The current SSTAR reactor design features a small, open-lattice cassette core (depicted in red) immersed in molten lead. A heat-exchange system (green) sends 550 °C CO² to a gas turbine.
In recent years, nuclear plant capacities have been trending larger, from an average of 854 MW for existing reactors, to 1,171 MW for plants on order, to 1,284 MW for those in the planning stage.
But one could ask if bigger is necessarily better. Smaller generators could enable broader use of clean, abundant energy to meet rapidly growing demand, in locations unsuitable for gigawatt-scale reactors. And surprisingly, significant evidence points to the cost-competitiveness of smaller plants.
Small-Scale Transportable Nuclear Reactors
Small reactors could one day revolutionize how clean, low-carbon-footprint nuclear energy is produced and delivered. These reactors, which operate below 100 MW, can be shipped anywhere in the world, including to developing nations that lack the capability to construct nuclear plants.
A complete SSTAR module would be capable of transport by ship or heavy-haul ground transporter. Modules would be mass produced at a central factory and shipped to remote sites or to countries that lack the technology infrastructure to build their own nuclear reactors.
Plants rated below 100 MW have played an important part in nuclear energy development. The first nuclear-electric power plant was the experimental EBR-I reactor which, in 1951, operated with an output of 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 though later upgraded to 200 MW, the world’s first commercial nuclear power station, Calder Hall in the U.K., began operations at 50 MW. The first commercial U.S. nuclear plant, the Shippingport Plant completed in 1957, maintained an output of 60 MW for 25 years. Small reactors continue to be used in training, isotope production, research, naval propulsion, and in some space applications.
The broad interest in small reactors appears to conflict with the trend toward ever-larger central power plants, but offsetting economies of scale is possible through simplicity of design, factory fabrication, and mass production.
One objection to smaller plants is the risk of diversion of fissionable material. In the 1990s, researchers at Lawrence Livermore National Laboratory concluded that transportable,
autonomous, sealed reactors with very long reactor lifetimes would eliminate the need to handle or process nuclear fuel and minimize the potential for its misuse. Several different reactor types were envisioned, among them one cooled by molten lead (small, sealed, transportable, autonomous reactor, or “SSTAR”) or light water (secure transportable autonomous light water reactor, STAR-LW).
The SSTAR design is particularly innovative. Its fast neutron energy spectrum and strong reactivity feedbacks enable it to adjust power output to match heat removal, a property known as autonomous load following. SSTAR reactors incorporate carbon dioxide heat exchangers to provide a compact system design, low primary system pressure, and separate coolant and working fluids.
Although conceptually exciting, SSTAR is years away from deployment. The design assumes development of advanced cladding and structural materials that will enable 15 to 30 years of continuous service at peak temperatures of about 650 °C. Other as-yet-unavailable enabling technologies for SSTAR include a qualified transuranic nitride fuel, whole-core cassette refueling, and a means for in-service inspection of components immersed in lead coolant. SSTAR will also require a novel regulatory framework.
Small reactors offer an opportunity for zero carbon-emission power generation in many regions of the world that lack elaborate technology infrastructures. To fulfill this promise, reactors must incorporate features to address operations, safety, and proliferation risk. STAR and SSTAR reactors provide good examples critical design features and viable alternatives to conventional energy generation.
Fulfilling Goals of Atoms for Peace
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.
Implicit in Eisenhower’s vision was the idea that atomic energy could be an important force to improve the socioeconomic condition of all of mankind. Today, although nuclear technology delivers about 16 percent of the world’s electricity, many observers would say that the vision of Atoms for Peace has not been fully achieved.
Although nuclear energy’s initial expansion was significant, the 436 power reactors that exist globally provide nuclear energy in only 30 of the world’s nearly 200 countries. 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.
[Adapted from “Nuclear’s Model T,” by Craig F. Smith, for Mechanical Engineering, July 2009.]
Small reactors could one day revolutionize how clean, low-carbon-footprint nuclear energy is produced and delivered.