Study on Performance of Europium Hafnate Ceramic for Neutron Absorbing

Several new types of neutron absorbers with high melting points and non-alpha emit characteristics have been proposed by France and Japan in recent years. After comparison of the fuel assembly reactivity inserted with different rare earth oxide, the absorption effect of Eu₂O₃ mix with HfO₂ is proven to be the best. The Eu₂HfO₅ ceramic derived from the mixture of Eu₂O₃ and HfO₂ with Mole ratio 1∶1 was fabricated shown fluorite phase. The preparation process was studied and multiple experiments on the physical characteristics of this ceramic material were performed. The measurement on density was examined by Archimedes’ method. The measurement of linear thermal expansion coefficient was carried out by push-rod technique. The specific heat capacity at constant pressure test was conducted by high temperature differential scanning calorimetry. The thermal diffusivity was measured with laser flash diffusivity analyzer. The thermal conductivity was then calculated from equation with density, specific heat capacity at constant pressure and thermal diffusivity, the results is 1.66 W/(m·℃) under room temperature. The Young’s modulus was done with Grindo Sonic MK7. The melting point was tested in a non-container laser suspension furnace. The flexural strength, compressive strength was examined on universal testing machine. The micro-structure was observed on scanning electron microscope (SEM) KYKY EM6900. The subcritical water oxidation test was done in autoclave at 360 ℃. The density measurement and the XRD spectrum show the compactness and phase purity for the Eu₂HfO₅ ceramic sample. SEM image demonstrates the grain size is around 5-15 μm. The melting point of 2 414 ℃ measured by the dual-spectroscope is consistent with the theoretical value 2 434 ℃. A weight gain of 0.6% is found after the subcritical water oxidation. The preparation process was also optimized and the density of the Eu₂HfO₅ sample was improved. The thermophysical properties such as specific heat capacity at constant pressure, thermal diffusivity, thermal conductivity and compressive strength was tested for the new sample. This new type neutron absorber Eu₂HfO₅ was irradiated with neutron flux up to 1×10²⁰ cm⁻² on CMRR. For comparison, test before and after the irradiation was performed on the same machine. The post examination of the irradiation shows that there is no significant change with respect to appearance and density. However flexural strength and compressive strength increases a bit. The analysis of neutron radiograph for Eu₂HfO₅ shows a decrease of neutron absorption ratio of 18.3% after irradiation while the computational simulation of neutron radiograph result gives an absorption ratio decrease of 2.3%. The comprehensive performance test data is obtained in this study, and irradiation experiment was performed which verifying the expected advantages for the fabricated Eu₂HfO₅ ceramic. The accumulated basic data is valuable for the subsequent application of the material.