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
Nuclear thermal propulsion (NTP) is an in-space propulsion method which directly transfers the heat from fission to a working fluid (ie propellant), which is heated to extremely high temperatures (2000-3000 K) and expanded through a nozzle to provide thrust. Through the use of a hydrogen (H 2) propellant, NTP is capable of high specific impulse (800-1100 seconds) and thrust (15-100 kN). This combination of high thrust and specific impulse is highly desirable for crewed interplanetary missions, to destinations such as Mars, primarily due to enabling reduced trip times which minimizes astronaut's exposure to deleterious health effects from microgravity, cosmic radiation, and prolonged confinement. High I sp and thrust can also enable longer surface stays, larger abort windows, and higher payload masses, ultimately maximizing return on scientific investment. NTP is a well-studied technology, with over 20 reactors tested during the nuclear engine for rocket vehicle application (NERVA)/Rover program (1955-1972). 1,2 Fuel systems for use in NTP systems must be able to withstand the demanding operating conditions of the engine, namely a high temperature (2000-3000 K) hydrogen (H 2) environment along with energetic neutron and gamma ray exposure, for short lifetimes (~45-110 minutes). 3 As a result of historic NTP fuel development programs, several legacy fuel forms have been developed and evaluated through both non-nuclear and nuclear screening testing. 4,5 Two NTP fuel systems with the greatest development status include
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