By James Pearce

Jul 26, 2016

Scientists make progress and face challenges in their studies of nuclear reactor materials such as advanced steels and composites.

by James Pearce

Developing materials that can endure the extreme environment inside a nuclear reactor has occupied scientists in Oak Ridge National Laboratory's materials science program for the last half century. Reaching this objective has proved to be no small challenge. Reactor components must endure intense neutron radiation, relatively high temperatures and massive thermal and mechanical stresses. These factors conspire to weaken and degrade components by damaging their microstructure and exploiting these defects.

For decades, ORNL researchers have produced progressively more advanced steels and composite materials designed to extend the life and improve the reliability of advanced fission reactor components like fuel rod cladding and structural materials. More recently, however, the ITER fusion energy project has stimulated interest in applying these materials in a fusion environment. ITER is a multibillion-dollar international R&D effort designed to demonstrate the feasibility of fusion power - and to enable studies of self-heating burning plasmas and the components that will be needed to build and maintain power-generating reactors (see sidebar, "ITER and the Future of Fusion").

Currently, more than half of the U.S. Department of Energy's funding for fusion materials research flows through ORNL, located in Oak Ridge, Tenn. Roger Stoller, Ph.D., manager of the Fusion Materials Program, notes that, since the early 1980s, ORNL has collaborated extensively with other national laboratories (Pacific Northwest National Laboratory and Lawrence Livermore National Laboratory) and universities (the University of California's Berkeley, Santa Barbara and Los Angeles campuses) to develop exactly these types of materials. The U.S. program is augmented by two long-standing and productive collaborations with Japan: one involves the Japan Atomic Energy Agency's Tokai laboratory, and the other a consortium of Japanese universities.

"Primarily, we're studying the properties of structural materials," Stoller explains. "We're looking at the effects of radiation damage on advanced steels and advanced silicon carbide fiber composites. These materials are of interest to researchers in the fields of both fusion and advanced fission reactors." One of the difficulties of this sort of research is that there is no source of neutrons - even in a fission reactor - that can match the energy and intensity of a fusion reactor's neutron flux.

The biggest difference between the fission and fusion environments - in terms of potential radiation damage - is the energy of their neutrons. The peak of the fission energy spectrum is about 1 megaelectron volt (MeV). In contrast, neutrons born from deuterium-tritium fusion reactions have energies of 14.1 MeV. Computer models indicate that the microstructural damage inflicted by these higher energy neutrons would be generally similar to that caused by lower energy fission neutrons (see Figure 1). However, these fusion neutrons will create much more hydrogen and helium due to nuclear transmutation reactions with elements in the steel components. These gases can interact with radiation-induced defects to form bubbles within the microstructure, making the steel more brittle and prone to cracking. Experiments conducted using ORNL's High Flux Isotope Reactor, a fission reactor, appear to confirm these predictions.

As their research progresses, Stoller and his colleagues continue to narrow the gap between components that work in theory and those they can be confident will perform inside an actual fusion reactor. The researchers do this with a combination of experiments conducted at HFIR, an array of analytical and characterization equipment and highly sophisticated computational models. To ensure a comprehensive examination of the issues surrounding component development, the ORNL Fusion Materials Program has four main emphases: low activation steels, silicon carbide composites, oxide-dispersion strengthened steels and friction stir welding.

Low Activation Steels

One of the main concerns for the people who design fusion reactors is having as little induced radioactivity in the structure as possible. "For instance, fusion reactors like ITER will be huge facilities containing hundreds of tons of steel components," says Stoller. "Therefore, we want to use materials that are as ‘green' as possible so that after a reasonable period of time, discarded components can be recycled - or at least be eligible for shallow land burial."

As a result of these environmental concerns, a considerable amount of the fusion program's work over the last 15 years has focused on replacing elements in conventional steel that become radioactive fairly easily and for long periods of time with materials that are more resistant to activation while still maintaining their desirable properties. Recently, much of the progress in this area has been achieved using ferritic steels.

Fusion materials researcher Mikhail Sokolov, Ph.D., notes that, because of their general lack of certain elements, ferritic steels are less susceptible to activation than austenitic stainless steels like those being used in the construction of ITER. "We can also tailor the content of ferritic steels to achieve even lower levels by replacing high activation elements with low activation elements that have similar properties," says Sokolov.

Sokolov recalls that, at the time ITER was being designed, the most promising candidates for structural applications were austenitic stainless steels. "We had not yet arrived at the point where we could qualify the ferritic steels for structural applications, and we still haven't fully reached that goal," he says. "However, the ITER project includes a program to produce test components made out of these steels to see how they perform in a fusion environment. The ITER project is not just about proving that we can construct a plasma reactor; it is also about proving that we can create materials that will withstand those conditions and testing different concepts for designing these materials. Our goal is to create materials that will withstand this harsh environment without creating highly radioactive waste."

"So far, we have been very successful in this area," Stoller adds. "In some cases the reduced-activation steels we have developed have had even better properties than the originals."

Silicon Carbide Composites

Another low activation material that has shown great promise for use in fusion environments is silicon carbide fiber composites. While most ceramics are inherently brittle, silicon carbide composites have proved to be remarkably durable. Stoller notes that this toughness results from the composites' unusual structure. Unlike normal ceramics, the structure of silicon carbide composites consists of a weave of fibers - like fiberglass in a bicycle frame - that gives it greater resistance to fracturing. These composites still develop microfractures under stress, but the fibers hold it together.

Yutai Katoh, Ph.D., another member of the fusion materials research group who has worked extensively with these composites, compares their behavior under stress to that of metals. "When stress is applied to normal ceramics, they fracture or break suddenly," he says. "Composite materials, on the other hand, can withstand greater stress. When they fail, they do so gradually, developing microfractures which eventually propagate throughout the composite. This behavior is similar to the deformation of metals under stress - they bend before they break." This ability to fail "gracefully," rather than catastrophically, is a particularly desirable trait because it allows damaged components to be detected before they fail. As a result, silicon carbide fiber composites are often considered for use in applications with high mechanical loads.

Katoh, who chairs ASTM International Subcommittee C28.07 on Ceramic Matrix Composites, a part of Committee C28 on Advanced Ceramics, adds that these composites are very resistant to radiation damage. When exposed to high energy neutron radiation, the materials develop a high density of "self-defects." These defects are simply flaws or irregularities in the microstructure of the material that steady the structure and protect against further damage. "It's kind of a self-stabilizing mechanism within the material," Katoh observes.

Silicon carbide composites are also attractive materials for fusion applications because they lose their radioactivity very quickly - sometimes in as little as a few days - when they are exposed to radiation. Using this sort of low activation material in a project would create less hazardous waste and would enable component maintenance to be done more quickly and easily.

Oxide-Dispersion Strengthened Steels

Another ORNL fusion program emphasis is developing a low activation steel variant that has been processed to include a very high density of oxide nanoparticles with a size of 2 to 5 nanometers. This distinctive nanostructure gives the oxide-dispersion strengthened steel very good high temperature strength, making it a good candidate for use for structural components in a fusion reactor (see Figure 2).

According to Stoller, the key to creating this steel is the preprocessing regimen that dissolves very small oxide particles evenly throughout the powder used to produce ODS steel. The powder is then put through an extrusion press, causing it to solidify. The result is a steel that includes a high concentration of nanoparticles. These particles enable the steel to withstand much higher temperatures than its conventional counterparts. "These materials also have very good radiation toughness at low temperatures, and they show less change in their toughness as radiation exposure increases," Stoller says.

"We try to maximize the density of these nanoparticles," says fusion materials researcher David Hoelzer, Ph.D. "We're currently getting a density of 1024 per cubic meter. The particles are only separated by 10 to 15 nanometers. It's a very concentrated set of particles."

When a neutron bombards this material and creates defects, the high density of nanoparticles can trap the defects and keeps them from migrating through the microstructure of the steel. "We're trying to eliminate the possibility of defects accumulating and causing radiation hardening and embrittlement," says Hoelzer.

Another aim of these particles is to prevent the development of small, high pressure helium bubbles that can migrate to grain boundaries in the material and cause the steel to literally fall apart. Testing of this steel at the HFIR under conditions similar to those in a fusion reactor has found that the yttrium-titanium oxide clusters do, in fact, trap the helium in small bubbles and prevent them from going to grain boundaries.

Hoelzer adds that ORNL's Fusion Materials Program currently has one of the most promising approaches to producing ODS steel. "Even groups in Europe, Japan and China are trying to reproduce our recipe," he says. "The trend is toward following what we're doing. Now our goal is to try to perfect the material and continue to test its durability in radiation environments and its trapping ability."

Friction Stir Welding

Of course, once a steel can withstand the rigors of a fusion reactor, the issue of how to work the steel into complex shapes arises. Normally, pieces of steel could be welded into virtually any configuration. However, in the case of ODS steel, standard welding - that is, melting pieces of steel together - would change the uniformly high concentration of nanoparticles, making the weld weaker and more prone to cracking.

To get around this problem, researchers have adopted the relatively new technique of friction stir welding. Originally developed for joining aluminum, the process has recently been successfully applied to several other metals, including ODS steel. Stir welding uses a small rotating tool to plow through the edges of adjacent pieces of material. The friction created by this process heats the edges to the point that they can be "stirred" together, plasticizing them without getting them so hot that they melt and lose their unique microstructure. Zhili Feng, Ph.D., leader of the ORNL materials joining group, observes that the advantage of friction stir welding over other alternatives, such as brazing and other friction-based alternatives, is that it can accommodate complex geometries while creating a weld with properties as good as those of the base metal.

"We have successfully demonstrated that we can stir weld ODS steel while maintaining the desirable characteristics of its microstructure," says Feng. "However, there are many more details of the process to work out. For example, stir welding produces some fine grains in the metal. We would like to develop a post-welding heat treatment to eliminate these small grains and reduce the potential for high temperature creep in the ODS steel."

The Challenge

Employing a full suite of experimental, analytical and computational resources, researchers in ORNL's Fusion Materials Program and their collaborators in the materials sciences community have made great progress in illuminating the extreme environment in which fusion reactor components will be required to perform. The program's advances in mitigating the effects of radiation damage in composite materials and ODS steels and fabricating radiation-resistant components, as well as its improvement and development of low activation materials, demonstrate the effectiveness of this multilateral approach.

The ongoing challenge for the program is twofold: to develop a more precise understanding of the mechanisms that control the microstructural behavior of advanced materials, and to apply that understanding to predicting the behavior of complex materials, improving existing materials and designing better ones.

 

James Pearce is a science writer at Oak Ridge National Laboratory.