Molecular Dynamics Simulation of the Impact of Fission Fragment Energy Deposition on Ion Tracks in Uranium Dioxide
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Molecular Dynamics Simulation of the Impact of Fission Fragment Energy Deposition on Ion Tracks in Uranium Dioxide Jonathan L. Wormald, Ayman I. Hawari Department of Nuclear Engineering, North Carolina State University, Raleigh, NC 27695, USA ABSTRACT In fission based nuclear reactors, uranium dioxide fuel is subject to an intense neutron environment that drives the fission chain reaction. In this process, fission fragments will be produced with an energy reaching 1 MeV/amu. These fragments will initially lose energy through inelastic interactions resulting in excitations of the electronic structure. The excitations subsequently transfer energy to the atomic lattice through electron-phonon (e-p) coupling resulting in a thermal spike which may enhance mobility of fuel atoms. Consequently, the enhanced mobility resulting from fission energy deposition is expected to promote annealing of lattice defects such as ion tracks. Classical molecular dynamics (MD) simulations of uranium dioxide were performed using the LAMMPS code to investigate the effects of fission enhanced mobility on ion tracks formed in the fuel. The MD model was composed of 10×60×60 unit cells, 432000 atoms, and used a Buckingham potential to describe interatomic interactions. A twotemperature model was used to capture the process of fission energy deposition in the electronic subsystem and its transfer to the atomic lattice through e-p coupling. Previous MD simulations demonstrated that fission-enhanced diffusion became more pronounced as the electronic system behavior was varied from metal-like to insulator-like, i.e., increasing the e-p coupling strength. In the present MD simulations, the annealing of an existing ion track (radius nearly 3.0 nm) due to the interaction with 18 keV/nm and 22 keV/nm fission fragments was observed. For a metallike system (weak e-p coupling), it was found that the track persisted with a radius of nearly 3.0 nm. For an insulator-like system (strong e-p coupling), it was found that the track can be reduced significantly in size approaching a radius of 1.4 nm. INTRODUCTION In fission based reactors the deposition of fission energy resulting from the intense neutron field represents the initial stage of heat and defect production within nuclear fuel elements. Fission fragments and other swift heavy ions (ions with energies > 1MeV/amu) lose energy predominantly through electronic stopping, resulting in electronic excitations which subsequently transfer energy to the lattice through electron-phonon coupling. This initial phase of interaction between the fission fragment and the crystal lattice of the fuel material is fundamental to the development of defects and the long term behavior of fuel under reactor conditions. Of particular importance to the further development of the concept of defect evolution in nuclear fuel is the understanding of fission induced defect recovery. Fission enhanced diffusion, where-by the electronic energy loss of a fission fragment in a material induces atomic mobility through the creation of a ther
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