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.

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.

The presence of CsI in nuclear fuel has long been debated. Its formation significantly decreases volatility, thereby reducing the rate at which iodine and cesium are released from the reactor core during a nuclear accident. A series of samples were investigated by Knudsen Effusion Mass Spectrometry (KEMS) in order to determine whether CsI is present in irradiated nuclear fuel. The examined samples were pure CsI, CsI exposed to gamma radiation, CsI-doped UO2 simulated fuel and irradiated LWR fuel samples. The CsI and CsI-doped samples were examined to establish boundary conditions for the detection of CsI by KEMS. These samples indicated that the presence of CsI in fuel is characterized by three mass spectrometric signals Cs+, I+ and CsI+, with a peak ratio of CsI+ and I+ of 1:0.7. The examinations of irradiated fuels showed none of these characteristics and hence no evidence that CsI is present in irradiated LWR nuclear fuel, at least after a storage period of years.