Results for “UMoC2”

DOE OSTI2020

Materials Data on UMoC2 by Materials Project

UMoC2 crystallizes in the orthorhombic Pnma space group. The structure is three-dimensional. U6+ is bonded to seven C4- atoms to form distorted UC7 pentagonal bipyramids that share corners with four equivalent UC7 pentagonal bipyramids, corners with three equivalent MoC5 trigonal bipyramids, edges with four equivalent UC7 pentagonal bipyramids, edges with seven equivalent MoC5 trigonal bipyramids, and faces with two equivalent UC7 pentagonal bipyramids. There are a spread of U–C bond distances ranging from 2.38–2.51 Å. Mo2+ is bonded to five C4- atoms to form MoC5 trigonal bipyramids that share corners with three equivalent UC7 pentagonal bipyramids, corners with four equivalent MoC5 trigonal bipyramids, edges with seven equivalent UC7 pentagonal bipyramids, and edges with two equivalent MoC5 trigonal bipyramids. There are a spread of Mo–C bond distances ranging from 2.15–2.21 Å. There are two inequivalent C4- sites. In the first C4- site, C4- is bonded to three equivalent U6+ and three equivalent Mo2+ atoms to form CU3Mo3 octahedra that share corners with nine equivalent CU4Mo2 octahedra, edges with nine CU3Mo3 octahedra, and a faceface with one CU4Mo2 octahedra. The corner-sharing octahedra tilt angles range from 14–58°. In the second C4- site, C4- is bonded to four equivalent U6+ and two equivalent Mo2+ atoms to form a mixture of edge, face, and corner-sharing CU4Mo2 octahedra. The corner-sharing octahedra tilt angles range from 14–58°.

36 MATERIALS SCIENCE↗

DOE OSTI2021

Microstructural and Micro-Chemical Evolutions in the Irradiated UCO Fuel Kernels of AGR-1 and AGR-2 TRISO Fuel Particles

AGR-1 and AGR-2 TRISO fuel particles were fabricated with slightly different fuel kernel chemical compositions, modified fabrication processes, different fuel kernel diameters, and changed 235U enrichments. To correlate those differences with the fuel kernel responses to neutron irradiations in terms of irradiated fuel microstructure, fission products chemical and physical states, and fission gas bubble evolutions, extensive microstructural and analytical characterizations were conducted. The studies used a state of art transmission electron microscopy (TEM) equipped with Energy-dispersive X-ray spectroscopy (EDS) of four silicon solid-state detectors which have super sensitivity and fast speed. The TEM specimens were prepared from selected AGR-1 and AGR-2 irradiated fuel kernels exposed to safety testing after irradiation. The particles were chosen to represent a representative irradiation conditions with a fuel burnup within the range from 10.8 to 18.6% FIMA, and the time-average volume-average temperatures vary from 1070 to 1287°C. The 235U enrichment was 19.74 wt.% for the AGR1 fuel kernels and 14.03 wt.% for the AGR-2 fuel kernels. The TEM results show that there were significant microstructural reconstructions in the irradiated fuel kernels for both the AGR-1 and AGR-2 fuels. There are four major phases including fuel matrix of UO2 and UC, U2RuC2, and UMoC2 in the irradiated AGR2 fuel kernel. Zr and Nb form solid solution in the UC phase. UMoC2 phase often shows a detectable concentration of Tc. Pd was found to mainly locate in the buffer layer or to be associated with fission gas bubble within the UMoC2 phase. The EDS maps qualitatively show that the rare-earth fission products (Nb, et al.) preferentially reside in the UO2 phase. In contrast, in the irradiated AGR1 fuel kernel, no U2RuC2 or UMoC2 precipitates were positively identified. Instead, there is a high number of rod-shape precipitates enriched with Ru, Tc, Rh, and Pd observed in the fuel kernel center and edge zone. The difference of microstructural and micro-chemical evolutions in irradiated fuel kernels between the AGR-1 and AGR-2 TRISO fuel particle may result from a combined factor of irradiation temperature, fuel geometry and chemical composition. However, the irradiation temperature probably play a more deterministic role. Limited electron energy loss spectroscopy (EELS) characterizations on the AGR2 fuel kernel show that there is nearly no carbon in the UO2 phase while a small fraction of oxygen was detected in the UC/UMoC2 phase.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗