# Atomistic-Object Kinetic Monte Carlo simulations of irradiation damage in tungsten

## TL;DR

This paper introduces a novel atomistic-object kinetic Monte Carlo simulation method for modeling irradiation damage in tungsten, capturing defect evolution with atomistic detail over long timescales.

## Contribution

The development of an off-lattice, atomistic-object KMC code that dynamically constructs defect configurations and incorporates elastic interactions for accurate long-term damage evolution.

## Key findings

- Faster evolution of interstitial clusters compared to molecular dynamics.
- Ability to simulate defect structures over seconds at atomistic detail.
- Insights into defect relaxation processes in tungsten irradiation.

## Abstract

We describe the development of a new object kinetic Monte Carlo code where the elementary defect objects are off-lattice atomistic configurations. Atomic-level transitions are used to transform and translate objects, to split objects and to merge them together. This gradually constructs a database of atomic configurations -- a set of relevant defect objects and their possible events generated on-the-fly. Elastic interactions are handled within objects with empirical potentials at short distances, and between spatially distinct objects using the dipole tensor formalism. The model is shown to evolve mobile interstitial clusters in tungsten faster than an equivalent molecular dynamics simulation, even at elevated temperatures. We apply the model to the evolution of complex defects generated using molecular dynamics simulations of primary radiation damage in tungsten. We show that we can evolve defect structures formed in cascade simulations to experimentally observable timescales of seconds while retaining atomistic detail. We conclude that the first few nanoseconds of simulation following cascade initiation would be better performed using molecular dynamics, as this will capture some of the near-temperature-independent evolution of small highly-mobile interstitial clusters. We also conclude that, for the 20keV PKA cascades annealing simulations considered here, internal relaxations of sessile objects difficult to capture using conventional object KMC with idealised object geometries establish the conditions for long timescale evolution.