Air oxidation of yttrium hydride as a high temperature moderator for thermal neutron spectrum fission reactors

Abstract Yttrium hydride (YHx) is an attractive moderator material for thermal neutron spectrum fission reactors requiring a small reactor core volume and has been selected as the neutron moderator for the Transformational Challenge Reactor (TCR), an advanced gas-cooled microreactor. Before YHx can be used in this application, it is important to understand the material response to off-normal conditions. In the present study, 550–650 °C isothermal dry air oxidation was performed to simulate a depressurized loss of force circulation (DLOFC) event. The oxidation was performed using thermogravimetric analysis (TGA) on bulk crack-free YHx coupons. Oxidation studies were also performed on Y coupons to elucidate the impact of H on oxidation. Both the chemistry and distribution of processing impurities were found to strongly affect oxidization behavior on a batch-to-batch basis. Regardless of batch, YHx oxidized at a significantly lower rate than Y at all temperatures, and the lower rate was directly correlated with increased hydride content. Metallic Y exhibited complex exponential kinetics, whereas YHx also exhibited complex kinetics but gained considerably less mass. According to literature reports on protonic and native-ion conductivities of Y2O3 and mass spectrometry analysis of gaseous reaction products formed during the oxidation of YHx, a mechanism for the reduced oxidation rate of yttrium hydride is suggested.