An innovative nuclear fuel doped with a redox buffer is being developed to control the partial oxygen pressure (pO$_2$) within the reactor and thereby limit stress corrosion cracking of the cladding. The selected redox buffer couple is Nb$_2$O$_5$/NbO$_2$. The doping of the fuel affects its densification and final properties. Therefore, a study is being conducted on the densification behavior of this type of fuel and the influence of dopant concentration. The dopant was mixed with UO$_2$, and the powder was then pressed into pellets, sintered under a reducing atmosphere. The linear shrinkage and oxygen release from the pellet were monitored during sintering. This method highlighted the reduction of niobium oxides during sintering, which causes oxygen release, influencing the densification behavior and microstructure of the doped fuel.