Investigation of High-Temperature Compatibility of Select Oxides with Yttrium Hydride for Micro and Space Reactor Applications

Yttrium hydride is a promising material for a high-temperature neutron moderator in advanced micro and space reactors due to its high hydrogen density and relative thermal stability compared to other metal hydrides. However, yttrium hydride desorbs hydrogen rapidly at temperatures above 800°C, which is below the operational temperature range of some reactor designs. A hydrogen barrier coating of oxide on the hydride surface may inhibit hydrogen loss at 800°C and beyond, but the high-temperature compatibility between yttrium hydride and many oxides is currently unknown. The high-temperature compatibility of Al₂O₃, MgO, and Y₂O₃ with YH_1.92 was examined by subjecting mixed oxide–hydride pellets to a 1200°C heat treatment then using a combination of diffractometry, microscopy, and spectroscopy to determine changes in the pellet composition as a result. Yttrium scavenged oxygen from both Al₂O₃ and MgO to form Y₂O₃, resulting in significant loss of YH_1.92. Yttrium reacted with reduced aluminum to form YAl₂ and several other compounds. Reduced magnesium volatilized above 1091°C and vacated the pellet. Only Y₂O₃ did not appreciably react with YH_1.92. Of the three oxides tested, only Y₂O₃ was compatible with YH_1.92 at 1200°C based on the experimental criteria.