Challenges in Developing Materials for Microreactors: A Case-Study of Yttrium Dihydride in Extreme Conditions

Abstract

The development of microreactor technology presents an efficient solution for providing portable electricity, catering to both human space exploration needs within our solar system and supplying power to remote Earth-bound areas. The miniaturization of nuclear reactors poses immediate new challenges for materials science with respect to the capability for controlling nuclear reactions via thermalization of highly-energetic neutrons. In a microreactor, neutron moderation takes place in compact geometries, therefore new moderator materials are required to exhibit high moderating power per unit of volume. This challenge is currently being addressed through the advancement of hydrogen technology and materials science, particularly focusing on development of transition metal hydrides, but to date, research on their irradiation response, specifically regarding phase stability, hydrogen in-lattice retention, and their dependence on irradiation temperature and dose, is limited. Herein, we present a detailed investigation on the response of yttrium dihydride (\ce{YH2}) -- the most suitable moderator candidate with respect to its high operational temperature -- to heavy ion irradiation. A combination of multiple experimental techniques indicate that \ce{YH2} is stable up to an irradiation dose of 2 dpa and below 800$^{\circ}$C, identified as a critical temperature for \ce{YH2}. Our study unravels that voids emerge as the major form of radiation damage. Below the critical temperature no phase change, precipitation or amorphization was observed to occur. Experimental results from ion beam and neutron scattering techniques with concomitant density functional theory calculations have allowed us to elaborate and propose new strategies to enhance the metal hydride performance in extreme environments.

Keywords: Hydrides, Microreactors, Radiation Damage, Phase Stability, Space Exploration