Examination of the Irradiation Growth Behavior of Gd203-U02 Fuel Rod in PWR

摘要

As an integral neutron absorber material for PWR fuel in Korea, Gd_2O_3 - UO_2 burnable absorber rods are widely employed in the 17x17 Westinghouse plants and the 16x16 OPR1000 (Optimized Power Reactor 1000 MW). During the poolside examination of Westinghouse 17x17 fuel assemblies, it was observed that most of Gd_20_3 - UO_2 rods have grown more than the UO_2 rods by approximately 5 mm after one cycle of irradiation, 7 mm after two cycles and 8 mm or less after thrée cycles of irradiation. The non - destructive hot cell examinations of some rods taken from the Westinghouse 17x17 fuel assembly irradiated for three cycles confirm that the average growth rates of the Gd_20_3 - UO_2 rods and the UO_2 rods are 0. 57 % and 0. 42 %, respectively. For thé Gd_20_3 - UO_2 rod, cladding diameter measurements in the hot cell show that cladding in axial blanket underwpent greater radial creep - down than that in the Gd_2O_3 - UO_2 pellet stack zone. It is also shown that the length of the Gd_2O_3 - UO_2 pellet stack zone in the Gd_20_3 - UO_2 rod increased significantly compared to the enriched UO_2 pellet zone in the UO_2 rod. Therefore, the higher growth rate of the Gd_20_3 - UO_2 rod is attributed to the large radial cladding creep - down at axial blankets resulting in the axial creep and early increase in length of the Gd_20_3 - UO_2 pellet stack zone contributing to the axial elongation.