High burnup structure formation in U-Mo fuels

Abstract

This study used electron microscopy techniques and phase field modeling to investigate the mechanisms responsible for developing the high burnup structure in U-Mo fuels. The results show that grain subdivision is initiated by polygonization; however, evidence of dynamic recrystallization is present with increasing fission densities. At 2.5 × 1021 fissions/cm3, LAGB misorientation of 4° was measured near the grain boundary of neighboring grains, which supports polygonization as the leading grain subdivision mechanism. However, with increasing fission densities, the formation of HAGB subgrain clusters are evident, marking the activation of another mechanism − dynamic recrystallization. Previously formed LAGBs transform into new HAGB grains, which is reflected in the increasing number frequency of HAGBs to LAGBs. At the same time, the HAGB subgrain clusters also undergo polygonization, suggesting that polygonization is a continuous mechanism. Based on the simulations performed in this study, polygonization may continue to occur until a mean grain diameter of ∼382 nm is achieved. Based on the results of this work, LAGB to HAGB transformation is associated with a strain-driven mechanism potentially caused by fission gas-induced plastic deformation and is consistent with recrystallization. Polygonization and dynamic recrystallization occurs in tandem with polygonization leading grain subdivision. Based on the fission densities evaluated in this study, authors postulate that the critical fission density at which dynamic recrystallization (DRX) becomes active is between 2.5 × 1021 fissions/cm3 and 3.5 × 1021 fissions/cm3.