This article discusses that the future of nuclear energy could lie in plants that can be factory built, shipped to a site, and operated 30 years without refueling. The scope and timing •of the “nuclear renaissance,” however, remain somewhat uncertain. All that is known is that in countries around the globe, including the United States, significant numbers of new nuclear energy projects are under way or in various stages of planning, and this activity represents a departure from that of recent decades. The broad interest in developing new small reactor system concepts seems to be in conflict with the trend toward ever-larger central station power plants, which is driven by the principle of economy of scale. The Secure Transportable Autonomous Reactor (STAR) concept and the Small Secure Transportable Autonomous Reactor (SSTAR) reactor in particular provide good examples of additional design features that could make the introduction of such reactors more readily accepted while offering the potential for economic performance that makes sense in comparison to other alternative sources of energy.
It has become commonplace to say that we are at the beginning of a global revitalization of the nuclear energy enterprise. The scope and timing of this “nuclear renaissance,” however, remain somewhat uncertain. What is known is that in countries around the globe, including the United States, significant numbers of new nuclear energy projects are under way or in various stages of planning, and this activity represents a departure from that of recent decades.
As in the past, there appears to be a continuing trend toward larger plant capacities as the industry leverages economy of scale to optimize the economic performance of the new plants. According to the World Nuclear Association, the average power rating of today’s 436 operating power reactors is 854 MW. In contrast, for the 112 new reactors on order or planned globally, the average size is 1,171 MW. And for the 31 plants under consideration or proposed in the U.S., the average is 1,284 MW.
The conclusion to be drawn from this seems clear: Although existing power plants include reactors with outputs as low as 12 MW, the major trend is toward larger plant size. This is easy to understand in light of the high capital costs associated with nuclear energy plants. Given the large capital outlay, and the fact that costs do not scale linearly, there seems to be a relentless pressure toward increasing plant size.
But is bigger necessarily better? While new conventional nuclear plants trend toward larger size, there has also been continuing and growing interest in small and medium size plants. Nuclear generating stations that can operate at that smaller scale could enable broader use of this source of clean, abundant energy in a rapidly growing world economy. Such plants could be installed in locations that would not be able to accept the large quantity of electricity generated by a gigawatt-scale reactor, and there is some indication that, properly designed, a small plant could be cost competitive with the larger ones currently planned.
Along with my colleagues, I have worked on developing the class of reactors known as the Secure Transportable Autonomous Reactor (STAR), and the Small Secure Transportable Autonomous Reactor (SSTAR) in particular, and I believe this concept could play an important role in a global renaissance in nuclear energy. Reactors of this kind could be built in a central locatiqn and shipped to locations around the world, even developing nations that do not have the capability to build nuclear plants themselves. Indeed, small nuclear power plants could be a key technology for curbing greenhouse gas emissions in emerging economies.
Craig F. Smith is the Lawrence Livermore National Laboratory Chair Professor at the Naval Postgraduate School, in Monterey. Calif. He is a Fellow of the American Nuclear Society and the American Association for the Advancement of Science.
It is true that most modern power plants exceed 1,000 MW in output and are trending toward even larger sizes. But smaller plants—those rating below 100 MW—have historically played an important part in nuclear energy development. The first nuclear power plant to generate electricity was the experimental reactor EBR-I that, in 1951, operated in Idaho with an output of merely 100 kW. Later, in 1954, the first plant to provide power to an electricity grid was the 5 MW Obninsk Atomic Power Station (APS-1) in the U.S.S.R. And the first commercial nuclear power station, Calder Hall in England, was operated in 1956 at 50 MW, though later upgraded to 200 MW. The first commercial nuclear power plant in the United States, the Shippingport Plant, began operating on December 2, 1957 and continued for 25 years of operation with an output level of 60 MW.
In addition to those pioneering efforts, small-size reactors are employed in training, isotope production, research, naval propulsion, and in some space applications.
But what role could small-scale nuclear reactors have in generating central station power? The International Atomic Energy Agency indicates that more than 50 new concepts and designs for advanced small or moderate-size reactors are under development in more than 15 IAEA member states. Proponents of such designs believe they have the potential to meet such needs as providing energy for islands that are not served by a national grid or for regions lacking the infrastructures and grid capacity needed for large plants. Small reactors could also power such energy-intensive industrial activities as water desalinization or the extraction of oil from tar sands.
The broad interest in developing new small reactor system concepts seems to be in conflict with the trend toward ever-larger central station power plants, which is driven by the principle of economy of scale. But there is the possibility to offset the advantages of economy of scale through a combination of factors that can serve to improve the economic performance of small systems.
Among these factors are the interrelated impacts of factory fabrication and mass production. Modern nuclear power plants are field-constructed using factory fabricated parts and components, and though individual plant designs offer some consistency from site to site, the experience in the United States has been that each individual plant is unique in many of its details. For some of the small reactor concepts being considered, however, the entire power plant would be built in a factory and shipped to the site; this would be expected to yield substantial economic benefits. Henry Ford long ago established the industrial benefits of mass production; and the practice of factory fabrication of complex mechanical systems, such as large passenger aircraft, is well-developed modern practice.
Another factor is the simplicity of design that can follow from the development of small nuclear power systems. Small reactors should not be considered miniature versions of large plants—to the contrary, there is great potential for plant simplification and alternate design features that can be obtained as a result of the lower power rating. An example of this is the possibility of eliminating main coolant pumps in reactors that can rely on natural convection circulation. That approach is difficult to achieve in a larger reactor, which must rely on forced coolant circulation.
Small plants can be operated differently from conventionally sized nuclear plants, which are typically used to provide base load power to a large regional or national grid. Especially if they are located on smaller, more isolated power distribution systems, small-scale plants would be expected to operate at varying power levels in addition to providing base load power. The ability to operate with a high level of autonomous load following not only facilitates meeting this goal, but it also reduces the operator burden. With small modular reactors, the option exists for incremental development of a site which in turn enables incremental financing that lowers financial risk and overall cost.
Finally, a small reactor size offers a substantially reduced footprint for both security and operations, and these factors act to reduce the operational complexity and cost of the plant.
Taken in combination, these factors would offset the loss of economy of scale of small systems; in fact, it is possible that they could eliminate the disadvantage in terms of cost per unit energy generated. Whether that would happen depends on such variables as the level of design simplification, the number of units produced, the degree of duplication in mass production, and the regulatory hurdles small reactors would face.
One frequently cited drawback to widespread use of nuclear power is the risk of fissionable material being diverted to produce weapons. In the 1990s, researchers at Lawrence Livermore National Laboratory began looking at reactor system designs intended to minimize the potential for nuclear weapons proliferation. The initial research effort concluded that this goal could be met by a sealed reactor that was transportable and autonomous in operation and that would have a very long reactor core lifetime. Such a reactor would eliminate the need for handling or processing fresh or spent nuclear fuel and otherwise minimize the potential for any possible misuse of the reactor.
In addition, several desired features were identified to address, in particular, the anticipated lack of existing industrial and human resource infrastructures in developing nations. Those requirements included, among others, the need to provide relatively small increments of electric power on distributed grids; the desire for simple controls, passive safety, and low maintenance characteristics; and the requirement for high reliability in power availability over long periods of time. The need for stability in energy prices and low investment risk was also stressed.
Continued research determined that certain design objectives for a reactor system would meet those requirements. For instance, to reduce the risk that nuclear material would be diverted, the core could operate for 15 to 30 years without refueling and reside in a sealed, tamper-proof vessel. The reactor would be small enough to enable factory fabrication and shipment. And the system should exhibit autonomous control to allow load following and reduction of operational burdens. This combination of desired features came to be known as the Secure Transportable Autonomous Reactor, or STAR.
To achieve these objectives, several different reactor types were envisioned, and research was launched in multiple parallel directions. For example, the Encapsulated Nuclear Heat Source effort led by the University of California sought to develop a modular encapsulated reactor system that is a self-contained fission power source cooled by heavy liquid metal coolant. A team led by the Westinghouse Corp. pursued the STAR-LW concept, a light-water cooled reactor variant that was the predecessor to the current system known as IRIS. In addition, many other small reactor concepts, such as the Japanese 4S (Super Safe, Small, and Simple) reactor, were influenced by the STAR effort. More recently, the IAEA has organized an effort focused on small reactors without on-site refueling (to which they assign the acronym SRWOR), and they identify no fewer than 30 such reactor concepts currently under consideration.
Researchers have continued to pursue the STAR concept in an effort to bring it closer to realization. A team that included Lawrence Livermore National Laboratory, Argonne National Laboratory, Los Alamos National Laboratory and the University of California developed a new reactor design designated the Small Secure Transportable Autonomous Reactor. SSTAR arose from the conclusion that the best approach to achieve the overall STAR objectives, and in particular the long core life characteristic, was through development of a fast spectrum reactor cooled by heavy liquid metal. Indeed, SSTAR is the only such advanced system designed from the start as a small system intended to meet the stringent STAR objectives. (The effort was supported by the Department of Energy under its Generation IV advanced reactor initiative.)
The resulting pre-conceptual design, developed primarily by the reactor design team at Argonne, is a 20 MW electric (45 MW thermal) transportable reactor system that features molten elemental lead as a coolant, natural circulation heat transfer, and power generation based on a 44 percent efficient supercritical C09 Brayton cycle energy conversion system. The compact active core is not accessible by the user but can be removed by the supplier as a single cassette and replaced by a fresh core.
The lead coolant is contained inside a reactor vessel surrounded by a guard vessel. An alloy of lead and bismuth had also been considered as a coolant because it has a lower melting point than pure lead. But lead was selected as the coolant in order to dramatically reduce the amount of an alpha particle-emitting polonium isotope (210Po) formed in the coolant—a problem with the alloy—and to eliminate dependency upon bismuth that might be a limited or expensive resource.
“A sealed reactor that was transportable and autonomous in operation and that had a very long reactor core lifetime would eliminate the need for handling or processing nuclear fuel and otherwise minimize the potential for any possible misuse of the reactor.”
The design has some interesting properties. The fast neutron energy spectrum and the strong reactivity feedbacks within the reactor core enable the reactor to adjust its power to match the heat removal, a property known as autonomous load following. And the system incorporates carbon dioxide heat exchangers inside the reactor vessel to provide a compact system design, low primary system pressure, and separate coolant and working fluids.
It’s an exciting concept, though one that is certainly not ready for immediate deployment. For one thing, the SSTAR design assumes that a number of advanced technologies will be successfully developed. One of those technologies is a yet-to-be-developed and qualified advanced cladding and structural materials that will enable service in lead for the 15 to 30 years core lifetime at peak temperatures of up to about 650 °C. Other technologies that need to be developed are qualified transuranic nitride fuel meeting performance requirements, a whole-core cassette refueling system, and a means for inservice inspection of components immersed in lead coolant. The hope is, though, that the promise of the SSTAR concept can provide a driver for the development of the advanced technologies incorporated in the design.
In addition to these technology advances, the deployment of reactors broadly into the developing world, whether of the SSTAR or any other type, will require a framework for regulation and governance that does not currently exist.
As the SSTAR concept has been developed, a parallel effort in Europe is being pursued to design a larger (600 MW) lead-cooled reactor system for central station electricity generation. The European Lead-cooled System, or ELSY, incorporates a number of innovative features and emphasizes the use of existing technologies to minimize the need for additional research and development prior to building an initial demonstrator reactor system. The initial ELSY design efforts demonstrate the great potential for system simplification and compactness that is a characteristic of lead-cooled reactor systems. The SSTAR and ELSY concepts are distinct and different in important ways, but they nevertheless share a number of common features and have the potential to benefit greatly from parallel and coordinated development efforts.
One of the great opportunities for small reactors is to bring zero carbon-emission nuclear power to many new regions of the world. To fulfill this need, small size is not enough by itself. Reactors suitable for use in remote areas and in nations lacking the elaborate technology infrastructures that we take for granted in the developed world need to include design features to address operations, safety, and proliferation risk management considerations. The STAR concept and the SSTAR reactor in particular provide good examples of additional design features that could make the introduction of such reactors more readily accepted while offering the potential for economic performance that makes sense in comparison to other alternative sources of energy.
In this way, the SSTAR reactors could finally fulfill the first promise of nuclear energy. During the Cold War, President Dwight D. Eisenhower laid out a new vision for peaceful uses of nuclear energy culminating in a program known as Atoms for Peace. In this vision, technology and assistance for peaceful civilian uses of nuclear energy would be provided to states that agreed to forgo the development of nuclear weapons.
“Small size is not enough by itself. Reactors suitable for use in remote areas and in nations lacking elaborate technology infrastructures need to include design features to address operations, safety and proliferation risk management considerations.”
Implicit in his vision was the idea that atomic energy could be an important force to improve the socioeconomic condition of all of mankind. The results were a remarkably rapid international deployment of nuclear reactors. Today, some 16 percent of the world’s electricity is generated by this technology.
Has the Atoms for Peace vision been fully achieved? Many would say no. Although the initial expansion of nuclear energy was significant, the 436 power reactors that exist globally provide nuclear energy in only 30 of the nearly 200 countries of the world. These 30 nations include many highly developed countries and a few of the largest developing nations, such as India and China, but a large part of the world has not been included in the initial global deployment of nuclear energy.
It is likely that small, autonomous reactors such as SSTAR could be the key to bringing the benefits of nuclear energy to the developing world while assuring safety, security, and proliferation resistance. I am not alone in believing that this is a promising new direction that should be vigorously pursued.