Development of a Thermodynamically Consistent Model for Radiation-Induced Defect Evolution in Structural Materials for Nigerian Nuclear Applications

The long-term structural integrity of nuclear plant components depends on understanding radiation-induced defect evolution in materials exposed to neutron and gamma irradiation over decades-long operational lifetimes. As Nigeria advances toward nuclear power plant deployment, developing indigenous capability to model and predict radiation damage in structural steels and zirconium alloys is a strategic scientific necessity. This study develops a thermodynamically consistent multiscale model for radiation-induced defect evolution in reactor pressure vessel steel and Zircaloy-4 cladding materials, integrating molecular dynamics simulation of primary damage events, kinetic Monte Carlo modelling of defect cluster evolution, and rate theory for long-term dose accumulation. The thermodynamic consistency of the multiscale coupling is enforced through free energy minimisation at each scale interface, a novel contribution extending existing non-equilibrium thermodynamic formulations for irradiated solids. Molecular dynamics simulations of primary knock-on atom cascades in Fe-Cr alloys are performed using LAMMPS with the Bonny-Pasianot embedded atom method potentials. Defect production and recombination rates are computed and passed to kinetic Monte Carlo modules. Rate theory predictions for embrittlement shift temperature and swelling rate are validated against published irradiation experiment data from French and Belgian research reactors. Keywords: radiation damage, defect evolution, molecular dynamics, nuclear materials, thermodynamic model