Prof. Jim Stubbins and his students have gained access to advanced experimental facilities that will allow them to identify atom-sized defects in materials used for nuclear reactor construction.
The U.S. Department of Energy has provided Stubbins a $238,000 grant from the Nuclear Energy University Programs (NEUP) to test samples in the reactor at North Carolina State University. Stubbins and his group will use the reactor’s positron beam to shoot positive electrons into material samples that have been irradiated in Idaho National Laboratory’s Advanced Test Reactor.
Stubbins explained that positive electrons injected into a material search for negative electrons. Combining, they decay very quickly, shooting gamma rays in the opposite directions of which can be detected. The positron lifetime is a signature of the presence or lack of defects in the crystal.
“If you shoot positive electrons in and there’s lot of defects, the annihilation process will take a while longer leading to a longer lifetime,” he said.
A material’s atoms usually position themselves in ordered crystal structure, but individual atoms can move when irradiated and struck by neutrons or ions leaving defects behind. Sometimes the material defects will recover. Other times, if a material is irradiated long enough, “holes” develop. “They get bigger and bigger, making the material stronger but more brittle,” Stubbins said.
“The question is, how do the defects heal and when do they start forming larger clusters?” he said. “We could ultimately change the material composition if we find one that supports the defects to heal for a long period of time.”
Stubbins’ technique was first successfully conducted 10 years ago at INL when Maria Okunewski, then a PhD candidate and now a Purdue University faculty member, worked with ion-irradiated samples.