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| Fig. 1: Illustration of a typical light water small modular nuclear reactor. [3] (Courtesy of the U.S. Government Accountability Office. Source: Wikimedia Commons) |
In 1996, nuclear energy supplied a record-time high of 17.6% of electricity used globally. [1] The nuclear accident in Fukushima in 2011 has greatly diminished overall support for nuclear energy; Japans 48 nuclear reactors have mostly stayed offline since then and has led Germany, Belgium, Spain, and Switzerland to slowly phase out their nuclear programs. In the United States, enthusiasm for nuclear energy has diminished ever since the Three Mile Island accident in 1979 when a reactor partially melted down in Pennsylvania. [1]
The recent push to reduce global CO2 emissions, however, has revitalized the idea of including nuclear energy into the global mix. Nuclear proponents support that achieving net-zero emissions will not be possible without the development of nuclear energy and are in particular support of the use of small modular reactors. Small modular reactors (SMRs) are scaled-down versions of traditional nuclear reactors that produce up to 300 MW of power which is about a third of what traditional reactors output. SMRs are the fraction of the size of traditional reactors which allow them to be modular and transported as units for easier installation. [1]
A couple things about the current state of nuclear energy are clear. Traditional nuclear power lacks the overall support of the public and is beset with problems: many existing plants are aging, plant construction is often delayed, and plant operation is costly. The smaller scale of SMRs has the potential to help accelerate the transition to renewable energy by better handling the intermittent nature of wind and solar by providing baseload power more quickly and reliably than what can be achieved with traditional reactors. According to companies like Rolls-Royce and NuScale, SMRs could also be useful in places where less robust grid systems may not support the development of traditional reactors. However, many of the problems that exist with nuclear reactors such as safety concerns, large costs, and dealing with radioactive waste do not disappear just because a reactor is made smaller and modular. This paper will discuss some of the benefits that SMRs may offer to the world during the current push for net-zero emissions but will also maintain light on the existing shortcomings of these systems.
SMRS have many potential benefits over traditional reactors that are inherent to the nature of their design small and modular. In general, current designs for SMRS are simpler than those of traditional reactors and their safety characteristics rely more on passive systems such as low power and operating pressure. [2] The potential for overheating and other accidents to occur is far lower, according to NuScale, since SMRs have fewer moving parts and rely on an integral system in which the fuel, steam, and generator are all in one vessel meaning that there are fewer pipes to break. NuScales technology also relies on the cores heat to drive the coolant flow which eliminates the need for coolant pumps and has built-in relief valves on the reactor vessel which open when power is lost and can provide passive cooling to allow the reactor to safely shut down. [2] The company also claims that natural circulation, convection, gravity, and self-pressurization passively cool the system without human intervention and can significantly lower the possibility of radioactive material being released into the environment in the case of an accident.
Constructing smaller reactors also makes it possible for SMRs to be mass manufactured at a central facility and transported easily to locations where installation may not be possible for larger, conventional reactors. Ideally, SMRs would have a much lower capital start-up cost as well and could be built more quickly or be installed incrementally to meet energy demands, making it possible to install nuclear power in remote locations or in the developing world where manufacturing facilities and upfront capital are not available. [3] SMRS would also be ideal in countries that cannot handle a power load as large as one that would come from a traditional reactor. The future of SMRs on large depends on how well it can supplement an electric grid that will increasingly rely on the intermittent energy produced by renewables. Nuclear has traditionally acted as a baseload source of energy since reactors try to spread their high fixed costs over the largest number of kilowatt-hours as possible. Unlike gas turbines which can be turned on and off in seconds to match the desired energy load, nuclear plants take an hour or more just to cut energy production in half. [1]
Significant drawbacks of SMRs are their high capital cost per Megawatt output and the risks that come with mass manufacturing them. Supporters of SMRs contend that their modular nature would allow them to be mass manufactured which would reduce overall capital requirements for construction. Although possible in theory, the reality is that there would need to be a standardized SMR design in order to realize savings through mass manufacturing, but there are currently dozens of designs. Traditional reactors are large because of economies of scale related to reactor construction and operation. [2] Currently, there is no reliable market for SMRs which creates a two-sided economic challenge to their implementation: without manufacturing facilities, SMRs cannot achieve the cost reductions that compensate for their poor economies of scale, and without cost reductions there will be no large number of orders to stimulate the investments needed to set up the initial supply chain. [4] Even if mass manufacturing is realized how proponents plan, SMRs would have to be manufactured for the price per kilowatt to be comparable to a traditional reactor. Each kilowatt hour of electricity produced by an SMR would cost anywhere from 15% to 70% more than the same amount coming from a traditional reactor. [5] For example, an 1100 megawatt plant would cost about 3 times more to construct as an 180 megawatt plant but would produce 6 times the amount of energy meaning that the capital costs per power output would be twice as much for smaller plants. [2] Recent experiences support the skepticism around mass manufacturing of SMRs; NuScale recently announced that their pilot project to construct 12 reactors would be delayed to 2030 and costs would rise from $4.2 billion to $6.1 billion. [1]
In addition, errors made in a mass manufacturing process could propagate through an entire fleet of reactors and lead to costly fixes and widespread safety issues. Designs for light water SMRs (Fig. 1), including NuScales, rely on pressurized water reactors which if not functioning properly can be very costly to fix. In the last decade, steam generators in similar systems have been needed to be replaced prematurely and led to the shut down of two nuclear plants in San Onofre, CA. [6]
Issues regarding long-lived radioactive waste and safety are also a concern with SMRs. Radioactive waste will continue to be generated by SMRs that operate with pressurized water reactors, yet there is still no solution for how to safely store such waste. In fact, SMRs based on light water designs will produce more waste per MWh of electricity produced and the United States government is already paying billions of dollars in fines for not fulfilling their waste disposal obligations. The United States has been searching for a permanent nuclear waste repository since the mid 1980s and much of the country's waste currently sits in cooling pools that potentially leave Americans at great risk of radioactive exposure. [7] In addition, NuScale claims that because SMRs produce less amounts of radioactive waste and can be sited underground, there does not need to be as tight security measures for SMRs. This type of claim from SMR proponents has led to sharp criticism from nuclear experts who believe that as long as terrorism threats exist, it is simply irresponsible to reduce security measures for nuclear reactors of any size. [2] Furthermore, proponents of SMRs like to claim that the natural circulation cooling in SMRs makes them inherently safe, but there are accident scenarios in which heat transfer conditions could be less than ideal or an error in the reactor design could occur. No design comes with zero safety risks or is 100% reliable and marketing SMRs as inherently safe is misleading. [2] Lastly, nuclear plants withdraw large amounts of water roughly 400 gallons of water are consumed per megawatt-hour of electricity generated which adds construction and operation costs to the plant as well. [8]
SMR designs are actively being pursued by both public and private sectors and are currently in the construction or licensing phase in countries such as the United States, South Korea, Argentina, Canada, China, and Russia. There are more than 70 commercial designs being developed globally with a wide spread of applications such as electricity generation, hybrid energy systems, heating, water desalination, and steam generation for industrial applications. [9] Although there are potential benefits of being small and modular, SMR technology has not yet been fully realized and cannot offer the outcomes that some advocates promise it will. Despite upfront capital costs of SMRs being lower than those of a traditional reactor, their economic competitiveness once deployed has not been determined and there is much variability and risk related to mass manufacturing SMRs. Furthermore, SMRs do not tackle the issues of nuclear waste disposal or safety. The world desperately needs technology that can accelerate the transition to a carbon-free future; SMRs may be able to play a role in that future transition by providing baseload power in a hybrid energy system with renewables but the technology today is not there and will require several improvements in its economic scalability, safety, and waste approach to be a viable solution.
© Amir Kader. 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] L. Parshley, "The Controversial Future of Nuclear Power in the U.S.," National Geographic, May 2021.
[2] E. Lyman, "Small Isn't Always Beautiful: Safety, Security, and Cost Concerns about Small Modular Reactors," Union of Concerned Scientists, September 2013.
[3] "Nuclear Reactors: Status and Challenges in Development and Deployment of New Commercial Concepts," U.S. Government Accountability Office, GAO-15-652, July 2015.
[4] B. Mignacca, and G. Locatelli, "Economics and Finance of Small Modular Reactors: A Systematic Review and Research Agenda," Renew.Sustain. Energy Rev. 118, 109519 (2019).
[5] C. P. Pannier and R. Škoda, "Comparison of Small Modular Reactor and Large Nuclear Reactor Fuel Cost," Energy Power Eng. 6, 82 (2014).
[6] M. L. Wald, "Nuclear Power Plant in Limbo Decides to Close," The New York Times, 7 Jun 13.
[7] "What Does the U.S. Do with Nuclear Waste?," Scientific American, 24 Jun 08.
[8] S. Abdul-Khabir, "Nuclear Reactor Water Usage and the Implications of Limited Water Availability," Physics 241, Stanford University, Winter 2013.
[9] "Advances in Small Modular Reactor Technology Developments," International Atomic Energy Agency, September 2020.
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