Influence of intra-granular void distribution on the grain sub-structure of UO2 pellets after high temperature compression tests

Abstract

The aim of this work is to study the role of intra-granular voids on the macroscopic behavior and the mi- crostructure of uranium dioxide (UO 2 ) for different strain conditions in the high temperature dislocational creep regime. Two batches (B1 and B2) of stoichiometric UO 2 pellets were fabricated by adapted powder metallurgy processes to obtain very close mean grain size and porosity but different fractions of intra-granular voids: they were 2.5 times more numerous in the second batch. The pellets were then compressed at 1773 K mostly in the dislocational regime for different strain levels and strain rates. Large Electron BackScattered Diffraction (EBSD) maps were acquired to quantify the sub-boundaries fraction in each deformed sample (with reliable detection of disorientation lines down to 0.25 °). Accurate-Electron Contrast Channeling Im- age (Accurate-ECCI) experiments were also performed to evidence the arrangement of dislocations in the sub-boundaries and highlight their interaction with intra-granular voids. The fractions of sub-boundaries and their disorientation increased in both batches with increasing strain levels and strain rates. This con- firms that during creep, UO 2 is subject to a dynamic recovery mechanism. Interestingly, for similar de- formation conditions, the pellets from batch B2 crept slower than those from batch B1. They also had a higher fraction of sub-boundaries which were more tortuous and located essentially close to the grain boundaries where the voids clustered. This suggests an influence of intra-granular voids on the creep rate, probably due to a void pinning effect of dislocation sub-boundaries. This effect should be taken into ac- count to optimize the microstructure and mechanical properties of UO 2 nuclear fuel, in order to improve its behavior under irradiation.