First-Principles Investigation of Helium Incorporation Effects on the Structural Stability and Electrochemical Performance of Thorium-Based Mixed Oxide Nuclear Fuels.

Helium accumulation is a major contributor to swelling, gas release, and mechanical degradation in oxide nuclear fuels under irradiation. This study employs first-principles density functional theory (DFT) to investigate helium behavior in thorium-based mixed oxide (MOX) fuels. A series of (Th1-xPux)O2 and (Th1-xUx)O2 solid solutions (x = 0, 0.25, 0.5, 0.75, and 1) was constructed, and the corresponding ground-state configurations were determined through total-energy minimization. The effects of 4.167 at.% helium incorporation on structural stability, electronic structure, elastic response, and thermal expansion were evaluated. Helium migration in ThO2, PuO2, and UO2 was further investigated at octahedral interstitial, metal-vacancy, and oxygen-vacancy sites. Positive helium incorporation energies indicated that helium incorporation is energetically unfavorable for all compositions. Vegard-like behavior was preserved for lattice constants and metal-oxygen bond lengths. The 2.06 eV band gap of UO2 disappeared after helium incorporation, whereas band-gap variations in most MOX compositions remained below 0.7 eV. Helium reduced the bulk moduli of (Th0.75U0.25)O2 and UO2 by 3.75% and 7.94%, respectively. Thermal expansion coefficients followed the order αL-UO2 > αL-PuO2 > αL-ThO2, with αL of UO2 nearly doubling. Metal vacancies acted as helium traps, whereas adjacent oxygen vacancies provided the lowest migration barrier of 0.42 eV. These results indicate that increasing ThO2 content improves the resistance of MOX fuels to helium-induced degradation.