Simulation by classical molecular dynamics of the influence of radiation effects on the fracture behavior of simplified
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Simulation by classical molecular dynamics of the influence of radiation effects on the fracture behavior of simplified nuclear glasses Le-Hai Kieu1, Jean-Marc Delaye1, Claude Stolz2 1 Service d’Études et Comportement des Matériaux de Conditionnement, DEN/DTCD/SECM, CEAEA Marcoule, BP 17171, 30207 Bagnols sur Cèze, France 2 Laboratoire de Mécanique des Solides, CNRS UMR7649, Ecole Polytechnique, 91128 Palaiseau, France ABSTRACT Classical molecular dynamics simulations were used to compare the fracture behavior of pristine and disordered specimens of a simplified nuclear glass. The disordered specimen is prepared in order to mimic the effects of accumulating displacement cascades. It is characterized by a decreasing Boron coordination and an increasing Na concentration in a modifying role. We observe an enhancement of the plasticity of the disordered glass and a decrease of the elastic limit, resulting in greater fracture toughness. The simulation findings are consistent with experimental results. INTRODUCTION Unrecyclable radioactive waste is destined for storage on a geological time scale in borosilicate glass matrices [1,2,3]. To limit the impact on the environment, it is necessary to ensure that the structural and mechanical properties of the containment matrices are not significantly modified by internal irradiation arising from radioactive decay nor by corrosion by the disposal environment. One aspect of this problem is to ensure the fracture resistance of the containment matrix, because exchanges between the waste package and the surrounding medium occur via the external surfaces of the glass. Good fracture resistance will limit these exchanges by limiting the accessible surface area. Atomistic modeling, and especially classical molecular dynamics, can be used to investigate the behavior of vitreous materials subjected to a variety of stresses. In this study, we have modeled a simple glass representative of the French nuclear containment glass, which we first subjected to displacement cascades before fracturing it to detect the impact of radiation effects on its fracture behavior. THEORY Displacement cascades Using interaction potentials developed a few years ago [4], we simulated several series of 600 eV displacement cascades in “SBN14” glass specimens (67.73% SiO2, 14.23% Na2O, 18.04% B2O3) up to saturation of their structural and macroscopic properties. We simulated a series of 100 displacement cascades in a cell containing 4000 atoms by maintaining the volume constant, i.e. 100 cascades were consecutively accumulated without
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modifying the cell volume, resulting in a gradual pressure increase in the cell [5]. One hundred intermediate configurations integrating between 1 and 100 cascades were thus progressively accumulated. We selected every fifth configuration, which was relaxed in the NPT ensemble by imposing zero pressure to determine a new equilibrium volume. All the equilibrium volumes obtained were used to measure the swelling produced by the accumulated cascades. The swelling follows an exponen
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