Characterization of cermet fuel for nuclear thermal propulsion (NTP)

“A manned flight to Mars is met with many technical challenges, not the least of which is the development of propulsion technology capable of moving a transit vehicle from Earth orbit to Mars orbit. NASA is investigating Nuclear Thermal Propulsion (NTP) as a way of reducing flight time and providing the option for a mid-mission abort. NTP, which uses a high temperature nuclear reactor to heat a propellant, requires advanced fuel materials capable of withstanding temperatures well in excess of 2000 K. Among the fuel options are ceramic metal (cermet) composites composed of refractory metals and Ultra-High Temperature Ceramics (UHTCs). The mechanical and thermal properties of MoW-HfN, a surrogate cermet for MoW-UN, were characterized over a wide range of elevated temperatures. Thermal diffusivity, the coefficient of thermal expansion (CTE), elastic modulus, and heat capacity were measured. Optical and scanning electron microscopy (SEM) were performed to characterize the microstructure and draw structure-property correlations. The thermal diffusivity was obtained though the laser flash method. Values ranged from about 0.18 cm2/s at room temperature and decreased down to 0.15 cm2/s at 1800 °C. The CTE was measured using push rod dilatometry up to 1600 °C, giving average values from 6.0-9.0×10-6 K-1. Four-point bend tests were conducted from 25-1600 °C revealing systematic strengthening with temperature up to about 1400 °C where strength began to decrease, likely due to the increased ductility of the MoW matrix. A scientific rationalization of the effective material properties is made using the rule-of-mixtures and other effective properties models.”--Abstract, page iii