T.-T.-G. Vo et alii, Frattura ed Integrità Strutturale, 34 (2015) 237-245; DOI: 10.3221/IGF-ESIS.34.25 237 Focussed on Crack Paths Modelling 3D crack propagation in ageing graphite bricks of Advanced Gas-cooled Reactor power plant Thi-Tuyet-Giang Vo University of Manchester – The Modelling and Simulation Centre, UK. Philippe Martinuzzi, Van-Xuan Tran EDF Energy R&D UK Centre – The Modelling and Simulation Centre, George Begg Building, University of Manchester, M139PL Manchester, UK. Neil McLachlan, Alan Steer EDF Energy Generation A BSTRACT . In this paper, crack propagation in Advanced Gas-cooled Reactor (AGR) graphite bricks with ageing properties is studied using the eXtended Finite Element Method (X-FEM). A parametric study for crack propagation, including the influence of different initial crack shapes and propagation criteria, is conducted. The results obtained in the benchmark study show that the crack paths from X-FEM are similar to the experimental ones. The accuracy of the strain energy release rate computation in a heterogeneous material is also evaluated using a finite difference approach. Planar and non-planar 3D crack growth simulations are presented to demonstrate the robustness and the versatility of the method utilized. Finally, this work contributes to the better understanding of crack propagation behaviour in AGR graphite bricks and so contributes to the extension of the AGR plants’ lifetimes in the UK by reducing uncertainties. K EYWORDS . 3D crack propagation; Advanced Gas-cooled Reactor; Graphite brick; eXtended Finite Element Method; Code_Aster; Strain energy release rate. I NTRODUCTION GR cores are composed of thousands of graphite bricks. After years of service, the heterogeneous irradiation and temperature fields gradually modify both the microstructure and internal stresses of the bricks. This modification may lead to crack initiation and propagation. One goal of the Plant Life EXtension (PLEX) program, developed by EDF Energy, aims at understanding the crack propagation in graphite bricks for the coming years. Part of the research program conducted at EDF Energy R&D UK Centre relies on studying the capabilities to model graphite brick crack propagation using the X-FEM in Code_Aster, an open source finite element software package developed by EDF R&D since 1989. X-FEM’s major advantage is that it represents the crack without explicitly meshing it. This method of crack propagation allows many steps of crack propagation to be performed using a single mesh and enables a more accurate calculation of the stress intensity factors and the strain energy release rate around the crack front. Recent developments in A