Mechanical Properties of Cubic (U,Zr)O2

Abstract

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.

Investigating the mechanical behaviour of Fukushima MCCI using synchrotron Xray tomography and digital volume correlation

“…Marchetti et al 36 also synthesised mixed samples containing different percentages of UO 2 and ZrO 2 to investigate their mechanical properties using micro-echography and high-frequency acoustic microscopy. Their results were in good agreement with Kitagaki et al 35 , reporting a mean Young's modulus equal to 203 GPa (±15 GPa). In addition to U-Zr containing solid samples, Marchetti et al 36 performed mechanical testing on nuclear fuel debris samples bored out of the damaged Three Mile Island (TMI) Unit 2 (TMI-2) reactor core 9 .…”

“…These three phases are directly associated with the presence of a silica sand-rich cement in the FDNPP MCCI. Both mixed U-Zr-O and other fuel debris samples reported previously 35,36 contained no phases relevant to concrete. In addition, the U-Zr-O phase comprises <14 wt% of MCCIsimulant batch mixture, while it is the dominant phase in both literature examples.…”

“…The wide variety of phases mixed in the batch to prepare a surrogate material, combined with the unknown composition of the real material and uncertainties related to sample preparation mimicking the meltdown conditions, have proven to be a considerably complex environment for research. However, numerous studies investigating the mechanical properties of multi-phase materials relevant to MCCI have been published [35][36][37] . Kitagaki et al 35 investigated the fracture toughness, Young's modulus and hardness of solid mixed (U, Zr)O 2 phases.…”

“…However, numerous studies investigating the mechanical properties of multi-phase materials relevant to MCCI have been published [35][36][37] . Kitagaki et al 35 investigated the fracture toughness, Young's modulus and hardness of solid mixed (U, Zr)O 2 phases. The weight percentages of UO 2 and ZrO 2 varied among the samples tested.…”