A multidisciplinary approach to optimizing the mechanical characterization and dismantling of nuclear-grade graphite

The large variability of the physical properties related to different types of nuclear-grade graphite makes its characterization for the decommissioning of nuclear power plants extremely problematic and time consuming. To facilitate this process, standardized procedures must be implemented taking into account the complexity of handling irradiated graphite. To this goal, the following study will present the mechanical tests chosen for a clear and easy characterization process. Virgin Atcheson Graphite Ordinary Temperature (AGOT), used as moderator and reflector in the L-54 M research reactor, has been used as a first attempt. Two standard mechanical tests are performed: uni-axial compression and four-point bending. Mechanical anisotropy of graphite was studied and verified, showing roughly a 30% difference between the machining directions parallel and perpendicular to the extrusion direction. A modified three-point bending test has been proposed, which may be used as a four-point bending substitute in case samples with non-standard geometries are needed. A correlation has been presented between the compressive, tensile strength and Young's modulus properties of graphite with Vickers microhardness with R2 respectively of 0.58, 0.53, and 0.91. Non-linear structural simulations of the tests have then been developed as supporting tools for retrieving additional mechanical information, such as Young's modulus. Finally, stress-strain curves have been used as input to develop a finite element method (FEM) which drastically improves the predictive capabilities by reducing the error from 102.2% to 27.6% for AGOT compressive strength, and from 16% to 0% AGOT tensile strength, when compared to in-built graphite models. The potential capabilities of this model to simulate potential graphite handling scenarios in the decommissioning process of nuclear power plants has been demonstrated.