Thermal expansion of UO2-Gd2O3 fuel pellets

In recent years, more consideration has been given to the application of UO2-Gd2O3 burnable poison fuel to LWRs in order to improve the core physics and to extend the burnup. It has been known that UO2 forms a single phase cubic fluorite type solid solution with Gd2O3 up to 20 - 30 wt.% above 1300 K. The addition of Gd2O3 to UO2 lattices changes the properties of the fuel pellets. The limited data on the thermal expansion of UO2-Gd2O3 fuel exist, but those are inconsistent. UO2-Gd2O3 fuel pellets were fabricated, and the linear thermal expansion of UO2 and UO2-(5, 8 and 10 wt.%)Gd2O3 fuel pellets was measured with a differential dilatometer over the temperature range of 298 - 1973 K. A sapphire rod of 6 mm diameter and 15.5 mm length was used as the reference material. After the preheating cycle, the measurement was performed in argon atmosphere. The results for UO2 pellets showed excellent agreement with the data in literatures. The linear thermal expansion of UO2-Gd2O3 fuel pellets showed the increase with increasing the Gd2O3 content. Consideration must be given to this excessive expansion in the fuel design of UO2-Gd2O3 pellets. The equations for the linear thermal expansion and density of UO2-Gd2O3 fuel pellets were derived by the method of least squares. (Kako, I.)