Hot hydrogen testing & cermet materials development supporting nuclear thermal propulsion

To significantly reduce travel time and generally increase mission architecture flexibility, crewed missions to Mars and beyond may choose to use nuclear thermal rockets, as they represent a key technological advancement over traditional chemical rockets. Rocket efficiency is measured in terms of specific impulse (Isp) with units of seconds; and nuclear rockets are expected to be capable of 100% increase in Isp as compared to chemical rockets, stemming from the greater energies released in nuclear fission as compared to that available when breaking chemical bonds in combustion. A critical step in the development of nuclear thermal propulsion (NTP) technology is creating nuclear fuel material with key characteristics, such as, low neutron cross sections, a high melting temperature, chemical compatibility with hydrogen propellant, and desirable thermomechanical properties over a wide range of engine operating temperatures. To accomplish this, the reactor’s uranium dioxide fuel particles, which can undergo reduction when exposed to hydrogen near the intended operating temperatures, are suspended within tungsten which is chemically compatible with hydrogen. This is known as a ceramic-metallic (CERMET) fuel of tungsten-coated uranium dioxide, and this research aims to manufacture and test subscale CERMET fuel using zirconium dioxide as a surrogate for uranium dioxide. To optimize the fuel sample microstructures prior to sintering, the larger zirconium dioxide particles are coated with approximately micron-sized tungsten particles using a polyethylene binder. The coating process was optimized by varying the weight percentage of the added binder. The resulting powder mixtures showing the best coating characteristics are then consolidated into a puck-shaped disk by direct current sintering. Both optimized and non-optimized specimens were tested in a hot hydrogen environment using the compact fuel element environmental test (CFEET) located at NASA/Marshall Space Flight Center in the Material and Processes Laboratory. Samples were tested in CFEET at temperatures exceeding 2,000°C in a pure hydrogen environment. Mass loss was determined and the microstructure assessed before and after testing.