Temperature dependences of the reduction kinetics and densification behavior of U3O8 pellets in Ar atmosphere

The reduction behavior of U3O8 during sintering in an Ar atmosphere has been investigated in the temperature range between 1200 and 1500 °C. The changes in stoichiometry (O/U ratio) and dimension of the U3O8 pellets were measured by TGA and TMA, respectively. During the reduction at 1400 and 1500 °C, the reduction rate was decreased abruptly at the O/U ratio below about 2.25 while a constant reduction rate was observed at 1200 and 1300 °C. The retardation was accompanied by a significant decrease of the activation energy for the reduction process, possibly due to the shift of the rate determining step. The sintered density, however, was similar irrespective of the sintering temperature. The microstructural observation by SEM demonstrated that the pores did not contribute to the densification due to their coalescence and entrapment in the grain boundaries, triple junctions, and inside the grains, resulting in the porous microstructures.