Modelling Crack Growth within Graphite Bricks due to Irradiation and Radiolytic Oxidation

During operation, nuclear graphite structures are exposed to gradients of neutron radiation and temperature. They are also prone to radiolytic oxidation, subsequent porosity changes and weight loss. These processes cause significant changes in the material properties of graphite and create complicated stress fields within graphite bricks. Significant efforts have been made to analyse these stresses over recent years. In this paper, the influence of stresses within a typical graphite brick, during operation, on crack growth is studied. A finite element (FE) mechanical constitutive model is implemented in ABAQUS as a user-subroutine (UMAT) in conjunction with a damage model to simulate crack growth within the graphite brick. The damage model is a linear traction separation model and it is applied with XFEM enrichment to model crack initiation and growth. The results indicate that the cracks initiate in the vicinity of the keyway roots where the maximum principal stress is at its peak. Complete failure was not observed from the simulation of the graphite brick under 30 years of irradiation conditions with a maximum brick temperature of 500 °C. Computed stresses and crack lengths were found to be largely dependent on the FE mesh refinement and the results of mesh sensitivity studies are also presented in this paper.