Defect evolution mechanism in U₃Si₂ from molecular dynamics simulations

Abstract

U₃Si₂ is proposed as a promising candidate for accident tolerant fuel (ATF) in recent years. Compared to previous extensively-used UO₂ fuel, U₃Si₂ has the advantage of a higher fissile density and thermal conductivity, which allows for extra coping time in case of accidents. The phase stability and thermophysical properties of U₃Si₂ have been studied previously with the purpose of evaluating its fuel performance. Nevertheless, the critical issue of irradiation damage in the U₃Si₂ fuel is still an underexplored area, and the evolution mechanism of irradiation-induced defects remains unclear. In this work, we perform molecular dynamics (MD) simulations to study defect evolution and defect properties in U₃Si₂. We simulate defect evolution by creating Frenkel defects directly in the system. We find that defects in U₃Si₂ prefer to form single or small defect clusters. With increasing defect concentration, amorphization is observed. To reveal the mechanism governing the observed defect evolution, we have carried out a systematic analysis of defect energetics and defect migration pathways in U₃Si₂. Our results indicate that V_Si and U_i are stable defects after evolution, consistent with their low formation energies. Defect diffusion exhibits an anisotropic nature in U₃Si₂, with the fastest diffusion along the c direction instead of in the a-b plane for both vacancy and interstitial-mediated diffusion. These results are helpful in the understanding of irradiation behavior of U₃Si₂.

Introduction

U₃Si₂ has recently received considerable interest as a candidate for accident tolerant fuel (ATF) in existing and future light-water reactors (LWRs) [1,2]. Compared to the most commonly used commercial UO₂ fuel, U₃Si₂ exhibits a higher fissile density and a higher thermal conductivity [3], both of which are beneficial in enhancing its fuel performance. On one hand, the high U density could reduce the requirement of fuel enrichment. On the other hand, the higher conductivity allows for better heat conduction and results in small temperature gradients across the fuel pellets. It also leads to rapid heat removal during accident scenarios.

To evaluate the fuel performance of U₃Si₂, previous researchers have been focused on its thermo-physical properties, including thermal and electrical conductivities [3,4], phase stabilities [5,6], point defect energies [7,8], grain boundary properties, and surfaces energies [9]. Inside reactors, U₃Si₂ would experience harsh conditions, including high temperatures and intense irradiations from energetic neutrons, resulting in severe radiation damage and corresponding microstructural changes. Besides, fission products, such as Xe gas, would lead to further degradation of fuel performance. Such changes depend critically on the defect evolution mechanism in U₃Si₂. Unfortunately, experimental data about the defect behavior in U₃Si₂ is rather limited; only some low-temperature irradiation test as dispersion fuel plates in research reactors is available [10]. Under neutron/ion irradiation, amorphization is found in U₃Si₂ [[11], [12], [13]]. At room temperature, it is demonstrated that the amount of damage resulting in amorphization is around 0.29–0.38 displacement per atom (dpa). At high temperatures, U₃Si₂ could be stabilized due to temperature-induced recovery. Recent irradiation experiments at a high temperature of 350 °C indicate that the crystal structure of U₃Si₂ is stable even up to a very high dose of 64 dpa [[14], [15], [16]]. Nevertheless, swelling and phase decomposition are observed in irradiated U₃Si₂ [13,14,17].

As a nuclear fuel, the properties of U₃Si₂ may get deteriorated by irradiation-induced microstructural changes due to defect accumulation. The effects have been observed in dispersion U₃Si₂ fuel. However, whether these observations are also operative in pellet fuel is not clear. In order to fully assess the performance of U₃Si₂, it is imperative to understand its defect evolution mechanism. In previous studies, the diffusion properties of point defects have been studied by ab initio calculations based on density functional theory (DFT) [7,8]. An anisotropic diffusion of point defects, which is attributed to the tetragonal crystal structure of U₃Si₂, was revealed by calculating the formation and migration energies of point defects. While such static calculations provide invaluable insight into the basic defect properties, a dynamic description would be highly helpful to predict defect evolution in U₃Si₂ at realistic conditions.

In this work, we employ molecular dynamics (MD) simulations to study the defect evolution mechanisms in the U₃Si₂ system. A newly developed empirical potential based on the modified embedded atom method (MEAM) is used [18]. To model defect evolution, we introduce different concentrations of Frenkel pair defects into the system and monitor the changes of defect configurations as a function of simulation time. We show that defects in U₃Si₂ tend to exist in the form of single defects or small defect clusters. With increasing defect concentration, amorphization of the system is observed. The most stable defects are Si vacancies and U interstitials, which can be understood by their low defect formation energetics. We also show that defect diffusion in U₃Si₂ is anisotropic: diffusion is faster along the c direction compared to that in the a-b plane.