the incomplete combustion of fuels containing carbon. Carbon monoxide can harm health and be fatal at high concentrations.
Activities or technologies that pull CO₂ out of the atmosphere, including enhancing natural carbon sinks and deploying engineered sinks.
Long-term storage of carbon in soils, sediment, biomass, oceans, and geologic formations after removal of CO₂ from the atmosphere or CO₂ capture from industrial and power generation processes.
carbon capture and storage
carbon capture, utilization, and storage
A binding ingredient in concrete responsible for most of concrete’s life-cycle emissions. Cement is made primarily of clinker mixed with other mineral components.
methane
Gases or particles that have a planet-warming effect when released to the atmosphere. Some climate pollutants also cause other forms of environmental damage.
A binding ingredient in cement responsible for most of the life-cycle emissions from cement and concrete production.
carbon monoxide
Neighbors, volunteer organizations, hobbyists and interest groups, online communities, early adopters, individuals sharing a home, and private citizens seeking to support the solution.
A solution that potentially lowers the benefit of another solution through reduced effectiveness, higher costs, reduced or delayed adoption, or diminished global climate impact.
A farming system that combines reduced tillage, cover crops, and crop rotations.
carbon dioxide
A measure standardizing the warming effects of greenhouse gases relative to CO₂. CO₂-eq is calculated as quantity (metric tons) of a particular gas multiplied by its GWP.
carbon dioxide equivalent
The process of cutting greenhouse gas emissions (primarily CO₂) from a particular sector or activity.
A solution that works slower than gradual solutions and is expected to take longer to reach its full potential.
Microbial conversion of nitrate into inert nitrogen gas under low-oxygen conditions, which produces the greenhouse gas nitrous oxide as an intermediate compound.
Greenhouse gas emissions produced as a direct result of the use of a technology or practice.
Ability of a solution to reduce emissions or remove carbon, expressed in CO₂-eq per installed adoption unit. Effectiveness is quantified per year when the adoption unit is cumulative over time.
Greenhouse gas emissions accrued over the lifetime of a material or product, including as it is produced, transported, used, and disposed of.
Solutions that work faster than gradual solutions, front-loading their impact in the near term.
Methane produced by microbes in the digestive tracts of ruminant livestock, such as cattle, sheep and goats.
environmental, social, and governance
exchange-traded fund
A process triggered by an overabundance of nutrients in water, particularly nitrogen and phosphorus, that stimulates excessive plant and algae growth and can harm aquatic organisms.
The scientific literature that supports our assessment of a solution's effectiveness.
A group of human-made molecules that contain fluorine atoms. They are potent greenhouse gases with GWPs that can be hundreds to thousands times higher than CO₂.
food loss and waste
Food discarded during pre-consumer supply chain stages, including production, harvest, and processing.
Food discarded at the retail and consumer stages of the supply chain.
Combustible materials found in Earth's crust that can be burned for energy, including oil, natural gas, and coal. They are formed from decayed organisms through prehistoric geological processes.
greenhouse gas
gigajoule or billion joules
The glass layers or panes in a window.
A measure of how effectively a gas traps heat in the atmosphere relative to CO₂. GWP converts greenhouse gases into CO₂-eq emissions based on their 20- or 100-year impacts.
A solution that has a steady impact so that the cumulative effect over time builds as a straight line. Most climate solutions fall into this category.
A gas that traps heat in the atmosphere, contributing to climate change.
metric gigatons or billion metric tons
global warming potential
hectare
household air pollution
Number of years a person is expected to live without disability or other limitations that restrict basic functioning and activity.
A unit of land area comprising 10,000 square meters, roughly equal to 2.5 acres.
hydrofluorocarbon
hydrofluoroolefin
Particles and gases released from use of polluting fuels and technologies such as biomass cookstoves that cause poor air quality in and around the home.
Organic compounds that contain hydrogen and carbon.
Human-made F-gases that contain hydrogen, fluorine, and carbon. They typically have short atmospheric lifetimes and GWPs hundreds or thousands times higher than CO₂.
Human-made F-gases that contain hydrogen, fluorine, and carbon, with at least one double bond. They have low GWPs and can be climate-friendly alternatives to HFC refrigerants.
internal combustion engine
Greenhouse gas emissions produced as a result of a technology or practice but not directly from its use.
Device used to power vehicles by the intake, compression, combustion, and exhaust of fuel that drives moving parts.
The annual discount rate that balances net cash flows for a project over time. Also called IRR, internal rate of return is used to estimate profitability of potential investments.
Individuals or institutions willing to lend money in search of a return on their investment.
internal rate of return
A measure of energy
International agreement adopted in 2016 to phase down the use of high-GWP HFC F-gases over the time frame 2019–2047.
A measure of energy equivalent to the energy delivered by 1,000 watts of power over one hour.
kiloton or one thousand metric tons
kilowatt-hour
A land-holding system, e.g. ownership, leasing, or renting. Secure land tenure means farmers or other land users will maintain access to and use of the land in future years.
Gases, mainly methane and CO₂, created by the decomposition of organic matter in the absence of oxygen.
leak detection and repair
Regular monitoring for fugitive methane leaks throughout oil and gas, coal, and landfill sector infrastructure and the modification or replacement of leaking equipment.
Relocation of emissions-causing activities outside of a mitigation project area rather than a true reduction in emissions.
The rate at which solution costs decrease as adoption increases, based on production efficiencies, technological improvements, or other factors.
Percent decrease in costs per doubling of adoption.
landfill gas
Greenhouse gas emissions from the sourcing, production, use, and disposal of a technology or practice.
low- and middle-income countries
liquefied petroleum gas
A measure of the amount of light produced by a light source per energy input.
square meter kelvins per watt (a measure of thermal resistance, also called R-value)
marginal abatement cost curve
Livestock grazing practices that strategically manage livestock density, grazing intensity, and timing. Also called improved grazing, these practices have environmental, soil health, and climate benefits, including enhanced soil carbon sequestration.
A tool to measure and compare the financial cost and abatement benefit of individual actions based on the initial and operating costs, revenue, and emission reduction potential.
A greenhouse gas with a short lifetime and high GWP that can be produced through a variety of mechanisms including the breakdown of organic matter.
A measure of mass equivalent to 1,000 kilograms (~2,200 lbs).
million hectares
Soils mostly composed of inorganic materials formed through the breakdown of rocks. Most soils are mineral soils, and they generally have less than 20% organic matter by weight.
A localized electricity system that independently generates and distributes power. Typically serving limited geographic areas, mini-grids can operate in isolation or interconnected with the main grid.
Reducing the concentration of greenhouse gases in the atmosphere by cutting emissions or removing CO₂.
Percent of trips made by different passenger and freight transportation modes.
megaton or million metric tons
A commitment from a country to reduce national emissions and/or sequester carbon in alignment with global climate goals under the Paris Agreement, including plans for adapting to climate impacts.
A gaseous form of hydrocarbons consisting mainly of methane.
Chemicals found in nature that are used for cooling and heating, such as CO₂, ammonia, and some hydrocarbons. They have low GWPs and are ozone friendly, making them climate-friendly refrigerants.
Microbial conversion of ammonia or ammonium to nitrite and then to nitrate under aerobic conditions.
A group of air pollutant molecules composed of nitrogen and oxygen, including NO and NO₂.
A greenhouse gas produced during fossil fuel combustion and agricultural and industrial processes. N₂O is hundreds of times more potent than CO₂ at trapping atmospheric heat, and it depletes stratospheric ozone.
Social welfare organizations, civic leagues, social clubs, labor organizations, business associations, and other not-for-profit organizations.
A material or energy source that relies on resources that are finite or not naturally replenished at the rate of consumption, including fossil fuels like coal, oil, and natural gas.
nitrogen oxides
nitrous oxide
The process of increasing the acidity of seawater, primarily caused by absorption of CO₂ from the atmosphere.
An agreement between a seller who will produce future goods and a purchaser who commits to buying them, often used as project financing for producers prior to manufacturing.
Productive use of wet or rewetted peatlands that does not disturb the peat layer, such as for hunting, gathering, and growing wetland-adapted crops for food, fiber, and energy.
A measure of transporting one passenger over a distance of one kilometer.
The longevity of any greenhouse gas emission reductions or removals. Solution impacts are considered permanent if the risk of reversing the positive climate impacts is low within 100 years.
A mixture of hydrocarbons, small amounts of other organic compounds, and trace amounts of metals used to produce products such as fuels or plastics.
Private, national, or multilateral organizations dedicated to providing aid through in-kind or financial donations.
An atmospheric reaction among sunlight, VOCs, and nitrogen oxide that leads to ground-level ozone formation. Ground-level ozone, a component of smog, harms human health and the environment.
passenger kilometer
particulate matter
Particulate matter 2.5 micrometers or less in diameter that can harm human health when inhaled.
Elected officials and their staff, bureaucrats, civil servants, regulators, attorneys, and government affairs professionals.
System in a vehicle that generates power and delivers it to the wheels. It typically includes an engine and/or motor, transmission, driveshaft, and differential.
People who most directly interface with a solution and/or determine whether the solution is used and/or available.
The process of converting inorganic matter, including carbon dioxide, into organic matter (biomass), primarily by photosynthetic organisms such as plants and algae.
Defined by the International Union for the Conservation of Nature as: "A clearly defined geographical space, recognised, dedicated and managed, through legal or other effective means, to achieve the long-term conservation of nature with associated ecosystem services and cultural values". References to PAs here also include other effective area-based conservation measures defined by the IUCN.
Very large or small numbers are formatted in scientific notation. A positive exponent multiplies the number by powers of ten; a negative exponent divides the number by powers of ten.
Small-scale family farmers and other food producers, often with limited resources, usually in the tropics. The average size of a smallholder farm is two hectares (about five acres).
soil organic carbon
Carbon stored in soils, including both organic (from decomposing plants and microbes) and inorganic (from carbonate-containing minerals).
Carbon stored in soils in organic forms (from decomposing plants and microbes). Soil organic carbon makes up roughly half of soil organic matter by weight.
Biologically derived matter in soils, including living, dead, and decayed plant and microbial tissues. Soil organic matter is roughly half carbon on a dry-weight basis.
soil organic matter
sulfur oxides
sulfur dioxide
The rate at which a climate solution physically affects the atmosphere after being deployed. At Project Drawdown, we use three categories: emergency brake (fastest impact), gradual, or delayed (slowest impact).
Climate regions between latitudes 23.4° to 35° above and below the equator characterized by warm summers and mild winters.
A polluting gas produced primarily from burning fossil fuels and industrial processes that directly harms the environment and human health.
A group of gases containing sulfur and oxygen that predominantly come from burning fossil fuels. They contribute to air pollution, acid rain, and respiratory health issues.
Processes, people, and resources involved in producing and delivering a product from supplier to end customer, including material acquisition.
metric tons
Technology developers, including founders, designers, inventors, R&D staff, and creators seeking to overcome technical or practical challenges.
Climate regions between 35° to 50° above and below the equator characterized by moderate mean annual temperatures and distinct seasons, with warm summers and cold winters.
A measure of how well a material prevents heat flow, often called R-value or RSI-value for insulation. A higher R-value means better thermal performance.
Individuals with an established audience for their work, including public figures, experts, journalists, and educators.
Low-latitude (23.4°S to 23.4°N) climate regions near the Equator characterized by year-round high temperatures and distinct wet and dry seasons.
United Nations
Self-propelled machine for transporting passengers or freight on roads.
A measure of one vehicle traveling a distance of one kilometer.
vehicle kilometer
volatile organic compound
Gases made of organic, carbon-based molecules that are readily released into the air from other solid or liquid materials. Some VOCs are greenhouse gases or can harm human health.
watt
A measure of power equal to one joule per second.
A subset of forest ecosystems that may have sparser canopy cover, smaller-stature trees, and/or trees characterized by basal branching rather than a single main stem.
year