| Summary: | Developing high-radiation-tolerant inert matrix fuel (IMF) with a long lifetime is important for advanced fission nuclear systems. In this work, we combined zirconia (ZrO<sub>2</sub>) with magnesia (MgO) to form ultrafine-grained ZrO<sub>2</sub>–MgO composite ceramics. On the one hand, the formation of phase interfaces can stabilize the structure of ZrO<sub>2</sub> as well as inhibiting excessive coarsening of grains. On the other hand, the grain refinement of the composite ceramics can increase the defect sinks. Two kinds of composite ceramics with different grain sizes were prepared by spark plasma sintering (SPS), and their radiation damage behaviors were evaluated by helium (He) and xenon (Xe) ion irradiation. It was found that these dual-phase composite ceramics had better radiation tolerance than the pure yttria-stabilized ZrO<sub>2</sub> (YSZ) and MgO. Regarding He<sup>+</sup> ion irradiation with low displacement damage, the ZrO<sub>2</sub>–MgO composite ceramic with smaller grain size had a better ability to manage He bubbles than the composite ceramic with larger grain size. However, the ZrO<sub>2</sub>–MgO composite ceramic with a larger grain size could withstand higher displacement damage in the phase transformation under heavy ion irradiation. Therefore, the balance in managing He bubbles and phase stability should be considered in choosing suitable grain sizes.
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