Advanced Protective Coatings for Gr-Based Nuclear Propulsion Fuel Elements

A protective coating for a graphite (Gr) containing fuel element used in a nuclear thermal propulsion system includes a first layer that is configured to resist hot hydrogen attacks. The first layer has a coefficient of thermal expansion that is higher than a coefficient of thermal expansion of the Gr containing substrate. The coating also includes a plurality of second layers located between the first layer and the substrate. The second layers are configured to mitigate the differences in coefficients of thermal expansion between the first layer and the substrate to minimize debonding and exposure of the substrate to hydrogen attack. Preferably, the protective coating can comprise an outermost first layer including zirconium carbide (ZrC), a second layer including niobium (Nb), a third layer including molybdenum (Mo), and a fourth layer including molybdenum carbide (Mo.sub.2C) located adjacent to the substrate.