Characterization of plutonium distribution in MIMAS MOX by image analysis

A better understanding of MOX fuel in-pile behaviour requires a very detailed characterization of the Pu distribution in the pellet before and after irradiation. Electron probe microanalysis (EPMA) can be used to determine the distributions of chemical elements with a spatial resolution of 1 μm. This paper describes the development of X-ray microanalysis techniques to produce semi-quantitative ‘maps’ of plutonium concentrations in order to rapidly characterize large areas of the fuel microstructure (1 mm2) with reasonable accuracy. A new segmentation technique based on statistical compatibility is then proposed, so as to finely describe the MIMAS MOX fuel microstructure. Two materials were finely characterized to demonstrate the reliability of this new method. In each case, the results demonstrate the good and reliable accuracy of this new characterization methodology. The analysis method used is currently able to identify three so-called phases (Pu-rich agglomerates, a coating phase and uranium-rich agglomerates), as well as to quantify the plutonium distribution and the plutonium content of these three phases. The impact of the fabrication process on the microstructure can be seen both in the surface distribution variations of the plutonium and in the local plutonium content variations.