RELATION OF COOLING RATE TO CRYSTAL STRUCTURE AND MICROSTRUCTURE OF (U, ZR)O₂

Abstract

Analysis of samples collected from the Fukushima Daiichi Nuclear Power Station provides information on the conditions at the time of the nuclear accident. The particle collected from the X-2 penetration of the Fukushima Daiichi unit 1 contained a fine precipitate of monoclinic ZrO₂ in a cubic UO₂-based matrix. The purpose of this study is to experimentally confirm at what cooling condition such a phase separation could be obtained, taking into account the temperature and time required for such a phase separation to occur. In this study, four patterns of cooling process were applied to U_0.8Zr_0.2O₂ to confirm the phase separation and phase transition.

U_0.8Zr_0.2O₂ cooled from 2200 °C at 600 °C/min or 10 °C/min showed no phase separation nor phase transition by observation using a scanning electron microscope and energy dispersive X-ray spectroscopy and a transmission electron microscope and energy dispersive X-ray spectroscopy. On the other hand, U_0.8Zr_0.2O₂ annealed at 1450 °C had fine phase separation. The phase of precipitated (Zr, U)O₂ transitioned from tetragonal to monoclinic by cooling at 0.5 °C/min. It was obtained that the possible condition for fine precipitates of monoclinic ZrO₂ to exist in a matrix of cubic UO₂, as in the particle collected from the X-2 penetration of the unit 1.