Fabrication And Characterization Of The Zr-Y Alloy Hydride Moderator

Zirconium hydride is used as an effective neutron moderator in compact nuclear reactors and plays a fundamental role in the development of energy system in aerospace and navigation industry due to its good thermal stability, high hydrogen atomic density, low thermal neutron absorption cross-section, negative prompt temperature coefficient of reactivity and superior thermal conductivity. However, the application of zirconium hydride suffers from the problems of hydrogen induced cracking and hydrogen loss at the reactor working temperature, which affects the moderating performance of the material.To tackle these two problems, the crack-free bulk Zr-Y alloy hydride moderator has been fabricated by hydrogenation process. The hydrogenation process was optimized using Zr-Y-H thermodynamic database built by CALPHAD technique combined with first principle calculation. The hydrogen content, phase structure, microstructure and element distribution of the Zr-Y alloy hydride were characterized. The hydrogen desorption behavior and mechanical properties of the alloy hydride were also studied.The microstructure of the Zr-Y alloy hydride shows that Y alloy exhibits a strong grain refining effect and inhibits the formation of ?-ZrHx banded twin structure, which can relieve the internal stress caused by martensitic phase transformation. The nanoindentation and Vickers hardness test shows that the nanohardness, elastic modulus, Vickers hardness and fracture toughness of the Zr-Y alloy hydride increased with the Y content, which enhance the resistance to deformation and crack propagation. The above two factors inhibit cracks formation during hydriding and working at the nuclear reactor, thus retaining the physical integrity of the hydride.TG/TDS analysis under Ar atmosphere revealed that zirconium hydrides containing higher Y content tend to have higher phase transformation temperatures and higher hydrogen content, suggesting that hydrogen could be retained by alloying zirconium hydride with yttrium. TG/TDS analysis under CO₂ atmosphere indicated that the initial decomposition temperature of the hydride, 603 "C, is much higher than that under Ar atmosphere, that is 360?. The Zr-Y alloy hydride can react with CO₂ and form an oxide layer on the surface which could be self-healed under oxidizing atmosphere. Furthermore, X-ray diffraction analysis suggests that yttrium addition could stabilize t-ZrO₂ phase in the surface oxide layer and thus a dense and continuous oxide layer can be formed to resist hydrogen diffusion towards the outside. In-situ oxidation at different temperature demonstrated that the thickness of the oxide layer and the intensity of diffraction peaks associated with t-ZrO₂ phase increased with the temperature, suggesting that higher temperature is beneficial for hydrogen resistance performance. Finally, a thicker and continuous oxide layer can be prepared by in-situ oxidation under CO₂ atmosphere at 700 ?, and the initial decomposition temperature of the hydride can be raised to 795 ?.In conclusion, alloying with Y can inhibit the cracks formation by improving the microstructure of the alloy and hydride and enhancing the mechanical properties against cracks. Moreover, the hydrogen loss at the reactor working temperature can be resisted without reducing the hydrogen content by stabilizing the hydride matrix and the surface oxide layer at the same time.