Post-irradiation studies on LWR-MOX fuel fabricated by the optimized co-milling process

A LWR-MOX fuel pin cross-section irradiated up to 4.4% local burnup was examined by scanning electron microscopy and X-ray microanalysis. The fuel had been fabricated by the optimized co-milling process (OCOM) of UO2 and PuO2 powders of the final composition of U0.7Pu0.3O2 and blending with UO2 powder down to a typical LWR composition of U0.949Pu0.052O2. The duplex structure of the highly burnt Pu rich agglomerates in the U rich fuel matrix remained. In the UO2 matrix, 5.3% PuO2 were observed at the fuel surface caused by the so-called rim effect. The cross-section averaged EOL PuO2 concentration in the total fuel was 2.9% PuO2. About 9% PuO2 were measured in the dense areas of the high-porous agglomerates. Here, the burnup was calculated from the Nd concentration to be 19%. The cross-section averaged EOL Xe concentration in the total fuel was 0.29% Xe, resulting in a Xe release of 43% of the generated Xe. The oxidation of the inner Zircaloy-4 region attained a thickness up to 20 μm in direct contact with the Pu-rich agglomerates. The outer oxide layer had a thickness of 22 μm.