Assessment of the hydrogen resistance of (U0.2Zr0.8)C following exposure up to 2327 °C

Abstract

Nuclear fuels able to withstand hydrogen exposure >2227 °C with minimal chemical and mechanical changes are required to enable nuclear thermal propulsion reactors for deep space exploration. Previously (U_0.2Zr_0.8)C was demonstrated to exhibit minimal mass loss, while maintaining structural integrity, when exposed to hydrogen at 2327 °C for 3 h. Here, various techniques were implemented for an in-depth characterization of that same sample. X-ray and neutron diffraction were used to assess for formation of secondary phases and to examine lattice parameter changes on the surface and the bulk of the material by probing the full volume of the 8 × 8 × 12 mm sample. In addition, nano-indentation and microstructural characterization were conducted to understand the impact of hydrogen exposure to the mechanical properties and internal microstructure of the material. The results indicate that: 1) no new phases were observed throughout the volume of the hydrogen-exposed sample, nor any lattice parameter evolution was reported suggesting the composition of the sample following hydrogen exposure remained unchanged; 2) the microstructure was not significantly altered, although a small reduction in the grain size (as-fabricated: 12.9 $\pm$ 2.98 $\mu$ m, hydrogen exposed: 8.6 $\pm$ 2.71 $\mu$ m) and an increase in porosity (as-fabricated: 97.82 % theoretical density (TD), hydrogen exposed: 89.31 % TD) were observed; 3) the hardness of the hydrogen-exposed material did increase by ∼8.5 % when compared to the as-fabricated material and the hardness of the hydrogen exposed sample was shown to decrease with increasing temperature, as expected based on experience with ZrC. This detailed post-characterization examination, which is the first of its kind for fuels exposed to pure hydrogen at 2327 °C, suggests (U_0.2Zr_0.8)C would be incredibly resistant against chemical, dimensional, and mechanical changes when exposed to high temperature hydrogen during operation of a nuclear thermal propulsion reactor, making it an attractive fuel choice.