Niobium carbide as a technology demonstrator of ultra‐high temperature ceramics for fully ceramic microencapsulated fuels

Abstract

The consolidation of Fully Ceramic Microencapsulated (FCM) fuels has been extended from matrices based on SiC to ultra-high temperature ceramics (UHTCs). Specifically, sintering conditions of NbC 1-x were compatible with hosting of microencapsulated fuel. NbC powder in as-received, chemically treated, and composite forms was consolidated. Elemental analysis, shrinkage of powder compacts, contents of ejected vapor, density, microstructure, and NbC lattice constants were analyzed. As-received NbC showed more shrinkage due to the presence of a liquid phase compared with the chemically treated powder. Removal of impurity metals was observed from chemical treatment and during sintering of as-received powders. An increase in true density during sintering was attributed to removal of compounds with lower density than NbC.Chemically treated powder showed reduced densification rate and absence of a liquid phase after sintering. Smaller grain sizes were observed in the NbC composite. The implications for NbC and other binary carbides as matrices for FCM fuels are discussed. K E Y W O R D Scarbides, niobium/niobium compounds, nuclear thermal propulsion, sinter/sintering, spark plasma sintering

Observed volatilization behavior of silicon carbide in flowing hydrogen above 2000 K

“…In particular, recent development efforts in the fabrication and verification testing of tristructural isotropic (TRISO) particles 18 and fully ceramic microencapsulated (FCM) fuel technology 19 show promising performance and design features which may be leveraged for higher performing and/or more reliable NTP fuel systems. Refractory carbides, including silicon carbide (SiC) and the group IV and V transition metal carbides are currently of interest for nuclear thermal propulsion (NTP) applications due to their high melting (or sublimation) temperature and desirable nuclear properties 20‐23. Through the use of dispersed coated fuel particles in an inert refractory carbide structural matrix, FCM has the potential to provide high operating temperature capability (>2000 K), high‐temperature hydrogen compatibility, and fuel particle retention, while limiting fission product loss and irradiation damage to the structural matrix.…”

“…Refractory carbides, including silicon carbide (SiC) and the group IV and V transition metal carbides are currently of interest for nuclear thermal propulsion (NTP) applications due to their high melting (or sublimation) temperature and desirable nuclear properties. [20][21][22][23] Through the use of dispersed coated fuel particles in an inert refractory carbide structural matrix, FCM has the potential to provide high operating temperature capability (>2000 K), high-temperature hydrogen compatibility, and fuel particle retention, while limiting fission product loss and irradiation damage to the structural matrix. Also, recent breakthroughs on advanced manufacturing of high-purity and crystalline carbide ceramics open design opportunities to realize high performance NTP systems.…”