Quantifying radiation damage to the network structure of radioactive waste-bearing materials

Leslie Dewan, Massachusetts Institute of Technology

The goal of this work is to examine the changes that occur in the network structures of high-level nuclear waste glasses and ceramics as the systems self-irradiate. Specifically, we use molecular dynamics (MD) techniques and topological analysis to quantify system amorphousness as a function of radiation dose. A wide range of ceramics and glasses has been proposed as potential nuclear waste storage media. The two waste-bearing materials under consideration in this study are sodium borosilicate glass and zircon. The majority of the damage in these systems occurs via ballistic collision cascades, which are initiated by energetic fission nuclei and alpha-emitting actinide nuclei. We simulate multiple series of these cascades using the LAMMPS MD software package. MD simulations of collision cascades are initiated ballistically, using 5 keV initial kinetic energy dissipated exclusively elastically. We then use the resulting atomic configurations' network topology as a metric for quantifying the system amorphization. The network topologies of the undamaged and damaged glass structures are determined by enumerating the local cluster atom complement at each atom site. The network structural changes are then related to other observable changes in the system, such as swelling, species segregation, and zones of radiation-induced devitrification. The resulting close correspondence between the ring statistics and these observables shows that topological methods can provide significant insight into structural changes in amorphous systems. This work further indicates that topological description can provide a more revealing assessment of network structural changes during irradiation than can conventional enumerations of atomic displacement, depolymerization indices, or coordination numbers.

Abstract Author(s): Leslie Dewan (MIT), Linn Hobbs (MIT), Jean-Marc Delaye (CEA, France)