Fabrication of UO₂-Gd₂O₃ Fuel Pellets

Abstract

The burnable poison Gadolinium oxide was incorporated into UO 2 in two of the 36 elements of the fuel assembly in the reload fuel of BWR Units I & II of Tarapur Atomic Power Station. This enabled loading of higher quantities of fuel and achieving a more flattened neutron flux distribution over a longer period of time in the nuclear reactor core. The UO 2 -Gd 2 O 3 pellets are made by powder pressing and sintering. In the early days of this author’s experience of the 1970s, the processing of UO 2 -Gd 2 O 3 turned out to be more complex than that of UO 2 alone. The small proportion of Gd 2 O 3 in the powder mixture (1.5%) is to be uniformly distributed in the UO 2 before and after sintering and substitutional solid solution formation must be complete prior to densification. The inadequacy of homogeneity in the powder and pressed pellets leads to severe defects in the sintering process. In this paper, the processing of U 2 -Gd 2 O 3 has been revisited. The defects in the product such as “free gadolinia”, low sintered density and bloating, caused by improper processing, have been brought out. The structural defect chemistry aspects of UO 2 -Gd 2 O 3 and diffusion processes relevant to sintering have also been discussed.