Early-stage helium behavior in : First-principles study of diatomic nucleation and vacancy-limited growth

¹ College of Materials Science and Metallurgical Engineering, Guizhou University - Guiyang, 550025, China
² Guizhou Key Laboratory for Mechanical Behavior and Microstructure of Materials - Guiyang, 550025, China
³ National & Local Joint Engineering Laboratory for High-performance Metal Structure Material and Advanced Manufacturing Technology - Guiyang, 550025, China

Received: 6 February 2026
Accepted: 20 April 2026

Abstract

Helium (He), although not the dominant fission gas responsible for macroscopic swelling in nuclear fuels, remains an important transmutation product because its accumulation can affect microstructural stability and mechanical integrity, especially under long-term irradiation and spent-fuel storage conditions. This study employs first-principles calculations to investigate the dissolution, clustering, trapping, and diffusion behavior of He in U₃Si₂ at the atomic scale. Results show that He prefers the octahedral interstitial site and exhibits a strong tendency to form stable diatomic clusters, indicating an early-stage nucleation mechanism. Vacancy defects further act as efficient traps for He and promote cluster growth. However, the growth of He clusters is ultimately limited by the available free volume of the vacancy, supporting a vacancy-limited growth model. Diffusion calculations reveal moderate barriers for interstitial migration (∼1 eV ) but significantly higher barriers (1.5 eV ) for vacancy-assisted pathways, indicating suppressed long-range transport in defective regions. These findings provide a more complete atomistic picture of early-stage He evolution in U₃Si₂ and offer useful guidance for understanding its irradiation response and long-term fuel performance.