Mechanical Properties of Cubic (U,Zr)O₂

Evaluation of fuel debris properties in the Fukushima Daiichi nuclear power plant (1F) is required to develop fuel debris removal tools. In the removal of debris resulting from the Three Mile Island unit 2 (TMI-2) accident, a core-boring system played an important role. Considering the working principle of core boring, hardness, elastic modulus, and fracture toughness were found to be important fuel debris properties that profoundly influenced the performance of the boring machine. It is speculated that uranium and zirconium oxide solid solution (U,Zr)O₂ is one of the major materials in the fuel debris from 1F. In addition, the Zr content of the fuel debris from 1F is expected to be higher than that of the debris from TMI-2 because the 1F reactors were boiling-water reactors. In this research, the mechanical properties of cubic (U,Zr)O₂ samples containing 10%–65% ZrO₂ are evaluated. The hardness, elastic modulus, and fracture toughness are measured by the Vickers test, ultrasonic pulse echo method, and indentation fracture method, respectively. In the case of (U,Zr)O₂ samples containing less than 50% ZrO₂, Vickers hardness and fracture toughness increased, and the elastic modulus decreased slightly with increasing ZrO₂ content. Moreover, all of those values of the (U,Zr)O₂ samples containing 65% ZrO₂ increased slightly compared to (U,Zr)O₂ samples containing 55% ZrO₂. ZrO₂ content affects fracture toughness significantly in the case of samples containing less than 10% ZrO₂. Higher Zr content (exceeding 50%) has little effect on the mechanical properties.