Modeling Radiation-Induced Alteration of the Network Structure of Alkali Borosilicate High-level Waste Glass
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Modeling Radiation-Induced Alteration of the Network Structure of Alkali Borosilicate High-level Waste Glass Leslie Dewan1, Linn W. Hobbs1 and Jean-Marc Delaye2 1 2
Massachusetts Institute of Technology, Cambridge, Massachusetts, USA CEA Valrhô-Marcoule, Bagnols-sur-Cèze, France.
ABSTRACT High-level nuclear waste glasses are subject to radiation-induced degradation over very long time scales. In such glasses, bond-breakage and atom displacements occur by both radiolysis (principally from energetic beta-decay electrons) and ballistic mechanisms involving collision cascades initiated by energetic fission nuclei and recoil of alpha-emitting actinide nuclei [1]. This study investigates collision-cascade-induced alteration of the glass network in a simplified sodium borosilicate model nuclear waste glass, using molecular dynamics (MD) codes and efficient topological assessment algorithms. Collision cascades were initiated ballistically (4 keV initial kinetic energy, dissipated elastically) and carried out using MD codes incorporating both two-body Buckingham and three-body Stillinger-Weber potentials verified in the GULP atomistic simulation package. Network topologies of the initial and resulting altered glass structures were determined by enumerating the primitive-ring-based local cluster atom complement at each atom site. The topological description is seen to provide a revealing assessment of network structural changes in the simulated radiation environment that can be potentially related to observable macroscopic changes, such as swelling, viscosity changes, and radiation-induced devitrification. INTRODUCTION Broadly speaking, the aim of this work is to investigate the changes to the network structure of high-level nuclear waste glass that occur as the glass system self-irradiates, and ultimately to determine how these changes correlate to larger-scale phenomena such as microcracking and species segregation. More specifically, this work uses MD simulations and topological analysis to quantify the effects of ballistic-induced displacements on the network structure of sodium borosilicate nuclear waste glass. Network Topology and Topological Algorithms This research uses topological algorithms to assess numerically the damage to the glass network structure. In this method, the glass network is represented as an undirected, unweighted graph [2,3]. Each node of the graph represents an atom, and each vertex represents a bond. Once the glass network has been mapped to this representation, one can readily examine the connectivity of the graph. There are a wide range of methods for examining graph connectivity, because connectivity analysis has long been an important problem in the field of computer science. There are therefore many different data sets one can collect that all provide a valid measure of a graphs connectivity. In this work, we enumerate the “primitive rings” that collectively form a node's “local cluster.” A primitive ring is a marginally-connected structural
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unit: it is a ring that cannot be broken
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