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| Fig. 1: Diagram of a NuScale small modular reactor showing the containment vessel and internal components. (Source: Wikimedia Commons) |
Small modular reactors (SMRs) is a technology which has been placing itself as a transformative approach to nuclear power generation, offering rated electrical capacities of 300 MW or less per module. [1] The industry has advertised SMRs as compact systems that promise enhanced safety features, reduced construction timelines, and flexible deployment options compared to conventional large-scale nuclear plants. [2] Many governments, including Canada, France, Japan, as well as companies such as Microsoft have been looking into and even investing in SMRs. [3,4] Yet, only two SMRs are currently reported to be in operation: one in China and one in Russia. [4] To understand the challenges to deploying SMRs, one has to consider both the efficiency, and more importantly, the safety of SMRs.
Current light water reactor SMRs typically achieve thermal efficiencies around 30-35%, while advanced high-temperature designs show potential for efficiencies exceeding 45-53%.
The maximum theoretical efficiency of any heat engine, including nuclear reactors, is governed by the Carnot efficiency principle. [5] This fundamental thermodynamic limit depends solely on the temperature difference between the hot reservoir (reactor core) and cold reservoir (condenser or heat sink). The Carnot efficiency is expressed as:
where temperatures are in absolute Kelvin. For a typical pressurized water reactor (PWR) operating at 300°C (573°K) with a condenser temperature of 40°C (313°K), the maximum theoretical efficiency is approximately 45%. However, real-world SMRs achieve lower efficiencies due to irreversibilities in the thermodynamic cycle, mechanical friction, heat losses, and design compromises necessary for safety and economic considerations. [6]
The actual thermal efficiency of a nuclear power plant is defined as the ratio of electrical work output to thermal energy input from fission:
Conventional light water reactor SMRs are constrained by material limitations and pressure vessel design requirements. To prevent boiling of the primary coolant and maintain adequate subcooling margins, pressures around 16 MPa are typical for PWR-type SMRs. These operational constraints limit achievable steam temperatures and consequently restrict thermal efficiency to approximately 33-35% for most water-cooled SMR designs currently under development. [7] Regulatory compliance and safety requirements may constrain SMR efficiency optimization, potentially limiting market competitiveness despite technological advances and theoretical limits.
A potentially larger issue of deploying SMRs is the large waste management troubles that arise from operating SMRs. The long-term hazards associated with this waste are substantial and enduring. Plutonium-239, a primary component of spent nuclear fuel, has a half-life of 24,110 years, meaning the radioactivity persists at dangerous levels for tens of thousands of years into the future. All nuclear waste must be isolated from the environment for hundreds of thousands of years, typically requiring disposal in deep-mined geologic repositories. [1,8] This extended hazard period creates scenarios where accidental or malicious dispersal of nuclear waste could render urban areas uninhabitable for centuries or millennia, presenting both environmental contamination risks and potential vectors for radiological terrorism.
Perhaps more concerning from a proliferation standpoint is the weapons-usable material contained within SMR spent fuel. Reactor-grade plutonium, while containing higher proportions of undesirable isotopes like plutonium-240 compared to weapons-grade material, remains viable for nuclear weapons construction. Historical precedent demonstrates this concern is not theoretical. India conducted its first nuclear test in 1974 using plutonium produced in the Canadian-supplied CIRUS reactor, which had been provided for peaceful purposes. [8] The CIRUS reactor produced most of the weapons-grade plutonium for India's nuclear weapons program, demonstrating how civilian nuclear technology can be diverted to military applications. [9] Similarly, North Korea constructed a plutonium-producing reactor at Yongbyon in the 1980s and conducted its first nuclear test in 2006 using a plutonium-based device. North Korea continues to increase its fissile material stockpile through both uranium enrichment and plutonium production, illustrating ongoing proliferation challenges associated with reactor-derived nuclear materials. [10]
Both security and technical concerns have proven to be a big hurdle for deploying SMRs. In 2023, the NuScale Power Module (Fig. 1), one of the most advanced light water reactor SMR designs and the first and only SMR to have its design certified by the U.S. Nuclear Regulatory Commission (NRC), was terminated. [11] NuScale was unable to obtain a license from the NRC to push forward with construction. [11] Additionally, realizing the full potential of advanced SMR designs required overcoming several technical challenges. NuScales cost of construction of SMRs turned out to be unreasonably high, being anywhere from 6% to 26% higher in comparison to large nuclear reactors. [11] Ironically enough, the construction timelines, cost, and effectiveness were concluded to be less than optimal a stark contrast from how SMRs were (and are still) heavily advertised.
As global energy systems transition toward low-carbon sources, SMRs represent a potentially valuable component of future energy portfolios. Yet, the cautionary tales of SMRs overpromise such as that with NuScale remind us to do thorough research before investing further into SMRs for commercial deployment. The efficiency performance of these systems, balanced against safety, cost, and operational feasibility, ultimately shapes their competitive position in an increasingly complex and decarbonizing energy market.
© Raphael Low. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. 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.
[1] "The Role of Nuclear Energy in a Low-carbon Energy Future," Nuclear Energy Agency, NEA No. 6887, 2013.
[2] B. Daigle, S. DeCarlo, and N. Lotze, "Big Change Goes Small: Are Small Modular Reactors (SMRs) the Future of Nuclear Energy?" U.S. International Trade Commission, Working Paper ICA-105, March 2024.
[3] "Small Modular Reactors: Advances in SMR Developments 2024," International Atomic Energy Agency, 2024.
[4] R. G. Eccles, "Microsoft Can Take the Lead in Small Modular Reactors For Powering AI," Forbes, 31 Aug 24.
[5] M. Feidt, ed. Carnot Cycle and Heat Engine Fundamentals and Applications (Multidisciplinary Digital Publishing, 2020).
[6] G. C. Masotti, S. Lorenzi, and M. E. Ricotti, "Dynamic Modelling and Control of a Small Modular Reactor in Load Following by Cogeneration Mode," Nucl. Eng. Des. 445, 114518 (2025).
[7] N. Norouzi, M. Fani, and S. Talebi, "Exergetic Design and Analysis of a Nuclear SMR Reactor Tetrageneration (Combined Water, Heat, Power, and Chemicals) With Designed PCM Energy Storage and a CO2 Gas Turbine Inner Cycle," Nucl. Eng. Technol. 53, 677 (2021).
[8] G. Perkovich, India's Nuclear Bomb: The Impact on Global Proliferation (University of California Press, 2001).
[9] R. S. Cline, "Prospects of an Indian Nuclear Test," U.S. Department of State, 23 Feb 72.
[10] S.-H. Choe, "North Korea's New Reactor Raises Fears of Increased Plutonium Production," New York Times, 22 Dec 23.
[11] T. Gardner and M. Mishra, "NuScale Ends Utah Project, in Blow to US Nuclear Power Ambitions" Reuters, 9 Nov 23.
