Comparing the impact of thermal stresses and bubble pressure on intergranular fracture in UO 2 using 2D phase field fracture simulations

UO 2 fuel fragmentation and pulverization during loss-of-coolant accidents (LOCAs) is an ongoing safety concern that has gained importance due to recent interest in increasing burnup limits for light water reactor fuel. In this work, we investigate the importance of bubble pressure on fragmentation using 2D phase field fracture simulations of UO 2 polycrystals. The model includes anisotropic single crystal elastic constants, encourages intergranular fracture, and includes bubble pressure and its impact on crack opening. An external stress is applied that is representative of the internal stresses observed in macroscale BISON simulations of LOCA tests. Cracks do not form under the external applied stress in UO 2 polycrystals without porosity. Crack nucleation and propagation do occur under the external applied stress in polycrystals with unpressurized 1 mu m radius intergranular voids, and crack propagation accelerates with increasing number of voids. Crack nucleation and propagation also occur in polycrystals without the external applied stress if the intergranular pores are pressurized, and crack propagation is faster with both pressurized pores and the external applied stress. Our 2D results indicate that bubble pressure may not be necessary to initiate fragmentation in polycrystals with intergranular porosity under LOCA conditions, though this should be verified using 3D simulations with fewer assumptions. (c) 2022 Elsevier B.V. All rights reserved.