Lightweight Zirconium Carbide Insulation for High Temperature Space Nuclear Propulsion Systems

The thermal insulator for fuel assembly structures of nuclear-thermal propulsion (NTP) reactors must withstand the high temperature (>2500°C) hydrogen operating environment and provide sufficient thermal resistance to ensure adequate heat retention within the fuel assemblies for propellant heating and to achieve the target specific impulse. The insulation package must possess low thermal conductivity within a hydrogen environment at the upper range of reactor operating temperatures. Zirconium carbide (ZrC) is a favored insulator material for the extreme temperature ranges of NTP reactors given its stability in hot hydrogen and acceptable neutronics characteristics. In previous work for BWX Technologies (BWXT), Ultramet established the initial feasibility of producing a low-density ZrC insulator by converting porous carbon structures (felt and open-cell foam) to ZrC through reaction of the carbon structures with a vapor containing zirconium at high temperature in a chemical vapor deposition reactor. However, significant materials optimization and properties testing remains to be performed to optimize the thermal conductivity and structural integrity of porous ZrC insulation. In this project, Ultramet will team with BWXT (the end user of the technology) to further develop the insulation through materials fabrication, thermal and mechanical properties testing of small development specimens, and thermal modeling. Mapping of thermal conductivity across various porous ZrC insulator architectures will be performed to identify insulators capable of achieving the target range of thermal conductivity while also exhibiting satisfactory mechanical properties and resiliency to be incorporated into the overall insulation package envisioned for high specific impulse NTP reactors.