Process and product control of gadolinium bearing oxide powder and pellets for LWR fuels

The utilities interest in extending reactor cycles or in obtaining a higher burnup has led to higher enrichment levels which in turn have called for solutions to the problem of neutron flux homogeneity. This problem was solved in many cases by using an integrated burnable neutron absorber of Gd203 in the UO2 matrix, as a solid solution. Gadolinia is an efficient neutron absorber, and has a burnup rate which matches the U235 depletion. The utilization of UO2 - Gd2O3 pellets started in the beginning of the 70's in the BWR's and a few years later in the PWR's. The use of UO2 - Gd2O3 called for extensive theoretical and experimental research on the following topics: Physical properties of the UO2 - Gd2O3 solid solution: lattice parameter, oxygen to uranium ratio, thermal conductivity, sinterability; Experiments in research reactors, in order to adjust the computer codes used in design and core management; Economic appraisal of the use of UO2 - Gd2O3 pellets; Experience gained in reactor operation and improvement in modelling; Manufacturing and Inspection process development. In fact, for this last topic, the main problem which occurred was the management of the density because of the sinterability behaviour of the solid solution. This problem was solved by using a moisture-controlled atmosphere in the sintering furnaces, or doping additions. This problem is well controlled and UO2 - Gd2O3 pellet manufacturing is now common place. 23 refs, 1 fig., 3 tabs