Issue Date: August 24, 2009
Coming Back To Nuclear Energy
For nearly 30 years, the inbox for new license applications at the U.S. Nuclear Regulatory Commission (NRC) collected nothing but dust. These days, the inbox has no room for dust.
“In the past two years, we have received applications to build and operate 28 new nuclear power plants in the U.S.,” NRC spokesman Scott Burnell says. The agency has recently received several letters of intent, according to Burnell, indicating that in the next two to three years, utility companies will be seeking permission to build additional nuclear power plants.
The recent upsurge in interest—which is being stimulated by a combination of economic, environmental, regulatory, and other factors—is reinvigorating research and development in the nuclear materials field. Scientists and engineers are working to understand the fundamentals of processes such as corrosion and radiation damage that degrade materials in the cores and adjacent structures of nuclear reactors. And they are also working to design new materials that withstand the deleterious effects of the punishing environment found there. The broad search for advanced materials for future nuclear power applications has yielded a variety of promising robust candidates, including novel types of steels and alloys as well as nanostructured metallic and ceramic composites.
In 1979, a series of malfunctions and operator errors at the Three Mile Island nuclear power facility near Harrisburg, Pa., led to a meltdown of one of the plant’s nuclear reactors. According to Mitchell Singer, a spokesman for the Washington, D.C.-based Nuclear Energy Institute, a multinational nuclear energy policy organization, nuclear-power worries sparked by that incident, coupled with unfavorable economic factors and utility market forecasts at that time, caused designs for new nuclear power plants to be wiped off the drawing board. Three decades later, the power of the atom is poised to make a comeback.
At the heart of the matter is a projected 21% increase in demand for electricity in the U.S. between now and 2030, Singer says, drawing on forecasts from the Energy Information Administration. The need for new power plants that can run nearly around the clock (unlike wind- or solar-based generators) and do not emit greenhouse gases or air pollutants (as coal- and gas-fired plants do) is driving utility companies to tap nuclear energy to meet some of our growing power demands.
Most of the world’s roughly 440 commercial nuclear reactors—about one-fourth of which are in the U.S.—are known as Generation II reactors and are based on 1970s technology and on materials developed through the 1960s. In total, they produce about 16% of the world’s electricity and 20% of the electric power in the U.S., according to the World Nuclear Association.
In many ways, this fleet of nuclear reactors performs “very impressively,” says Steven J. Zinkle, director of the Materials Science & Technology Division at Oak Ridge National Laboratory. But to successfully power the planet through the 21st century and beyond, more advanced and even better performing reactors will be needed, Zinkle says. Compared with today’s reactors and the soon-to-be-constructed Generation III reactors, a future generation of reactors (Generation IV) is being designed now to remain in service longer and be more economical, safer, and more efficient in terms of thermodynamics and the use of uranium resources.
Achieving those gains, however, will come at a price. As Zinkle explains, future reactors will likely run at higher temperatures (to yield greater thermodynamic efficiency), with more corrosive coolants, and under more intense radiation. “All of those attributes place increased demands on the materials used in the cores of these reactors,” Zinkle stresses. But even outside of the cores, materials degradation processes can shorten the lifetimes of numerous reactor components, many of which are common to new and old reactors alike.
All nuclear power systems are designed to harness the intense heat released in nuclear reactions to do useful work. In principle, nuclear power generators are similar to coal-, gas-fired, and other types of power plants in that heat is used to produce steam, which drives the rotary motion of a turbine to generate electricity.
The fuel in most nuclear reactors takes the form of short (less than 1 inch) uranium dioxide pellets, which are loaded into long metal tubes made of a corrosion-resistant and neutron-permeable zirconium alloy. The tubes are bundled together into fuel-rod assemblies. Hundreds of these fuel rods, which are often on the order of 15 feet in length, are immersed in a heat-transfer fluid medium, known as a coolant, in the core of a reactor. Reactors are typically categorized according to coolant type, which varies with the reactor design.
Far and away, light-water reactors are the most common commercial type. These Generation II reactors are powered by fuel that has been enriched in 235U, a fissile uranium isotope. When a neutron collides with a 235U nucleus, that event can trigger a fission reaction that splits the nucleus into two smaller nuclei and liberates heat and additional neutrons. The product neutrons can fission other nuclei, which sets off a chain reaction that maintains the fuel-rod assemblies at high temperature. The chain reaction and the temperature in the reactor are mediated by inserting and withdrawing control rods into the reactor core. These rods, which play a critical safety role, are made from neutron-absorbing materials such as cadmium, boron, and hafnium.
In light-water reactors, ordinary (nondeuterated) water serves as the coolant, which transfers heat from the fuel rods to other parts of the power plant. At the same time, the water in the reactor core also serves as a neutron moderator, a material that reduces the kinetic energy of the emitted neutrons to increase the probability that they will cause fission reactions.
In the most common type of light-water reactor, the water in contact with the fuel rods circulates at over 320 °C as a liquid under pressure (over 2,200 psi or 150 atm) through a closed cycle known as the primary loop. The water flows to a steam generator, where it transfers heat via a set of intricate heat exchangers to a second closed water circuit, and then flows again past the hot fuel rods to continue the cycle. In the secondary loop, boiling water turns into steam, drives the turbine to generate electricity, cools, condenses, and cycles again past the heat exchangers. To prevent the possibility of spreading radioactive material, water in the primary loop is separated from the fuel pellets by the metal fuel-rod tubing (known as cladding), and the water supplies in the two loops are isolated from each other.
In a commercial reactor environment, where metal parts are constantly in contact with hot flowing water and steam, “corrosion is a fact of life,” says Paul A. Sherburne, a technical consultant at the Lynchburg, Va., facility of French power plant manufacturer Areva. And life is expected to be even tougher for components in future reactors, such as ones incorporating lead or lead-bismuth coolant and running at 800 °C and the so-called Very High Temperature Reactor that is designed to run at 1,000 °C with helium coolant.
Yet despite the threat of corrosion, some materials hold up quite well in commercial reactors. Recent studies of those materials’ properties, including mechanisms of damage initiation and propagation and surface chemical reactivity, have uncovered the basis of their robustness. The studies have also pointed to important criteria for selecting new materials from which to make long-lasting replacement parts, a key step in extending a reactor’s service lifetime.
For example, nickel-based alloys and stainless steels are widely used in light-water reactors in and out of the reactor cores. According to Krishnamurti (Ken) Natesan, a senior metallurgist and research group leader at Argonne National Laboratory, microscopic cracks caused by corrosion and mechanical stress in steam generator tubes are ushering in the replacement of tubes made from alloy 600 with ones made from alloy 690. Both materials are Ni-Cr-Fe-based, but the Cr-rich alloy 690 (roughly 28 versus 15% by weight Cr) exhibits better resistance to stress-corrosion cracking.
Surface chemistry plays a key role in providing that protection. As Sherburne explains, in a low-oxygen environment, a micrometer-thick passivation layer forms spontaneously on the surfaces of the Cr-rich alloy. The film, a Ni-Cr-Fe-oxide with a spinel crystal structure, adheres tightly to the underlying metal and serves as a protective barrier between the water and the metal.
Potential trouble spots aren’t confined to homogeneous regions of metal parts. Welded sections call for critical examination because high temperatures used in the welding process and differences in the properties of the base and filler metals that are fused together at weld joints can affect the weldment’s behavior in a nuclear reactor. Presently, “welding is more of an art than a science,” Natesan says, but his group is working to understand the materials science consequences of that common industrial process.
By exposing metal samples to high temperatures and radiation in test reactors and then analyzing with microscopy, Natesan’s group found key clues related to damage processes. In one case, they observed that in a sample of alloy 690 welded with a common filler material (alloy 152, a Co-Cr-Mn metal), micrometer-sized cracks preferentially initiate and propagate along the interface between the two metals in the so-called heat-affected zone.
In related work, the group found that a minute difference in the oxygen content of two specimens of 304L stainless steel—a material used in reactor-core support structures—made a huge difference in the materials’ responses to radiation. A sample with 0.047 wt% of oxygen became a brittle agglomeration of micrometer-sized grains. In contrast, a lower oxygen sample (0.008 wt%) retained its ductility and ability to tolerate stress without cracking.
The harsher conditions in the cores of other types of reactors present additional challenges and call for other types of materials. With regard to nuclear fuel, for example, although all fuel-and-cladding combinations are designed to retain hazardous radionuclides and maintain the fissionable material in a useful form that can be cooled, the requirements and designs vary widely.
Unlike the uranium dioxide pellets in zirconium-alloy tubes used in light-water reactors, multicoated millimeter-sized spheres (kernels) of UO2 or uranium oxycarbide (a mixture of UO2, UC, and UC2) lie at the heart of the 1,000 °C helium-cooled reactors.
Douglas Crawford, a manager at Global Nuclear Fuel, in Wilmington, N.C., notes that years of test-reactor studies aimed at designing a rugged fuel form that can reliably generate that high level of heat have led to a design in which a fuel kernel is coated with layers of porous carbon, dense pyrolytic carbon (a material similar to graphite), and silicon carbide. The layers collectively accommodate and contain the fission products, which include krypton and xenon; provide structural strength; and efficiently transmit heat. For loading the fuel into reactors, the coated kernels are typically packed into balls or blocks of graphite.
After some fraction of a nuclear fuel is consumed by fission, fuel performance degrades and the remainder of the fissionable material cannot be used reliably without reprocessing. For these coated fuels, “burnups” of about 9% (9% of fissile atoms consumed) have been demonstrated successfully, Crawford says. Higher burnups and higher operating temperatures would make these fuels more attractive commercially. But increasing those parameters remains challenging, he says, because doing so may weaken the fuel and release fission products.
Advanced reactors do not demand only novel fuel forms. They also call for novel structural materials. Standard strategies for toughening materials for high-temperature applications appear unlikely to offer much help for use in nuclear reactors, where intense radiation can make materials unstable.
That’s one reason why new types of metals known as oxide-dispersion-strengthened (ODS) alloys are drawing considerable attention these days. ODS metals contain uniformly dispersed nanometer-sized particles composed of oxides of titanium and yttrium. Zinkle notes that these types of nanocomposites provide a pronounced increase in high-temperature strength and radiation resistance relative to traditional alloys. “They have the potential to make a radical improvement,” he says, but notes that they also pose fabrication and embrittlement challenges.
An altogether different type of materials challenge is designing analytical methods and experiments that address tough-to-probe materials issues. Liquid lead and a mixture known as lead-bismuth eutectic (LBE) are high-performance coolants that challenge reactor designers because those liquids corrode most steels and many other materials. According to Peter Hosemann, a staff scientist at Los Alamos National Laboratory, one strategy for protecting against corrosion calls for using the liquid metal to transport dissolved oxygen to reactor surfaces, where it can form a passivating oxide film. But the details of that process and the films it forms are poorly understood and tough to probe.
So Hosemann and coworkers developed a combined atomic-force/magnetic-force microscopy method to analyze oxide films grown on stainless steels in an LBE test reactor at high temperature. The study showed the method’s utility by revealing the structures of multiple films of varying composition and magnetic character (J. Nucl. Mater. 2008, 376, 289).
Another thrust in nuclear materials analysis is developing computational tools to model the behavior of candidate materials under reactor conditions. At Massachusetts Institute of Technology, materials science professor Michael J. Demkowicz developed a model to help search for materials that resist lattice damage induced by collisions with energetic particles. Normally, such collisions create vacancies and interstitials (“extra” atoms in a lattice). Clusters of these kinds of defects can weaken a material.
Demkowicz found that in a crystal composed of abruptly alternating nanolayers of copper and niobium, vacancies and interstitials accumulate at the interfaces, where in effect, they cancel out one another and heal the lattice damage (Phys. Rev. Lett. 2008, 100, 136102). Demkowicz acknowledges that that particular nanocomposite is not suitable for nuclear reactor applications because the material would become radioactive. The damage-resistance concept, however, can be used as a criterion to search for structurally related materials suitable for nuclear energy applications, he says.
Years of research have generated a wide assortment of materials with advanced properties that hold promise for nuclear power technology. But in this field, the road from discovery to implementation is long. “Unfortunately, the next generation of nuclear power plants may not take full advantage of the numerous advances materials science has made in the past two decades,” Zinkle says.
The reason is twofold: Current (Generation II) nuclear power plants provide electricity very reliably. So the thinking goes, “If it ain’t broke, don’t fix it.” The other reason is that nuclear power plant designs are subject to tough regulation due to safety concerns, which leads to a long and costly qualification process before new materials are approved by NRC.
Nonetheless, nuclear science marches on, albeit at a somewhat slower pace than it did 30 years ago, in the view of James B. Roberto, director of strategic capabilities at Oak Ridge National Laboratory. With no new reactor start-ups for so many years, the scale of nuclear research dropped off from its heyday, Roberto says. But now it’s beginning to make a comeback. He adds, “I’d say there is a high probability that we’re headed toward a nuclear renaissance in the U.S.”
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