Fully anharmonic calculations of the free energy of migration of point defects in UO2 and PuO2

TL;DR

This study demonstrates that anharmonic effects significantly influence defect migration free energies in UO2 and PuO2, challenging harmonic approximation assumptions and improving the accuracy of diffusion modeling in nuclear materials.

Contribution

It introduces a fully anharmonic calculation approach for defect migration free energies using PAFI, comparing different potentials and revealing the importance of anharmonic effects.

Findings

Anharmonic effects cause significant deviations from harmonic estimates.

Migration barriers decrease with increasing temperature.

PuO2 and UO2 have similar jump frequencies despite different enthalpies.

Abstract

Calculating diffusion rates of point defects in materials typically relies on the harmonic approximation to estimate migration free energies. However, anharmonic effects can have a large impact on diffusion properties, and explicitly accounting for them is usually computationally demanding and difficult to achieve in practice. In this work, we investigate the role of anharmonic effects on defect migration in UO2 and PuO2 using the potential of average force integration (PAFI) method. Fully anharmonic migration free energies are computed for several cation and anion defect types, using the Cooper-Rushton-Grimes (CRG) potential and a recently developed machine learning spectral neighbour analysis potential (SNAP) for UO2. Results are systematically compared to harmonic estimates based on attempt frequencies and the Debye approximation. Our results reveal that the validity of the harmonic approximation strongly depends on the defect type and the underlying potential, with significant deviations observed in several cases. In particular, defect migration barriers are found to decrease strongly with increasing temperature (up to 1 eV between 0 and 1200 K), and anharmonic contributions can substantially modify migration entropies and, consequently, diffusion coefficients. Comparing defect migration in UO2 and PuO2 using the CRG potential reveals that PuO2 has lower migration enthalpies at 0~K for all considered defects, but this is compensated by higher attempt frequencies, resulting in similar overall jump frequencies in UO2 and PuO2. These findings provide insight into the limitations of commonly used approximations and highlight the importance of anharmonic effects for predictive modeling of diffusion in nuclear fuels as well as in other classes of materials.