From U Michigan: “Toward safer, long-life nuclear reactors”

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University of Michigan

12/15/2016 [Why so long to get this into social media?]
Kate McAlpine

In findings that could change the way industries like nuclear energy and aerospace look for materials that can stand up to radiation exposure, University of Michigan researchers have discovered that metal alloys with three or more elements in equal concentrations can be remarkably resistant to radiation-induced swelling.

The big problem faced by metals bombarded with radiation at high temperatures—such as the metals that make up nuclear fuel cladding—is that they have a tendency to swell up significantly. They can even double in size.

“First, it may interfere with other parts in the structure, but also when it swells, the strength of the material changes. The material density drops,” said Lumin Wang, U-M professor of nuclear engineering and radiological sciences. “It may become soft at high temperatures or harden at low temperatures.”

This happens because when a particle flies into the metal and knocks an atom out of the crystal structure, that displaced atom can travel quickly through the metallic crystal. Meanwhile, the empty space left behind doesn’t move very fast. If many atoms are ousted from the same area, those empty spaces can coalesce into sizable cavities.

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To control the formation of these cavities, and the attendant swelling, most recent research has focused on creating micro- and nano-structures inside the metal as specially designed “sinks” to absorb small defects in a way that preserves the integrity of the material. But Wang and his colleagues are kicking it old school, looking at alloys that don’t have breaks in the crystal structure of the atoms.

Colleagues at Oak Ridge National Laboratory in Tennessee created samples of a variety of nickel-based alloys. These were then exposed to radiation in a facility at the University of Tennessee. The most successful alloys were concentrated solid solutions—crystals made of equal parts nickel, cobalt and iron; or nickel, cobalt, iron, chromium and manganese.

“These materials have many good properties such as strength and ductility, and now we can add radiation tolerance,” said Chenyang Lu, a U-M postdoctoral research fellow in nuclear engineering and radiological sciences and the leading author of the report in Nature Communications.

In an experiment proposed by Wang, UT researchers exposed the samples to beams of radiation that created two levels of damage, similar to what may accumulate in a reactor core over several years and over several decades. These experiments were done at a temperature of 500 Celsius or 932 Fahrenheit—a temperature at which nickel-based alloys are usually prone to swelling.

These samples were analyzed at U-M’s Center for Material Characterization with a transmission electron microscope. The team found that compared to pure nickel, the best alloys had more than 100 times less radiation damage.

To explain what was special about these alloys, the team worked closely with the group of Fei Gao, a theoretician and U-M professor of nuclear engineering and radiological sciences. Gao’s group performed computer simulations at the level of individual atoms and showed that the radiation tolerance in this group of alloys can be attributed to the way that the displaced atoms travel within the material. The explanation was further confirmed by another set of experiments conducted by the team at the University of Wisconsin.

“In simplified terms, if there are a lot of atoms of different sizes, you can consider them bumps or potholes,” Wang said. “So this defect won’t travel so smoothly. It will bounce around and slow down.”

Because the displaced atoms and the holes in the crystal structure stayed near one another, they were much more likely to find one another. In effect, this repaired many of the vacancies in the complicated alloys before they could join together into larger cavities.

“Based on this study, we now understand how to develop a radiation-tolerant matrix of an alloy,” Wang said.

The study, titled Enhancing radiation tolerance by controlling defect mobility and migration pathways in multicomponent single phase alloys, appears in Nature Communications.

The work was supported as part of the Energy Dissipation to Defect Evolution Center, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences.

See the full article here .

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The University of Michigan (U-M, UM, UMich, or U of M), frequently referred to simply as Michigan, is a public research university located in Ann Arbor, Michigan, United States. Originally, founded in 1817 in Detroit as the Catholepistemiad, or University of Michigania, 20 years before the Michigan Territory officially became a state, the University of Michigan is the state’s oldest university. The university moved to Ann Arbor in 1837 onto 40 acres (16 ha) of what is now known as Central Campus. Since its establishment in Ann Arbor, the university campus has expanded to include more than 584 major buildings with a combined area of more than 34 million gross square feet (781 acres or 3.16 km²), and has two satellite campuses located in Flint and Dearborn. The University was one of the founding members of the Association of American Universities.

Considered one of the foremost research universities in the United States,[7] the university has very high research activity and its comprehensive graduate program offers doctoral degrees in the humanities, social sciences, and STEM fields (Science, Technology, Engineering and Mathematics) as well as professional degrees in business, medicine, law, pharmacy, nursing, social work and dentistry. Michigan’s body of living alumni (as of 2012) comprises more than 500,000. Besides academic life, Michigan’s athletic teams compete in Division I of the NCAA and are collectively known as the Wolverines. They are members of the Big Ten Conference.

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