Effect of High Gd2O3 Content on the Thermal Conductivity of UO2-Gd2O3 Fuels

Thermal conductivity of UO 2 -Gd 2 O 3 fuels is strongly affected by both temperature and Gd 2 O 3 content, but experimental data for high Gd loadings remain limited. This study evaluated the thermophysical properties of UO 2 -Gd 2 O 3 fuels containing 8-20 wt% Gd 2 O 3 , developed for long-cycle operation and boron-free SMR cores. Specific heat, thermal expansion, and thermal conductivity were measured to examine the effects of high Gd additions. The specific heat values agreed with those calculated by Neumann-Kopp’s law for specific heat capacity within ±7% and increased with Gd 2 O 3 content. Linear thermal expansion, obtained from ΔL/L 0 measurements, showed slightly higher coefficients at higher Gd 2 O 3 contents, and the temperature-dependent densities were used in the thermal conductivity analysis. Thermal conductivity decreased with increasing temperature and Gd 2 O 3 content, and the 20 wt% sample exhibited an ∼18% lower conductivity than pure UO 2 at 1200 °C. The conductivity was normalized to 95% of theoretical density (K 95 ), and the original Hirai model showed increasing deviations at high Gd loadings. To address this, the Hirai model constants D 0 and D 1 were recalibrated using the present data, and the modified Hirai model reproduced the measured K 95 values within ±6% across all compositions and temperatures.