When Atoms Get Out of Line

Author Rebecca McDonald

How materials are impacted by the extreme conditions of a nuclear reactor

The inside of a nuclear reactor is a treacherous place. Neutrons, escaping from unstable, radioactive atoms, collide with neighboring atoms, causing their nuclei to burst apart, which releases more neutrons that pummel additional neighbors in a cascade of fission reactions. The whole process is carefully controlled using multiple strategies, including strong containment vessels, to ensure safety. These containers, however, are not completely spared from the ricocheting atoms and rogue neutrons, and over time they accumulate damage and can no longer be used.

Scientists at Los Alamos National Laboratory are leading a multi-lab effort to study the steel used to make these containers. Instead of simply surveying the damaged steel from an old reactor, they are trying to understand the fundamental science about what is happening to the atoms that comprise the steel.

“The reactors of the future could be even safer and more economical if they use materials that are optimized for harsh conditions,” says Blas Uberuaga, Los Alamos physicist and director of the multi-lab effort called FUTURE (Fundamental Understanding of Transport Under Reactor Extremes). “We want to gain a mechanistic understanding of how radiation, corrosion, and heat impact materials, and our expectation is that this understanding will translate into real systems down the road.”

A nuclear future?

Currently, 440 nuclear reactors distributed among dozens of countries produce about ten percent of the world’s electricity. Although nuclear energy is a reliable source of electricity that does not rely on burning fossil fuels or release significant carbon emissions, the extent to which nuclear energy will be included in the future energy needs of the United States is currently in flux. Most existing reactors were built in the 1970s and 1980s, and many have reached or are nearing the limit of their intended operational lifetimes. Furthermore, concerns about safety and the buildup of radioactive waste have kept the production of new reactors at bay for decades.

Today, with accelerated concerns about increasing energy needs and climate change, proponents of nuclear power are convinced that new reactors should be a vital part of the energy portfolio. To this end, many private companies are charging forward with new reactor strategies and designs. In addition, multiple Department of Energy programs are dedicated to modeling and testing alternative fuels, coolants, and next-generation reactors, as well as researching the fundamentals of nuclear power to enable innovation.

With next-generation reactors, many things are possible. For instance, new types of fuel or reprocessing strategies could help increase efficiency and possibly repurpose radioactive waste, and coolants other than water could make reactors safer by reducing the chances of a meltdown. However, the materials that make reactors will still be subjected to extreme environments that will limit the lifetimes of their safe usage. Critically, in many cases, reactor materials are simultaneously exposed to more than one such extreme environment, such as irradiation and corrosion. FUTURE is specifically targeting the coupling between these multiple extreme environments.

Nuclear energy does not rely on burning fossil fuels or release significant carbon emissions.

“There will always be corrosion, but it’s a question of how much and how fast,” says Peter Hosemann, professor at the University of California at Berkeley and deputy director of FUTURE. FUTURE is a Department of Energy-funded Energy Frontiers Research Center that combines expertise at six U.S. institutions: Los Alamos National Laboratory, The University of California at Berkeley, Pacific Northwest National Laboratory, North Carolina State University, Bowling Green State University, and the University of Virginia. Through a combination of modeling and experimentation, scientists in the FUTURE project are working together to pinpoint exactly how irradiation impacts corrosion in different types of materials and corrosive environments. With this information, they can help the nuclear power community make more informed choices about the best materials to use in next-generation reactors.

Solids aren’t so solid after all

Uranium-235 is the fuel that powers most of today’s nuclear power reactors. For the reactor to work, small pellets of uranium modestly enriched with this isotope are encased in metal rods that are bound together in a steel assembly, surrounded by a coolant such as water, and contained in additional steel vessels. The energy released from the fission reactions within the fuel rods heats the water and, through a number of heat exchanges, steam eventually drives a turbine generator to create electricity. The fission reactions are controlled, but ultimately the metal fuel rods and steel containment vessels deteriorate from radiation exposure and corrosion damage and are no longer safe to use.

Steel is a common structural material used in many parts of nuclear reactors. It is an alloy of iron and carbon but often includes chromium, nickel, or other elements—together they make steel a strong and versatile material. However, this solid compound that is commonplace as the skeleton of skyscrapers and bridges is not as inert as it looks; when in proximity to nuclear fission reactions and coolants, the lattice of steel atoms can be both physically and chemically disrupted.

The physical disruption happens when an incoming neutron from a nuclear fission reaction strikes the steel, forcing a few atoms out of order through elastic collisions. This irradiation can result in an empty space, or vacancy, in one part of the lattice, and a build-up of displaced atoms, called interstitials, in another part of the lattice. Furthermore, this disruption makes nearby atoms shift and shuffle, causing the vacancies and interstitials to essentially migrate until they reach a trap, such as a grain boundary. (Grain boundaries are places where small crystals of the material meet and don’t match up perfectly.)

During this shifting and movement many things can happen: the vacancies can combine with other vacancies to become larger voids, or the vacancies can heal if they encounter out-of-place interstitial atoms, thus shrinking the voids and re-joining the lattice structure. Interstitials can also aggregate, forming large disk-like clusters of extra atoms called dislocation loops within the materials. Moreover, neutrons can also induce nuclear reactions within the steel that can create new elements that were not originally in the lattice—including gases that make bubbles in the metal or unstable atoms that can even initiate further radioactive decay. All of these defects can ultimately lead to macroscopic changes in the properties of the material and, in many cases, cause a loss in performance such that the reactor may not optimally operate as long as expected.

Chemical deterioration, or corrosion, is also a significant concern, and understanding how corrosion happens to different materials is vital. Rust is a familiar type of corrosion that appears when iron atoms combine with oxygen to form layers of iron oxide on the outermost part of a metal. As rust layers thicken and spread, the material structure is compromised. However, some oxide layers are helpful; many types of steels are considered “stainless” when they include a thin, protective chromium oxide or other oxide layer on their surface.

Various types of stainless steel are used in reactors, and most currently operating reactors use water as a coolant. When fuel assemblies are submerged in water, any defects in the metal lattice provide opportunities for the metal atoms to interact with water’s oxygen and hydrogen atoms, thus accelerating the possibility of corrosion. In this oxidizing environment, the thin oxide layers can protect the metal as long as they are the optimum thickness. Other coolants that can be used instead of water include liquid metals, molten salts, or even gases, but these coolants can introduce other types of corrosion.

“We know that corrosion can occur when there are defects in the material, and we know radiation increases defects,” explains Uberuaga. “However, we want a deeper understanding of when irradiation will influence corrosion and how.”

To gain this understanding from such a complex environment, the FUTURE scientists are tackling the challenge from many angles and breaking it down into easier-to-interrogate pieces. For instance, instead of using steel for experiments, they are examining simpler, surrogate materials like iron-chromium binary alloys. They are conducting separate experiments to study void generation and oxide formation, while other studies are combining these different effects in one experiment. Finally, the scientists are using advanced multi-scale models to predict and validate their results.

Reactor vessel
A reactor vessel submerged in water at a nuclear power plant. Most nuclear reactors currently in operation use water to cool and moderate the nuclear fission.

Watching atoms one at a time

Although the FUTURE project research is spread out among all the partner laboratories, a number of experiments are taking advantage of the Ion Beam Materials Lab (IBML) at Los Alamos, which is one of very few accelerator facilities dedicated solely to materials research. Here, particle accelerators can simulate the conditions of a reactor by using high-energy protons or heavy ions to induce radiation damage. This is better than using neutrons because it’s easier to control, less expensive, and doesn’t cause the material to become excessively radioactive. Once a sample is damaged, various techniques can be employed to examine the results.

In recent work, FUTURE scientists used ion beams at the IBML to irradiate samples of iron-chromium and then examined the defects using transmission electron microscopy (TEM). They also took some of the irradiated samples to the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) research center in Germany to study the defects using an advanced, complementary technique called positron annihilation spectroscopy (PAS). Together PAS and TEM data could give the scientists a more complete understanding of the damage, and right now, HZDR is one of only a few places worldwide with this kind of positron capability.

Positrons are anti-electrons: electrons and positrons have the same mass but electrons are negatively charged and positrons are positively charged. When a positron meets an electron, they annihilate one another and release energy called annihilation radiation, which can be detected via spectroscopy. This is a useful tool for looking at vacancies and voids in an irradiated sample because positrons trapped in the voids take longer to annihilate, making it possible to deduce how much of the material is consumed with voids.

The TEM and PAS analyses of the Los Alamos samples revealed details about how vacancies accumulated into clusters, but it also revealed how preexisting voids from the material’s synthesis had shrunk in size as a result of irradiation. Together, these data reinforce the idea that the damage is dynamic and can change over time—but this change can happen quickly after radiation is lifted; in some cases the damage state relaxes within a few seconds or minutes. For this reason, the FUTURE team—with help from Los Alamos science investment funds—is developing a new, unique positron capability at the Lab to measure the damage as it happens.

We want a mechanistic understanding of how radiation, corrosion, and heat impact materials.

Bowling Green State University Professor Farida Selim leads the point defects thrust area of FUTURE. Selim arrived in Los Alamos in January 2021 to work with Yongqiang Wang, Director of the IBML, to set up a positron beam between two of the existing accelerators. The plan is to use an ion beam to introduce displacement damage in a sample (simulating radiation), a second beam to implant helium (simulating gas formation), and finally the positron beam to simultaneously allow extremely sensitive detection of the damage as it is created.

To enable PAS, the positrons emitted by a radioactive sodium-22 source are first slowed down and their energy is equalized, then they are transported through a magnetic field and bunched into very short pulsed beams (roughly a 100-picosecond (ps) pulse duration—100 trillionths of a second). By measuring how long the positrons live before they are annihilated, the scientists can infer the size of the individual vacancies and vacancy clusters. Each positron lives only about 100 ps, but they survive longer—up to a few hundred picoseconds—when encountering vacancies or voids, where there are fewer electrons with which to annihilate. The positron beams can also be accelerated at different energies, allowing annihilations to occur at different depths so that damage variation as a function of depth can be mapped out.

“The positron technique could allow us to see just one missing atom,” explains Selim. “This is compared to electron microscopy which can only measure large defects, the size of at least several hundred if not thousands of atoms.” When finished, the capability in Los Alamos will be the first of its kind in the world, offering a unique way to characterize the propagation of physical damage in multiple materials in situ.

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