Thermal stability test of UO2-doped pellet manufactured at INB

Abstract

The thermal stability test of UO2-doped pellet manufactured at INB was carried out in order to analyze the resintering behavior. This analysis is fundamental for predicting dimensional behavior during irradiation. INB commonly performs resintering test to qualify its production lots, and the same methodology was applied to UO2-doped pellets. In this preliminary study, three sets of experiments have been made: 1) without any chemical additive (Z test, the standard UO2 pellets - undoped); 2) UO2 pellets doped with 0.1, 0.2 and 0.3 wt% of Al2O3; and 3) 0.1, 0.2 and 0.3 wt% of Nb2O5. The preliminary results showed an increase in sintered density in all resintering experiments. So as to obtain the percentage increase, the theoretical densities (g/cm3 and %TD) were calculated based on the undoped UO2 pellets. All samples increased in a range of 0.27 to 0.32 %TD the out-pile densification during the resintering process. However, the Z(Nb)3 test showed the lowest value of 0.08 %TD, which is not in agreement with the INB specification limits. The sintered density of this test (0.3 wt% niobia) was 96.15% TD. This fact might be related to the competitive mechanism between Kirkendall effect, forming porosity owing to niobium solubilization on UO2 matrix, and densification process as a result of uranium diffusivity. Thus, the densification was only 0.08 %TD in Z(Nb)3 sample. All the other samples were in agreement with INB specification.