The results of the study were published in the Journal of Nuclear Materials, in a paper titled “The application of synchrotron micro-computed tomography to characterize the three-dimensional microstructure in irradiated nuclear fuel,” and were also described in a press release issued by Argonne and Purdue University on January 19.
Technique: Micro-computed tomography detects an X-ray beam as it emerges on the other side of the sample. From multiple images taken as a sample is rotated, the internal features of a sample can be imaged based on how the X-ray beam was altered as it passed through the sample.
At Argonne, the Purdue research team worked with scientists at beamline 1-ID-E, a high-brilliance X-ray source at the APS, to examine the sample. The research marked the first time that synchrotron X-ray micro-computed tomography was used to analyze the morphology of the microstructure of irradiated nuclear fuel in three dimensions, according to the research team.
The sample: The subject of investigation was a tiny piece of uranium-zirconium (U-10Zr) from a fuel pin that spent two years at full power in the Fast Flux Test Facility at the Hanford Site, near Richland, Wash., before it was extracted in the early 1990s.
The sample was prepared at Idaho National Laboratory. A cube of the material about 100 microns across—about the width of a human hair—was milled from a fuel pin using a focused ion beam with scanning electron microscopy. “We had to wait decades for this fuel to radiologically cool, or decay,” said Maria Okuniewski, an assistant professor of materials engineering at Purdue University and the paper’s lead author. “It was literally the coolest specimen that we could remove, based on the permissible safety guidelines at both INL and APS.”
Findings: Okuniewski and her colleagues wanted to characterize swelling caused by the accumulation of gaseous fission by-products, which limits the useful life of nuclear fuels.
The study revealed the presence of pores and of three distinct uranium phase regions: poor, intermediate, and rich. The researchers determined that 7.2 percent of the fuel specimen was porous. Five growth stages of pore evolution were observed, including nucleation, growth, coalescence, interconnected porosity, and extended/interconnected porosity. The research also found that the release of fission gases might continue to occur beyond the thresholds assumed in previous analyses.
“We’re always striving within the nuclear community to figure out ways that we can improve the fuel performance codes,” Okuniewski said. “This is one way to do that. Now we have three-dimensional insight that we previously didn’t have at all.”
