The early stages of U3O8 formation on unirradiated CANDU UO2 fuel oxidized in air at 200–300°c

Abstract

This paper describes the use of X-ray powder diffraction (XRD) to detect the early stages of U3O8 formation by air oxidation of unirradiated CANDU UO2 fuel. Expressions are derived to describe both the U3O8 detection limit and the appearance of U3O8 as a significant minor phase (as determined by XRD), as functions of time and temperature. These expressions are compared with visual “time-to-powder” data reported for British advanced gas-cooled reactor fuel. The XRD measurements permit detection of U3O8 formation earlier than the visual detection of powder. The more rapid formation of U3O8 on rough than polished UO2 surfaces, and the dependence of U3O8 formation on individual UO2 grain orientation, are discussed.