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All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Materials Data on UWC2 by Materials Project

UWC2 crystallizes in the orthorhombic Pnma space group. The structure is three-dimensional. U6+ is bonded in a 6-coordinate geometry to six C4- atoms. There are a spread of U–C bond distances ranging from 2.32–2.68 Å. W2+ is bonded in a 3-coordinate geometry to five C4- atoms. There are a spread of W–C bond distances ranging from 2.08–2.39 Å. There are two inequivalent C4- sites. In the first C4- site, C4- is bonded to three equivalent U6+, two equivalent W2+, and one C4- atom to form a mixture of distorted edge and corner-sharing CU3W2C octahedra. The corner-sharing octahedra tilt angles range from 16–22°. The C–C bond length is 1.51 Å. In the second C4- site, C4- is bonded in a 5-coordinate geometry to three equivalent U6+, three equivalent W2+, and one C4- atom.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on UWC2 by Materials Project

UWC2 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 WC5 trigonal bipyramids, edges with four equivalent UC7 pentagonal bipyramids, edges with seven equivalent WC5 trigonal bipyramids, and faces with two equivalent UC7 pentagonal bipyramids. There are a spread of U–C bond distances ranging from 2.39–2.52 Å. W2+ is bonded to five C4- atoms to form WC5 trigonal bipyramids that share corners with three equivalent UC7 pentagonal bipyramids, corners with four equivalent WC5 trigonal bipyramids, edges with seven equivalent UC7 pentagonal bipyramids, and edges with two equivalent WC5 trigonal bipyramids. There are a spread of W–C bond distances ranging from 2.14–2.22 Å. There are two inequivalent C4- sites. In the first C4- site, C4- is bonded to four equivalent U6+ and two equivalent W2+ atoms to form a mixture of corner, edge, and face-sharing CU4W2 octahedra. The corner-sharing octahedra tilt angles range from 14–58°. In the second C4- site, C4- is bonded to three equivalent U6+ and three equivalent W2+ atoms to form a mixture of corner, edge, and face-sharing CU3W3 octahedra. The corner-sharing octahedra tilt angles range from 14–58°.

36 MATERIALS SCIENCE↗
NASA NTRS1972

Some observations on uranium carbide alloy/tungsten compatibility

Chemical compatibility between both pure and thoriated tungsten and uranium carbide alloys was studied at 1800 C for up to 3300 hours. Alloying with zirconium carbide appeared to widen the homogeneity range of uranium carbide, making additional carbon available for reaction with the tungsten. Reaction layers were formed both by vapor phase reaction and by physical contact, producing either or both UWC2 and W2C, dependent upon the phases present in the starting fuel alloy. Formation of UWC2 results in slow growth of the reaction layer with time, while W2C reaction layers grow rapidly, allowing equilibrium to be reached in less than 2500 hours at 1800 C. The presence of a thermal gradient had no effect on the reactions observed nor did the presence of thoria in the tungsten clad.

Phillips, W. M.↗
NASA NTRS1971

Compatibility of buffered uranium carbides with tungsten.

Results of compatibility tests between tungsten and hyperstoichiometric uranium carbide alloys run at 1800 C for 1000 and 2500 hours. These tests compared tungsten-buffered uranium carbide with tungsten-buffered uranium-zirconium carbide. The zirconium carbide addition appeared to widen the homogeneity range of the uranium carbide, making additional carbon available for reaction. Reaction layers could be formed by either of two diffusion paths, one producing UWC2, while the second resulted in the formation of W2C. UWC2 acts as a diffusion barrier for carbon and slows the growth of the reaction layer with time, while carbon diffusion is relatively rapid in W2C, allowing equilibrium to be reached in less than 2500 hours at a temperature of 1800 C.

Phillips, W. M.↗
NASA NTRS1972

Some observations on uranium carbide alloy/tungsten compatibility.

Results of chemical compatibility tests between both pure tungsten and thoriated tungsten run at 1800 C for up to 3300 hours with uranium carbide alloys. Alloying with zirconium carbide appeared to widen the homogeneity range of uranium carbide, making additional carbon available for reaction with the tungsten. Reaction layers were formed both by vapor phase reaction and by physical contact, producing either or both UWC2 and W2C, depending upon the phases present in the starting fuel alloy. Formation of UWC2 results in slow growth of the reaction layer with time, while W2C reaction layers grow rapidly, allowing equilibrium to be reached in less than 2500 hours at 1800 C. Neither the presence of a thermal gradient nor the presence of thoria in the tungsten clad affect the reactions observed.

Phillips, W. M.↗