Sintering and microstructure development of SFR MOX nuclear fuel
Julie Simeon
Abstract
For the manufacture of FNR (Fast Neutron Reactor) MOX (Mixed Oxide) fuel, (U,Pu)O2-x, consolidation of green compacts shaped from the raw powders, is performed by sintering at high temperature. Thus, the detailed study of the sintering is based on coupling of dilatometric tests in controlled reducing atmospheres to multi-scale microstructural characterizations in order to plot sintering maps and to establish predictive sintering models.In this study, three batches of powders were obtained by cryogenic granulation with target Pu/(U+Pu) contents of 15, 26 and 33 %mol. A single sintering trajectory was obtained from the relative density and grain size of each sample (raw and sintered). It is independent not only of the heat cycle (heating rate, sintering temperature and soak time) but also of the plutonium content and the oxygen stoichiometry.A batch with a Pu/(U+Pu) content of 26 %mol was prepared by direct co-milling (based on an industrial method for nuclear fuel fabrication). The trajectory observed for this batch differs from that of the three others.For all batches (granulated and co-milled), calculations of the diffusion coefficients from dilatometry data and the sintering map show that the densification is governed by the self-diffusion of plutonium at the grain boundaries. Furthermore, transmission electron microscopy results, in agreement with results obtained by X-ray diffraction, electron microprobe and Raman spectroscopy, show that grain growth is controlled by grain boundaries.A single sintering trajectory allows the calculation of the densification activation energy through methods exploiting the data collected by dilatometry. Thus, the activation energy of densification is evaluated at 454 ± 64 kJ/mol for all batches resulting from freeze granulation and at 548 ± 33 kJ/mol for the co-milled batch. These values allow the establishment of predictive sintering models that were used to lower the maximum temperature and duration required during sintering without affecting the fuel with the specifications.
Keywords
Microstructure, Sintering, Nuclear Fuel