(408e) Improving the Stability of Nuclear Thermal Propulsion Fuel Elements with Tungsten Atomic Layer Deposition

Crewed missions to Mars will require engines with high specific impulse to achieve sufficient fuel efficiency for a return trip. Nuclear thermal propulsion (NTP) engines can achieve double the specific impulse of conventional chemical rockets by utilizing fission reactions to heat hydrogen propellant to around 2700 K before it is expelled through a supersonic nozzle. Fuel elements for NTP engines are typically composed of uranium oxide fuel pellets embedded in a refractory metal matrix and are susceptible to degradation at the temperatures required for NTP. Applying a barrier coating to the fuel pellets before fabrication of the cermet (ceramic-metallic) fuel element has been demonstrated to limit uranium hydride formation, prevent segregation of uranium oxide within the refractory metal matrix, and improve the final density of the fabricated cermet, all of which improve the durability of the fuel element. However, there is a need for finer control over the coating-fuel interface and an improved understanding of how coating properties influence performance in high-temperature, hydrogen-rich environments.

In this work, we utilize particle atomic layer deposition (ALD) to synthesize a tungsten coating on yttria-stabilized zirconia (YSZ) ceramic particles as a surrogate for UO₂. Particle ALD is performed by sequentially dosing WF₆ and Si₂H₆ into a fluidized bed of YSZ powder, which deposits a tungsten metal coating by self-limited surface reactions to create a nearly conformal 350 nm coating (Figure 1). Thermal treatments are used to evaluate the stability of the coated powders in a dilute hydrogen environment up to 1773 K. Further characterization of the powder by SEM-EDS after high-temperature hydrogen exposure is used to elucidate microstructural changes to the coating and fuel surrogate. To evaluate the ALD tungsten coating under more realistic NTP conditions, a test matrix is developed to compare the performance of the W-coated YSZ particles to the uncoated baseline YSZ particles in a variety of refractory metal matrices using spark-plasma sintering for consolidation. The consolidated cermet samples are then treated with pure hydrogen at >2000 K in MSFC’s CFEET facility to simulate conditions approximating NTP engine operation. Microstructural analysis with SEM and XRD, as well as characterization of surrogate fuel mass loss is used to evaluate the effectiveness of the ALD tungsten coating in preventing cermet degradation. This is the first instance of an ALD coating being incorporated into an NTP cermet via SPS, as well as the first test of such a coating under realistic operating conditions. This research advances our understanding of how refractory metal ALD coatings behave under high-temperature and introduces a new technique to improve the durability of NTP engines for deep-space exploration.