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

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 degrees 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. (C) 2021 Elsevier B.V. All rights reserved.

Automated summary

Yttrium hydride (YHx) is an attractive moderator material for thermal neutron spectrum fission reactors due to its ability to achieve a small reactor core volume. In oxidation studies, YHx showed significantly lower oxidation rates compared to metallic yttrium (Y), with the lower rate directly correlated with increased hydride content.