Thermal conductivity variation in uranium dioxide with gadolinia additions

Abstract

By combining experimental observations on Gd doped fuel with a theoretical understanding, the variation in thermal conductivity with Gd concentration and accommodation mechanism has been modelled. Four types of Gd accommodation mechanisms have been studied. In UO 2−x , isolated substitutional Gd 3+ ions are compensated by oxygen vacancies and {2Gd U : V •• O } × defect clusters. In UO 2 , isolated substitutional Gd 3+ ions are compensated by U 5+ ions and {Gd U : U • U } × defect clusters. The results indicate that defect clusters can be considered as less effective phonon scatterers and therefore result in less thermal conductivity degradation. The thermal conductivity predicted for UO 2 with {Gd U : U • U } × defect clusters is in good agreement with experimental data for UO 2 with 5 wt% Gd 2 O 3 . This supports the previous theoretical results that Gd is accommodated through defect clusters {Gd U : U • U } × in UO 2 in the presence of excess oxygen.

Thermal Energy Transport in Oxide Nuclear Fuel

“…An additional complexity for ionic materials is the fact that scattering due to charge compensating defects must also be considered. For example, Figure 31 shows the significant impact that a particular charge compensating mechanism has on the thermal conductivity of UO2 doped with 5 wt% Gd [635]. If Gd impurities are compensated by U 5+ substituted at a U 4+ site the thermal conductivity is significantly higher than if oxygen vacancies (VO) are used.…”

“…For example, Figure 31 shows the significant impact of a particular chargecompensating mechanism on the thermal conductivity of UO 2 doped with 5 wt % Gd. 635 If U 5+ substituted at U 4+ sites is used to compensate for Gd impurities, the thermal conductivity is significantly higher than if oxygen vacancies (V O ) are used, despite the lower concentration of V O needed to chargecompensate 5 wt % Gd. By reducing the number of phonon scattering centers, binding increases the thermal conductivity with respect to a random distribution of defects.…”

“…First, the defect-free lattice thermal conductivity should be determined as a baseline. Subsequently, defects can be introduced to the system by adding new atoms at interstitial sites, removing atoms to create vacancies, or substituting one atom for another to create impurity defects. ,,− The now-defective system is then allowed to equilibrate before the thermal conductivity assessment of the system is repeated, taking care to treat finite-size effects. This process can be repeated with varying defect concentrations.…”

“…Broadly speaking, impurity defects have a lesser impact on the thermal conductivity than interstitials and vacancies, and among the fission products, Xe has the most significant impact on the thermal conductivity of UO 2 . ,, An additional complexity for ionic materials is the fact that scattering due to charge-compensating defects must also be considered. For example, Figure shows the significant impact of a particular charge-compensating mechanism on the thermal conductivity of UO 2 doped with 5 wt % Gd . If U 5+ substituted at U 4+ sites is used to compensate for Gd impurities, the thermal conductivity is significantly higher than if oxygen vacancies (V O ) are used, despite the lower concentration of V O needed to charge-compensate 5 wt % Gd.…”

“…(a) Thermal conductivity of UO 2 containing 5 wt % Gd calculated using NEMD by Qin et al Each data set corresponds to a different charge-compensating mechanism, as shown by (b) the schematic of atomic configurations. Adapted with permission from ref . Copyright 2020 Elsevier B.V.…”