@article{wormald_hawari_2022, title={Modeling fission spikes in nuclear fuel using a multigroup model of electronic energy transport}, volume={566}, ISSN={["1873-4820"]}, DOI={10.1016/j.jnucmat.2022.153797}, abstractNote={In fission based nuclear reactors the fuel is subject to an intense neutron environment that drives the fission chain reaction. Due to this process fission fragments are created with energies reaching 1 MeV/amu that lose energy primarily through inelastic interactions with the electronic structure producing electronic excitations . Subsequently, these excitations thermalize through electron-phonon interactions resulting in the formation of a high temperature thermal spike and associated pressure spike. This process promotes atomic mobility that is expected to evolve lattice defects, including the annealing of latent ion tracks. In this work, a multigroup model for electron energy transport is developed and applied to molecular dynamics simulations in the LAMMPS code to examine fission energy deposition and fission effects in nuclear fuel. This technique utilizes MCNP Monte Carlo electron transport calculations to determine the initial injection of fission energy. To provide a more predictive approach than semi-empirical two-temperature models, the electron-phonon interactions are defined to include multiphonon energy transfer as a function of atomic and electron temperature, and are evaluated from electronic structure calculations using the VASP density functional theory code and PHONON lattice dynamics code. Application of this model to fission energy deposition in uranium dioxide predicts ion track formation and fission enhanced atomic mobility behavior within reasonable agreement of experimental trends. Furthermore, simulations of fission fragment interactions with latent ion tracks demonstrate an annealing effect due to this enhanced mobility.}, journal={JOURNAL OF NUCLEAR MATERIALS}, author={Wormald, J. L. and Hawari, A. I.}, year={2022}, month={Aug} }
@article{wormald_hawari_zerkle_2020, title={Impact of magnetic structure and thermal effects on vibrational excitations and neutron scattering in uranium mononitride}, volume={143}, ISSN={["0306-4549"]}, DOI={10.1016/j.anucene.2020.107447}, abstractNote={Uranium mononitride (UN) is a nuclear fuel material of interest in the design of advanced reactors. Phonon spectra and dispersion relations of UN in its anti-ferromagnetic and paramagnetic structures were calculated using ab initio lattice dynamics. Subsequently, the dynamic structure factors of uranium and nitrogen in UN were generated in the incoherent approximation using the phonon expansion method for inclusion in the US National ENDF/B-VIII.0 database of neutron thermal scattering law evaluations. Phonons from spin polarized density functional theory simulations demonstrate good agreement with inelastic neutron scattering measurements; however, phonons from non-spin polarized simulations deviate from experiment, indicating a previously unexplored impact of magnetism on the vibrational characteristics. An oscillatory behavior due to multi-phonon scattering, previously observed in neutron scattering experiments at low temperatures, was captured in the dynamic structure factor. Moreover, calculated dynamic structure factors at 296 K and 1200 K demonstrate that the oscillatory behavior is present at elevated temperatures.}, journal={ANNALS OF NUCLEAR ENERGY}, author={Wormald, J. L. and Hawari, A. and Zerkle, M. L.}, year={2020}, month={Aug} }
@article{wormald_hawari_2015, title={Examination of the impact of electron-phonon coupling on fission enhanced diffusion in uranium dioxide using classical molecular dynamics}, volume={30}, ISSN={["2044-5326"]}, DOI={10.1557/jmr.2014.405}, number={9}, journal={JOURNAL OF MATERIALS RESEARCH}, author={Wormald, Jonathan L. and Hawari, Ayman I.}, year={2015}, month={May}, pages={1485–1494} }
@inproceedings{wormald_hawari_2014, title={Exploring fission enhanced diffusion of uranium in uranium dioxide using classical molecular dynamics simulations}, DOI={10.1002/9781118889879.ch21}, booktitle={TMS 2014 Supplemental Proceedings}, author={Wormald, J. L. and Hawari, A. I.}, year={2014}, pages={155–162} }