We show that a homogeneous single-phase mixed oxide is obtained by blending and sintering commercial UO₂ and nanocrystalline PuO₂ powder; whereas, under the same conditions, conventional PuO₂ powder obtained from oxalate precipitation and composed of platelets, yields two-phase mixed oxide. The use of nanometric PuO₂ results in a larger final grain size.

We compare typical reductive sintering conditions for U, Pu mixed oxides (4 h at 1700 °C in Ar/6 % H2 + 1200 ppm H2O) with lower temperature and mildly oxidative conditions (2 h at 1200 °C in CO/CO2 = 1/9) and report on the resulting microstructures and homogeneity. We show that lower temperature and mildly oxidative conditions, without cover gas change, can give close-to stoichiometric, crack-free MOX pellets with a relative density of ∼ 95 %, and we propose ways to improve the homogenisations of PuO2 and UO2 and increase the grain size.