Effect of AlOOH on the microstructure of UO2 pellets

A mechanism of reducing the open porosity and coarsening the grain size of UO2 by the addition of AlOOH has been investigated. X-ray diffraction experiments showed that oxygen atoms released by the dissociation of AlOOH during sintering moved into the UO2 matrix, resulting in the transformation of UO2 to U4O9 at high concentrations of AlOOH. The excess oxygen atoms raised the grain size due to the increase of uranium vacancies through Schottky equilibrium. It is suggested that the Al-containing components produced by AlOOH dissociation were located at the grain boundaries with small pores. The presence of the Al-containing components at the grain boundaries hampered the movement of pores and led to the decrease of open porosity.