Development of CAP process for fabrication of ThO2–UO2 fuels Part II: Characterization and property evaluation

Coated agglomerate pelletization (CAP) process is being developed for the fabrication of ThO2–UO2 mixed oxide fuel pellets. The procedure for the fabrication of ThO2–4%UO2 and ThO2–20%UO2 pellets by this process has been described in detail in part I of this series of papers. The present part II deals with the characterization of the above pellets by optical microscopy, scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). The microstructure of the ThO2–4%UO2 and ThO2–20%UO2 pellets showed a duplex grain structure. Thermal expansion and thermal conductivity were measured in the temperature range from room temperature to 1500 °C. It was found that the thermal conductivity decreased by the addition of UO2 to ThO2 at any temperature. Addition of 20%UO2 to ThO2 caused very large decrease in thermal conductivity. The thermal expansion of ThO2–20%UO2 pellet was different from that of ThO2 and ThO2–4%UO2, e.g. it increased more rapidly with increasing temperature in the temperature range of 1000–1500 °C.