Strengthening materials to withstand irradiation
As we seek to decarbonize the global energy portfolio to combat the growing threats of climate change, nuclear energy is unquestionably a necessary and significant contributor of low-carbon baseload power. Maintaining and sustaining our existing nuclear fleet is a critical part of this, as is the construction of new nuclear plants — including microreactors and small modular reactors, as well as advanced reactors (such as molten salt reactors and lead-cooled fast reactors).
If we want to see these new reactors come online, we cannot go another 60 years without qualifying new materials. We need to take advantage of technological advancements across the board — accelerated, high-throughput testing and characterization of materials; automation of materials synthesis and inspection; use of artificial intelligence (AI) to improve predictions and reduce the number of experiments needed; and data mining with AI to extract data points buried in historic/legacy reports.
Materials are a central issue limiting nuclear power production. When I previously worked at a major power company, every time we had to shut down a reactor for some unforeseen reason, it was due to a materials-related issue. This made me realize that the neutrons in a reactor are always going to work — we understand the physics of fission and power generation.
The challenge with nuclear environments is that the irradiation and high temperatures cause gradual changes in a material, its structure and its properties. So even if you start out with a material that has the desired properties, it will degrade during its service in a nuclear reactor.
To keep the current fleet of reactors running — and continue to produce power for our clean energy future — we need to ensure the materials will remain reliable after 50, 60 or even 100 years of duty in high temperatures and high radiation fields.
To deploy new reactors that have better neutron economy/efficiency, we need to design and qualify materials that will withstand the more extreme irradiation doses, higher temperatures, and different corrosive coolants (including molten salts and molten lead).
And to responsibly recycle used nuclear fuel and ultimately dispose of nuclear waste, we need materials that will withstand salt corrosion and/or underground environments, coupled with low levels of radiation and moderate temperatures.
A manufacturing fix
Most materials used in reactors are made by conventional manufacturing and joining methods that have been around for decades — or in some cases, centuries — such as casting, forging, and arc welding. In the past decade or so, there has been a manufacturing revolution (with 3D printing a relatable example for the layperson). So we now are asking this question: Are materials made by these advanced manufacturing methods more resilient against degradation than conventionally manufactured materials?
My research group is studying an advanced manufacturing method known as powder metallurgy with hot isostatic pressing (PM-HIP). Unlike metal casting, which involves pouring molten metal into a mold, PM-HIP entails pouring metal powders into a mold and applying high pressure and high temperature — just shy of the melting temperature of the metal — to form a dense component.
This method provides a more uniform structure for the starting material (compared with castings or forgings), making it more resilient against irradiation. We now are combining PM-HIP with electron beam welding to determine whether this more advanced, state-of-the-art welding technique also is more resistant to irradiation than historic arc welding.
Seeing at the electron level
Our researchers combine electron microscopy and mechanical testing during experimentation. An optical microscope uses visible light to magnify specimens through a series of lenses, and an electron microscope uses electrons, which have a much shorter wavelength than visible light.
Material behavior at the macroscale level is governed by atomic/microscopic level features in the material. Through electron microscopy, we can see these tiny features and structures inside a material that govern its overall properties and behavior. We can check these characteristics before, during and after irradiation to understand how they evolve.
In mechanical testing, we push, pull and poke materials at varying size/length scales — sometimes only a few hundred nanometers in size — to assess how strong, how tough, how “stretchy” (ductile), and how resistant to cracking/breaking a material is. We do this before and after irradiation to check how irradiation affects these attributes.
Then we use electron microscopy during and/or after mechanical testing to more directly link structure changes to mechanical property changes. We also use computational tools and models to help provide a theoretical explanation to bolster our experimental inferences.
Meeting stringent qualification requirements
The nuclear power industry is extremely interested in deploying PM-HIP materials in current and future reactors. But anything that goes into constructing or operating a nuclear reactor — for example, any component of the structure, any part of the fuel assemblies, or the fuel itself — needs to be “qualified” by governing standards bodies and regulators. Given these hurdles, it’s not surprising that no new materials have been qualified for nuclear reactors since the 1960s.
Engineers need to provide sufficient evidence that PM-HIP materials, or any new material, will perform satisfactorily in nuclear reactor environments before these materials can become qualified. We are looking at performance after irradiation to understand whether the PM-HIP alloys withstand irradiation as effectively as (or better than) conventionally fabricated (i.e., cast or forged) materials.
Our microstructure characterization and mechanical testing of these PM-HIP alloys after irradiation is showing that they typically are more resistant to irradiation than their cast or forged counterparts. The alloys don’t tend to form as many defects under irradiation, and tend to remain stronger and more ductile for longer time periods.
Thus, we are providing data that is enabling additional PM-HIP alloys to become qualified, and extending these qualifications for irradiation-facing components. These breakthroughs are crucial for maintaining our existing nuclear baseload and constructing new, novel reactors for a sustainable future.
Janelle P. Wharry, PhD
School of Materials Engineering
College of Engineering