Designer Fuels
Nuclear reactors continue to provide 20 percent of the electricity used in the U.S.
Enthusiasm for nuclear power has waxed and waned for decades as people have tried to balance its real and perceived risks and benefits. Despite shifting support for this technology, nuclear reactors continue to provide 20 percent of the electricity used in the U.S. and 14 percent of the electricity used worldwide—without generating carbon emissions. In the last decade, however, as both energy costs and concern over greenhouse gases have increased, nuclear research and development has enjoyed a resurgence in the U.S.
One sign of this change is a renewed interest in advanced nuclear fuel formulations. Scientists working with power companies have found that fine tuning a reactor's fuel mix has several advantages. The most obvious benefit is increased efficiency. By 2015, advanced fuels will help to increase the output of existing nuclear plants by 9.2 gigawatts—the equivalent of nine new power plants. Advanced fuels also help power plants operate more safely, reduce the production of long-lived radioactive waste products, and make it harder to divert used fuel for use in nuclear weapons.
Advancing the art
"The 'advanced' part of advanced fuels means we help make the nuclear fuel cycle safer and more efficient," says Jeff Binder, head of ORNL's Fuel Cycle and Isotopes Division. "We do this by developing fuels that are more tolerant to accidents, devising less costly ways to process used fuel and designing fuels that incorporate recycled waste products."
One priority for Binder's staff is developing fuels that are more tolerant to accident conditions like those experienced recently at Japan's Fukushima Daiichi Nuclear Power Plant. "Devising fuels and cladding materials that are less likely to degrade when a reactor core loses coolant for a period of time is of the utmost importance," Binder says. For example, current light water reactor fuel is encased in a zirconium-based alloy metal cladding. When a reactor loses coolant, this cladding reacts with the steam and high temperatures that are present in a severe accident and produces hydrogen, creating the potential for an explosion. "If a completely ceramic-composition fuel could be devised," Binder explains, "this would eliminate hydrogen production during nuclear accidents that involve a loss of coolant to the reactor core. The production of significant amounts of hydrogen in several of the reactors at Fukushima contributed significantly to the severity and consequences of the accident."
The other top priority for the Fuel Cycle and Isotopes Division is developing fuel cycle technologies that enable nuclear utilities to wring more power out of the same amount of fuel. "We only use about three or four percent of the energy potential of the fuel," he says, "so part of the economic motive for advanced fuel cycle work is to use this resource more efficiently. Also, because enriched uranium is fairly inexpensive, there hasn't been a lot of incentive from a cost perspective to recycle spent fuel. But there is growing interest to do so for both nonproliferation and waste management reasons."
Another incentive for reprocessing used fuel is that it can be a source of isotopes used in medical and industrial applications. Binder notes that there are a number of potential synergies between fuel cycle research and the production of nuclear isotopes. Both technologies depend on the ability of researchers to design complex materials, anticipate how they will change when placed in a reactor and devise ways of chemically separating the resulting materials. ORNL is one of the few places in the nation that has both the range of expertise and the facilities to accomplish this.
Managing the actinides
Another pressing problem Binder's group is addressing is separating long-lived waste products known as actinides (including plutonium, americium, curium and neptunium) from used fuel more efficiently. Although these materials account for only about one percent of nuclear waste, they are extremely difficult to handle and dispose of. Separating actinides from the rest of the waste and from each other makes them much easier to process and allows researchers to consider how they can be routed back into the fuel cycle. Binder explains that part of his team's job is to develop fuels that incorporate these by-products, so they can be sent back into the reactor to be burned or transformed into more easily handled waste.
Considerable research goes into changing the composition of reactor fuel. Scientists must consider how the redesigned fuel will affect reactor operation and how characteristics of the reactor, such as coolant flow, need to change to ensure safe, economical reactor operation. ORNL's recently dedicated Consortium for Advanced Simulation of Light Water Reactors will accelerate researchers' ability to consider these factors by applying the power of the laboratory's supercomputers to the study of advanced fuel performance and a range of related issues.
Chemical fingerprints
Binder's group also applies its expertise to supporting the laboratory's national security and nuclear nonproliferation efforts. Working with ORNL's Global Security Directorate, fuel cycle researchers investigate increasingly sensitive ways of detecting chemical evidence of small-scale nuclear fuel reprocessing. This involves extremely precise analyses of isotopes—in concentrations down to the parts per billion level—to generate distinctive chemical "fingerprints" of materials. These forensic clues can provide indications of attempts to divert both new and used nuclear fuel to the production of nuclear weapons.
Techniques like these can be used from a distance to analyze the chemistry of smokestack emissions, or by nuclear inspectors to determine whether a plant normally dedicated to legitimate activities has been diverted to producing materials for use in nuclear weapons. Scientists can apply similar analyses to tracing the origin of a nuclear material or to deducing the origin of a nuclear device or dirty bomb–either before or after detonation.
Bridging the gap
The advantages of applying fuel cycle and isotope technologies effectively will become increasingly important as the nation's power companies bridge the gap between current nuclear plants and the generation of reactors that will take their place. In addition to maximizing the efficiency of our current power plants, this area of research and development has the potential to make the next-generation nuclear fuels cleaner, safer, more durable and less susceptible to proliferation. These advanced fuels also hold the promise of a waste stream that is safer to store, simpler to process and easier to recycle.—Jim Pearce