Microstructural evolution of a uranium-10 wt.% molybdenum alloy for nuclear reactor fuels

Abstract

Low-enriched uranium-10 wt.% molybdenum (LEU-10wt.%Mo) is of interest for the fabrication of monolithic fuels to replace highly-enriched uranium (HEU) dispersion fuels in high performance research and test reactors around the world. In this work, depleted uranium-10wt.%Mo (DU-10wt.%Mo) is used to simulate the solidification and microstructural evolution of LEU-10wt.%Mo. Electron backscatter diffraction (EBSD) and complementary electron probe microanalysis (EPMA) reveal significant microsegregation present in the metastable γ-phase after solidification. Homogenization is performed at 800 and 1000 °C for times ranging from 1 to 32 h to explore the time-temperature combinations that will reduce the extent of microsegregation, as regions of higher and lower Mo content may influence local mechanical properties and provide preferred regions for γ-phase decomposition. We show for the first time that EBSD can be used to qualitatively assess microstructural evolution in DU-10wt.%Mo after homogenization treatments. Complementary EPMA is used to quantitatively confirm this finding. Homogenization at 1000 °C for 2 to 4 h reduces the regions that contain 8 wt.% Mo or lower, whereas homogenization at 1000 °C for longer than 8 h effectively saturates Mo chemical homogeneity, but results in substantial grain growth. The appropriate homogenization time will depend upon additional microstructural considerations, such as grain growth and intended subsequent processing. Higher carbon LEU-10wt.%Mo generally contains more inclusions within the grains and at grain boundaries after solidification. The effect of these inclusions on microstructural evolution (e.g. grain growth) during homogenization and as potential γ-phase decomposition nucleation sites is unclear, but likely requires additional study.

Formation mechanism of gas bubble superlattice in UMo metal fuels: Phase-field modeling investigation

“…Microstructure and defects in irradiated UMo fuels is complicated. Mo usually depletes at grain boundaries and enriches at the center of the grain because of congruent growth during solidification [48].…”

Production of Uranium-Molybdenum Alloy as a Candidate for Nuclear Research Reactor Fuel

“…Two phases are present: gamma phase and −U 2 Mo phase. The uranium is alloyed with molybdenum to stabilize the gamma phase at room temperature [8]. The metastable cubic gamma phase of U−Mo alloy could be retained under 560 °C either by rapid cooling from gamma phase field or by adding sufficient quantities of molybdenum in the alloy [9].…”

Observed Changes in As-Fabricated U-10Mo Monolithic Fuel Microstructures After Irradiation in the Advanced Test Reactor

“…Depending on how a fuel plate is fabricated, it is possible for the U-10Mo alloy fuel microstructure to consist of elongated fuel grains in the rolling direction, areas of transformed c-(U,Mo) phase, areas of chemical banding, and impurity phases in different areas of the microstructure. 3,[10][11][12] Figure 1 shows examples of these microstructural features. The elongated grains results from the rolling process.…”