Study on chemical interaction between UO2 and Zr at precisely controlled high temperatures

Abstract

ABSTRACT Interaction tests between UO2 and Zr were performed at precisely controlled high temperatures between 1840 and 2000°C (±2.5°C for 1900, 1920, 1930, 1940, and 2000°C, ±10°C for 1840 and 1890°C, respectively), to understand the interaction mechanism in detail. A Zr rod was inserted in a UO2 crucible and then heat-treated at a fixed temperature in an Ar-gas flow for 10 min. After heating in the range of 1890 to 1930°C, the Zr rod was deformed to a round shape, in which the post-analysis detected the significant diffusion of U into the Zr region and the formation of a dominant α-Zr(O) matrix and a small amount of U-Zr-O precipitates. An abrupt process of liquefaction was observed in the sample heated at around 1940°C or higher. Furthermore, the post-analysis results suggested the detailed liquefaction mechanism: (1) the higher oxygen concentration in the α-Zr(O) matrix suppressed the liquefaction progress, due to the variation in the equilibrium state; (2) the U-Zr-O melt formation progressed by the selective dissolution of Zr from the matrix, corresponding to the reverse variation of the oxygen concentration in the matrix; (3) the selective diffusion of U could occur via the U-Zr-O melt.