NEWS
Ion Beams Speed Qualification for Nuclear Materials
By mimicking years of neutron damage in days, ion beam testing could dramatically reduce the time and cost of qualifying materials for advanced nuclear reactors.
Written by Judy Murray
QUALIFYING MATERIALS FOR ADVANCED NUCLEAR REACTORS is one of the slowest and most expensive steps in bringing new designs online. Researchers at the University of Michigan (U-M) are using ion beams to simulate years of radiation damage in days, potentially accelerating that process and cutting costs.
For years, the accepted approach relied on neutron irradiation in test reactors, followed by post‑irradiation analysis that demonstrates core structural materials will maintain their integrity through the life of the core. It is a process that is trusted and reliable but one that is extraordinarily slow.
Taking a Different Tack
“Damage accumulates slowly, and reaching end‑of‑life exposure levels for core components of advanced reactor designs can take decades in test reactors, in addition to the problem that few accessible test reactors exist today,” explained Gary Was, Professor Emeritus, U-M. He noted that this qualification process also produces highly radioactive material, which complicates critical testing for properties such as fracture toughness, fatigue, strength, ductility, and dimensional stability.
Ion irradiation has been used for decades to study radiation effects in materials. In 1989, Was and his team were studying irradiation‑assisted stress corrosion cracking when they discovered they could reproduce the same degradation that occurred with neutron exposure via ion irradiation. “This led us to consider the idea that we could use ion irradiation as a surrogate for radiation exposure in a nuclear reactor, where neutrons cause the damage,” Was said.
Three beam lines converge on a multi-beam target chamber in the Michigan Ion Beam Laboratory. Simultaneous exposure to hydrogen, helium, and heavy ion beams is necessary to emulate the damage that will occur in future fusion reactor materials. Photo: Ovidiu Toader, Michigan Ion Beam Laboratory, University of Michigan
The goal was to use ion irradiation to produce an irradiated microstructure with mechanical properties that could be measured to show that it is like neutron irradiated material from a nuclear reactor. Ion irradiation works by bombarding a material with charged particles, typically ions of elements already present in the alloy. Because large ions create a lot of damage very rapidly, the accelerator yields damage rates approximately 1,000 times higher than a reactor, using ions of energy around 9 MeV.
According to Was, at these rates, defects would simply recombine if the irradiations were conducted at reactor temperature. “The key is that you have to raise the temperature when using ion irradiation to allow defects to migrate and organize into the same features observed under neutron irradiation,” he explained.
Another consideration is that elements are transmuted into other elements in a reactor. The team figured out that injecting helium from a second accelerator simulates transmutation of this critical element, which plays an important role in the microstructure his team wants to emulate.
Validating the Results
Verification hinges on rigorous characterization. Using transmission electron microscopy, the team measures the size, number, density, and distribution of cavities, loops, and precipitates. “We plot the size distributions for ion‑irradiated and neutron‑irradiated materials, and if they lay on top of each other, we’ve matched the microstructure,” Was said.
The team also developed techniques using nanoindentation to extract measurements of the yield strength because these materials always harden under irradiation—in some instances, four times the yield strength of the original material.
“We can measure this and compare it to what is produced in a reactor,” he explained. “When you know what the microstructure is, there are models that can be applied using the known information about the microstructure to determine the yield strength. It turns out that the models and measurements are in very good agreement.”
Leading Research
Support for ion beam research at the university has come mainly from the United States Department of Energy Office of Nuclear Energy, augmented by funding from the Electric Power Research Institute (EPRI) and others. Technology advancement is also due in part to financing from TerraPower, an advanced nuclear energy company founded by Bill Gates in 2006, which provided access to the BOR-60 fast experimental reactor at the Research Institute of Atomic Reactors in Dimitrovgrad, Russia.
“We irradiated a number of alloys in that reactor over a range of temperatures and damage levels, retrieved samples, and characterized them to verify we can produce the same microstructure as that in a reactor, which is important to ensure the integrity of the alloys for their intended purpose,” Was explained. Ion irradiations benchmarked against neutron irradiations and development of models for both microstructure and mechanical properties constitute the framework for accelerating materials qualification for core components, termed QUICC (Qualification Under Ion irradiation of Core Components).
Central to the success of the approach was the expansion of the Michigan Ion Beam Laboratory (MIBL), which began in 2015. Today, MIBL, which houses three accelerators and provides exceptional performance flexibility, is considered one of the best facilities of its kind in the world.
Work at MIBL is not only industry-leading, it has fed directly into standards development. The team has provided content for ASTM E521 “Standard Practice for Neutron Radiation Damage Simulation by Charged-Particle Irradiation,” now undergoing its third update. Multiple revisions were driven by this program, and Was, who believes his group has put more effort than any other into determining how to do this work properly to obtain valid results, said more contributions are anticipated as the technology advances.
Judy Murray is an independent writer in Houston.
The Michigan Ion Beam Laboratory (MIBL). Here, three accelerators are combined to produce radiation damage that mimics both fission and fusion reactor damage. Photo: Ovidiu Toader, Michigan Ion Beam Laboratory, University of Michigan
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