Charge compensation mechanisms in U_1-xGd_xO₂ and Th_1-xGd_xO_2-x/2 studied by X-ray Absorption Spectroscopy

The charge compensation mechanisms in U_1-xGd_xO₂ and Th_1-xGd_xO_2-x/2 have been systematically studied using X-ray Absorption Spectroscopy (XAS) upon gradually increasing the Gd content. Gd doped nuclear fuels are widely used for optimizing the fresh core neutronics, yet when Gd³⁺ is substituted into U⁴⁺ or Th⁴⁺ lattice position in UO₂ or ThO₂, respectively, charge must be compensated for charge neutrality. In U_1-xGd_xO₂ the general hypothesis has been that the U⁴⁺ will oxidise to U⁵⁺/U⁶⁺ while in Th_1-xGd_xO_2-x/2 the fixed Th⁴⁺ valence requires generation of O vacancies. Our XAS results for a series of technologically relevant Gd contents (x = 0.04 to 0.14) in U_1-xGd_xO₂ clearly demonstrate that upon increasing the Gd content U⁵⁺ is formed inducing slight increase in the U coordination number and contraction for the U-O distances while the Gd local environment remains virtually unchanged. For the Th_1-xGd_xO_2-x/2 larger Gd fractions were applied (x = 0.10 to 0.34). Nonetheless, both Gd and Th local environments show changes upon increasing the Gd content; the average Gd-O and Th-O distances decrease gradually and the Gd and Th coordination numbers follow the expected trend considering the O vacancy formation to obtain charge neutrality. Finally, comparison to Gd₂O₃ allowed us to propose that one of the Gd L₃-edge XANES resonance features is directly connected to the generation of O vacancies.